Air processing system

Abstract

A system 100 for processing polluted air, the system comprising: a primary air flow path 101 with a sealing mechanism 141, 142, a first physisorption medium 110 through which the primary airflow passe

Description

BACKGROUND Known ambient air processing systems may operate to reduce the concentration of unwanted, and potentially harmful, chemical compounds in the ambient air surrounding the environmental conditioning units that they process. Often, this reduction in concentration of such chemical compounds is effected by adsorption media (e.g. a chemisorbent or physisorbent medium can be used). Catalytic units for catalysing the degradation of such chemical compounds within ambient air processing systems are also known. In most enclosed environments, the ambient air contains nitrogen oxides and volatile organic compounds. SUMMARY There are challenges with known air processing systems that rely on adsorption media to remove unwanted chemical compounds from the air that they process. Firstly, where a physisorption medium is used to adsorb the unwanted chemical compounds, the adsorption of the chemical compounds is non-permanent under the conditions the environmental conditioning units operate in, meaning that adsorbed compounds can be subsequently desorbed and released back into the ambient air surrounding the unit. This can occur spontaneously and rapidly at certain temperatures, pressures and humidities. Secondly, where a chemisorption medium is used to adsorb the unwanted chemical compounds, the adsorption of the chemical compounds is permanent under the conditions the air processing systems operate in, meaning that the adsorbent loses its adsorption efficiency as the loading of the adsorbent increases, and will eventually stop adsorbing chemical compounds. Therefore, with chemi sorbents, there are two known solutions to this problem: a first solution is to use a greater quantity of adsorbent (so that the maximum loading of the absorbent exceeds the expected loading over the lifetime of the environmental conditioning unit). However, if a greater quantity of adsorbent is used, this can lead to problems such as higher pressure drop across the system, which may be undesirable as it may impair proper functioning of the environmental conditioning unit (e.g. reducing the flowrate of air through the unit and / or increasing the power requirements of airflow generating means). A second solution is that the unit can be configured to allow the adsorbent to be replaceable. However, this solution results in its own problems: replacing the adsorbent may be inconvenient to a user. Furthermore, the user may not know when to replace the adsorbent. Additionally, designing the system to allow access for replacement may result in increased complexity for the system. It is possible to provide a catalyst in the airflow path so that unwanted chemical compounds have their degradation catalysed when passing through the air processing system. However, providing such a catalyst in the airflow path also results in an increase in the pressure drop across the air processing system and the associated problems discussed above. The present inventors have realised that some or all of these problems can be reduced or resolved by use of a system comprising first and second adsorption media. Accordingly, in a first aspect there is provided a system for processing ambient air containing one or more air pollutants, the system comprising: a primary airflow path between an air inlet of the system and an air outlet of the system; a first adsorption medium configured to adsorb the one or more air pollutants by physisorption, through which the primary airflow path passes; a second adsorption medium configured to adsorb one or more air pollutants by chemisorption and / or catalyse the degradation of the air pollutant(s); and an actuatable sealing mechanism arranged for sealing of the primary airflow path; wherein the system is configured to actuate the sealing mechanism during operation to switch the system between: a first state in which the system is configured to allow an airflow to pass along the primary airflow path and through the first adsorption medium; and a second state in which the primary airflow path is sealed by the sealing mechanism by creation of a sealed volume that contains the whole of the first adsorption medium and the whole of the second adsorption medium such that the first and second adsorption media are arranged in fluid connection to thereby allow adsorption of one or more air pollutants desorbed from the first adsorption medium by the second adsorption medium The term “sealed volume” is used herein to define a volume into or out of which no substantial mass transfer (e.g. fluid flow) occurs. By providing an arrangement in which the system comprises a first adsorption medium through which a primary airflow path passes, and a second adsorption medium fluidly connected to the first adsorption medium and arranged to adsorb air pollutants desorbed from the first adsorption medium by chemisorption and / or catalyse the degradation of the air pollutants (e.g. catalyse the oxidation and / or reduction of the air pollutants), it is possible to provide a relatively low flow restriction system for the reduction or removal of one or more air pollutants from the ambient air. Specifically, in this arrangement, the first adsorption medium is able to act as a temporary storage medium for the one or more air pollutants, which can then be permanently captured (by chemisorption) or destroyed (by catalysts) by the second adsorption medium, when the one or more pollutants desorb from the first adsorption medium. Because the first adsorption medium is only required to act as a temporary storage medium, it can therefore have a lower thickness (i.e. lower volume per unit area of the primary airflow path) in comparison to conventional arrangements where the primary airflow path flows through all adsorption media in the system, thereby allowing for reduced flow restriction in systems according to the invention in comparison to convention systems. Furthermore, by providing an arrangement in which the whole of the first adsorption medium and the whole of the second adsorption medium are both contained within a single sealed volume (e.g. a sealed chamber) together in the second state, the system can be mechanically simple whilst still allowing for effective reduction or removal of one or more air pollutants from the ambient air. In particular, this arrangement allows the first and second adsorption media to remain in fluid connection in the second operational state and obviates the need for provision of e.g. one or more valves along the fluid connection path between the first and second adsorption media. The term ‘chemisorption’ as used herein is taken to mean adsorption that involves a chemical reaction between the adsorbent’s surface and the adsorbate; new chemical bonds are generated at the adsorbent surface. The bond between the adsorbate and adsorbent in chemisorption may be ionic or covalent in nature. In contrast, the term ‘physisorption’ as used herein is taken to mean adsorption that leaves the chemical species of the adsorbate and adsorbent surface intact and chemically unaltered. The energetic threshold separating the binding energy of "physisorption" from that of "chemisorption" is approximately 0.5 eV per adsorbed species. The term ‘degradation’ as used herein to define that the compound to be degraded undergoes a change of chemical species to one or more different compounds. The term ‘degradation’ may encompass oxidation and / or reduction of the compound. It will be noted that the term ‘adsorbent medium’ is used herein to describe both media which adsorb compounds by physisorption or chemisorption, as well as media which are configured to catalyse the degradation of one or more air pollutants contained in the airflow. Catalysis (e.g heterogeneous catalysis) involves a cycle of molecular adsorption, reaction, and desorption occurring at the catalyst surface. Accordingly, in the present disclosure, catalytic media are referred to more generally as ‘adsorption media’ in view of the (at least temporary) adsorption that occurs during the process of heterogeneous catalysis. The second adsorption medium may or may not be fluidly connected to the first adsorption medium when the system is in the first state. However, in some arrangements, the second adsorption medium remains in fluid connection with the first adsorption medium in both the first and second states, e.g. they may be arranged to be in fluid connection in all operational states of the system. In such arrangements, the system may be configured such that no valving arrangement is located between the first and second adsorption medium on the primary airflow path. The system may find particular utility as part of an environmental conditioning unit. The term “environmental conditioning unit” is used herein to refer to a system for conditioning ambient air. Conditioning ambient air may include e.g. altering the composition of the ambient air e.g. by removing or reducing the concentration of one or more impurities or pollutants present in the air. Ambient air can alternatively be referred to as atmospheric air and is distinguished from e.g. exhaust gases from a vehicle. The system / environmental conditioning unit may be configured to not be suitable for conditioning exhaust gases from a vehicle i.e. not configured for use in automotive applications. The first adsorption medium may comprise e.g. activated carbon, a metal organic framework, a porous polymeric material, or a zeolite. The first adsorption medium may be configured not to adsorb the air pollutants by chemisorption and not to catalyse the degradation of air pollutants (e.g. may be configured to adsorb the air pollutants solely by physisorption). Accordingly, the first adsorption medium is only able to adsorb compounds via physisorption, which is readily reversible (i.e. the compounds can be desorbed). The second adsorption medium may be a chemisorption medium. The chemisorption medium may comprise one or more of one or more hindered amines; one or more acid impregnations on a porous support (for adsorption of basic gases, e.g. ammonia); one or more base impregnations on a porous support (for adsorption of acidic gases, e.g. nitrous oxides); one or more functionalised polymers, e.g. molecularly imprinted polymers; one or more functionalised metal organic frameworks, or potassium carbonate. Alternatively, the second adsorption medium may be a catalyst, for example, a manganese oxide-based catalyst such as cryptomelane,. The manganese-oxide based catalyst may be doped with one or more transition metals selected from the group consisting of Ti, Cr, Fe, Co, Fe, Zn, Ce, and Ni. The manganese oxide-based catalyst may have the formula: Mnz / y O2 wherein: J and L are transition group metals other than manganese; J and L are different transition group metals to each other; x is greater than zero; y is greater than zero; and x + y is less than 1. J and L may be transition group metals selected from the group consisting of: Ti, Cr, Fe, Co, Zn, Ce and Ni. The manganese oxide-based catalyst may have the formula: Mnz / 1_z O2 wherein: J is a transition group metal other than manganese; x is greater than zero and less than one. / may be a transition group metal selected from the group consisting of: Ti, Cr, Fe, Co, Zn, Ce and Ni. The second adsorption medium may be disposed on a substrate. The substrate may be, for example, a polymer substrate, a metallic substrate, a foam substrate, a paper substrate, an activated carbon substrate, a zeolite substrate, a metal organic framework substrate, or a ceramic substrate. The one or more air pollutants may be selected from a group consisting of: nitrous oxides; nitrogen radicals; volatile organic compounds; volatile inorganic compounds; carbon monoxide; formaldehyde; ozone; ozone amines; sulphur compounds; thiols; and chlorinated hydrocarbons. The first adsorption medium may have an adsorption capacity of may have a specific adsorption capacity of from 1 to 10 mg of VOCs per gram of adsorbent material. For example, the specific adsorption capacity may be less than or equal to 8 mg / g and / or greater than or equal to 2 mg / g. In some embodiments, the specific adsorption capacity may be at least 5 mg of VOCs per gram of adsorbent material. The first adsorbent, and / or the second adsorbent (where it is configured to adsorb VOCs by physisorption), may have a total adsorption capacity of 20 mg or more, 30 mg or more, or 40 mg or more. In some embodiments, the total adsorption capacity may suitably be around 40 mg of VOCs. It has been found that providing an adsorbent material with specific or total adsorption capacity in the ranges described can allow for suitable performance of the exhaust treatment unit (in particular, can allow for effective use of the unit for a suitable time period without the need to perform any refresh or regeneration of the adsorbent in that time period). Where the second adsorption medium is configured to adsorb the air pollutants by chemisorption, the second adsorption medium may have a specific adsorption capacity of from 2 to 100 mg of VOCs per gram of chemisorbent material. For example, the specific adsorption capacity may be less than or equal to 80 mg / g and / or greater than or equal to 4 mg / g. In some embodiments, the specific adsorption capacity may be at least 25 mg of VOCs per gram of adsorbent material. The second adsorbent (where it is configured to adsorb VOCs by chemisorption) may have a total adsorption capacity of 200 mg or more, 300 mg or more, or 400 mg or more. It has been found that providing a chemisorbent material with specific or total adsorption capacity in the ranges described can allow for suitable performance of the exhaust treatment unit (in particular, it can allow for effective use of the unit for a suitable time period without the need to replace the chemisorbent second adsorption medium in that time period). Part, or all, of the second adsorption medium may be disposed off of the primary airflow path. Preferably all of the second adsorption medium is disposed off of the primary airflow path. Accordingly, air flowing along the primary airflow path does not flow through the second adsorption medium and thus the pressure drop across the air processing system along the primary airflow path is not (substantially) altered (e.g. is not increased) by the presence of the second adsorption medium within the system. The primary airflow path may pass through the first adsorption medium and not pass through the second adsorption medium. This allows substantially all of the air flowing along the primary airflow path to interact with the first adsorption medium and have air pollutants removed from that air, whilst also avoiding an increase in the pressure drop across the system when the second adsorption medium is present. The system may be configured to be switchable between: a first operating mode in which the system is in the first state and an airflow is generated along the primary airflow path; and a second operating mode in which the system is in the second state and the system is configured to promote desorption of one or more air pollutants from the first adsorption medium, and / or to promote adsorption of said air pollutants onto the second adsorption medium and / or increase the rate of catalysis of the degradation of said air pollutants by the second adsorption medium within the single sealed volume. Accordingly, the system can remove air pollutants from the airflow using the first adsorption medium as a temporary storage medium and subsequently transfer those adsorbed pollutants to the second adsorption medium when the system is in a state (the second state) in which pollutants desorbed from the first adsorption medium cannot be released into the ambient air surrounding the system. The system may be configured to switch between the first operating mode and the second operating mode according to a predetermined timing schedule. For example, the system may be configured to switch from the first operating mode to the second operating mode between once an hour and once every 48 hours, preferably approximately once every 24 hours. The system may operate in the second mode for a predetermined time period (e.g. 10 minutes, 30 minutes, 1 hour or any other suitable time period) before switching back to the first mode. Accordingly, the adsorption capacity of the first adsorption medium can be regenerated at a regular time interval. Additionally, or alternatively, the system may be configured to switch between the first operating mode and the second operating mode in response to an operational instruction input by a user - for example, the system may be configured to switch from the first mode to the second mode in response to an instruction to stop driving an airflow along the primary airflow path. The environmental conditioning unit may be further configured to switch from the second operating mode to the first operating mode in response to an instruction to start driving an airflow along the primary airflow path. Accordingly, the adsorption capacity of the first adsorption medium can be regenerated at a time where it is convenient based on the instructed operation of the system. The primary airflow path may be at least partly defined by a conduit through (part of) the system, the conduit comprising one or more sidewalls. The first adsorption medium may occupy substantially the entire internal cross-sectional area defined by the conduit in a plane perpendicular to the longitudinal axis of the conduit. Accordingly, substantially all of the air flowing along the primary airflow path will flow through the first adsorption medium, increasing the interaction of the airflow with the first adsorption medium and thus the fraction of air pollutants in the airflow that are adsorbed onto the first adsorption medium. The system may further comprise a secondary airflow path, wherein: the secondary airflow path passes through the first adsorption medium; and the second adsorption medium is disposed downstream of the first adsorption medium on the secondary airflow path. Accordingly, the system may be configured such that an airflow can be driven along the secondary airflow path to promote the transfer of air pollutants adsorbed onto the first adsorption medium onto the second adsorption medium when the system is in the second state and operating under the second operating mode. Where the primary airflow path is at least partially defined by a conduit, a sidewall of the conduit, or a portion thereof, may be defined by a surface of the second adsorption medium. Multiple sidewalls of the conduit may be defined by a surface of the second adsorption medium. The system may further comprise first and second sealing members, the first sealing member being provided upstream of the first adsorption medium along the primary airflow path relative to the airflow along the primary airflow path, the second sealing member being provided downstream of the first adsorption medium along the primary airflow path. The first and second sealing members may be actuatable to together seal the primary flow path to provide the single sealed volume. This is a simple manner in which to provide a system with the ability to form a single sealed volume containing both the first and second adsorption media. The first and second sealing members may be provided in any convenient manner. In some embodiments, the first and / or second sealing members may constitute a valve operable to either allow or prevent flow along the primary airflow path. The system may comprise a mechanical actuator configured to actuate each sealing member, for example, a single mechanical actuator may be provided to actuate both the first and second sealing members simultaneously such that the first and second sealing members both open and close together. It is also contemplated than in other arrangements, the system may comprise separate mechanical actuators configured to actuate respective sealing members. A surface defining the sealed volume may be elastically deformable such that the capacity of the sealed volume is variable with elastic deformation of the surface. Such an arrangement can allow the pressure within the sealed volume to remain (approximately) constant even as the temperature and / or the number of moles of gaseous molecules in the sealed volume varies. The first and / or second sealing members may be elastically deformable in order to provide this volume adjustment. The system may be configured such that the volume of the sealed volume may be variable by at least ±20% by elastic deformation of the surface. This percentage volume change may allow the system to maintain the pressure in the sealed volume at a substantially constant pressure when the temperature of air within the sealed volume increases from a first predetermined temperature (e.g. 20°C) to a second predetermined temperature (e.g. 80°C). The system may be configured such that the volume of the sealed volume is variable by at least ±10%, at least ±15%, at least ±25%, at least ±30%, or at least ±35% by elastic deformation of the surface. The elastically deformable surface of the sealed volume may be formed of a material which is able to provide the desired change in the volume (e.g. ±20%) of the sealed volume without exceeding the fatigue limit of the material. The material may be e.g. an elastomeric material such as a rubber or silicone elastomer. The system may further comprise a movable member disposed in, or defining a surface of, the sealed volume. The position of the movable member may be controllable such as to control the capacity of the sealed volume. This can allow the pressure within the sealed volume to be controlled by moving the movable member. The movable member may be the elastically deformable sealing member and / or the conduit. The movable member may be a piston. The movable member may be a movable wall. The movable member may be positioned upstream of the first adsorption medium within the sealed volume relative to the primary airflow path. The system may further comprise an actuator configured to move the movable member. The system may comprise a first movable member provided upstream of the first adsorption medium and a second movable member provided downstream of the first adsorption medium, relative to the primary airflow path. This arrangement can allow flow to be induced through the first adsorption medium. However, in other arrangements, a single movable member may be provided at any suitable location to allow for control of the capacity of the sealed volume during use of the system. The system may be configured to promote desorption of air pollutants adsorbed onto the first adsorption medium by moving the movable member to increase the volume of, and thereby reduce the pressure in, the sealed volume. This can reduce the time required for the transfer of air pollutants adsorbed onto the first adsorption medium to the second adsorption medium. The system may further comprise a pressure relief valve configured to provide pressure relief to the sealed volume. This arrangement may prevent the sealed volume from becoming over pressurised and rupturing during use. The pressure relief valve may be e.g. a spring-loaded pressure relief valve, although other arrangements are contemplated. Where a pressure-relief vale is provided, the system may further comprise a third adsorption medium disposed over an inlet of the pressure relief valve, the third adsorption medium configured to: adsorb the air pollutants by physisorption; adsorb the air pollutants by chemisorption; and / or catalyse the degradation of the air pollutants; for example, the third adsorption medium may have the same composition as the first adsorption medium or the second adsorption medium. This may prevent one or more air pollutants from being re-released from the system via the pressure-relief valve. Where the third adsorption medium is configured to adsorb the air pollutants by chemisorption or catalyse the degradation of the air pollutants, the third adsorption medium may be heated. This can increase the rate of chemisorption or catalysis. The system may further comprise at least one concentration sensor disposed in the sealed volume, the concentration sensor being configured to measure the concentration of one or more of the air pollutants within the sealed volume when the system is in the second state. This concentration measurement can be used to estimate or otherwise determine the concentration of those air pollutants in the airflow passing through the system when it is in the first state and operating under the first operating mode. The at least one concentration sensor may include an oxidising gas sensor, a reducing gas sensor, a metal oxide sensor, an ultrasonic CO2 sensor, or any combination thereof. In some embodiments, the concentration sensor may comprise a plurality of sub-sensors. Each of said sub-sensors may be an oxidising gas sensor, a reducing gas sensor, a metal oxide sensor or an ultrasonic CO2 sensor. The concentration sensor may be an electronic nose (also referred to as an ‘e-nose’). Accordingly, the concentration and / or presence of certain air pollutants in the airflow can be established. Where the concentration comprises a plurality of sub-sensors / ‘e-nose’ and the system comprises a movable member that is controllable such as to control the volume of the sealed volume, the system may be configured to vary the pressure in the sealed volume and make a determination of the identity of one of more of the air pollutants based on an analysis of the sensor response to the pressure variation. Accordingly, the identity and the concentration of individual air pollutant species can be established simultaneously. The system may further comprise an air flowrate sensor configured to measure the flowrate of ambient air along the primary airflow path. The system may be configured such that, on switching to the second state, the system estimates the concentration of the one or more air pollutants measured by the concentration sensor in the ambient air based on: the measured concentration within the sealed volume, the measured flowrate and the time elapsed since the system was last switched into the first state from the second state. Accordingly, it may be possible to obtain an accurate estimate of the concentration of the air pollutant(s) in the ambient air without directly measuring the concentration of the air pollutant(s) in the ambient air, and instead measuring the concentration of said air pollutant(s) in the sealed volume. In other words, the system may be configured to indirectly measure the concentration of the air pollutant(s) in the ambient air. The concentration value measured by the sensor in the sealed volume will be higher than in the concentration in the ambient air and thus a wider range of sensors may be suitable for the measurement by this indirect measurement method, in comparison with known systems which attempt to directly measure the concentration of the air pollutant(s) in the ambient air, where sensors having much high sensitivity may be required. The system may be configured to promote desorption of air pollutants adsorbed onto the first adsorption medium by heating the first adsorption medium. This can reduce the time required for the transfer of air pollutants adsorbed onto the first adsorption medium to the second adsorption medium. The system may be configured to heat the first adsorption medium to a temperature less than or equal to 120°C. By way of example, the system may be configured to heat the first adsorption medium to a temperature less than or equal to 110°C, less than or equal to 100°C, less than or equal to 90°, less than or equal to 80°C, less than or equal to 70°C, or less than or equal to 60 °C. The system may be configured to heat the first adsorption medium to a temperature greater than or equal to 50°C. By way of example, the system may be configured to heat the first adsorption medium to a temperature greater than or equal to 60°C, greater than or equal to 70°, greater than or equal to 80°C, greater than or equal to 90°C, greater than or equal to 100°C, or greater than or equal to 110°C. The system may be configured to heat the first adsorption medium when the system is in the second state. Accordingly, the desorption of air pollutants can be promoted when the first and second adsorption media are in fluid connection within the single sealed volume, facilitating the transfer of the desorbed air pollutants onto the second adsorption medium. The system may be configured not to heat the first adsorption medium when the system is in the first state. In this manner, it is possible to reduce desorption of air pollutants from the first adsorption medium into an airflow along the primary airflow path that then returns to the ambient air surrounding the system, thereby reducing re-emission of previously-adsorbed air pollutants into the ambient air. Heating of the first adsorption medium may be effected through direct heating of the medium, for example, by a (resistive) heating element disposed within the first adsorption medium, and / or by alternative methods such as heating of the air within the sealed volume. The system may be configured to heat the second adsorption medium. Where the second adsorption medium is configured to adsorb air pollutants by chemisorption, heating the second adsorption medium may promote the chemisorption of air pollutants onto the second adsorption medium. Where the second adsorption medium is configured to catalyse the degradation of air pollutants, heating the second adsorption medium may increase the turnover frequency of the catalyst, i.e. may increase the rate of catalysis of the degradation of air pollutants by the second adsorption medium. The system may be configured to heat the second adsorption medium simultaneously with heating the first adsorption medium, such that desorption of air pollutants from the first adsorption medium is promoted simultaneously with increasing the rate of adsorption / catalysis of air pollutants at the second adsorption medium. The system may be configured to heat the second adsorption medium when the system is in the second state. Accordingly, the adsorption / catalysis of air pollutants at the second adsorption medium can be promoted when the first and second adsorption media are in fluid connection within a single sealed volume, facilitating the transfer of the desorbed air pollutants onto the second adsorption medium. The system may be configured not to heat the second adsorption medium when the system is in the first state. This can avoid the indirect heating of the first adsorption medium by conduction and / or convection when the system is in the first state, and thus, it is possible to reduce desorption of air pollutants from the first adsorption medium into an airflow along the primary airflow path that then returns to the ambient air surrounding the system, thereby reducing re-emission of previously-adsorbed air pollutants into the ambient air. Heating of the second adsorption medium may be effected through direct heating of the medium, for example, by a (resistive) heating element disposed within the first adsorption medium, and / or by alternative methods such as heating of the air within the sealed volume. The system may be configured to heat the second adsorption medium to a temperature greater than or equal to 50°C and less than or equal to 300°C, preferably to a temperature greater than or equal to 60°C and less than or equal to 120°C. The system may be configured to heat the second adsorption medium to a temperature greater than or equal to 70°C, greater than or equal to 80°C, greater than or equal to 90°C, greater than or equal to 100°C, greater than or equal to 110°C, greater than or equal to 120°C, greater than or equal to 130°C, greater than or equal to 140°C, greater than or equal to 150°C, greater than or equal to 160°C, greater than or equal to 170°C, greater than or equal to 180°C, greater than or equal to 190°C, or greater than or equal to 200°C. The system may be configured to heat the second adsorption medium to a temperature less than or equal to 70°C, less than or equal to 80°C, less than or equal to 90°C, less than or equal to 100°C, less than or equal to 110°C, less than or equal to 120°C, less than or equal to 130°C, less than or equal to 140°C, less than or equal to 150°C, less than or equal to 160°C, less than or equal to 170°C, less than or equal to 180°C, less than or equal to 190°C, or less than or equal to 200°C. The first adsorption medium and second adsorption medium may be in direct physical contact, for example, the first and second adsorption media may be provided on the same substrate and / or arranged in a layered structure adjacent one another. Alternatively, the first adsorption medium and second adsorption medium may be closely spaced (e.g. arranged with a spacing of no more than 1 cm between the media, such as a spacing of 5 mm or less, or 1 mm or less between the media). By arranging the first and second adsorption media to be in direct contact, or to be positioned close together, the time required for the transfer of air pollutants desorbed from the first adsorption medium onto the second adsorption medium can be reduced, by reducing the distance the air pollutants need to diffuse and / or advect between the first and second adsorption media. In a second aspect there is provided an environmental conditioning unit comprising: a body comprising an air intake and an air exhaust; airflow generating means configured to generate an airflow between the air intake and the air exhaust; and an air processing system according to the first aspect provided downstream of the air intake and upstream of the air exhaust. Any one or more of the optional features set out in relation to the first aspect may be equally applied to the environmental conditioning unit of the second aspect. The airflow generating means may be a motor-driven impeller. The environmental conditioning unit may be a fan assembly. The environmental conditioning unit may further comprise a particulate filter, e.g. a HEPA filter, positioned upstream of the first adsorption medium on the primary airflow path. Accordingly, the environmental conditioning unit can remove both solid and gaseous air pollutants from the air flowing therethrough. The environmental conditioning unit may be configured to draw ambient air from the surroundings into the unit, pass that air through the air processing system along the primary airflow path, and expel that air back to the surroundings. The environmental conditioning unit may be configured to reduce the concentration of one or more air pollutants in the airflow passing through the environmental conditioning unit by an amount of up to 100%. That is, the concentration of one of more air pollutants in the airflow may be up to 100% lower when measured downstream of the environmental conditioning unit as compared with a measurement taken upstream of the environmental conditioning unit (i.e. may be reduced to a concentration of substantially Oppm). . The environmental conditioning unit may be configured to reduce the concentration of one or more air pollutants in the airflow passing through the environmental conditioning unit by an amount greater than or equal to 50%, greater than or equal to 55%, greater than or equal to 60%, greater than or equal to 65%, greater than or equal to 70%, greater than or equal to 75%, or greater than or equal to 80%. The environmental conditioning unit may be configured to reduce the concentration of one or more air pollutants in the airflow passing through the exhaust line by an amount less than or equal to 90%, less than or equal to 85%, less than or equal to 80%, less than or equal to 75%, less than or equal to 70%, less than or equal to 65%, or less than or equal to 60%. The environmental conditioning unit being configured to reduce the concentration of one or more air pollutants in the airflow passing through the environmental conditioning unit by an amount between 60% and 80% may provide a suitable balance of the ease of manufacture of the environmental conditioning unit, the pressure drop across the system, the mass of first and / or second adsorbent, and the efficiency of the system in removing air pollutants in the airflow through the environmental conditioning unit. In a third aspect there is provided a method of removing one or more air pollutants from an airflow using a system according to the first aspect or an environmental conditioning unit according to the second aspect, wherein the method comprises the steps of actuating the sealing mechanism to place the system into the first state in which the system is configured to allow an airflow to pass along the primary airflow path and through the first adsorption medium; generating an airflow along the primary airflow path when the system is in the first state, such that air pollutants in the airflow are adsorbed onto the first adsorption medium; actuating the sealing mechanism to place the system into the second state in which the primary airflow path is sealed by the sealing mechanism by creation of a sealed volume that contains the whole of the first adsorption medium and the whole of the second adsorption medium such that the first and second adsorption media are arranged in fluid connection, and allowing adsorption of one or more air pollutants desorbed from the first adsorption medium by the second adsorption medium. Any one or more of the optional features set out in relation to the first aspect or second aspect may be equally applied to the method of the third aspect. The method may further comprise the steps of: operating the system in a first operating mode in which is the system is in the first state and an airflow is generated along the primary airflow path; and operating the system in a second operating mode in which the system is in the second state and the desorption of one or more air pollutants from the first adsorption medium and / or the adsorption of said pollutants onto the second adsorption within the single sealed volume is promoted. Accordingly the method can remove air pollutants from the airflow using the first adsorption medium as a temporary storage medium and subsequently transfer those adsorbed pollutants to the second adsorption medium when the system is in a state in which pollutants desorbed from the first adsorption medium cannot be released into the ambient air surrounding the system. The method may comprise switching between operating the system in the first operating mode and operating the second operating mode based on a predetermined timing schedule. Accordingly, the adsorption capacity of the first adsorption medium can be regenerated at a regular time interval. Additionally, or alternatively, the method may comprise: receiving an instruction to stop driving an airflow along the primary airflow path; and switching from operating the system in the first operating mode to operating the system in the second operating mode in response to receiving said instruction. Equivalently, the method may comprise: receiving an instruction to start driving an airflow along the primary airflow path; and switching from operating the system in the second operating mode to operating the system in the first operating mode in response to receiving said instruction. Accordingly, the adsorption capacity of the first adsorption medium can be regenerated at a time where it is convenient based on the instructed operation of the system. The method may further comprise moving the movable member(s) (where present) such as to vary the volume of the sealed volume. Following actuating the sealing mechanism to place the system into the second state, the method may further comprise moving the movable member (where present) to increase the volume of the sealed volume, thereby decreasing the pressure within the sealed volume, and subsequently moving the movable member to decrease the volume of the sealed volume, thereby increasing the pressure within the sealed volume. This sequence of decreasing and then increasing the pressure of the sealed volume can promote the desorption of air pollutants from the first adsorption medium and subsequent adsorption of said air pollutants onto the second adsorption medium. The method may further comprise the steps of measuring the flowrate of air along the primary airflow path when an airflow along the primary airflow path is generated with the system in the first state; measuring the time elapsed since the system was last switched into the first state from the second state until the system is switched into the second state; measuring the concentration of one or more air pollutants within the sealed volume when the system is in the second state; and estimating the concentration of said air pollutant(s) in the ambient air based on the flowrate, the elapsed time and the concentration of said air pollutant(s) measured within the sealed volume. The method may further comprise heating the first adsorption medium. The step of heating the first adsorption medium may be conducted (only) when the system is in the second state. The heating of the first adsorption medium may involve heating the first adsorption medium to within one or more of the optional temperature ranges for the first adsorption medium described above with respect to the first aspect. The method may further comprise heating the second adsorption medium. The step of heating the second adsorption medium may be conducted (only) when the system is in the second state. The steps of heating the first adsorption medium and the second adsorption medium may be conducted contemporaneously and / or simultaneously. The heating of the second adsorption medium may involve heating the second adsorption medium to within one or more of the optional temperature ranges for the second adsorption medium described above with respect to the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa schematic of an environmental conditioning unit; Figure 2 is a schematic of a first air processing system; Figure 3 is a schematic of a second air processing system; Figure 4 is a schematic of a third air processing system; Figure 5 is a schematic of a fourth air processing system; Figure 6 is a schematic of a fifth air processing system; Figure 7A is a schematic of a sixth air processing system in a first state; Figure 7B is a schematic of the sixth air processing system in a second state; Figure 8 is a flowchart for a method of estimating the concentration of air pollutants in an airflow using an air processing system or an environmental conditioning unit; and Figure 9 is a flowchart for a method of removing one or more air pollutants from an airflow using an air processing system or an environmental conditioning unit. DETAILED DESCRIPTION Figure 1 is a schematic of an environmental conditioning unit 1. The environmental conditioning unit 1 comprises a body 2 having an air inlet 3 through which air can be drawn into the unit 1 and an air exhaust 6 through which air can be expelled from the unit 1. In order to generate an airflow through the body 2, the unit 1 comprises an airflow generating means 5 such as a motor driven impeller (although other airflow generating means 5 are contemplated). The environmental conditioning unit 1 further comprises an air processing system 100 positioned downstream of the air inlet 3 and upstream of the air exhaust 6. The air processing system 100 is configured to remove one or more air pollutants (e.g. nitrous oxides; nitrogen radicals; volatile organic compounds; volatile inorganic compounds; carbon monoxide; formaldehyde; ozone; ozone amines; sulphur compounds; thiols; and chlorinated hydrocarbons) from the airflow passing therethrough. In the case of the environmental conditioning unit 1 in Figure 1, the air processing system 100 is provided downstream of the airflow generating means 5 such that air is blown through the air processing system 100 by the airflow generating means 5. However, an alternative configuration (not shown in Figure 1) is for the air processing system 100 to be provided upstream of the airflow generating means 5 such that the airflow generating means 5 draws air through the air processing system 100. The environmental conditioning unit 1 further comprises an (optional) particle filter 4 (e.g. a HEPA filter). The filter is provided upstream of the airflow generating means 5 such that the airflow generating means 5 draws air through the filter 4. The filter removes solid particles from the airflow drawn into the body 2 via the air inlet 3 and prevents them from reaching the airflow generating means 5 and air processing system 100. Where the air processing system 100 is positioned upstream of the airflow generating means 5 and the unit 1 comprises a filter 4, the filter 4 is typically still positioned upstream of the air processing system 100. The environmental conditioning unit 1 is intended for use with ambient air (i.e. approximately room temperature and pressure), such that ambient air from the surroundings is drawn into the unit 1 and expelled back to the surroundings. In some configurations, the environmental conditioning unit may be configured to heat and / or cool the ambient air as it passes through the unit, before it is expelled back to the surroundings. In some configurations, the environmental conditioning unit may be configured to humidify and / or dehumidify the ambient air as it passes through the unit, before it is expelled back to the surroundings. Figure 2 is a schematic of a first air processing system 100. The first air processing system 100 comprises a primary airflow path 101 between an air inlet and an air outlet of the system. The primary airflow path 101 in the first air processing system 100 is defined by a conduit 160 that extends through the system 100, the conduit having one or more sidewalls that constrain the airflow through the system 100. The primary airflow path 101 is substantially parallel to the longitudinal axis of the conduit 160. Disposed along the primary airflow path 101 is a first adsorption medium 110. The configuration of the first air processing system 100 means that the primary airflow path 101 passes through the first adsorption medium 110 and, in the case of the first system 100, substantially all of the air flowing through the conduit 160 passes through the first adsorption medium 110 because the first adsorption medium 110 occupies the entire internal cross-sectional area defined by the conduit 160 in a plane perpendicular to the longitudinal axis of the conduit 160. The first adsorption medium 110 is configured to adsorb one or more air pollutants in the air flowing through the system 100 by physisorption, and preferably without adsorbing the air pollutants by chemisorption. Typical adsorbents used as the first adsorption medium 110 are activated carbon and zeolites. Because the interactions through which air pollutants are physisorbed onto the first adsorption medium 110 have a low binding energy (< 0.5 eV), the storage of air pollutants on the first adsorption medium 110 is temporary in nature: the desorption of air pollutants can be promoted by, for example, heating the first adsorption medium 110 to temperatures of only, for example, 80°C. The system 100 further comprises a second adsorption medium 120 configured to adsorb one or more air pollutants by chemisorption and / or catalyse the degradation of the air pollutant(s). All of the second adsorption medium 120 is disposed off of the primary airflow path 101 such that air flowing along the primary airflow path 101 does not flow through the second adsorption medium 120. In the case of the first air processing system 100, the second adsorption medium 120 extends around the primary airflow path 101 in a circumferential manner such that a portion of the sidewalls of the conduit 160 is defined by a surface of the second adsorption medium 120 - although as Fig. 1 is a schematic cross-section, this single continuous medium appears visually as two separate ‘blocks’ of the second adsorption media in this figure. In this manner, the presence of the second adsorption medium 120 does not (substantially) alter the pressure drop across the system 100. Nevertheless, the first adsorption medium 110 and second adsorption medium 120 are concentrically arranged, and furthermore are arranged to be in direct physical contact. It can also be appreciated that a portion of the second adsorption medium 120 is positioned upstream of the first adsorption medium 110 and another portion of the second adsorption medium 120 is positioned downstream of the first adsorption medium 110. Because the second adsorption medium 120 is configured to adsorb one or more air pollutants by chemisorption and / or catalyse the degradation of the air pollutant(s), the air pollutants that interact with the second adsorption medium 120 are permanently (at least at the temperatures of < 200°C at which the system 100 is operated) adsorbed onto the second adsorption medium 120 and / or have their degradation (e.g. oxidation or reduction) catalysed, respectively. Accordingly, by the airflow along the primary airflow path 101 passing through the first adsorption medium 110, the air pollutants in the airflow can be temporarily stored on the first adsorption medium 110 and subsequently those air pollutants can be desorbed from the first adsorption medium 110 and air pollutants desorbed from the first adsorption medium 110 can be adsorbed onto the second adsorption medium 120 for permanent storage and / or to have their degradation catalysed (and the resulting reaction products subsequently desorbed). In order to avoid the air pollutants that are desorbed from the first adsorption medium 110 to be subsequently adsorbed onto the second adsorption medium from being re-emitted to the surroundings by the airflow passing through the system 100, the system 100 further comprises an actuatable sealing mechanism arranged for sealing of the primary airflow path 101. In the case of the first system 100, the actuatable sealing mechanism comprises a first sealing member 141 positioned upstream of the first adsorption medium 110 (and upstream of the second adsorption member 120) along the primary airflow path 101 and a second sealing member 142 positioned downstream of the first adsorption medium 110 (and downstream of the second adsorption member 120) along the primary airflow path 101. Actuation of the sealing mechanism is intended to seal and unseal the primary airflow path 101 such as to switch the system 100 between: a first state in which the system 100 is configured to allow an airflow to pass along the primary airflow path 101 and through the first adsorption medium 110 (the first state shown by the dashed line positions of the first sealing member 141 and second sealing member 142 in Figure 1); and a second state in which the primary airflow path 101 is sealed by the sealing mechanism by creation of a sealed volume that contains the whole of the first adsorption medium 110 and the whole of the second adsorption medium 120 such that the first and second adsorption media 110, 120 are arranged in fluid connection to thereby allow adsorption of one or more air pollutants desorbed from the first adsorption medium 110 by the second adsorption medium 120 (the second state shown by the solid line positions of the first sealing member 141 and second sealing member 142 in Figure 1). The sealed volume is defined by the movable members, and by an outer perimeter of the conduit & the second adsorption medium 120. Accordingly, a mechanically simple mechanism for preventing the re-emission of air pollutants into the surroundings from the system 100 during the transfer of air pollutants from the first adsorption medium 110 to the second adsorption medium 120 is provided. Corresponding to the first states and second states of the system 100 are first and second operating modes of the system 100. The system 100 is configured to be switchable between: a first operating mode in which the system 100 is in the first state and an airflow is generated along the primary airflow path 101; and a second operating mode in which the system 100 is in the second state and the system 100 is configured to promote desorption of one or more air pollutants from the first adsorption medium 110, and to promote adsorption of said air pollutants onto the second adsorption medium 120 and / or increase the rate of catalysis of the degradation of said air pollutants by the second adsorption medium within the single sealed volume. One manner in which the desorption of air pollutants from the first adsorption medium 110 can be promoted by the system 100 is for the temperature of the first adsorption medium 110 to be increased. Typically, in the first operating mode, the temperature of the first adsorption medium 110 is approximately ambient air temperature (e.g. between 15°C and 30°C). In the second operating mode, the first adsorption medium 110 may be heated to a temperature greater than or equal to 50°C and less than or equal to 300°C, typically in the range 60°C to 120°C, in order to promote desorption of air pollutants adsorbed thereon. A convenient manner in which to heat the first adsorption medium 110 is via resistive heating (e.g. direct resistive heating of the physisorbent or resistive heating of a substrate that the physisorbent is supported by). An additional manner in which the desorption of air pollutants from the first adsorption medium 110 can be promoted by the system 100 is by the manipulation of the pressure within the sealed volume; this is discussed further in relation to Figure 5. Where the second adsorption medium 120 is a chemisorption medium, one manner in which the adsorption of air pollutants onto the second adsorption medium 120 can be promoted by the system 100 is for the temperature of the second adsorption medium 120 to be increased. Because there is an activation energy requirement for chemisorption of the air pollutants onto the second adsorption medium 120 (unlike physisorption, where there is no activation energy for adsorption to occur), increasing the temperature of the second adsorption medium 120 increases the rate of chemisorption. Typically, in the first operating mode, the temperature of the second adsorption medium 120 is approximately ambient air temperature (e.g. between 15°C and 30°C). In the second operating mode, the second adsorption medium 120 may be heated to a temperature greater than or equal to 50°C and less than or equal to 300°C, typically in the range 60°C to 120°C, in order to promote chemisorption of air pollutants onto the second adsorption medium 120. An additional manner in which the chemisorption of air pollutants onto the second adsorption medium 120 can be promoted by the system is by the manipulation of the pressure within the sealed volume; this is discussed further in relation to Figure 5. Where the second adsorption medium 120 is a catalyst, one manner in which the rate of catalysis of the degradation of said air pollutants and thus removal of air pollutants desorbed from the first adsorption medium 110 from the air within the sealed volume can be increased by the system 100 is for the temperature of the second adsorption medium 120 to be increased. Increasing the temperature of the catalyst increases the turnover frequency of the catalyst. Typically, in the first operating mode, the temperature of the catalyst is approximately ambient air temperature (e.g. between 15°C and 30°C) and the turnover frequency will be low. In the second operating mode, the second adsorption medium 120 may be heated to a temperature greater than or equal to 50°C and less than or equal to 300°C, typically in the range 60°C to 120°C, in order to increase the turnover frequency. The composition of the catalyst may be such that the maximum turnover frequency at a given concentration of air pollutants is within the range of temperatures that the catalyst is heated to in the second operating mode. A convenient manner in which to heat the second adsorption medium 120 is via resistive heating (e.g. direct resistive heating of the chemi sorb ent / cataly st or resistive heating of a substrate that the chemi sorb ent / cataly st is supported by). Figure 3 is a schematic of a second air processing system 200. The second air processing system 200 is similar in structure and operation to the first air processing system 100. Where features and components of the second system 200 are the same or similar to the first system 100, equivalent reference signs (e.g. 110 and 210, 120 and 220) are used. A detailed description of the features and components of the second system 200 that are the same as those of the first system 100 is omitted, and reference can be made back to the description of Figure 2. The second air processing system 200 may be incorporated into an environmental conditioning unit 1 such as that illustrated in Figure 1. The second system 200 primarily differs from the first system 100 in the positioning of the second adsorption medium 220 with respect to the first adsorption medium 210 and the second sealing member 242. In the second system 200, the second adsorption medium 220 is not in direct physical contact with the first adsorption medium 210. Rather, the second adsorption medium 220 is still provided off of the primary airflow path 201, but is also positioned along a secondary airflow path 202 passing through the first adsorption medium 210 and reaches the second adsorption medium 220 downstream of the first adsorption medium 210. Specifically, the schematic in Figure 3 has the second adsorption medium 220 disposed in a recess in a sidewall on one side of the conduit 260 that defines the primary airflow path 201. Accordingly, air pollutants desorbed from the first adsorption medium 210 can be transferred to the second adsorption medium 220 by flowing along the secondary airflow path 202. The system is configured such that an airflow can be generated along the secondary airflow path 202 when the system is in the second state and operating under the second operating mode. For example, the system may rely on the heating of the first adsorption medium to promote desorption of air pollutants therefrom to drive an airflow along the secondary flow path 202 as a result of thermal gradients within the single sealed volume. Alternatively, the system may comprise a movable member (not shown in Figure 2) disposed upstream of the first adsorption medium 210 that can be used to alter the pressure gradients within the sealed volume - this is discussed further in relation to Figure 5. Moreover, in the second system 200, the arrangement of the second adsorption medium 220 and the second sealing member 242 adjacent thereto provides the system 200 with an additional feature over the first system 100. Figure 2 illustrates how, when the second sealing member 242 is in the position corresponding to the first state (i.e. the dashed line position of the second sealing member 242), the sealing member 242 extends across an opening to the recess containing the second adsorption medium 220 such that air flowing along the primary airflow path 201 when the system 200 is operating in the first operating mode cannot be diverted along the secondary airflow path 202 and interact with the second adsorption medium 220 (i.e. the second adsorption medium 220 is sealed from the air flowing along the primary airflow path in the first operating mode and is not fluidly connected to the first adsorption member 210). However, when the system 200 is switched into the second state, the second sealing member 242 moves such that the first and second adsorption media 210, 220 are in fluid communication and contained within a single sealed volume (i.e. the solid line position of the second sealing member 242). This arrangement can allow the pressure drop across the system 200 to be reduced in comparison to an arrangement where the positioning of the second sealing member 242 is as shown in Figure 3 but the second sealing member 242 does not act to fluidly isolate the first and second adsorption media 210, 220 when the system 200 is in the first state. Figure 4 is a schematic of a third air processing system 300. The third air processing system 300 is similar in structure and operation to the first air processing system 100. Where features and components of the third system 300 are the same or similar to the first system 100, equivalent reference signs (e.g. 110 and 310, 120 and 320) are used. A detailed description of the features and components of the third system 300 that are the same as those of the first system 100 is omitted, and reference can be made back to the description of Figure 2. The third air processing system 300 may be incorporated into an environmental conditioning unit 1 such as that illustrated in Figure 1. The third system 300 primarily differs from the first system 100 in the provision of an elastically deformable portion of the surface defining the sealed volume in the third system 300. Specifically, the third system 300 comprises a balloon 371. The balloon 371 is positioned upstream of the first adsorption medium 310 on a sidewall of the conduit 360. The balloon 371 is able to expand and contract elastically (i.e. elastically deform) such that when the system 300 is in the second state, the volume of the sealed volume is variable. In this manner, the pressure within the sealed volume can remain (approximately) constant even as the temperature and / or the number of moles of gaseous molecules in the sealed volume varies. Where the system 300 is configured to promote the desorption of air pollutants from the first adsorption medium 310 by heating the first adsorption medium 310 (as discussed in relation to Figure 2), the surface defining the sealed volume having an elastically deformable portion (e.g. the balloon 371) can be useful in avoiding overpressure of the sealed volume. Overpressure is particularly undesirable, because rupturing of the sealed volume due to overpressure when operating the system 300 in the second operating mode could lead to a substantial quantity of air pollutants suddenly being re-released into the surroundings of the system 300. By way of example, even disregarding the effect of desorption of air pollutants from the first adsorption medium 310 on the pressure inside the sealed volume, heating the first adsorption medium 310 and the air within the sealed volume from 20°C to 80°C would result in approximately a 20% increase in the pressure of the sealed volume if the volume thereof was not variable. If the effect of desorbing air pollutants from the first adsorption medium 310 were accounted for, the resulting increase in pressure would be greater still. As well as for the purpose of avoiding overpressure of the sealed volume, maintaining a lower pressure within the sealed volume may be desirable from the perspective of promoting the net desorption of air pollutants from the first adsorption medium 310, because the rate of physisorption of air pollutants increases with increasing partial pressure of air pollutants. The balloon 371 in Figure 4 comprises a plurality of folds such that the ability of the balloon 371 to elastic deform is provided by both the stretching of the balloon material and the expansion of the folds. Figure 5 is a schematic of a fourth air processing system 400. The fourth air processing system 400 is similar in structure and operation to the third air processing system 300. Where features and components of the fourth system 400 are the same or similar to the third system 300, equivalent reference signs (e.g. 310 and 410, 320 and 420) are used. A detailed description of the features and components of the fourth system 300 that are the same as those of the first and third systems 100, 300 is omitted, and reference can be made back to the description of Figures 2 and 4. The fourth air processing system 400 may be incorporated into an environmental conditioning unit 1 such as that illustrated in Figure 1. The fourth system 400 primarily differs from the third system 300 in relation to the manner in which the volume of the sealed volume can vary. The fourth system 400 comprises a movable member defining a surface of the sealed volume, wherein the position of the movable member can be controlled such as to control the volume of the sealed volume. This is in contrast to the third system 300 where the balloon 371 allows the volume of the sealed volume to vary but does not provide for the active control of the volume of the sealed volume. Specifically, the fourth system 400 comprises a pair of such movable members, in the form of pistons: a first piston 472a disposed upstream of the first adsorption medium 410 and a second piston 472b disposed downstream of the second adsorption medium 410. Both the pistons 472a, 472b are provided on a sidewall of the conduit 460. Each of the pistons 472a, 472b is associated with an actuator (not shown) that is operable to control the position of that piston in order to vary the volume of the sealed volume. In addition to the pistons 472a, 472b being usable to maintain the pressure within the sealed volume in a similar manner to the balloon 371 of the third system 300, the pistons 472a, 472b can also be used to control the volume of the sealed volume. This facilitates the controlling of the pressure of the sealed volume in order to promote desorption of air pollutants from the first adsorption medium 410 and adsorption of said air pollutants onto the second adsorption medium 420. By way of example, when the system 400 is in the second state, one or both of the pistons 472a, 472b can be actively moved by their respective actuator such as to increase the volume of the sealed volume and thereby decrease the pressure in the sealed volume (assuming that the number of moles of gas remains constant). This reduction in pressure in turn promotes the net desorption of air pollutants from the first adsorption medium 410 because the rate of adsorption of air pollutants onto the first adsorption medium is positively correlated to the partial pressure of the air pollutants in the air within the sealed volume. Similarly, when the system 400 is in the second state and after air pollutants have been desorbed from the first adsorption medium 410 (i.e. when the concentration of air pollutants in the sealed volume is high), one or both of the pistons 472a, 472b can be actively moved by their respective actuator such as to decrease the volume of the sealed volume and thereby increase the pressure in the sealed volume (assuming that the number of moles of gas remains constant). This increase in pressure in turn promotes the net adsorption of air pollutants onto the second adsorption medium 420 because the rate of adsorption of air pollutants onto the second adsorption medium 420 (whether it comprises a catalyst and / or a chemisorbent) is positively correlated to the partial pressure of the air pollutants in the air within the sealed volume. Increasing the pressure within the sealed volume in order to promote the adsorption of air pollutants onto the second adsorption medium 420 is particularly effective when coupled with the heating of the first adsorption medium 410. In addition to increasing the adsorption rate onto the second adsorption medium 420, increasing the pressure within the sealed volume also promotes the re-adsorption of air pollutants previously desorbed from the first adsorption medium 410 back onto the first adsorption medium 410; however, by also heating the first adsorption medium 410 (which promotes the desorption of air pollutants therefrom) when increasing the pressure within the sealed volume, it is possible to counteract the effect of the increased pressure on the first adsorption medium 410 and thereby increase the adsorption rate onto the second adsorption medium 420 whilst reducing or maintaining (or only increasing to a lesser extent) the adsorption rate onto the first adsorption medium 410. Accordingly, a sequence of operations to promote the transfer of air pollutants adsorbed onto the first adsorption medium 410 to the second adsorption medium 420 when operating the fourth system 400 in the second operating mode may comprise: (i) moving the piston(s) 472 to increase the volume of the sealed volume, (ii) heating the first adsorption medium 410, and (iii) moving the piston(s) 472 to decrease the volume of the sealed volume. However, it can be appreciated that step (ii) above may be conducted prior to step (i) and / or may be conducted concurrently with step (i) and / or step (ii). Another possible use of movable members such as the pistons 472a, 472b is for inducing an airflow through the first adsorption medium 410. Specifically, because (a) the first piston 472a is provided upstream of the first adsorption medium 410 and the second piston 472b is provided downstream of the first adsorption medium 410 and (b) the first adsorption medium 410 occupies the entire cross section of the conduit 460, it is possible to induce an airflow through the first adsorption medium 410 by moving the first piston 472a to increase the pressure in the portion of the sealed volume upstream of the first adsorption medium 410 and / or moving the second piston 472b to decrease the pressure in the portion of the sealed volume downstream of the first adsorption medium 410, thereby creating a pressure differential across the first adsorption medium 410 that induces a flow therethrough. The ability to induce an airflow through the first adsorption medium 410 when the system 400 is in the second state (i.e. when an airflow cannot be provided along the primary airflow path 401 due to the sealing mechanism) is useful in (a) inducing mixing within the sealed volume to promote further desorption of air pollutants from the first adsorption medium 410 and adsorption of air pollutants onto the second adsorption medium 420; and (b) providing a flowrate along a secondary airflow path in systems configured like the second system 200 illustrated in Figure 3. Figure 6 is a schematic of a fifth air processing system 500. The fifth air processing system 500 is similar in structure and operation to the third air processing system 300. Where features and components of the fifth system 500 are the same or similar to the third system 300, equivalent reference signs (e.g. 310 and 510, 320 and 520) are used. A detailed description of the features and components of the fifth system 300 that are the same as those of the first and third systems 100, 300 is omitted, and reference can be made back to the description of Figures 2 and 4. The fifth air processing system 500 may be incorporated into an environmental conditioning unit 1 such as that illustrated in Figure 1. The fifth system 500 primarily differs from the third system 300 in relation to the manner in which increases in pressure of the sealed volume can be managed. As discussed in relation to Figure 4, when the system is in the second state, the pressure of the sealed volume may increase due to increases in temperature and / or the number of moles of gas in the sealed volume. However, rather than having a surface defining the sealed volume that is elastically deformable as in the third system 300, the fifth system employs a pressure relief valve (PRV) 575. An inlet side of the PRV 575 is connected to the sealed volume when the system 500 is in the second state and an outlet side of the PRV 575 feeds to the surroundings of the system 500. Accordingly, if the pressure within the sealed volume reaches a predetermined pressure (the set pressure of the PRV 575), the PRV 575 opens to vent a portion of the air contained within the sealed volume until the pressure within the sealed volume falls below the set pressure. Provided that the set pressure of the PRV 575 is below than the design pressure of the system 500 / sealed volume, overpressure of the sealed volume can be avoided. However, it is not desirable to vent air from the sealed volume that contains a high concentration of air pollutants into the surroundings through the PRV 575. Accordingly, the fifth system 500 further comprises a third adsorption medium 530 disposed over an inlet of the PRV 575 such that air flowing through the PRV 575 from the sealed volume passes through the third adsorption medium 530 Depending on the composition of the third adsorption medium 530, it is configured to adsorb the air pollutants in the air passing therethrough by physisorption, adsorb the air pollutants in the air passing therethrough by chemisorption, and / or catalyse the degradation of the air pollutants in the air passing therethrough. Accordingly, the air released to the surroundings through the PRV 575 contains a lower concentration of air pollutants than would be the case if the third adsorption medium 530 were not present. Figures 7A and 7B are schematics of a sixth air processing system 600 in first and second states respectively. The sixth air processing system 600 is similar in structure and operation to the first air processing system 100. Where features and components of the sixth system 600 are the same or similar to the first system 100, equivalent reference signs (e.g. 110 and 610, 120 and 620) are used. A detailed description of the features and components of the sixth system 600 that are the same as those of the first system 100 is omitted, and reference can be made back to the description of Figure 2. The sixth air processing system 600 may be incorporated into an environmental conditioning unit 1 such as that illustrated in Figure 1. The sixth system 600 primarily differs from the first system 100 in that details of how the first sealing member 641 and second sealing member 642 are opened and closed are provided. Specifically, the sixth system 600 comprises an actuator 680 configured to actuate the sealing mechanism during operation to switch the system 600 between the first state and the second state. Figure 7A illustrates the sixth system 600 in the first state and Figure 7B illustrates the sixth system 600 in the second state. The actuator 680 includes a stepper motor 686 connected to a rod 685. The stepper motor 686 is configured to move the rod laterally between a first position illustrated in Figure 7A and a second position illustrated in Figure 7B. The rod 685 extends between the lateral positions of the first sealing member 641 and second sealing member 642. Extending between the rod 685 and the first and second sealing members 641, 642 are respective first and second linkages 681, 682. The first and second linkages 681, 682 are fixed to the rod 685 such that lateral movements of the rod between the first position and second position result in corresponding lateral movements of the linkages 681, 682. The first and second sealing members are pivotably connected to the conduit 660 to allow them to pivot between positions in which the first airflow path is open and closed. Additionally, the first and second linkages 681, 682 are slidably and pivotably connected to the first and second sealing member 641, 642, respectively, at a position offset from the pivoted connections of the sealing members 641, 642 to the conduit 660. Consequently, lateral movements of the linkages 681, 682 result in the pivoting of the first and second sealing members 641, 642 relative to the conduit 660, thereby allowing the sealing members, 641, 642 to move between open and closed positions as the rod 685 moves laterally between the first position and second position. Specifically, the pivoted connection between each linkage 681, 682 and its respective sealing member 641, 642 is able to slide relative to the respective linkage 681, 682. Whilst the actuator 680 in Figures 7A and 7B operates through lateral movement of the rod 685 by the stepper motor 686 in a direction substantially parallel to the conduit 660, it can be appreciated that an alternative configuration in which the rod 685 is moved vertically (in the orientation of figures 7A and 7B, i.e. towards and away from the conduit 660) by the stepper motor in order to effect the movement of the sealing members 641, 642 could be used. In this alternative configuration, the linkages 681, 682 would be pivotably, but not slidably) connected to the sealing members and the linkages 681, 682 would be slidably and / or pivotably connected to the rod 685 such that the linkages 681, 682 could either slide laterally or rotate relative to the rod 685. It can be appreciated that the actuator 680 in the sixth system 600 may be incorporated into any of the first to fifth systems 100 - 500 discussed above. Figure 8 is a flowchart for a method of estimating the concentration of air pollutants in an airflow using an air processing system such as one of the first, second, third, fourth, or fifth systems 100 - 500 described above. Although not illustrated in Figures 2-7, the above described systems 100 - 500 may comprise a concentration sensor disposed within the system such that it is contained within the sealed volume when the system is in the second state. The concentration sensor is configured to measure the concentration of one or more of the air pollutants within the system when the system is in the second state. Depending on the type(s) of air pollutant(s) that it is desired to measure the concentration of, one or more different sensors can be disposed within the sealed volume. The concentration sensor may be an oxidising gas sensor, a reducing gas sensor, a metal oxide sensor, an ultrasonic CO2 sensor or may comprise a plurality of sub-sensors. Each of said sub-sensors may be an oxidising gas sensor, a reducing gas sensor, a metal oxide sensor or an ultrasonic CO2 sensor. By way of example, it is possible to measure the concentration of Formaldehyde using an electrochemical sensor, the concentration of other VOCs and nitrogen oxides can be measured using a metal oxide sensor, and carbon dioxide concentration can be measured using a photoacoustic sensor. By using the method illustrated in Figure 8, it is possible to estimate the concentration of air pollutants in the air flowing through the system when it is being operated in the first operating mode by measuring the concentration of air pollutants within the sealed volume when the system is being operated in the second operating mode. Because the concentration of air pollutants within the sealed volume when the system is being operated in the second operating mode is orders of magnitude higher than the concentration of air pollutants in the air flowing through the system in the first operating mode, a wider range of sensors are suitable for the measurement (because the required sensor precision is lower). In particular, the higher concentration of air pollutants to be measured by using the method of Figure 8 allows a wider range of e-nose sensor modules to be used; typically the concentration of air pollutants within ambient air is too low for detection by most e-noses. The method of Figure 8 commences with switching the system to operate in the first operating mode (step S710). At this point, a timer is started (step S720) so that the duration of time that the system has been operating in the first operating mode since it was last operating in the second operating mode can be determined at a later stage. When the system is operating in the first operating mode, an airflow is driven along the primary airflow path. When the system is operating in the first operating mode, the volumetric flowrate of air through the system along the primary airflow path is recorded (step S730) so that, in combination with the timing, it is possible to estimate the volume of air that has passed along the primary airflow path and thus through the first adsorption medium. The volumetric flowrate may be measured, for example, by a velocity flow meter or a differential pressure meter. Alternatively, it may be estimated based on the speed and / or power consumption of the airflow generating means within the environmental conditioning unit. Additionally, the recording of the flowrate may comprise a single measurement of the flowrate when the system is operating in the first operating mode (for example, if the flowrate is constant when in the first operating mode), or may comprise recording the flowrate at regular intervals or continuously (for example, if the flowrate is known to vary - intentionally or unintentionally - when in the first operating mode). It can be appreciated that more frequent recording of the flowrate will provide a more accurate estimate of the air volume that has passed along the primary airflow path. Subsequently, the system is switched to operate in the second operating mode (step S740), at which point the flow of air along the primary airflow path ceases. Accordingly, the timer is then stopped (step S750) so that the duration of time that the system has been operating in the first operating mode since it was last operating in the second operating mode is recorded. When the system is operating in the second operating mode, the desorption of one or more air pollutants from the first adsorption medium occurs, the desorbed air pollutants entering the sealed volume that the one or more concentration sensors are disposed in. As the desorption of air pollutants occurs, the concentration of these air pollutants in the air within the sealed volume is measured by the sensor(s) (step S760). Based on the recorded flowrate(s), the recorded time the system was operating in the first operating state and the concentration measurements of one or more air pollutants within the sealed volume, the concentration of said one or more air pollutants within the airflow in the first operating mode can be estimated. The volume of air within the sealed volume is known based on the geometry of the system, and accordingly, assuming the air in the sealed volume is well-mixed (i.e. the concentration of air pollutants within the air contained in the sealed volume is uniform), the concentration measurements can be converted into an estimate of the amount (e.g. number of moles) of air pollutants adsorbed onto the first adsorption medium in the time the system was operating in the first operating state (assuming the loading of the first adsorption medium at step S710 was zero). Typically, the concentration measurement taken is the maximum concentration measurement recorded by the sensor whilst the system is operating in the second operating condition. Based on the recorded flowrate(s) and time, the volume of air that has passed along the primary airflow path between steps S710 and S740 can be estimated. Accordingly, an estimate of the mean concentration of said air pollutant(s) within the airflow along the primary airflow path can be obtained by dividing the amount of air pollutants adsorbed onto the first adsorption medium by the volume or air that has passed along the primary airflow path (step S770). The above-described method for estimating the concentration of one or more air pollutants within the airflow in the first operating state may be particularly effective when the system it is implemented in is like that of the fourth system 400 described above in relation to Figure 5. Two synergistic effects arise from combining the fourth system 400 with the method of Figure 8: Firstly, the ability of the fourth system 400 to use the pistons 472a, 472b to alter the volume of the sealed volume allows the process of desorption of the air pollutants from the first adsorption medium 410 to be decoupled from the process of adsorption of said desorbed air pollutants onto the second adsorption medium 420. This is because reducing the pressure of the sealed volume (by increasing its volume) using the pistons 472a, 472b promotes desorption from the first adsorption medium 410 and hinders adsorption onto the second adsorption medium 420. Consequently, the estimated amount of air pollutants adsorbed onto the first adsorption medium based on the maximum concentration measurement from the sensor(s) within the sealed volume will be closer to true amount of air pollutants adsorbed onto the first adsorption medium than would otherwise be the case, since the instances of adsorption of air pollutants onto the second adsorption medium 420 before all of the air pollutants have desorbed from the first adsorption medium 410 is reduced. Secondly, where the fourth system 400 comprises a sensor array such as an e-nose, the ability of the fourth system 400 to use the pistons 472a, 472b to alter the pressure within the sealed volume can allow the system 400 to determine the identity of the air pollutants alongside their concentration. Specifically, e-noses typically contain an array of sub-sensors, each of which is sensitive to a range of air pollutants / VOCs, but each in a different manner, which allows the concentration of individual air pollutants and / or sub-groups of air pollutants to be determined despite each sub-sensor being sensitive to all the air pollutants. By varying the pressure within the sealed volume using the pistons 472a, 472b, and by analysing the sensor response to the pressure variation, further data regarding the interaction of the air pollutants with the sub-sensors at different pressures can be obtained to further inform the determination of which air pollutants are present, and in what concentrations. Figure 9 is a flowchart for a method of removing one or more air pollutants from an airflow using an air processing system, for example, one of the first, second, third, fourth or fifth systems 100- 500 described above, or an environmental conditioning unit as described in relation to Figure 1. Although the method is presented in Figure 9 as a sequence of steps S810- S850 starting from step S810, it can be appreciated that because step S850 loops back to return to step S810, the method could be commenced from any of steps S810 - S850. In Figure 9 the method commences with actuating the sealing mechanism of the system to place the system into the first state (step S810). In the first state, the system is configured to allow an airflow to pass along the primary airflow path, which extends between an air inlet of the system and an air outlet of the system. A first adsorption medium is disposed within the system such that the primary airflow path passes through the first adsorption medium. The first adsorption medium is configured to adsorb one or more air pollutants in the air flowing through it by physisorption. Subsequently, with the system in the first state, an airflow is generated along the primary airflow path (step S820), such that air pollutants in the airflow passing through the first adsorption medium are then adsorbed onto the first adsorption medium (step S830). The airflow may be generated, for example, by an airflow generating means within an environmental conditioning unit that the system is within, as described above in relation to Figure 1. After a period of passing an airflow along the primary airflow path and adsorbing air pollutants therefrom by the first adsorption medium, the sealing mechanism is actuated to place the system into a second state (step S840). This switching may occur according to a predetermined timing schedule e.g. the system may be switched to operate in the second state for a predetermined period (such as 10 minutes, 30 minutes, 1 hour or any other suitable time period) once every 24 hours. Additionally, or alternatively, the system may be configured to switch from the first operating mode to the second operating mode in response to an operational instruction input by a user - for example, in response to an instruction to stop driving an airflow along the primary airflow path. Additionally, or alternatively, to the two options discussed above, the system may be configured to switch from the first operating mode to the second operating mode in response to the airflow through the system being switched off and / or the power to the system being switched off. In the second state, the primary airflow path is sealed by the sealing mechanism by the creation of a sealed volume that contains the whole of the first adsorption medium and the whole of a second adsorption medium such that the first and second adsorption media are arranged in fluid connection. The second adsorption medium is configured to adsorb one or 5 more air pollutants by chemisorption and / or catalyse the degradation of the air pollutants. Once the system is in the second state, the desorption of air pollutants from the first adsorption medium and the adsorption of said air pollutants onto the second adsorption medium can be allowed (step S850) without the possibility of air pollutants desorbed from 10 the first adsorption medium leaving the sealed volume and entering the surroundings when the system is in the second state. As described in relation to Figures 1-5, this transfer of air pollutants from the first adsorption medium to the second adsorption medium may be actively promoted as part of step S850, for example, by heating the first and / or second adsorption medium.

Claims

1. A system for processing ambient air containing one or more air pollutants, the system comprising:a primary airflow path between an air inlet of the system and an air outlet of the system;a first adsorption medium configured to adsorb the one or more air pollutants by physisorption, through which the primary airflow path passes;a second adsorption medium configured to adsorb one or more air pollutants by chemisorption and / or catalyse the degradation of the air pollutants; andan actuatable sealing mechanism arranged for sealing of the primary airflow path; wherein the system is configured to actuate the sealing mechanism during operation to switch the system between:a first state in which the system is configured to allow an airflow to pass along the primary airflow path and through the first adsorption medium; anda second state in which the primary airflow path is sealed by the sealing mechanism by creation of a sealed volume that contains the whole of the first adsorption medium and the whole of the second adsorption medium such that the first and second adsorption media are arranged in fluid connection to thereby allow adsorption of one or more air pollutants desorbed from the first adsorption medium by the second adsorption medium.

2. The system according to claim 1, wherein part, or all, of the second adsorption medium is disposed off of the primary airflow path.

3. The system according to claim 1 or 2, wherein the system is configured to be switchable between:a first operating mode in which the system is in the first state and an airflow is generated along the primary airflow path; anda second operating mode in which the system is in the second state and the system is configured to promote desorption of one or more air pollutants from the first adsorption medium, and / or to promote adsorption of said pollutants onto the second adsorptionmedium and / or increase the rate of catalysis of the degradation of said air pollutants by the second adsorption medium within the single sealed volume.

4. The system according to any preceding claim, wherein the primary airflow path is at least partly defined by a conduit through the system, the conduit comprising one or more sidewalls.

5. The environmental conditioning unit according to claim 4, wherein a sidewall of the conduit, or a portion thereof, is defined by a surface of the second adsorption medium.

6. The system according to any preceding claim, wherein:the system further comprises first and second sealing members, the first sealing member being provided upstream of the first adsorption medium along the primary airflow path, the second sealing member being provided downstream of the first adsorption medium along the primary airflow path; andthe first and second sealing members are actuatable to together seal the primary flow path to provide the single sealed volume.

7. The system according to any preceding claim, wherein a surface defining the sealed volume is elastically deformable such that the volume of the sealed volume is variable with elastic deformation of the surface.

8. The system according to claim 7 as dependent on claim 6, wherein the first and / or second sealing member is elastically deformable.

9. The system according to claim 7 or 8, wherein the volume of the sealed volume is variable by at least ±20% by elastic deformation of the surface.

10. The system according to any preceding claim, wherein:the system further comprises a movable member disposed in, or defining a surface of, the sealed volume; andthe position and / or volume of the movable member is controllable such as to control the volume of the sealed volume.

11. The system according to claim 10, wherein the system is configured to promote desorption of air pollutants adsorbed onto the first adsorption medium by moving the movable member to increase the volume of, and thereby reduce the pressure in, the sealed volume.

12. The system according to any of claims 1 to 11, further comprising a pressure relief valve configured to provide pressure relief to the sealed volume.

13. The system according to claim 12, the system further comprising a third adsorption medium disposed over an inlet of the pressure relief valve, the third adsorption medium configured to:adsorb the air pollutants by physisorption, adsorb the air pollutants by chemisorption, and / or catalyse the degradation of the air pollutants.

14. The system according to any preceding claim, the system further comprising at least one concentration sensor disposed in the sealed volume, the concentration sensor being configured to measure the concentration of one or more of the air pollutants within the sealed volume when the system is in the second state.

15. The system according to claim 14, wherein:the system further comprises an air flowrate sensor configured to measure the flowrate of ambient air along the primary airflow path; andon switching to the second state, the system is configured to estimate the concentration of the one or more air pollutants measured by the concentration sensor in the ambient air based on the measured concentration, the measured flowrate and the time elapsed since the system was last switched into the first state from the second state.

16. The system according to any preceding claim, wherein the system is configured to promote desorption of air pollutants adsorbed onto the first adsorption medium by heating the first adsorption medium.

17. The system according to any preceding claim, wherein the system is configured to heat the second adsorption medium.

18. The system according to any preceding claim, wherein the first adsorption medium and second adsorption medium are in direct physical contact.

19. An environmental conditioning unit comprising:a body comprising an air intake and an air exhaust;airflow generating means configured to generate an airflow between the air intake and the air exhaust; anda system according to any one of claims 1 to 18 provided downstream of the air intake and upstream of the air exhaust.

20. A method of removing one or more air pollutants from an airflow using a system according to any one of claims 1 to 18, wherein the method comprises the steps ofactuating the sealing mechanism to place the system into the first state in which the system is configured to allow an airflow to pass along the primary airflow path and through the first adsorption medium;generating an airflow along the primary airflow path when the system is in the first state, such that air pollutants in the airflow are adsorbed onto the first adsorption medium;actuating the sealing mechanism to place the system into the second state in which the primary airflow path is sealed by the sealing mechanism by creation of a sealed volume that contains the whole of the first adsorption medium and the whole of the second adsorption medium such that the first and second adsorption media are arranged in fluid connection, andallowing adsorption of one or more air pollutants desorbed from the first adsorption medium by the second adsorption medium.

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