A method and system for measuring the surface tension of cloud droplets in aerosol activation to cloud processes

Abstract

The present application belongs to the technical field of aerosol measurement, and particularly relates to a method and system for measuring the surface tension of cloud droplets in the process of aerosol activation into clouds. The method comprises: building a system for measuring the surface tension of cloud droplets, including a cloud condensation nucleus counter, electrode plates, a high-voltage power supply, a condensation nucleus particle counter, and a differential mobility analyzer; applying a high-voltage electric field to cause positive and negative ions in the droplets to split under the traction of the electric field; aerosol samples are converted into droplets by absorbing water vapor in a cloud chamber, and the droplets split under the traction of the high-voltage electric field; the number concentration of the droplets in the outflow sample is measured by the condensation nucleus particle counter; the critical voltage at which the droplets split is obtained by fitting the graph of the number concentration of the droplets versus the voltage intensity; and the surface tension of the droplets is calculated according to the quantitative relationship between the critical voltage and the surface tension of the droplets. The present application can be used in atmospheric chemical transport simulation and climate models to evaluate the influence of the surface tension of cloud droplets on the number concentration and particle size of the cloud droplets.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of aerosol measurement, and particularly relates to a method and system for measuring the surface tension of cloud droplets in the activation and cloud formation process of aerosols. BACKGROUND

[0002] Clouds play a very important role in global energy balance and weather and climate. Aerosol particles in the atmosphere can act as cloud condensation nuclei (CCN) and ice nuclei (IN) to affect the cloud droplet number concentration and effective radius, thereby affecting the optical and radiative properties of the cloud, such as optical thickness and albedo, and changing the cloud precipitation efficiency and life cycle, thus indirectly changing the global radiative energy balance. Aerosol-cloud interaction mainly manifests in two aspects. On the one hand, under a certain liquid water path, the increase of aerosol concentration increases the cloud droplet number concentration and reduces the cloud droplet effective radius, thereby increasing the cloud albedo. On the other hand, the increase of aerosol can weaken the cloud precipitation efficiency and increase the cloud life cycle, thereby cooling the earth-atmosphere system. According to the IPCC assessment report, aerosol-cloud interaction is the largest uncertainty in the current global radiative forcing assessment. The cooling effect caused by this interaction can offset the warming effect caused by greenhouse gases to a great extent.

[0003] Due to the lack of understanding of the cloud formation process, the influence of aerosols on the concentration of CCN, the number concentration and particle size of cloud droplets cannot be accurately quantified. The concentration of CCN mainly depends on the water vapor supersaturation, the particle size, the composition of aerosols and the surface tension of cloud droplets in the activation (cloud formation) process. At present, many studies focus on the influence of aerosol particle size and composition on CCN, and there is a relatively accurate understanding. However, the specific value of the surface tension of cloud droplets is currently unknown due to the lack of a suitable measurement method. For a long time, the surface tension of cloud droplets has been assumed to be equal to the surface tension of pure water. This assumption may have a great impact on the concentration of CCN and the particle size of cloud droplets because the surface tension of the droplet is different from that of pure water due to the widespread presence of organic matter in atmospheric aerosols. For example, it was found that when primary marine ultrafine aerosol particles (<100 nm) are used as CCN, the use of the surface tension of pure water to estimate the concentration of CCN can cause a 10-fold underestimate. When the cloud formation process of atmospheric common organic aerosols is used as CCN, the use of the surface tension of pure water will cause a 40%-60% underestimate of the cloud droplet size. The great deviation of the CCN concentration and the cloud droplet size caused thereby may have an important influence on global radiative forcing and regional and global climate prediction. Therefore, it is of great significance to measure the surface tension of cloud droplets in the cloud formation process of aerosols.

[0004] Current methods for measuring liquid surface tension are mainly for bulk solution (tens of milliliter level) or need a large amount of solution to generate millimeter level droplets. A small number of measurement methods for micrometer level small droplets such as atomic force microscope method and optical tweezer method are only suitable for determining the relative humidity less than 100%, and cannot be used under the condition of water vapor supersaturation. Since the cloud droplet size is very small (micron level), and the cloud droplet is difficult to exist stably under the condition of water vapor supersaturation, it is extremely difficult to measure the surface tension of the micrometer level cloud droplet under the condition of water vapor supersaturation, and therefore there is no report on the measurement of the surface tension of the cloud droplet in the clouding process. In order to better evaluate the influence of aerosol on clouding and climate, it is urgent to develop a method for measuring the surface tension of the cloud droplet in the process of aerosol activation and clouding. SUMMARY

[0005] The purpose of the present application is to provide a method and system for measuring the surface tension of the cloud droplet in the process of aerosol activation and clouding, which is convenient to measure, simple to operate and reliable in performance.

[0006] The method for measuring the surface tension of the cloud droplet in the process of aerosol clouding provided by the present application comprises the following specific steps:

[0007] (1) A system for measuring the surface tension of the cloud droplet is built, which comprises a cloud condensation nucleus counter, an electrode plate and a high-voltage power supply, and an agglomerated nucleus particle counter and a differential mobility analyzer for selecting single particle size aerosol; the cloud condensation nucleus counter comprises a lateral optical particle counter (OPC) inside, different temperature and water vapor supersaturation clouding conditions are generated in the artificial cloud chamber inside, so that the aerosol is activated into cloud droplets, and the number concentration and particle size of the activated cloud droplets are measured by the optical particle counter (OPC); the electrode plate is arranged at the airflow outlet end of the cloud chamber and connected with the high-voltage power supply, and is used for generating high-voltage electric field of different intensity;

[0008] (2) A high-voltage electric field is applied, and the positive and negative ions in the droplets are split under the action of the electric field of different intensity, and the voltage at the time of splitting is called critical voltage; the aerosol sample is absorbed by the cloud chamber to become droplets, i.e. cloud droplets, and after splitting under the action of the high-voltage electric field, the number concentration of the droplets in the sample flowing out of the outlet end is measured by the agglomerated nucleus particle counter; the critical voltage at which the droplets split is analyzed by fitting the relationship diagram of the number concentration of the droplets changing with the voltage intensity;

[0009] (3) The principle of the present application is based on the fact that droplets will split into multiple small droplets under a certain high-voltage electric field, and the critical electric field strength of the splitting has a quantitative relationship with the surface tension. Previous studies have pointed out (Beroz et al. 2019, Phys. Rev. Lett., 122, 244501) that there is a quantitative relationship between the critical voltage of droplet splitting under a high-voltage electric field and its surface tension. The surface tension is calculated as follows:

[0010] (1)

[0011] where R represents the droplet radius, which is given by the optical particle counter in the cloud condensation nucleus counter; ε is the vacuum permittivity; E0 is the critical voltage, which is determined by observing the change in the concentration of cloud droplet residues by scanning the voltage; and γ is the cloud droplet surface tension to be solved.

[0012] According to the above formula, the droplet radius, critical voltage, and constant term are known, and the surface tension of the cloud droplet can be calculated.

[0013] In the present application, the aerosol mode is divided into: single-particle-size aerosol mode (single particle size) and multi-particle-size aerosol mode (also known as polydisperse aerosol, containing aerosol particles of multiple particle sizes). Taking the single-particle-size aerosol mode as an example, the specific operation steps are as follows:

[0014] (1) The prepared aerosol sample to be tested is introduced into a differential mobility analyzer (DMA; TSI3080) for particle size selection to obtain aerosol of a single particle size;

[0015] (2) The screened aerosol sample of a single particle size is introduced into a cloud chamber in a cloud condensation nucleus counter at a certain flow rate, the supersaturation in the cloud chamber is set to 0.1-1%, and the temperature is set to 5-40℃; the aerosol particles are allowed to condense and grow in the cloud chamber to generate cloud droplets;

[0016] (3) Different voltages V (1~25kV) are set by a high-voltage power supply, and the voltage is increased at a speed of 0.1 kV / s, while the number concentration N of droplets at the outlet end of the cloud condensation nucleus counter is measured by an agglomerated nucleus particle counter to obtain data of N and V changing with time;

[0017] (4) The change of N with V is analyzed, and the N(V) function change graph is obtained by S curve fitting to identify the critical voltage corresponding to the obvious increase of N (i.e. the splitting of droplets); according to the critical voltage and the droplet radius measured by the optical particle counter, the surface tension of the aerosol activated into cloud droplets under the condition can be calculated.

[0018] Further, the cloud droplet surface tension is determined by changing the supersaturation, temperature and aerosol particle size: the cloud condensation nucleus counter is used to determine the cloud droplet surface tension under different supersaturation conditions (0.1% to 1%); or the differential mobility analyzer is used to screen aerosol particles of different sizes (30 to 600 nm) to determine the surface tension of the aerosol activated into cloud droplets; or the temperature in the cloud condensation nucleus counter is changed (5 to 40℃) to determine the cloud droplet surface tension under different temperature conditions. Thus, the influence of different factors on the cloud droplet surface tension is studied.

[0019] The aerosol clouding process cloud droplet surface tension determination system provided by the application comprises a cloud condensation nucleus counter, electrode plates, a high-voltage power supply, a condensation nucleus particle counter and a differential mobility analyzer for selecting single-particle-size aerosol. The cloud condensation nucleus counter comprises a lateral optical particle counter (OPC), which can generate clouding conditions of different temperatures and different water vapor supersaturation in the artificial cloud chamber, so that the aerosol is activated into cloud droplets, and the number concentration and particle size of the activated cloud droplets are measured by the optical particle counter (OPC); the electrode plates are arranged at the airflow outlet end of the cloud chamber and are connected with the high-voltage power supply, and are used to generate high-voltage electric fields of different intensities.

[0020] The application has the following beneficial effects:

[0021] (1) The cloud droplet surface tension determination system is based on the commercial cloud condensation nucleus counter and the condensation nucleus particle counter, which is convenient to connect with the high-voltage power supply at any time, simple to operate and maintain, and suitable for wide use.

[0022] (2) The cloud droplet surface tension determination system can quantitatively measure the surface tension of the cloud droplets activated from the aerosol for the first time, makes up for the lack of current measurement means, and provides an observation means for future related research;

[0023] (3) The cloud droplet surface tension determination system can measure aerosol particles of different compositions, particle sizes and morphologies, set different water vapor supersaturation and temperature conditions in the cloud chamber, measure the surface tension of the cloud droplets activated from the aerosol, and be used to study the factors affecting the cloud droplet surface tension.

[0024] (4) The cloud droplet surface tension determination system can comprehensively consider the factors affecting the cloud droplet surface tension, combine the observation of the cloud droplet surface tension under different actual atmospheric conditions, establish a parameterization scheme of the cloud droplet surface tension, and be used in atmospheric chemical transport simulation and climate model to evaluate the influence of the cloud droplet surface tension on the cloud droplet number concentration and particle size. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a schematic diagram of the working principle of the cloud chamber in the cloud condensation nucleus counter.

[0026] Figure 2 System for measuring the surface tension of cloud droplets in the process of aerosol activation into cloud (single-particle mode).

[0027] Figure 3 System for measuring the surface tension of cloud droplets in the process of aerosol activation into cloud (multi-particle mode).

[0028] Figure 4 Schematic diagram of droplet splitting in high-voltage electrode. Positive and negative ions in the droplet migrate under the action of the electric field, stretch the particles, and split at a specific critical voltage.

[0029] Figure 5 Data processing method for obtaining critical electric field. The horizontal axis represents the electric field strength V (unit: kV), and the vertical axis represents the droplet number concentration N (unit: cm -3 ), and the red dotted line represents a sudden increase in N at the critical voltage, i.e., droplet splitting. DETAILED DESCRIPTION

[0030] Figure 1 Schematic diagram of the working principle of the cloud chamber in the cloud condensation nucleus counter. It is mainly based on the theory that the heat dissipation ratio in the gas is slow in water vapor. A thermal instability and supersaturated water vapor condition is created in the column, so that aerosol is activated into cloud droplets. The water vapor diffuses from the warm and wet wall to the center line faster than the heat diffusion speed. C point along the center line, the heat diffusion (red line, A point) on this column is higher than the mass diffusion (blue line, B point). Assuming that the wall is saturated with water vapor, and the temperature at B point is higher than that at A point (i.e., the water vapor partial pressure at B point is greater than that at A point), the actual water vapor partial pressure at C point is equal to that at B point, and its temperature is lower than that at B point. Therefore, relative to the saturated water vapor pressure at B point, the thermal condition makes C point have more water vapor, i.e., C point is supersaturated, so that water vapor condenses on aerosol particles in the sampling airflow to form droplets, simulating the formation of cloud droplets in the atmosphere.

[0031] The application will be further described below in conjunction with Figures 2-5 and examples.

[0032] The present application provides an aerosol clouding process cloud droplet surface tension measurement system, comprising a cloud condensation nucleus counter (Droplet Measurement Technologies, CCNC), an electrode plate and a high-voltage power supply, and a condensation particle counter (TSI, CPC) and a differential mobility analyzer (DMA, TSI) for selecting single particle size aerosol. The cloud condensation nucleus counter includes a lateral optical particle counter (OPC), which can generate different temperature and different water vapor supersaturation clouding conditions in the artificial cloud chamber, so that the aerosol is activated into cloud droplets, and the activated cloud droplet number concentration and particle size are measured by the optical particle counter (OPC); the electrode plate is arranged at the airflow outlet end of the cloud chamber and connected with the high-voltage power supply, for generating high-voltage electric field of different intensity.

[0033] The use mode of the system is divided into single particle size aerosol mode (single particle size) and multi-particle size aerosol mode (also known as polydisperse aerosol, containing aerosol particles of multiple particle sizes), taking the single particle size aerosol mode as an example (the single particle size aerosol mode is used as an example Figure 2 ), the implementation is as follows:

[0034] (1) The aerosol sample to be measured is introduced into the differential mobility analyzer (DMA; TSI3080) for particle size selection, and single particle size aerosol is obtained; the aerosol flow rate is controlled to be 0.6 L / min (LPM), the sheath gas flow rate is set to be 6 LPM, and aerosol particles of a certain particle size (such as 100 nm) are screened;

[0035] (2) The screened single particle size aerosol sample is introduced into the cloud chamber in the cloud condensation nucleus counter at a flow rate of 550 mL / min, the supersaturation in the cloud chamber is set to be 0.1-1%, and the temperature is set to be 5-40℃; the aerosol particles are condensed and grown in the cloud chamber to generate cloud droplets;

[0036] (3) Different voltages V (1~25kV) are set by the high-voltage power supply, and the voltage is increased at a speed of 0.1 kV / s, and the condensation particle counter measures the liquid droplet number concentration N at the outlet end of the cloud condensation nucleus counter sample, and obtains the data of N and V changing with time;

[0037] (4) The N-V change graph is analyzed, and the N(V) function change graph is obtained by S curve fitting Figure 5 ), the critical voltage corresponding to the obvious increase of N (that is, the splitting of liquid droplets) is distinguished; according to the critical voltage and the liquid droplet radius measured by the optical particle counter, the surface tension of the aerosol activated into cloud droplets under the condition can be calculated;

[0038] (5) Determine the surface tension of cloud droplets by changing supersaturation, temperature and aerosol particle size: Change the supersaturation conditions (0.1%~1%) in the cloud condensation nucleus counter and measure the surface tension of cloud droplets under different supersaturation conditions; or use a differential electromobility analyzer to screen the particle size (30~600 nm) of different sample aerosols and measure the surface tension of cloud droplets activated by aerosols of different particle sizes; or change the temperature (5~40℃) in the cloud condensation nucleus counter and measure the surface tension of cloud droplets under different temperature conditions; thereby studying the influence of different factors on the surface tension of cloud droplets.

[0039] In the multi-particle-size aerosol mode ( Figure 3 ),compared to Figure 2 Aerosol samples do not need to be screened for particle size that meets the conditions using a differential electromobility analyzer. They can start directly from step (2) above, and the remaining steps are the same.

[0040] The specific usage process of this invention is as follows:

[0041] (1) Preparation stage: Sample preparation: Select the aerosol particles to be tested, and screen out the particles that meet the required particle size using a differential electromobility analyzer or other equipment; Instrument start-up: Prepare the cloud condensation nucleus counter, high voltage power supply, and condensation nucleus particle counter and connect them correctly in sequence, confirm that the sample inlet and outlet of the cloud condensation nucleus counter are unobstructed and the instrument can be used normally; Then, set the supersaturation and temperature conditions in the cloud chamber of the cloud condensation nucleus counter (in this embodiment, they are set to 0.1%~1% and 5~40 ℃ respectively); Set the distance between the two circuit boards of the power supply to 1cm, and set the scanning voltage to 1~25 kV, changing at a rate of 0.1 kV / s; The condensation nucleus particle counter is in the start-up state; After the instrument runs stably, it can start working;

[0042] (2) Working stage: A certain amount of aerosol sample of the same particle size is introduced into the cloud chamber of the cloud condensation nucleus counter to cause condensation and growth process inside, generating large droplets (cloud droplets); wait for the condensation nucleus particle counter to record the change of droplet number concentration over time; at the same time, record the sequence of scanning voltage over time for subsequent analysis.

[0043] (3) Analysis stage: Extract the time series of scanning voltage and droplet number concentration, and use data analysis software such as Excel or Igor to fit the functional relationship between droplet number concentration and scanning voltage; obtain an S-shaped curve (e.g., Figure 5 The horizontal axis represents the scanning voltage, and the vertical axis represents the droplet number concentration. A sudden increase in droplet number concentration occurs at a certain position on the horizontal axis, indicating that the droplets have undergone significant splitting under that voltage condition. Figure 4, positive and negative ions in the droplet migrate under the action of electric field, making the particle stretch, and splitting at a certain critical voltage, which is called critical voltage; the midpoint of the rising curve is the critical voltage by fitting the curve with sigmoid function; according to the quantitative relationship between the critical voltage of the droplet splitting under high voltage electric field and the surface tension in formula (1), the corresponding cloud droplet surface tension under the condition of the particle size and the supersaturation and temperature of the cloud chamber can be obtained;

[0044] (4) End stage: After the aerosol sample and the generated droplets are measured by the condensation nucleus particle counter, they flow to the outlet pump; after the analysis data is extracted, the instrument can end the working state; the connected instrument can be used continuously or some parts can be taken out for preservation and maintenance.

Claims

1. A method for determining the surface tension of cloud droplets in an aerosol cloud formation process, characterized in that The specific steps are: (1) Build a cloud droplet surface tension measurement system, the measurement system includes cloud condensation nucleus counter, electrode plate and high voltage power supply, and condensation nucleus particle counter and differential mobility analyzer for selecting single particle size aerosol; The cloud condensation nucleus counter includes a lateral optical particle counter, which generates different temperature and different water vapor supersaturation cloud forming conditions in the artificial cloud chamber, so that the aerosol is activated into cloud droplets, and the activated cloud droplet concentration and particle size are measured by the optical particle counter; The electrode plate is arranged at the airflow outlet end of the cloud chamber and is connected with the high voltage power supply for generating high voltage electric field of different intensity; (2) Apply high voltage electric field, so that the positive and negative ions in the droplet are split under the action of electric field of different intensity, and the voltage at the time of splitting is called critical voltage; The aerosol sample is absorbed by the cloud chamber to become liquid droplets, that is, cloud droplets, which are split by the action of high voltage electric field, and the liquid droplet concentration in the sample outlet is measured by the condensation nucleus particle counter; The critical voltage of the liquid droplet splitting is obtained by fitting the relationship diagram of the liquid droplet concentration and the voltage intensity; (3) According to the quantitative relationship between the critical voltage of the liquid droplet splitting under the high voltage electric field and the surface tension thereof, the surface tension is calculated as follows: （1） Wherein, R represents the droplet radius, which is given by the optical particle counter in the cloud condensation nucleus counter; ε is the vacuum dielectric constant; E0 is the critical voltage, which is determined by observing the change of the concentration of the cloud droplet residue by scanning the voltage; γ is the cloud droplet surface tension to be solved; According to the above formula, the droplet radius, critical voltage and constant term are known, and the surface tension of the cloud droplet can be calculated.

2. The method according to claim 1, wherein The specific operation steps are: (1) Put the prepared aerosol sample to be measured into the differential mobility analyzer to select the particle size, and obtain the single particle size aerosol; (2) Put the screened single particle size aerosol sample into the cloud chamber in the cloud condensation nucleus counter at a certain flow rate, and set the supersaturation in the cloud chamber to 0.1-1% and the temperature to 5-40℃; The aerosol particles are condensed and grown in the cloud chamber to form cloud droplets; (3) Set different voltages V by the high voltage power supply, and increase at a speed of 0.1 kV / s, while measuring the liquid droplet concentration N at the outlet end of the cloud condensation nucleus counter by the condensation nucleus particle counter, and obtaining the data of N and V changing with time; (4) Analyze the change graph of N with V, and obtain the N(V) function change graph by S curve fitting, and identify the critical voltage corresponding to the obvious increase of N, that is, the splitting of the liquid droplet; According to the critical voltage and the droplet radius measured by the optical particle counter, the surface tension of the aerosol activated into cloud droplets under the condition is calculated.

3. The method according to claim 2, wherein Change the supersaturation, temperature and aerosol particle size to measure the surface tension of cloud droplets: change the supersaturation condition in the cloud condensation nucleus counter to measure the surface tension of cloud droplets under different supersaturation; Or using differential mobility analyzer to screen different sample aerosol particle size, determine the surface tension of aerosol activated into cloud droplet of different particle size; Or change the temperature in the cloud condensation nucleus counter, determine the surface tension of cloud droplet under different temperature conditions; Thus study the influence of different factors on the surface tension of cloud droplet.

4. A system for the determination of the surface tension of cloud droplets in an aerosol cloud generation process for the determination method according to one of claims 1 to 3, characterized in that The system for measuring the surface tension of cloud droplet comprises a cloud condensation nucleus counter, an electrode plate and a high-voltage power supply, and a condensation nucleus particle counter and a differential mobility analyzer for selecting single particle size aerosol; The cloud condensation nucleus counter comprises a lateral optical particle counter, and different temperature and different water vapor supersaturation cloud forming conditions are generated in the artificial cloud chamber, so that the aerosol is activated into cloud droplet, and the activated cloud droplet number concentration and particle size are measured by the optical particle counter; The electrode plate is arranged at the airflow outlet end of the cloud chamber and is connected with the high-voltage power supply, and is used for generating high-voltage electric field of different intensity.

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