Trace Gas Measurements Research Articles

Assessing the ability to derive rates of polar middle-atmospheric descent using trace gas measurements from remote sensors

Abstract. We investigate the reliability of using trace gas measurements from remote sensing instruments to infer polar atmospheric descent rates during winter within 46–86 km altitude. Using output from the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM) between 2008 and 2014, tendencies of carbon monoxide (CO) volume mixing ratios (VMRs) are used to assess a common assumption of dominant vertical advection of tracers during polar winter. The results show that dynamical processes other than vertical advection are not negligible, meaning that the transport rates derived from trace gas measurements do not represent the mean descent of the atmosphere. The relative importance of vertical advection is lessened, and exceeded by other processes, during periods directly before and after a sudden stratospheric warming, mainly due to an increase in eddy transport. It was also found that CO chemistry cannot be ignored in the mesosphere due to the night-time layer of OH at approximately 80 km altitude. CO VMR profiles from the Kiruna Microwave Radiometer and the Microwave Limb Sounder were compared to SD-WACCM output, and show good agreement on daily and seasonal timescales. SD-WACCM CO profiles are combined with the CO tendencies to estimate errors involved in calculating the mean descent of the atmosphere from remote sensing measurements. The results indicate errors on the same scale as the calculated descent rates, and that the method is prone to a misinterpretation of the direction of air motion. The “true” rate of atmospheric descent is seen to be masked by processes, other than vertical advection, that affect CO. We suggest an alternative definition of the rate calculated using remote sensing measurements: not as the mean descent of the atmosphere, but as an effective rate of vertical transport for the trace gas under observation.

The Small Whiskbroom Imager for atmospheric compositioN monitorinG (SWING) and its operations from an unmanned aerial vehicle (UAV) during the AROMAT campaign

Abstract. The Small Whiskbroom Imager for atmospheric compositioN monitorinG (SWING) is a compact remote sensing instrument dedicated to mapping trace gases from an unmanned aerial vehicle (UAV). SWING is based on a compact visible spectrometer and a scanning mirror to collect scattered sunlight. Its weight, size, and power consumption are respectively 920 g, 27 cm × 12 cm × 8 cm, and 6 W. SWING was developed in parallel with a 2.5 m flying-wing UAV. This unmanned aircraft is electrically powered, has a typical airspeed of 100 km h−1, and can operate at a maximum altitude of 3 km. We present SWING-UAV experiments performed in Romania on 11 September 2014 during the Airborne ROmanian Measurements of Aerosols and Trace gases (AROMAT) campaign, which was dedicated to test newly developed instruments in the context of air quality satellite validation. The UAV was operated up to 700 m above ground, in the vicinity of the large power plant of Turceni (44.67∘ N, 23.41∘ E; 116 ma.s.l.). These SWING-UAV flights were coincident with another airborne experiment using the Airborne imaging differential optical absorption spectroscopy (DOAS) instrument for Measurements of Atmospheric Pollution (AirMAP), and with ground-based DOAS, lidar, and balloon-borne in situ observations. The spectra recorded during the SWING-UAV flights are analysed with the DOAS technique. This analysis reveals NO2 differential slant column densities (DSCDs) up to 13±0.6×1016 molec cm−2. These NO2 DSCDs are converted to vertical column densities (VCDs) by estimating air mass factors. The resulting NO2 VCDs are up to 4.7±0.4×1016 molec cm−2. The water vapour DSCD measurements, up to 8±0.15×1022 molec cm−2, are used to estimate a volume mixing ratio of water vapour in the boundary layer of 0.013±0.002 mol mol−1. These geophysical quantities are validated with the coincident measurements.

Sources of nitrous oxide and other climate relevant gases on surface area in a dairy free stall barn with solid floor and outside slurry storage

Livestock production systems in agriculture are one of the major emitters of greenhouse gases. So far, the focus of research in the dairy farm sector was primarily on ruminal methane (CH4) emissions. Emissions of nitrous oxide (N2O) usually arise from solid manure or in deep litter free stall barns. Release of N2O occurs as a result of interactions between organic material, nitrogen and moisture. Data of N2O emissions from modern dairy barns and liquid manure management systems are rare. Thus, the goal of this research was to determine the main sources of trace gas emissions at the dairy farm level, including N2O. Areas such as the scraped surface area where dry and wet conditions alternate are interesting. Possible sources of trace gases within and outside the barn were localised by measuring trace gas concentration rates from different dairy farm areas (e.g., areas covered with urine and excrement or slurry storage system) via the closed chamber technique. The results indicate typical emission ratios of carbon dioxide (CO2), CH4 and N2O in the various areas to generate comparable equivalent values. Calculated on the basis of nitrogen excretion from dairy cows, total emissions of N2O were much lower from barns than typically measured in fields. However, there were also areas within the barn with individual events and unexpected release factors of N2O concentrations such as urine patches, polluted areas and cubicles. Emission factors of N2O ranged from 1.1 to 5.0 mg m−2 d−1, respectively, for cleaned areas and urine patches. By considering the release factors of these areas and their proportion of the entire barn, total emission rates of 371 CO2-eq. LU−1 a−1, 36 CO2-eq. LU−1 a−1, and 1.7 kg CO2-eq. LU−1 a−1 for CO2, CH4 and N2O, respectively, were measured for the whole barn surface area. The CH4 emissions from surface area were stronger climate relevant comparing to N2O emissions, but compared to CH4 emissions from slurry storage or ruminal fermentation (not measured) even insignificant.

Role of ambient ammonia in particulate ammonium formation at a rural site in the North China Plain

Abstract. The real-time measurements of NH3 and trace gases were conducted, in conjunction with semi-continuous measurements of water-soluble ions in PM2.5 at a rural site in the North China Plain (NCP) from May to September 2013 in order to better understand chemical characteristics of ammonia and the impact of secondary ammonium aerosols on formation in the NCP. Extremely high NH3 and NH4+ concentrations were observed after a precipitation event within 7–10 days following urea application. Elevated NH3 levels coincided with elevated NH4+, indicating that NH3 likely influenced particulate ammonium mass. For the sampling period, the average conversion ∕ oxidation ratios for NH4+ (NHR), SO42- (SOR), and NO3- (NOR) were estimated to be 0.30, 0.64, and 0.24, respectively. The increased NH3 concentrations, mainly from agricultural activities and regional transport, coincided with the prevailing meteorological conditions. The high NH3 level with NHR of about 0.30 indicates that the emission of NH3 in the NCP is much higher than needed for aerosol acid neutralisation, and NH3 plays an important role in the formation of secondary aerosols as a key neutraliser. The hourly data obtained were used to investigate gas–aerosol partitioning characteristics using the thermodynamic equilibrium model ISORROPIA-II. Modelled SO42-, NO3-, and NH3 values agree well with the measurements, while the modelled NH4+ values largely underestimate the measurements. Our observation and modelling results indicate that strong acids in aerosol are completely neutralised. Additional NH4+ exists in aerosol, probably a result of the presence of a substantial amount of oxalic and other diacids.

An exploratory study on "people allergic to me" syndrome based on measurement of trace gases released from skin surface and microbial species in nasal cavity of a patient

An exploratory study on "people allergic to me" syndrome based on measurement of trace gases released from skin surface and microbial species in nasal cavity of a patient

Performances of a HGCDTE APD based direct detection lidar at 2 μm. Application to dial measurements

A lidar receiver with a direct detection chain adapted to a HgCdTe APD based detector with electric cooling is associated to a 2.05 μm Ho :YLF pulsed dual wavelength single mode transmitter to provide the first atmospheric lidar measurements using this technology. Experiments confirm the outstanding sensitivity of the detector and hightligth its huge potential for DIAL measurements of trace gas (CO2 and H2O) in this spectral domain. Performances of coherent vs direct detection at 2.05 μm is assessed.

Investigations into the use of multi-species measurements for source apportionment of the Indianapolis fossil fuel CO2 signal

Current bottom up estimates of CO2 emission fluxes are based on a mixture of direct and indirect flux estimates relying to varying degrees on regulatory or self-reported data. Hence, it is important to use additional, independent information to assess biases and lower the flux uncertainty. We explore the use of a self-organizing map (SOM) as a tool to use multi-species observations to partition fossil fuel CO2 (CO2ff) emissions by economic source sector. We use the Indianapolis Flux experiment (INFLUX) multi-species observations to provide constraints on the types of relationships we can expect to see, and show from the observations and existing knowledge of likely sources for these species that relationships do exist but can be complex. An Observing System Simulation Experiment (OSSE) is then created to test, in a pseudodata framework, the abilities and limitations of using an SOM to accurately attribute atmospheric tracers to their source sector. These tests are conducted for a variety of emission scenarios, and make use of the corresponding high-resolution footprints for the pseudo-measurements. We show here that the attribution of sector-specific emissions to measured trace gases cannot be addressed by investigating the atmospheric trace gas measurements alone. We conclude that additional a priori information such as inventories of sector-specific trace gases are required to evaluate sector-level emissions using atmospheric methods, to overcome the challenge of the spatial overlap of nearly every predefined source sector. Our OSSE additionally allows us to demonstrate that increasing the (already high) data density cannot solve the co-localization problem.

A review of four decades of atmospheric trace gas measurements at Cape Point, South Africa

South Africa’s Cape Point (CPT) trace gas observatory, operated by the South African Weather Service (SAWS), has been monitoring mole fractions (mol/vol) of ambient greenhouse gases (GHGs), e.g. ca...

Detection of nitrous oxide by resonant photoacoustic spectroscopy based on mid infrared quantum cascade laser

Atmospheric greenhouse gases have great influence on the climate forcing, which is important to human being and also for natural systems. Nitrous oxide (N2O), such as carbon dioxide and methane, is an important greenhouse gas. It plays an important role in the atmospheric environment. Therefore, sensitive measurement of N2O concentration is of significance for studying the atmospheric environment. In this paper, a photoacoustic spectroscopy (PAS) system based on 7.6 m mid infrared quantum cascade laser combined with resonant PAS technique is established for the sensitive detection of N2O concentration. The PAS has been regarded as a highly sensitive and selective technique to measure trace gases. Compared with laser absorption spectroscopy, the PAS offers several intrinsic attractive features including ultra-compact size and no cross-response of light scattering. In addition, the signal of PAS is recorded with low-cost wavelength-independent acoustic transducer. The performance of the developed system is optimized and improved based on the traditional photoacoustic spectroscopic detection. Dual beam enhancement method is used to increase the effective optical power which effectively improves the detection sensitivity of the system. The N2O absorption line at 1307.66 cm-1 is chosen as the target line, and an operation pressure of 50 kPa is selected for reducing cross-talking from H2O absorption line. By detecting the photoacoustic signals of a certain concentration of N2O at different modulation frequencies and modulation amplitudes, the optimal modulation frequency and modulation amplitude of the system are determined to be 800 Hz and 90 mV, respectively. Different concentrations of N2O gas are detected under the optimized parameters, and calibration curve of the system, that is, the curve of photoacoustic signal versus concentration of N2O is obtained, which shows good linearity. The experimental results show that the minimum detection limit of the system is 150 ppb at a pressure of 50 kPa with an integration time of 30 ms. The system noise can be further reduced by increasing the averaging time. A minimum detection limit of 37 ppb is achieved by averaging signals 100 times, and the signal of N2O in the atmosphere is obtained.

Highly precise and real-time measurements of 13CO2/12CO2 isotopic ratio in breath using a 2 μm diode laser

Real-time breath gas analysis with high accuracy, precision and time resolution, as a promising, non-invasive, fast and reliable tool, is important in medical diagnostics. Especially stable isotopologues of carbon dioxide is applied to multiple research areas including the diagnosis of gastrointestinal diseases. Helicobacter pylori (H. pylori) is one of the most frequent bacterial infectious diseases in human beings and is now recognized as one of the key risk factors for chronic gastritis, peptic ulcers, stomach cancer and lymphoma. In contrast to traditional invasive tests, the most reliable non-invasive method in the diagnosis of the H. pylori infection is considered to be 13C-urea breath test which is implemented by measuring the 13CO2/12CO2 isotope ratio in human breath. Tunable diode laser absorption spectroscopy (TDLAS) has the advantages of fast response, low drift, good gas selectivity and high detection sensitivity, and it is very convenient to develop a high precision, real-time and online measurement system. A precision laser spectrometer for the measurement of CO2 isotope abundance in human breath (with CO2 concentration of 4%-5%) or high concentration gas is designed and evaluated based on TDLAS technology. The spectrometer contains a novel compact dense-pattern multipass cell with a small volume of 280 cm3 and an effective optical path length of 26. 4 m. The cell is in conjunction with a fiber-coupled distributed feedback diode laser operating at 2.008 μm. Wavelength modulation spectroscopy approach is used. The mass flow, pressure and temperature of the cell are actively controlled, and able to keep long-term stability. The influence of laser power fluctuation is eliminated by fitting the baseline with cubic polynomial to normalize the raw spectrum. Moving window regression is used to remove the influence of frequency drift on measuring isotope abundance. The system measurement precision is improved by wavelet denosing and Kalman filtering. The experimental results demonstrate that moving window regression method not only extends the stability time of the system but also improves the measurement precision of isotope abundance well, the wavelet denoising improves the signal-to-noise ratio by 2 times that by the method of multi-spectral average, the stability time of the system is 100 s given by Allan variance, and the measurement precision of CO2 isotope ratio is 0. 067‰ after Kalman filtering. The use of small multi-pass cell and the default of denoising devices make the system more portable and improve the real-time and online measurement performance of the system. In addition to the measurement of 13CO2/12CO2 isotope ratio in human breath, by replacing different lasers, the spectrometer can also be used to measure trace gas concentration and the stable isotope abundance of many gas molecules in atmosphere. Therefore, the spectrometer will have broad applications in the areas of medical diagnosis, carbon cycle study and environmental monitoring.

Investigating Local and Remote Terrestrial Influence on Air Masses at Contrasting Antarctic Sites Using Radon‐222 and Back Trajectories

AbstractWe report on the first summer of high‐sensitivity radon measurements from a two‐filter detector at Jang Bogo Station (Terra Nova Bay) and contrast them with simultaneous observations at King Sejong Station (King George Island). King Sejong radon concentrations were characteristic of a marine baseline station (0.02–0.3 Bq m−3), whereas Jang Bogo values were highly variable (0.06–5.2 Bq m−3), mainly due to emissions from exposed coastal ground (estimated mean flux 0.09–0.11 atoms cm−2 s−1) and shallow atmospheric mixing depths. For wind speeds of ≤3.5 m s−1 the influence of local radon emissions became increasingly more prominent at both sites. A cluster analysis of back trajectories from King Sejong (62°S) revealed a fairly even distribution between air masses that had passed recently over South America, the Southern Ocean, and Antarctica, whereas at Jang Bogo (75°S) 80% of events had recently passed over the Ross Ice Shelf and West Antarctica, 12% were synoptically forced over Cape Adare, and 8% were associated with subsidence over the Antarctic interior and katabatic flow to the station. When cross‐checked against radon concentrations, only half of the back trajectories ending at Jang Bogo that had indicated distant contact with nonpolar southern hemisphere continents within the past 10 days showed actual signs of terrestrial influence. A simple‐to‐implement technique based on high‐pass filtered absolute humidity is developed to distinguish between predominantly katabatic, oceanic, and near‐coastal air masses for characterization of trace gas and aerosol measurements at coastal East Antarctic sites.

Source apportionment of particulate matter and trace gases near a major refinery near the Houston Ship Channel

From February 7 to 27, 2015, a mobile air quality laboratory was deployed to a location proximate to a major refinery, the Port of Houston, and several neighborhoods to conduct measurements of atmospheric trace gases and particulate matter. Two statistical models were utilized to apportion the sources of pollution impacting this site and the denizens of the nearby neighborhoods. Positive matrix factorization (PMF) was performed on the organic signal of the aerosol mass spectra, resulting in five factors totaling an average of 4.1 μg/m3 of the organic aerosol: hydrocarbon-like (0.67 μg/m3), cooking (0.35 μg/m3) biomass burning (1.14 μg/m3), low-volatility oxidized (1.15 μg/m3), and semi-volatile oxidized (0.78 μg/m3). Principal component analysis was performed on daytime and nighttime data, including concentrations from PMF output, of other PM1 components, and of trace gases. This generated five daytime and five nighttime factors that explained 74.5% and 73.0% of the variance, respectively. The most important factors impacting this site were from mobile source exhaust and petrochemical aromatic compound emissions. Together these two factors also constitute most of the observed carcinogens.

The novel HALO mini-DOAS instrument: inferring trace gas concentrations from airborne UV/visible limb spectroscopy under all skies using the scaling method

Abstract. We report on a novel six-channel optical spectrometer (further on called mini-DOAS instrument) for airborne nadir and limb measurements of atmospheric trace gases, liquid and solid water, and spectral radiances in the UV/vis and NIR spectral ranges. The spectrometer was developed for measurements from aboard the German High-Altitude and Long-Range (HALO) research aircraft during dedicated research missions. Here we report on the relevant instrumental details and the novel scaling method used to infer the mixing ratios of UV/vis absorbing trace gases from their absorption measured in limb geometry. The uncertainties of the scaling method are assessed in more detail than before for sample measurements of NO2 and BrO. Some first results are reported along with complementary measurements and comparisons with model predictions for a selected HALO research flight from Cape Town to Antarctica, which was performed during the research mission ESMVal on 13 September 2012.

A CO2Profile Retrieving Method Based on Chebyshev Fitting for Ground-Based DIAL

The vertical profile of atmospheric CO2 is of great scientific significance in identifying carbon sinks and sources, and estimating CO2 emissions or uptakes. Differential absorption Light Detection And Ranging (DIAL), has been widely accepted as the most promising technique to sense atmospheric CO2. The classical method to retrieve measurements, generated from range-resolved detection, is derived from differentiating the measured column content, but its performance in dealing with aerosol backscatter signals is poor. To address this issue, this paper proposes a derivative method, which is based on Chebyshev fitting to the measured differential absorption optical depth. We created a performance evaluation model to assess the performance of the proposed method. Simulations revealed that the error of a single CO2 profile in data retrieval can be reduced to less than 4 ppm in 6000 m. The precision of long-term mean CO2 profile is expected to be less than 1 ppm. We believe that this novel method can be used in other applications also, e.g., trace gas measurements collected using DIAL, especially when the signal-to-noise-ratio of received signal is small.

Linking NO and N2O emission pulses with the mobilization of mineral and organic N upon rewetting dry soils

Drying and rewetting of soils triggers a cascade of physical, chemical, and biological processes; understanding these responses to varying moisture levels becomes increasingly important in the context of changing precipitation patterns. When soils dry and water content decreases, diffusion is limited and substrates can accumulate. Upon rewetting, these substrates are mobilized and can energize hot moments of intense biogeochemical cycling, leading to pulses of trace gas emissions. Until recently, it was difficult to follow the rewetting dynamics of nutrient cycling in the field without physically disturbing the soil. Here we present a study that combines real-time trace gas measurements with high-resolution measurements of diffusive nutrient fluxes in intact soils. Our goal was to distinguish the contribution of different inorganic and organic nitrogen (N) forms to the rewetting substrate flush and the production of nitric oxide (NO) and nitrous oxide (N2O). Diffusive flux of N-bearing substrates (NO2−, NO3−, NH4+ and amino acids) was determined in situ in hourly resolution using a microdialysis approach. We conducted an irrigation experiment in a semi-arid California grassland at the end of the dry season, and followed soil N flux and N trace gas emissions over the course of 30 h post-wetting. Upon rewetting, both inorganic and organic N diffused through the soil, with inorganic N contributing most to the rewetting N flush. Emissions of NO and N2O rapidly increased and remained elevated for the duration of our measurements, whereas diffusive soil N flux was characterized by large temporal variation. Immediately after rewetting, NO3− contributed 80% to the total diffusive N flux but was consumed rapidly, possibly due to fast microbial uptake or denitrification. Ammonium flux contributed only ∼10% to the initial diffusive N flux, but it dominated total N diffusion 27 h post-wetting, coinciding with peak N-gas emissions. This suggests nitrification may control most of the N trace gases produced during the late stages of a rewetting pulse. Nitrite contributed only 1% to total N diffusion and did not show a clear temporal pattern. Amino acids contributed roughly as much as NH4+ to the initial diffusive N flux, but the organic N pulse was short-lived, indicating that organic N did not contribute substantially to N-gas formation shortly after rewetting at our study site. Our results support the hypothesis that in semi-arid environments N-bearing substrates concentrate during dry periods and, upon rewetting, can lead to pulses of NO and N2O when they react chemically or are transformed by microorganisms.

A ppb level sensitive sensor for atmospheric methane detection

A high sensitivity sensor, combining a multipass cell and wavelength modulation spectroscopy in the near infrared spectral region was designed and implemented for trace gas detection. The effective length of the multipass cell was about 290 meters. The developed spectroscopic technique demonstrates an improved sensitivity of methane in ambient air and a relatively short detection time compared to previously reported sensors. Home-built electronics and software were employed for diode laser frequency modulation, signal lock-in detection and processing. A dual beam scheme and a balanced photo-detector were implemented to suppress the intensity modulation and noise for better detection sensitivity. The performance of the sensor was evaluated in a series of measurements ranging from three hours to two days. The average methane concentration measured in ambient air was 2.01ppm with a relative error of±2.5%. With Allan deviation analysis, it was found that the methane detection limit of 1.2ppb was achieved in 650s. The developed sensor is compact and portable, and thus it is well suited for field measurements of methane and other trace gases.

Application of Gauss's theorem to quantify localized surface emissions from airborne measurements of wind and trace gases

Abstract. Airborne estimates of greenhouse gas emissions are becoming more prevalent with the advent of rapid commercial development of trace gas instrumentation featuring increased measurement accuracy, precision, and frequency, and the swelling interest in the verification of current emission inventories. Multiple airborne studies have indicated that emission inventories may underestimate some hydrocarbon emission sources in US oil- and gas-producing basins. Consequently, a proper assessment of the accuracy of these airborne methods is crucial to interpreting the meaning of such discrepancies. We present a new method of sampling surface sources of any trace gas for which fast and precise measurements can be made and apply it to methane, ethane, and carbon dioxide on spatial scales of ∼ 1000 m, where consecutive loops are flown around a targeted source region at multiple altitudes. Using Reynolds decomposition for the scalar concentrations, along with Gauss's theorem, we show that the method accurately accounts for the smaller-scale turbulent dispersion of the local plume, which is often ignored in other average mass balance methods. With the help of large eddy simulations (LES) we further show how the circling radius can be optimized for the micrometeorological conditions encountered during any flight. Furthermore, by sampling controlled releases of methane and ethane on the ground we can ascertain that the accuracy of the method, in appropriate meteorological conditions, is often better than 10 %, with limits of detection below 5 kg h−1 for both methane and ethane. Because of the FAA-mandated minimum flight safe altitude of 150 m, placement of the aircraft is critical to preventing a large portion of the emission plume from flowing underneath the lowest aircraft sampling altitude, which is generally the leading source of uncertainty in these measurements. Finally, we show how the accuracy of the method is strongly dependent on the number of sampling loops and/or time spent sampling the source plume.

Historic global biomass burning emissions for CMIP6 (BB4CMIP) based on merging satellite observations with proxies and fire models (1750–2015)

Abstract. Fires have influenced atmospheric composition and climate since the rise of vascular plants, and satellite data have shown the overall global extent of fires. Our knowledge of historic fire emissions has progressively improved over the past decades due mostly to the development of new proxies and the improvement of fire models. Currently, there is a suite of proxies including sedimentary charcoal records, measurements of fire-emitted trace gases and black carbon stored in ice and firn, and visibility observations. These proxies provide opportunities to extrapolate emission estimates back in time based on satellite data starting in 1997, but each proxy has strengths and weaknesses regarding, for example, the spatial and temporal extents over which they are representative. We developed a new historic biomass burning emissions dataset starting in 1750 that merges the satellite record with several existing proxies and uses the average of six models from the Fire Model Intercomparison Project (FireMIP) protocol to estimate emissions when the available proxies had limited coverage. According to our approach, global biomass burning emissions were relatively constant, with 10-year averages varying between 1.8 and 2.3 Pg C yr−1. Carbon emissions increased only slightly over the full time period and peaked during the 1990s after which they decreased gradually. There is substantial uncertainty in these estimates, and patterns varied depending on choices regarding data representation, especially on regional scales. The observed pattern in fire carbon emissions is for a large part driven by African fires, which accounted for 58 % of global fire carbon emissions. African fire emissions declined since about 1950 due to conversion of savanna to cropland, and this decrease is partially compensated for by increasing emissions in deforestation zones of South America and Asia. These global fire emission estimates are mostly suited for global analyses and will be used in the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations.

Intercomparison of open-path trace gas measurements with two dual-frequency-comb spectrometers

Abstract. We present the first quantitative intercomparison between two open-path dual-comb spectroscopy (DCS) instruments which were operated across adjacent 2 km open-air paths over a 2-week period. We used DCS to measure the atmospheric absorption spectrum in the near infrared from 6023 to 6376 cm−1 (1568 to 1660 nm), corresponding to a 355 cm−1 bandwidth, at 0.0067 cm−1 sample spacing. The measured absorption spectra agree with each other to within 5 × 10−4 in absorbance without any external calibration of either instrument. The absorption spectra are fit to retrieve path-integrated concentrations for carbon dioxide (CO2), methane (CH4), water (H2O), and deuterated water (HDO). The retrieved dry mole fractions agree to 0.14 % (0.57 ppm) for CO2, 0.35 % (7 ppb) for CH4, and 0.40 % (36 ppm) for H2O at ∼ 30 s integration time over the 2-week measurement campaign, which included 24 °C outdoor temperature variations and periods of strong atmospheric turbulence. This agreement is at least an order of magnitude better than conventional active-source open-path instrument intercomparisons and is particularly relevant to future regional flux measurements as it allows accurate comparisons of open-path DCS data across locations and time. We additionally compare the open-path DCS retrievals to a World Meteorological Organization (WMO)-calibrated cavity ring-down point sensor located along the path with good agreement. Short-term and long-term differences between the open-path DCS and point sensor are attributed, respectively, to spatial sampling discrepancies and to inaccuracies in the current spectral database used to fit the DCS data. Finally, the 2-week measurement campaign yields diurnal cycles of CO2 and CH4 that are consistent with the presence of local sources of CO2 and absence of local sources of CH4.

Intercomparison of Open-Path Trace Gas Measurements with Two Dual Frequency Comb Spectrometers.

We present the first quantitative intercomparison between two open-path dual comb spectroscopy (DCS) instruments which were operated across adjacent 2-km open-air paths over a two-week period. We used DCS to measure the atmospheric absorption spectrum in the near infrared from 6021 to 6388 cm-1 (1565 to 1661 nm), corresponding to a 367 cm-1 bandwidth, at 0.0067 cm-1 sample spacing. The measured absorption spectra agree with each other to within 5×10-4 without any external calibration of either instrument. The absorption spectra are fit to retrieve concentrations for carbon dioxide (CO2), methane (CH4), water (H2O), and deuterated water (HDO). The retrieved dry mole fractions agree to 0.14% (0.57 ppm) for CO2, 0.35% (7 ppb) for CH4, and 0.40% (36 ppm) for H2O over the two-week measurement campaign, which included 23 °C outdoor temperature variations and periods of strong atmospheric turbulence. This agreement is at least an order of magnitude better than conventional active-source open-path instrument intercomparisons and is particularly relevant to future regional flux measurements as it allows accurate comparisons of open-path DCS data across locations and time. We additionally compare the open-path DCS retrievals to a WMO-calibrated cavity ringdown point sensor located along the path with good agreement. Short-term and long-term differences between the two systems are attributed, respectively, to spatial sampling discrepancies and to inaccuracies in the current spectral database used to fit the DCS data. Finally, the two-week measurement campaign yields diurnal cycles of CO2 and CH4 that are consistent with the presence of local sources of CO2 and absence of local sources of CH4.