Optical remote sensor for real time measurement of atmospherictrace gas slant column densities, apparatus and methods

Abstract

A device for measuring atmospheric trace-gas slant column density is disclosed. The device includes a diffuse reflector that receives direct sunlight and provides diffusely reflected sunlight to a front focusing lens. The lens collects and focuses the reflected sunlight into a fiber optic bundle having a common end and at least three fiber bundle legs. The bundle generates a homogenized irradiance distribution and delivers substantially proportional optical power to each leg. Each leg feeds an optical channel including a collimating lens, an ultra-narrow bandpass optical filter, a focusing lens, and a photodetector that produces a photocurrent signal representative of intensity in a selected wavelength band. An electronics module converts the photocurrent signals into digital values, and a processing unit acquires the values simultaneously and continuously to determine a trace-gas slant column density within the device's field of view.

Description

PATENTAttorney Docket No.: JL002PCTOPTICAL REMOTE SENSOR FOR REAL TIME MEASUREMENT OF ATMOSPHERICTRACE GAS SLANT COLUMN DENSITIES, APPARATUS AND METHODSCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority to U.S. Provisional Patent Application No. 63 / 723,614, filed on 22 November 2024. The disclosure of the application above is incorporated herein by reference in its entirety.BACKGROUND OF THE DISCLOSUREField of the Disclosure

[0002] The present disclosure is directed at a method and a system for measuring total column densities of a trace gas in the atmosphere using a passive illumination source and detecting the relative transmission intensities at key wavelengths in the near infrared spectrum.Description of Related Art

[0003] Trace gas concentrations in the atmosphere have a significant impact on the Earth’s climate. Methane, for example, may only constitute roughly 2 parts per million of air on a molecular basis, but it is believed to be responsible for up to 25% of the current global greenhouse gas warming. Methane emissions are the second largest source of greenhouse gas emissions after CO2. Over a 20-year horizon, methane has more than 80 times the global warming potential of Carbon Dioxide (CO2) on an equivalent mass basis. It should be appreciated by those skilled in the art that it takes a lot less methane to have the same global warming effect as that of CO2.

[0004] Human activities are believed to be responsible for over 50% of methane emissions and upstream oil and gas operations are recognized to be a major contributor. Published estimates range from 0.5% to 2.5% of all-natural gas production is lost to the atmosphere as fugitive emissions.PATENTAttorney Docket No.: JL002PCT

[0005] The National Oceanic and Atmospheric Administration (NOAA) has tracked global mean atmospheric concentrations of methane since 1983 and has charted a steady increase in methane levels with 2024 levels more than 15% higher than those in 1983. That is a historically rapid rise in the total atmospheric concentration of this potent greenhouse gas.

[0006] Reducing methane emissions in the atmosphere is widely regarded as the single most effective short-term strategy to slow global warming but progress has been elusive. The Global Methane Pledge was launched in 2021 with the goal of reducing methane by at least 30% from 2020 levels by 2030 but as of the end of 2024, levels were some 3% higher than those in 2020. As of 2024, over 150 countries have signed on to the Global Methane Pledge demonstrating broad support for the initiative, but meaningful abatement has yet to occur. One key challenge is effective measurement and tracking.

[0007] NOAA global reporting is based largely on analyzing thousands of air samples collected annually around the globe and processed in a central laboratory. Satellite based measurements are becoming more readily available but are still very limited with regard to temporal and spatial resolution. So called “top-down” modeling is often used to predict likely emission sources and changes over time, but regulators and environmental scientists recognize there is a need for better “bottom-up” measurement efforts and a means of reconciling those with the top-down predictions.

[0008] There are a small number of ground-based spectrometers capable of measuring differential absorbances to calculate total column methane density measurements that extend up through the troposphere, but these are expensive systems that require labor intensive data analysis efforts. To date, there exists no low cost, autonomous robust fixed position remote sensor that can measure total column densities of methane in the atmosphere.

[0009] Most of the prior art technologies are based on periodic gas sampling or complex spectrometers either fitted into satellites or ground based inside controlled environments. A low-cost autonomous remote sensor capable of measuring total slant column densitiesPATENTAttorney Docket No.: JL002PCT of methane in the atmosphere is needed and could be deployed across the country and throughout the world to provide regional tracking of methane emissions on a daily basis.SUMMARY OF THE DISCLOSURE

[0010] The disclosed system differs from prior photometers and DOAS spectrometers by using simultaneous, continuous, multi-channel ultra narrow-band measurements with sub-5 nm separation, enabling high-precision trace gas detection in a compact, low-cost, and field-deployable package.

[0011] The system is comprised of front end optics designed to collect near infrared (NIR) radiation from the Sun, a fiber bundle and homogenizing element to convey the input NIR radiation into three or more individual legs with appreciably identical optical information in terms of relative intensity as a function of wavelength, a set of individual optical narrow band pass filters to limit transmission to wavelength bands of interest, individual detectors for each leg to convert optical intensity into photocurrent, and a computer to collect the measured data and calculate parameters of interest including column densities of a target trace gas, and software to create geographic mapping of a target trace gas in air concentrations from a regional deployment.

[0012] The system measures relative transmission intensities at three or more wavelength bands to determine the differential absorbance related to a trace gas, (e.g. methane) present in a cone centered on the Sun and extending from the surface of the Earth up to the outer edge of the atmosphere with a narrow angular field of view defined by the perimeter of the Sun. The system can provide a continuous string of measurements for total slant column gas densities from sunrise to sunset. These measurements provide a total methane content for a column extending from the location of the sensor to the edge of the atmosphere with that effective total path length being highly variable but predictable based on date, time, and physical location. The system provides an east to west slice of total column densities that can cover a span of 60 miles or more with a single unit. The system can also provide a low-cost regional methane atmospheric concentration map by integrating multiple units spread out over a north-south and east-west grid, covering 300PATENTAttorney Docket No.: JL002PCT square miles or more per individual node. Individual systems are networked together with software that analyses individual measurements associated with various Sun angles and azimuths throughout the day and interpolates to create regional concentration maps at the end of each day in a fully automated process.

[0013] A system of one or more computers can be configured to perform particular operations or actions by virtue of having software, firmware, hardware, or a combination of them installed on the system that in operation causes or cause the system to perform the actions. One or more computer programs can be configured to perform particular operations or actions by virtue of including instructions that, when executed by data processing apparatus, cause the apparatus to perform the actions.

[0014] In one general aspect, a trace gas measurement system is disclosed that may include a diffuse reflector positioned to receive direct sunlight from the Sun and to provide diffusely reflected sunlight. The trace gas measurement system may also include a front focusing lens arranged with an optical axis directed toward the diffuse reflector and configured to collect the diffusely reflected sunlight and to focus the diffusely reflected sunlight. The trace gas measurement system may furthermore include a fiber optic bundle having a common end optically coupled to the front focusing lens and optically coupled to at least three separate fiber bundle legs, the fiber optic bundle configured to produce a homogenized irradiance distribution from the diffusely reflected sunlight, the fiber optic bundle being further configured to provide substantially proportional optical power distribution from the homogenized irradiance distribution among the fiber bundle legs. The trace gas measurement system may in addition include for each fiber bundle leg, an optical channel having a collimating lens configured to receive a portion of the homogenized irradiance distribution from the corresponding fiber bundle leg and produce a collimated beam, an ultra-narrow bandpass optical filter positioned to receive the collimated beam and configured to transmit a corresponding wavelength band, a focusing lens positioned to receive light transmitted by the ultra-narrow bandpass optical filter, and a photodetector positioned to receive light from the focusing lens and generate a photocurrent signal representative of an intensity in the corresponding wavelength band.PATENTAttorney Docket No.: JL002PCTThe trace gas measurement system may moreover include an electronics module having at least one analog-to-digital converter configured to convert the photocurrent signals from the photodetectors into digital values. The trace gas measurement system may also include a processing unit configured to acquire the digital values from the at least three optical channels substantially simultaneously and continuously over time, and determine a trace gas slant column density in a field of view. Other embodiments of this aspect include corresponding computer systems, apparatus, and computer programs recorded on one or more computer storage devices, each configured to perform the actions of the methods.

[0015] Implementations may include one or more of the following features. The trace gas measurement system may include a homogenizing rod optically coupled to the front focusing lens and the fiber optic bundle and configured to output the homogenized irradiance distribution to the fiber optic bundle. The trace gas measurement system where the diffuse reflector may include a Lambertian target. The trace gas measurement system where each ultra-narrow bandpass optical filter has a full-width half-maximum (FWHM) between about 2 nm and about 4 nm and where center wavelengths of at least two of the filters are separated by less than about 5 nm. The trace gas measurement system where the trace gas may include methane and the ultra-narrow bandpass optical filters are centered on wavelengths within a methane absorption region between about 1650 nm and about 1680 nm, including a first filter centered on a wavelength on a first side of a methane absorption cluster, a second filter centered substantially over the methane absorption cluster near about 1666 nm, and a third filter centered on a wavelength on an opposite side of the methane absorption cluster, such that the second filter experiences stronger methane absorbance than the first filter and third filter. The trace gas measurement system where the photodetectors may include extended -range Indium Gallium Arsenide (InGaAs) photodetectors and the at least one analog-to-digital converter has a resolution of at least 24 bits and is configured to sample each optical channel at a rate of at least about 1 Hz. The trace gas measurement system where the fiber optic bundle may include a high-count randomized fiber optic bundle and is configured such that, after calibration, an optical power balance among the at least three optical channelsPATENTAttorney Docket No.: JL002PCT is maintained within about 0.1 %. The trace gas measurement system where the diffuse reflector is positioned substantially normal to a central axis of the front focusing lens such that the system does not require direct-Sun tracking within a solar disk perimeter to maintain uniform irradiance at the front focusing lens. The trace gas measurement system may include a pan-and-tilt stage supporting at least the diffuse reflector and the front focusing lens, and a controller configured to reposition the pan-and-tilt stage based on calculated solar elevation and azimuth derived from geographic location, date, and time so that a front face of the diffuse reflector remains illuminated by the Sun throughout a day. The trace gas measurement system where the electronics module further may include a communication interface selected from the group having of a cellular modem and a wired network interface, the communication interface being configured to transmit trace gas slant column density data and associated metadata to a remote server. The trace gas measurement system where the processing unit is further configured to compute photocurrent ratios between at least two of the optical channels, apply calibration coefficients determined using a trace gas reference cell having a known trace gas partial pressure and path length to relate the photocurrent ratios to total trace gas, and correct for water vapor absorbance by applying modeled transmission coefficients derived from HITRAN-based simulations combined with measured filter transmission profiles. The trace gas measurement system where the processing unit is further configured to compute a solar zenith angle and azimuth from a geographic location, a date, and a time, determine an air-mass factor as a function of the solar zenith angle, normalize the trace gas slant column density by the air-mass factor and barometric pressure to produce a dry-air-normalized trace gas concentration, and assign a geographic position corresponding to a center of mass of an associated slanted air column as a function of solar elevation, solar azimuth, and an atmospheric density- versus-elevation model for each air-mass-normalized trace gas concentration data.

[0016] A regional trace gas mapping system comprising a plurality of trace gas measurement systems each deployed at a different geographic location and configured to produce a time stamped trace gas data having time-stamped trace gas slant column density or air-mass-normalized trace gas concentration data, and a server inPATENTAttorney Docket No.: JL002PCT communication with the plurality of trace gas measurement systems and configured to receive the time-stamped trace gas data and associated location assignments from each of the plurality of trace gas measurement systems, and generate a two-dimensional regional trace gas concentration map by interpolating between the measured concentrations with assigned locations based on air column center of mass analysis. The regional trace gas mapping system where each trace gas measurement system effectively covers a span of tens of miles in an east-west direction and hundreds of square miles in area, and where the server is further configured to format the regional trace gas concentration map for display within a geographic information system.

[0017] A method is disclosed wherein the method to comprises to obtain, throughout a day, trace gas concentrations or trace gas slant column densities that have been normalized for air mass, for each normalized trace gas measurement, assigning a geographic position corresponding to a center of mass of an associated slanted air column as a function of solar elevation, solar azimuth, and an atmospheric density- versus-elevation model, transmitting the normalized trace gas measurements and corresponding geographic positions from each trace gas measurement system to a central server, and at the central server, interpolating the normalized trace gas measurements between the geographic position assignments by the plurality of trace gas measurement systems to produce a two-dimensional regional trace gas concentration map. The method may include spacing the plurality of trace gas measurement systems at east-west and north-south intervals such that each trace gas measurement system effectively covers hundreds of square miles, and updating the regional trace gas concentration map at least daily using measurements acquired during the corresponding day. The method may include formatting the regional trace gas concentration map for display within a geographic information system and storing or displaying the map for use in tracking temporal trends in trace gas concentration over the region. Implementations of the described techniques may include hardware, a method or process, or a computer tangible medium.PATENTAttorney Docket No.: JL002PCT

[0018] In one general aspect, the method may include collecting near-infrared sunlight from the Sun after reflection from a diffuse reflector using a front focusing lens and producing a collected sunlight. The method may also include directing the collected sunlight into a homogenizing element and producing, at an exit face of the homogenizing element, a homogenized irradiance distribution. The method may furthermore include coupling the homogenized irradiance distribution into a fiber optic bundle having a common end and at least three fiber bundle legs so that each fiber bundle leg receives a corresponding portion of the homogenized irradiance. The method may in addition include for each fiber bundle leg, forming an optical channel by producing a collimated light by collimating light from the fiber bundle leg, passing the collimated light through a corresponding ultra-narrow bandpass optical filter that isolates a wavelength band within a trace gas absorption region in the near infrared, focusing light transmitted by the corresponding ultra-narrow bandpass optical filter, and detecting the focused light with a corresponding photodetector to generate a photocurrent signal. The method may moreover include substantially simultaneously converting the photocurrent signals from the at least three photodetectors into digital values using at least one analog-to-digital converter. The method may also include determining a trace gas slant column density in a field of view defined by the Sun elevation and azimuth, the front focusing lens and the diffuse reflector by computing differential absorbance from relative transmission intensities across the wavelength bands of the ultra-narrow bandpass optical filters based on the digital values. Other embodiments of this aspect include corresponding computer systems, apparatus, and computer programs recorded on one or more computer storage devices, each configured to perform the actions of the methods.

[0019] Implementations may include one or more of the following features. The method may include performing a calibration procedure using a reference cell containing trace gas at a known partial pressure and having a known path length, measuring photocurrent ratios or logarithms of photocurrent ratios for at least one channel aligned with a trace gas absorption cluster and at least one adjacent reference channel at different trace gas contents in the reference cell, and deriving calibration coefficients or an empirical curve that relates the photocurrent ratios or logarithms of photocurrent ratios to total trace gasPATENTAttorney Docket No.: JL002PCT expressed as an equivalent length of pure trace gas at standard conditions. The method may include modeling water vapor absorbance using HITRAN-based spectral data combined with measured filter transmission profiles for the ultra-narrow bandpass optical filters, computing expected contributions of trace gas and water vapor to measured photocurrent ratios for at least two ratios selected from 13 / l2and l3 / li, and solving a set of equations that include contributions from trace gas and water vapor to obtain a watervapor-corrected trace gas slant column density. The method may include determining a solar zenith angle and azimuth for each set of measurements from geographic location, date, and time, computing an air-mass factor as a function of the solar zenith angle, and normalizing the trace gas slant column density by the air-mass factor to obtain a verticalequivalent trace gas column or concentration. The method may include measuring barometric pressure and relative humidity, and converting the normalized trace gas column into a dry-air-normalized trace gas concentration using the barometric pressure and relative humidity, and assigning a geographic position corresponding to a center of mass of an associated slanted air column as a function of solar elevation, solar azimuth, and an atmospheric density-versus-elevation model for each air-mass-normalized trace gas concentration data. The method may include repeatedly performing the collecting, directing, coupling, forming, detecting, converting, and determining steps at a sampling rate of at least about 1 Hz during daylight hours, and generating a time series of trace gas concentrations or slant column densities from sunrise to sunset for a fixed geographic location. The method may include monitoring bulk transmission levels or absolute photocurrent magnitudes in the at least three optical channels, detecting common-mode transients associated with clouds or other atmospheric changes, and applying at least one quality-control criterion based on the bulk transmission levels or transients to select or reject individual measurements for inclusion in the time series. Implementations of the described techniques may include hardware, a method or process, or a computer tangible medium.PATENTAttorney Docket No.: JL002PCTBRIEF DESCRIPTION OF THE DRAWINGS

[0020] So that the manner in which the above-recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

[0021] Figure 1 is a graphical representation of solar irradiance vs wavelength.

[0022] Figure 2 is a schematic representation of methane absorbance vs wavelength and ultra narrow bandpass transmission vs wavelength in accordance with the present disclosure.

[0023] Figure 3 is a schematic representation of the optical components that comprise a methane remote sensor device in accordance with the present disclosure.

[0024] Figure 4 is a schematic representation of independent channel tracking during normal solar irradiance transients.

[0025] Figure 5 is a schematic representation of a methane remote sensor device in accordance with the present disclosure.

[0026] Figure 6 is a graphical representation of transmission ratios for the three channels vs methane content for a calibration cell in accordance with the present disclosure.

[0027] Figure 7 is a graphical representation of measured methane concentration vs time of day prior to air-mass factor correction for a methane remote sensor in accordance with the present disclosure.

[0028] Figure 8 is a schematic representation of a field mounted methane remote sensor in accordance with the present disclosure.PATENTAttorney Docket No.: JL002PCT

[0029] Figure 9 is a schematic representation of a network of methane remote sensors used for regional methane concentration mapping in accordance with the present disclosure.DETAILED DESCRIPTION

[0030] In the following detailed description of the embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the examples described herein may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the disclosure.

[0031] The present disclosure provides for methods and systems for measuring trace gas such as methane in the atmosphere using Sunlight as a passive light source. Referring to FIG. 1 , there is shown a prior art graphical representation of the spectrum of solar radiation from the Sun as it relates to the Earth. The plot shows the Sun irradiance against wavelength across the spectrum. As one skilled in the art will recognize, the amount of energy available in the NIR range, specifically between about 1500 nm and 1750 nm, is not appreciably absorbed by normal atmospheric constituents such as H2O and CO2 and therefore is efficiently transmitted to the Earth’s surface. Now referring to FIG. 2, there is shown a plot of the absorbance peaks for methane (CFL), 4, for wavelengths of light between 1650 and 1680 nm. As can be seen, the methane absorbance in this region is characterized by a number of isolated narrow peaks and a cluster of closely packed peaks centered around 1666 nm. This is a useful area of focus for the current disclosure. On a second Y-axis (right hand side), the plot also shows the expected transmission for three custom narrow bandpass optical filters consistent with the trace gas measurement system (TGMS) described in this disclosure.

[0032] The data from a TGMS of the current disclosure is shown in the figure with filter 1 , is positioned on the left side with filter 2, in the center, and filter 3, is on the right. As will be disclosed in further detail herein below, filters 1 , 2, 3 can comprise ultranarrow band filters. Filter 2 is positioned on top of the methane absorbance peak cluster with filters 1PATENTAttorney Docket No.: JL002PCT and 3, positioned over adjacent bands where there is significantly lower absorbance related to methane. By measuring the simultaneous transmission for these three wavelength bands, the TGMS disclosed here is able to generate a differential signal that can be used to measure the slant column density of a trace gas such as methane in the field of view of the optical system. The center filter 2 isolates a band with relatively high total absorbance of the target gas compared to the adjacent filters 1 and 3. As one skilled in the art should recognize, using adjacent wavelength bands for reference with center wavelength separation of less than 5 nm minimizes wavelength dependent effects such as Rayleigh scattering that can mask the differential absorbance effect. Additionally, measuring adjacent wavelength bands enables the TGMS of the current disclosure to correct for secondary gas absorbance effects by another gas such as water vapor. The use of ultranarrow bandpass filters, 1 ,2,3, minimizes non-absorbance based differential effects and simultaneously minimizes temperature and pressure effects that slightly broaden absorbance peaks but not beyond the transmission band of the filter. In one such embodiment, the TGMS of the current disclosure having 2 to 4 nm full width half maximum (FWHM) bandpass filters provide an optimal balance to minimize non-absorbance wavelength dependent effects while desensitizing the measurement against temperature and pressure effects.

[0033] Referring next to FIG 3, there is shown a schematic representing the key optical components of a TGMS 1 consistent with the current disclosure. A front focusing lens, 6, collects Sunlight 31 from Sun 30 reflected off a diffusely reflecting surface such as a Lambertian surface, 5, positioned normal to the front lens center line. The Lambertian surface 5 provides a diffuse reflection with uniform radiance at all angles. The front focusing lens, 6, focuses light into a homogenizing rod, 7, designed to provide uniform irradiance at the exit face 7a. The common end 8a of a high-count randomized fiber optic bundle 8, is butted up to the exit face 7a of the homogenizing rod 7. The high-count randomized fiber optic bundle 8 has three or more legs 4a, 4b, 4c in optical communication therewith designed to proportionally split the irradiance.PATENTAttorney Docket No.: JL002PCT

[0034] Each fiber bundle leg 4a, 4b, 4c comprises a channel and is then positioned next to a respective collimating lens (9,13,17) followed by narrow bandpass filters (10,14,18), followed by respective focusing lenses (11 ,15,19) and photodetectors (12,16,20). As will be disclosed in more detail hereinafter, electronics and a microprocessor as part of electronics module 21 are connected to the photodetectors 12,16,20 and used to measure intensity and perform real time trace gas concentrations in the TGMS 1 of the current disclosure.

[0035] The Lambertian surface, 5, is designed to provide a Sun tracking mechanism that is insensitive to alignment and provides uniform input to the front focusing lens, 6. It should be noted that, direct Sun tracking optics of the prior art pose significant challenges to proportional optical power distribution. If the field of view for a direct Sun tracking system encompasses more than the perimeter of the Sun (Sun solar disk and aureole, for example), a high precision differential measurement necessitates the proportional distribution of both the high intensity direct Sun rays and the orders of magnitude lower intensity rays from the aureole. If the direct Sun tracking optics are designed to maintain the field of view within the Sun’s perimeter, the system must be mechanically very precise, typically with feedback positioning that is expensive and intolerant to windy conditions. The Lambertian surface 5 of the present disclosure provides a diffuse reflector that uniformly projects the Sun’s rays 31 into the front focusing lens 6, in a manner that is insensitive to the Sun angle to the surface. Low-cost pan & tilt control with basic Sun elevation and azimuth programing is more than sufficient to achieve extremely precise differential measurements.

[0036] The homogenizing rod 7, and high-count fiber bundle 8, act to further homogenize and proportionally distribute the optical power. The homogenizing rod, also known as a lightpipe, uses total internal reflection to make a non-uniform light source produce a highly uniform and homogenized output. Common forms include hexagonal lightpipes and single large core fiber rods. In an alternative embodiment, the high count fiber bundle can be produced with a highly randomized and uniform distribution of the individual leg fibers at the common end in a manner sufficient to achieve the necessary homogenization.PATENTAttorney Docket No.: JL002PCT

[0037] Note, it is not necessary to have all legs 4a, 4b, 4c transmit the exact same total radiation. A leg-to-leg scale factor can be simply determined with a calibration procedure. The objective is to have each leg share repeatable proportional representation across the entire field of view to avoid any differential transmission related to variations in light intensity across the field of view. The low-cost system described here is capable of maintaining an optical distribution balance better than + / - 0.1 % across the legs enabling differential-based absorbance measurements of trace gases with parts per million concentrations. An overall precision in the parts per billion range is achievable with a of TGMS 1 in accordance with the current disclosure.

[0038] Each individual fiber bundle leg 4a, 4b, 4c is connected to an optical filter assembly with a front collimating lens, 9,13,17, a narrow band pass filter 10,14,18, a focusing lens 11 ,15,19, and a photodetector 12,16,20. Each narrow band pass filter, 10,14,18, isolates a different wavelength band as shown in FIG 2 (1 , 2, 3). The photodetectors 12,16,20, which could be extended range Indium Gallium Arsenide (InGaAs) are each connected to an electronic circuit of electronics module 21 , designed to convert the detector photocurrent into a signal that can be measured with a high resolution (e.g. 24 bit) analog to digital converter (also comprised within electronics module 21 ). A field computer, either stand alone or part of electronics module 21 , records the measured values and performs real time calculations including determining the total slant column density for the target gas.

[0039] A key design element of the disclosed TGMS 1 is the use of optical filters 10, 14, 18, with full width half maximum (FWHM) bandpass in the 2-4 nm range. This range provides for system tolerance related to blue-shift associated with non-collimated input and absorbance peak broadening due to temperature and pressure. For spectrometer systems with wavelength resolution better than 0.5 nm, careful post processing is required to correct for a wide range of effects such as atmospheric temperature and pressure fluctuations that can broaden absorbance peaks and complicate differential measurements. With bandpass filters 10,14,18 in the 2-4 nm FWHM range, the TGMS 1PATENTAttorney Docket No.: JL002PCT can achieve good absorbance contrast while being insensitive to modest peak broadening and center wavelength shifts that fall within the bandpass.

[0040] Another key design element of the disclosed system is the ability to measure all channels continuously enabling simultaneous differential measurements. Most prior art systems and methods related to slant column density measurement of trace gas is based on interferometer-based spectrometers. Interferometers require mirror movement to sweep through a range of wavelengths so there is some time delay between transmission measurements across the desired range. Common mode changes in bulk transmission of sunlight regularly can occur at very high rates, and any time lag can complicate differential readings. Some interferometers can scan rapidly but there is a tradeoff in cost, speed, and accuracy. The ability to measure multiple channels continuously enables the use of best practices in analog and digital filtering to reduce noise and eliminate common mode transient effects. Simultaneous differential measurements of continuous transmission measurements are superior and less costly to what can be achieved with a repeated scanning approach or a filter photometer with a spinning wheel.

[0041] Referring now to Fig 4, there is shown data collected from an embodiment of TGMS 1 under clear blue sky conditions with no discernable clouds. Individual channel photocurrents are plotted vs time in seconds. The measurement frequency is 5 Hz. Over the 10 second interval, common mode transients can be over 0.25% of signal in 200 milliseconds for what would be considered ideal conditions. Any passive device using sunlight to measure ppm level trace gas concentrations has to accommodate these relatively high common mode transients. Fig 4 demonstrates the continuous measurements and simultaneous sampling of TGMS 1 enable the system’s three channels to track the transients to better than 0.1 % even without digital filtering.

[0042] Another aspect of the disclosed system is the ability to accurately track bulk transmission levels. Differential transmission measurements are required for slant column density measurements, but bulk individual channel transmissions levels can be used to track environmental conditions such as cloud cover and overall transients. These can inPATENTAttorney Docket No.: JL002PCT turn be used to ensure that condition stability requirements are met for high measurement confidence.

[0043] Yet another aspect of the disclosed system is to use discreet optical filters, such as narrow bandpass filters 10,14,18 FIG. 3, to reduce system cost and complexity. Most prior art related to atmospheric trace gas analysis is based on the use of spectrometers. Fine wavelength resolution is critical for differential transmission measurements, and prior multichannel photometers and radiometers were not capable of achieving the required resolution. DOAS (differential optical absorption spectroscopy) type instruments have been the primary tool for trace gas analysis. The TGMS 1 of the current disclosure overcomes the prior limitations in filter resolution by employing plasma deposition thin- film interference ultra narrow band pass filters for fine resolution wavelength band isolation. The near proximity positioning of the wavelength bands disclosed herein also provides far better reference measurements than have been described in any Sun photometer or multichannel radiometers of the prior art. The design of focusing lens 6 with homogenizing rod 7 and high-count randomized fiber optic bundle 8 achieves excellent homogenization so individual channels are able to provide continuous and simultaneous differential measurements that are free from variations due to optical alignment differences or non-uniform ities across the field of view. The design features described in this disclosure enable the TGMS 1 to perform differential measurements that heretofore could only be performed by expensive interferometer-based spectrometers. In fact, the novel features described herein enable the disclosed TGMS 1 to make trace gas specific measurements that are technically superior and far less costly than those previously performed by spectrometers of the prior art.

[0044] Referring to FIG 5, there is shown a complete TGMS 1 unit including Lambertian target 5, pan and tilt body 22, and a cellular modem 23, for remote control and reporting. The pan and tilt body 22, can be programmed to move throughout the day to ensure the front face of the Lambertian target 5, is exposed to the Sun. Open-source code is readily available to calculate Sun elevation and azimuth as a function of position (latitude and longitude), date, and time. Repositioning every 30 minutes or so is all that is needed forPATENTAttorney Docket No.: JL002PCT the TGMS 1 described herein. Note, that in certain embodiments, the Lambertian target 5, can be fitted with a hood to minimize stray light. The disclosed TGMS 1 can be programed to run autonomously and a cellular modem 23, or a known hardwired network connection can be added to provide real time feedback, enable remote configurations, perform remote diagnostics, and collect data for regional mapping of trace gas concentrations.

[0045] As part of the TGMS 1 of the current disclosure, trace gas concentration calculations are based on differential absorbance measurements for the three channels to determine water vapor corrected methane slant column densities; applying air-mass normalization to account for Sun elevation; and using barometric pressure plus a relative humidity correction to determine a dry air total. The differential absorbance measurements for methane are based on controlled environment calibration testing with a test cell. Since the water vapor absorbance is relatively low and only measurable over long path lengths, a model was developed using HITRAN (High Resolution Transmission, a worldwide standard for simulating atmospheric molecular transmission) overlayed with the actual system filter transmission properties 1 ,2,3.

[0046] Shown in FIG 6 is actual data for an embodiment of TGMS 1 disclosed herein using a calibration cell where the methane concentration in air was carefully controlled. The plot shows the log of the photocurrent ratio of measured photocurrent (I) relative to a reference (Io) for the three channels with unique bandpass filters vs the total methane in centimeters. Total methane in cm can be determined by multiplying methane partial pressure times total cell length. Methane slant column densities can be measured as an equivalent length as if it were isolated into a separate layer of pure methane gas at standard conditions. The -Log(l2 / I2o) data depicted by line 26, relates to the channel with the bandpass filter that spans the strongest methane absorbance cluster filter 2 (FIG 2). The -Log(l1 / I1 o) and -Log(l3 / I3o) data depicted by lines 24,25, relate to the adjacent reference filter bandpass channels 1 , 3 (FIG. 2). The Y axis covers the range of methane that is to be expected for slant column density measurements for an approximate 2 ppm atmospheric concentration and a range of Sun angles from solar noon to late afternoon.PATENTAttorney Docket No.: JL002PCTThe total air-mass traversed by the Sun’s rays in the morning and afternoon can be several times that at solar noon.

[0047] In a narrow-band filter photometer, such as the system of TGMS 1 described herein, the Beer-Lambert law is strictly valid only for infinitesimally narrow passbands. For finite-width filters, the measured transmission represents an integral over varying absorption strengths, so the apparent absorption coefficient becomes concentration - dependent. At low optical depths, a single empirical coefficient can approximate the response, but at higher gas concentrations or long paths, stronger sub-bands saturate while weaker ones transmit more light, producing a nonlinear departure from Beer- Lambert behavior. In the systems and methods described herein, the log of the photocurrent ratio is not expected to vary exactly linearly with concentration but it will be a close approximation for methane. A linear approximation or an empirical curve fit can be used.

[0048] Generally speaking, the intensity distributions through the three channels will not be perfectly balanced in the TGMS 1 system described herein due to expected differences in fiber count in individual legs and other subtle differences in channel optics. A simple scale factor can be determined through a calibration process that can compensate thereby aligning the I1o, I2o, and I3o values for the three channels, In this case, channel to channel ratios such as -Log(l3 / I2) and -Log(l3 / I1 ) can be used to determine the methane concentration in the field of view. The common mode variations in intensity are shared by I1o, I2o, and I3o and therefore can be dropped.

[0049] Absorbance due to water vapor is relatively small compared to methane absorbance for the system described here but it is not insignificant. Measuring three bands / channels provides a means of correcting for the water vapor absorbance. Since the monochromatic absorption coefficients for water vapor over the range covered by the three filters 1 ,2,3, (FIG. 2) are very small and absorbance is only measureable over long path lengths, modeling using HITRAN data is the preferred approach. Combining HITRAN data for water vapor with the filter transmission profiles yields expected total transmission (photocurrent) ratios as a function of total water.PATENTAttorney Docket No.: JL002PCT

[0050] As part of the current disclosure, the measured photocurrent ratios ) and ( ) can be represented as follows: (Equation 1 )®= 1 + 4®„+ A®B,(Equation 2)

[0051] Where A (-) and A(-) are the differences to the normalized ratio due to methane and water absorbance, repspectively. Furthermore, the relationships:A I —) = f (total methane) (Equation 3)

[0052] can be determined from the calibration procedure described by FIG 6. The following relationships:A (-) = f (total water) (Equation 4)

[0053] can be determined from the modeling exercise using HITRAN data. This approach yields six unknowns and six independent equations that can be solved for total methane.

[0054] Referring to FIG 7, there is shown methane concentration in ppb vs time depicting actual data from an embodiment of a TGMS 1 in accordance with the current disclosure. The concentration shown is based on a vertical air-mass calculated from barometric pressure. The results represent “uncorrected” concentrations because the total air-mass changes with the angle of the Sun. Typically, an air-mass factor (AMF) is used to adjust the results for the increased air-mass associated with increasing zenith angles. A simple and reliable AMF model is just 1 / cos(zenith angle) which works well for moderate Sun angles. For higher angles more sophisticated models can be used. The overall curvaturePATENTAttorney Docket No.: JL002PCT seen in FIG 7 matches the shape of the AMF. For reference, solar noon for the location and date of the test was at 12:25 which corresponds well with the minimum 70 in the measured results. When the measured results as shown in FIG 7 are corrected by dividing by the AMF, the curve is flattened over time showing a concentration that is representative of a normalized value for the Sun angle and azimuth at that time for that position. The concentration associated with a particular time can then be assigned to a single 2-dimensional geographic position by determining the center of mass for the slanted air column with the zenith angle and azimuth for that particular time. The data shown in FIG 7 was collected at a 1 Hz rate with approximately 9000 individual measurements taken over the 2.5Hrs shown. Overall measurement precision is better than 10 ppb.

[0055] Now referring to FIG 8, there is shown the span 81 of atmosphere that can be covered with a single unit 27 of an embodiment of TGMS 1. It is known that 80-90% of the atmosphere’s mass is in the troposphere that extends from 4 to 12 miles above the Earth depending on lattitude and time of year. For morning and afternoon Sun elevations, the single unit 27 field of view 28, can extend to the top of the troposphere some 50 miles east and west of the system’s ground location. On a typical day, the system can provide thousands of individual measurements of normalized methane concentrations sweeping from east to west. Typically, the concentration measurement is asigned to a position (lattitude and longitude) representing the center of mass for an air column aligned with the field of view 28. That is determined by Sun elevation and azimuth coupled with barometric pressure and a simple air density vs elevation model. At the end of each day, the system 27, produces an array of concentration measurements extending east to west along the path defined by the Sun’s elevation angle and azimuth.

[0056] The array of high resolution methane concentration measurements can be combined with a plurality of other strategically positioned TGMS 1 to provide a regional mapping by interpolating north and south of system locations.

[0057] Referring next to FIG 9 there is shown an example layout with three TGMS1 single unit systems 27. East-west spacing 91 can be 50 miles, for example with north-PATENTAttorney Docket No.: JL002PCT south spacing 92 every 10 miles. At the end of each day, server based software collects data from each single unit system 27, and creates a 2 dimensional concentration map by interpolating north and south. The mapping can be loaded into a commercial geographic information system (GIS) software to provide near real time (daily) mapping for a region. Each single unit system 27, in the example effectively covers 500 square miles. A network of just 150 single unit systems could cover the entire Permian basin and provide daily trending of total basin methane concentrations.

[0058] Prior art existing Sun photometers and multi-channel radiometers are typically designed for aerosol optical thickness, water vapor, or broadband irradiance measurements rather than trace gas retrieval. Their filter bands are usually separated by tens to hundreds of nanometers, providing insufficient spectral resolution to isolate the fine absorption features of methane and other trace gases. In addition, many of these systems rely on precise direct-Sun tracking, which increases mechanical complexity and makes deployment in windy or harsh environments more difficult.

[0059] Prior art Differential Optical Absorption Spectroscopy (DOAS) instruments, both ground-based and satellite-borne, can resolve individual molecular absorption lines by using diffraction gratings or interferometers with high spectral resolution. However, these systems are typically bulky, expensive, and require stable thermal and mechanical environments. Field deployment often demands controlled enclosures, frequent calibration, and expert operation. As a result, DOAS instruments are not well suited for dense, low-maintenance monitoring networks intended to operate autonomously for long periods.

[0060] In addition, prior art satellite-based spectrometers provide valuable global coverage but suffer from coarse spatial and temporal resolution, variable surface albedo, and low signal levels due to long optical paths and diffuse reflection from the Earth’s surface. Clouds, aerosols, and viewing geometry further limit data availability, particularly for localized emissions and rapid temporal changes. Satellites typically revisit a given region only once per day or less, making it difficult to capture short-lived plumes or verifyPATENTAttorney Docket No.: JL002PCT the effectiveness of mitigation actions at facility scale. Furthermore, data processing can take weeks or months. Same day concentration mapping is not currently possible.

[0061] In contrast, and inventively, methods and apparatus disclosed herein as part of the TGMS1 employ three or more ultra-narrow bandpass filters having full-width halfmaximum values in the 2-4 nm range and positioned on and near a strong methane absorption cluster in the near infrared. The system uses a homogenizing optical train comprising high-count randomized fiber optic bundle 8, fiber bundle legs 4a, 4b, 4c and a Lambertian target 5 to deliver highly uniform, simultaneous optical signals to multiple detector channels. This architecture enables continuous differential absorption measurements with parts-per-billion precision using inexpensive components and a mechanically tolerant pointing scheme, which has not been described or suggested in the prior art.

[0062] Disclosed is a filter photometer design using ultra narrow band pass filters and a novel homogenizing and signal splitting design that has the ability to make differential absorbance measurements suitable for trace gas total column density determinations. A multichannel photometer that measures continuously and simultaneously with exceptional common mode rejection. A very compact, low-cost, low power system that measures total column methane density and methane concentrations real time (daily).

[0063] A system that can determine favorable conditions for measurement in real time based on bulk transmission levels.

[0064] A system that is largely immune to temperature and pressure effects by employing narrow bandpass filters that are broad relative to the temperature and pressure effects on absorbance peak broadening while narrow enough to produce excellent transmission contrast.

[0065] A “Sun photometer” or “radiometer” that does not need to track the Sun. A simple Lambertian target dramatically reduces positioning sensitivity and enhances uniform distribution.PATENTAttorney Docket No.: JL002PCT

[0066] Software that combines data from a regional network of systems to provide a high- resolution concentration map for a region that is updated on a daily basis.

Claims

PATENTAttorney Docket No.: JL002PCTWHAT IS CLAIMED IS:CLAIMS1. A trace gas measurement system, the system comprising: a diffuse reflector positioned to receive direct sunlight from the Sun and to provide diffusely reflected sunlight; a front focusing lens arranged with an optical axis directed toward the diffuse reflector and configured to collect the diffusely reflected sunlight and to focus the diffusely reflected sunlight; a fiber optic bundle having a common end optically coupled to the front focusing lens and optically coupled to at least three separate fiber bundle legs, the fiber optic bundle configured to produce a homogenized irradiance distribution from the diffusely reflected sunlight, , the fiber optic bundle being further configured to provide substantially proportional optical power distribution from the homogenized irradiance distribution among the fiber bundle legs; for each fiber bundle leg, an optical channel comprising: a collimating lens configured to receive a portion of the homogenized irradiance distribution from the corresponding fiber bundle leg and produce a collimated beam; an ultra-narrow bandpass optical filter positioned to receive the collimated beam and configured to transmit a corresponding wavelength band; a focusing lens positioned to receive light transmitted by the ultra-narrow bandpass optical filter; and a photodetector positioned to receive light from the focusing lens and generate a photocurrent signal representative of an intensity in the corresponding wavelength band; an electronics module comprising at least one analog-to-digital converter configured to convert the photocurrent signals from the photodetectors into digital values; and a processing unit configured to:PATENTAttorney Docket No.: JL002PCT acquire the digital values from the at least three optical channels substantially simultaneously and continuously over time; and determine a trace gas slant column density in a field of view.

2. The system of claim 1 , further comprising a homogenizing rod optically coupled to the front focusing lens and the fiber optic bundle and configured to output the homogenized irradiance distribution to the fiber optic bundle.

3. The system of claim 1 , wherein the diffuse reflector comprises a Lambertian target.

4. The system of claim 1 , wherein each ultra-narrow bandpass optical filter has a full-width half-maximum (FWHM) between about 2 nm and about 4 nm and wherein center wavelengths of at least two of the filters are separated by less than about 5 nm.

5. The system of claim 1 , wherein the trace gas comprises methane and the ultranarrow bandpass optical filters are centered on wavelengths within a methane absorption region between about 1650 nm and about 1680 nm, including: a first filter centered on a wavelength on a first side of a methane absorption cluster; a second filter centered substantially over the methane absorption cluster near about 1666 nm; and a third filter centered on a wavelength on an opposite side of the methane absorption cluster, such that the second filter experiences stronger methane absorbance than the first filter and third filter.

6. The system of claim 1 , wherein the photodetectors comprise extended-range Indium Gallium Arsenide (InGaAs) photodetectors and the at least one analog-to-PATENTAttorney Docket No.: JL002PCT digital converter has a resolution of at least 24 bits and is configured to sample each optical channel at a rate of at least about 1 Hz.

7. The system of claim 1 , wherein the fiber optic bundle comprises a high-count randomized fiber optic bundle and is configured such that, after calibration, an optical power balance among the at least three optical channels is maintained within about ±0.1 %.

8. The system of claim 1 , wherein the diffuse reflector is positioned substantially normal to a central axis of the front focusing lens such that the system does not require direct-Sun tracking within a solar disk perimeter to maintain uniform irradiance at the front focusing lens.

9. The system of claim 1 , further comprising a pan-and-tilt stage supporting at least the diffuse reflector and the front focusing lens, and a controller configured to reposition the pan-and-tilt stage based on calculated solar elevation and azimuth derived from geographic location, date, and time so that a front face of the diffuse reflector remains illuminated by the Sun throughout a day.

10. The system of claim 1 , wherein the electronics module further comprises a communication interface selected from the group consisting of a cellular modem and a wired network interface, the communication interface being configured to transmit trace gas slant column density data and associated metadata to a remote server.

11. The system of claim 1 , wherein the processing unit is further configured to: compute photocurrent ratios between at least two of the optical channels; apply calibration coefficients determined using a trace gas reference cell having a known trace gas partial pressure and path length to relate the photocurrent ratios to total trace gas; andPATENTAttorney Docket No.: JL002PCT correct for water vapor absorbance by applying modeled transmission coefficients derived from HITRAN -based simulations combined with measured filter transmission profiles.

12. The system of claim 1 , wherein the processing unit is further configured to: compute a solar zenith angle and azimuth from a geographic location, a date, and a time; determine an air-mass factor as a function of the solar zenith angle; normalize the trace gas slant column density by the air-mass factor and barometric pressure to produce a dry-air-normalized trace gas concentration; and assign a geographic position corresponding to a center of mass of an associated slanted air column as a function of solar elevation, solar azimuth, and an atmospheric density-versus-elevation model for each airmass-normalized trace gas concentration data.

13. A regional trace gas mapping system comprising: a plurality of trace gas measurement systems according to claim 1 , each deployed at a different geographic location and configured to produce a time stamped trace gas data comprising time-stamped trace gas slant column density or air-mass-normalized trace gas concentration data; and a server in communication with the plurality of trace gas measurement systems and configured to: receive the time-stamped trace gas data and associated location assignments from each of the plurality of trace gas measurement systems; and generate a two-dimensional regional trace gas concentration map by interpolating between the measured concentrations with assigned locations based on air column center of mass analysis.PATENTAttorney Docket No.: JL002PCT14. The regional trace gas mapping system of claim 13, wherein each trace gas measurement system effectively covers a span of tens of miles in an east-west direction and hundreds of square miles in area, and wherein the server is further configured to format the regional trace gas concentration map for display within a geographic information system.

15. A method of determining atmospheric trace gas slant column density, the method comprising: collecting near-infrared sunlight from the Sun after reflection from a diffuse reflector using a front focusing lens and producing a collected sunlight; directing the collected sunlight into a homogenizing element and producing, at an exit face of the homogenizing element, a homogenized irradiance distribution; coupling the homogenized irradiance distribution into a fiber optic bundle having a common end and at least three fiber bundle legs so that each fiber bundle leg receives a corresponding portion of the homogenized irradiance; for each fiber bundle leg, forming an optical channel by: producing a collimated light by collimating light from the fiber bundle leg; passing the collimated light through a corresponding ultra-narrow bandpass optical filter that isolates a wavelength band within a trace gas absorption region in the near infrared; focusing light transmitted by the corresponding ultra-narrow bandpass optical filter; and detecting the focused light with a corresponding photodetector to generate a photocurrent signal;PATENTAttorney Docket No.: JL002PCT substantially simultaneously converting the photocurrent signals from the at least three photodetectors into digital values using at least one analog- to-digital converter; and determining a trace gas slant column density in a field of view defined by the Sun elevation and azimuth, front focusing lens and the diffuse reflector by computing differential absorbance from relative transmission intensities across the wavelength bands of the ultra-narrow bandpass optical filters based on the digital values.

16. The method of claim 15, further comprising: performing a calibration procedure using a reference cell containing trace gas at a known partial pressure and having a known path length; measuring photocurrent ratios or logarithms of photocurrent ratios for at least one channel aligned with a trace gas absorption cluster and at least one adjacent reference channel at different trace gas contents in the reference cell; and deriving calibration coefficients or an empirical curve that relates the photocurrent ratios or logarithms of photocurrent ratios to total trace gas expressed as an equivalent length of pure trace gas at standard conditions.

17. The method of claim 15, further comprising: modeling water vapor absorbance using HITRAN-based spectral data combined with measured filter transmission profiles for the ultra-narrow bandpass optical filters; computing expected contributions of trace gas and water vapor to measured photocurrent ratios for at least two ratios selected from l3 / l2and l3 / li; andPATENTAttorney Docket No.: JL002PCT solving a set of equations that include contributions from trace gas and water vapor to obtain a water-vapor-corrected trace gas slant column density.

18. The method of claim 15, further comprising: determining a solar zenith angle and azimuth for each set of measurements from geographic location, date, and time; computing an air-mass factor as a function of the solar zenith angle; and normalizing the trace gas slant column density by the air-mass factor to obtain a vertical-equivalent trace gas column or concentration.

19. The method of claim 18, further comprising: measuring barometric pressure and relative humidity; and converting the normalized trace gas column into a dry-air-normalized trace gas concentration using the barometric pressure and relative humidity; and assigning a geographic position corresponding to a center of mass of an associated slanted air column as a function of solar elevation, solar azimuth, and an atmospheric density-versus-elevation model for each air- mass-normalized trace gas concentration data.

20. The method of claim 15, further comprising: repeatedly performing the collecting, directing, coupling, forming, detecting, converting, and determining steps at a sampling rate of at least about 1 Hz during daylight hours; and generating a time series of trace gas concentrations or slant column densities from sunrise to sunset for a fixed geographic location.

21. The method of claim 20, further comprising:PATENTAttorney Docket No.: JL002PCT monitoring bulk transmission levels or absolute photocurrent magnitudes in the at least three optical channels; detecting common-mode transients associated with clouds or other atmospheric changes; and applying at least one quality-control criterion based on the bulk transmission levels or transients to select or reject individual measurements for inclusion in the time series.

22. A method of generating a regional trace gas concentration map, the method comprising: deploying a plurality of trace gas measurement systems at different geographic locations, each trace gas measurement system comprising an optical train with a diffuse reflector, a front focusing lens, a homogenizing element, a fiber optic bundle having at least three fiber bundle legs, ultranarrow bandpass optical filters, and corresponding photodetectors configured to perform differential absorbance measurements of trace gas; for each trace gas measurement system, performing the method of claim 13 to obtain, throughout a day, trace gas concentrations or trace gas slant column densities that have been normalized for air mass; for each normalized trace gas measurement, assigning a geographic position corresponding to a center of mass of an associated slanted air column as a function of solar elevation, solar azimuth, and an atmospheric density-versus-elevation model; transmitting the normalized trace gas measurements and corresponding geographic position assignments from each trace gas measurement system to a central server; and at the central server, interpolating the normalized trace gas measurements between the geographic position assignments of the plurality of trace gas measurement systems to produce a two-dimensional regional trace gas concentration map.PATENTAttorney Docket No.: JL002PCT23. The method of claim 22, further comprising: spacing the plurality of trace gas measurement systems at east-west and north-south intervals such that each trace gas measurement system effectively covers hundreds of square miles; and updating the regional trace gas concentration map at least daily using measurements acquired during the corresponding day.

24. The method of claim 22, further comprising formatting the regional trace gas concentration map for display within a geographic information system and storing or displaying the map for use in tracking temporal trends in trace gas concentration over the region.

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