Recent improvements of long-path DOAS measurements: impact on accuracy and stability of short-term and automated long-term observations

Abstract. Over the last few decades, differential optical absorption spectroscopy (DOAS) has been used as a common technique to simultaneously measure abundances of a variety of atmospheric trace gases. Exploiting the unique differential absorption cross section of trace-gas molecules, mixing ratios can be derived by measuring the optical density along a defined light path and by applying the Beer–Lambert law. Active long-path (LP-DOAS) instruments can detect trace gases along a light path of a few hundred metres up to 20 km, with sensitivities for mixing ratios down to ppbv and pptv levels, depending on the trace-gas species. To achieve high measurement accuracy and low detection limits, it is crucial to reduce instrumental artefacts that lead to systematic structures in the residual spectra of the analysis. Spectral residual structures can be introduced by most components of a LP-DOAS measurement system, namely by the light source, in the transmission of the measurement signal between the system components or at the level of spectrometer and detector. This article focuses on recent improvements by the first application of a new type of light source and consequent changes to the optical setup to improve measurement accuracy. Most state-of-the-art LP-DOAS instruments are based on fibre optics and use xenon arc lamps or light-emitting diodes (LEDs) as light sources. Here we present the application of a laser-driven light source (LDLS), which significantly improves the measurement quality compared to conventional light sources. In addition, the lifetime of LDLS is about an order of magnitude higher than of typical Xe arc lamps. The small and very stable plasma discharge spot of the LDLS allows the application of a modified fibre configuration. This enables a better light coupling with higher light throughput, higher transmission homogeneity, and a better suppression of light from disturbing wavelength regions. Furthermore, the mode-mixing properties of the optical fibre are enhanced by an improved mechanical treatment. The combined effects lead to spectral residual structures in the range of 5-10×10-5 root mean square (rms; in units of optical density). This represents a reduction of detection limits of typical trace-gas species by a factor of 3–4 compared to previous setups. High temporal stability and reduced operational complexity of this new setup allow the operation of low-maintenance, automated LP-DOAS systems, as demonstrated here by more than 2 years of continuous observations in Antarctica.

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Recent improvements of Long-Path DOAS measurements: impact on accuracy and stability of short-term and automated long-term observations

10.5194/amt-2019-69 ◽

2019 ◽

Cited By ~ 1

Author(s):

Jan-Marcus Nasse ◽

Philipp G. Eger ◽

Denis Pöhler ◽

Stefan Schmitt ◽

Udo Frieß ◽

...

Keyword(s):

Light Source ◽

Optical Density ◽

Measurement Accuracy ◽

Trace Gases ◽

Detection Limits ◽

Trace Gas ◽

Light Sources ◽

Light Path ◽

Mixing Ratios ◽

Spectral Residual

Abstract. Over the last decades, Differential Optical Absorption Spectroscopy (DOAS) has been used as a common technique to simultaneously measure abundances of a variety of atmospheric trace gases. Exploiting the unique differential absorption cross section of trace gas molecules, mixing ratios can be derived by measuring the optical density along a defined light path and by applying the Beer-Lambert law. Active long-path (LP-DOAS) instruments can detect trace gases along a light path of a few hundred metres up to 20 km with sensitivities for mixing ratios down to ppbv and pptv levels, depending on the trace gas species. To achieve high measurement accuracy and low detection limits, it is crucial to reduce instrumental artefacts that lead to systematic structures in the residual spectra of the analysis. Spectral residual structures can be introduced by most components of a LP-DOAS measurement system, namely by the light source, in the transmission of the measurement signal between the system components or at the level of spectrometer and detector. This article focuses on recent improvements by the first application of a new type of light source and consequent changes to the optical setup to improve measurement accuracy. Most state-of-the-art LP-DOAS instruments are based on fibre optics and use xenon arc lamps or light emitting diodes (LEDs) as light sources. Here we present the application of a Laser Driven Light Source (LDLS), which significantly improves the measurement quality compared to conventional light sources. In addition the lifetime of LDLS is about an order of magnitude higher than of typical Xe-arc lamps. The small and very stable plasma discharge spot of the LDLS allows the application of a modified fibre configuration. This enables a better light coupling with higher light throughput, higher transmission homogeneity, and a better suppression of light from disturbing wavelength regions. Furthermore, the mode mixing properties of the optical fibre are enhanced by an improved mechanical treatment. The combined effects lead to spectral residual structures in the range of 5–10 · 10−5 RMS (in units of optical density). This represents a reduction of detection limits of typical trace gas species by a factor of 3–4 compared to previous setups. High temporal stability and reduced operational complexity of this new setup allow the operation of low-maintenance automated LP-DOAS systems as demonstrated here by more than two years of continuous observations in Antarctica.

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Trace gases and organic aerosol at a rural site in Vietnam during large scale biomass burning

10.5194/egusphere-egu21-181 ◽

2021 ◽

Author(s):

Simone M. Pieber ◽

Dac-Loc Nguyen ◽

Hendryk Czech ◽

Stephan Henne ◽

Nicolas Bukowiecki ◽

...

Keyword(s):

Air Quality ◽

Chemical Composition ◽

Biomass Burning ◽

Large Scale ◽

Trace Gases ◽

Organic Aerosol ◽

Trace Gas ◽

Rural Site ◽

Air Masses ◽

Mixing Ratios

Open biomass burning (BB) is a globally widespread phenomenon. The fires release pollutants, which are harmful for human and ecosystem health and alter the Earth's radiative balance. Yet, the impact of various types of BB on the global radiative forcing remains poorly constrained concerning greenhouse gas emissions, BB organic aerosol (OA) chemical composition and related light absorbing properties. Fire emissions composition is influenced by multiple factors (e.g., fuel and thereby vegetation-type, fuel moisture, fire temperature, available oxygen). Due to regional variations in these parameters, studies in different world regions are needed. Here we investigate the influence of seasonally recurring BB on trace gas concentration and air quality at the regional Global Atmosphere Watch (GAW) station Pha Din (PDI) in rural Northwestern Vietnam. PDI is located in a sparsely populated area on the top of a hill (1466 m a.s.l.) and is well suited to study the large-scale fires on the Indochinese Peninsula, whose pollution plumes are frequently transported towards the site [1]. We present continuous trace gas observations of CO2, CH4, CO, and O3 conducted at PDI since 2014 and interpret the data with atmospheric transport simulations. Annually recurrent large scale BB leads to hourly time-scale peaks CO mixing ratios at PDI of 1000 to 1500 ppb around every April since the start of data collection in 2014. We complement this analysis with carbonaceous PM2.5 chemical composition analyzed during an intensive campaign in March-April 2015. This includes measurements of elemental and organic carbon (EC/OC) and more than 50 organic markers, such as sugars, PAHs, fatty acids and nitro-aromatics [2]. For the intensive campaign, we linked CO, CO2, CH4 and O3 mixing ratios to a statistical classification of BB events, which is based on OA composition. We found increased CO and O3 levels during medium and high BB influence during the intensive campaign. A backward trajectory analysis confirmed different source regions for the identified periods based on the OA cluster. Typically, cleaner air masses arrived from northeast, i.e., mainland China and Yellow sea during the intensive campaign. The more polluted periods were characterized by trajectories from southwest, with more continental recirculation of the medium cluster, and more westerly advection for the high cluster. These findings highlight that BB activities in Northern Southeast Asia significantly enhances the regional OA loading, chemical PM2.5 composition and the trace gases in northwestern Vietnam. The presented analysis adds valuable data on air quality in a region of scarce data availability.&#160;REFERENCES[1] Bukowiecki, N. et al. Effect of Large-scale Biomass Burning on Aerosol Optical Properties at the GAW Regional Station Pha Din, Vietnam. AAQR. 19, 1172&#8211;1187 (2019).[2] Nguyen, D. L, et al. Carbonaceous aerosol composition in air masses influenced by large-scale biomass burning: a case-study in Northwestern Vietnam. ACPD., https://doi.org/10.5194/acp-2020-1027, in review, 2020.

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Impacts of updated spectroscopy on thermal infrared retrievals of methane evaluated with HIPPO data

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-7-10059-2014 ◽

2014 ◽

Vol 7 (9) ◽

pp. 10059-10107

Author(s):

M. J. Alvarado ◽

V. H. Payne ◽

K. E. Cady-Pereira ◽

J. D. Hegarty ◽

S. S. Kulawik ◽

...

Keyword(s):

Nitrous Oxide ◽

Trace Gases ◽

Thermal Infrared ◽

Trace Gas ◽

Radiative Transfer Model ◽

Transfer Model ◽

Spectroscopic Parameters ◽

Mixing Ratios ◽

Aircraft Observations ◽

The Impact

Abstract. Errors in the spectroscopic parameters used in the forward radiative transfer model can introduce altitude-, spatially-, and temporally-dependent biases in trace gas retrievals. For well-mixed trace gases such as methane, where the variability of tropospheric mixing ratios is relatively small, reducing such biases is particularly important. We use aircraft observations from all five missions of the HIAPER Pole-to-Pole Observations (HIPPO) of the Carbon Cycle and Greenhouse Gases Study to evaluate the impact of updates to spectroscopic parameters for methane (CH4), water vapor (H2O), and nitrous oxide (N2O) on thermal infrared retrievals of methane from the NASA Aura Tropospheric Emission Spectrometer (TES). We find that updates to the spectroscopic parameters for CH4 result in a substantially smaller mean bias in the retrieved CH4 when compared with HIPPO observations. After an N2O-based correction, the bias in TES methane upper tropospheric representative values for measurements between 50° S and 50° N decreases from 56.9 to 25.7 ppbv, while the bias in the lower tropospheric representative value increases only slightly (from 27.3 to 28.4 ppbv). For retrievals with less than 1.6 DOFS, the bias is reduced from 26.8 to 4.8 ppbv. We also find that updates to the spectroscopic parameters for N2O reduce the errors in the retrieved N2O profile.

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Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) – applicability and corrections

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-1-481-2008 ◽

2008 ◽

Vol 1 (1) ◽

pp. 481-507 ◽

Cited By ~ 1

Author(s):

U. Platt ◽

J. Meinen ◽

D. Pöhler ◽

T. Leisner

Keyword(s):

Absorption Spectroscopy ◽

Broad Band ◽

Gas Absorption ◽

Trace Gas ◽

Optical Absorption Spectroscopy ◽

Light Sources ◽

Light Path ◽

Gas Concentration ◽

Mirror Reflectivity ◽

Gas Measurements

Abstract. Atmospheric trace gas measurements by cavity assisted long-path absorption spectroscopy are an emerging technology. An interesting approach is the combination of CEAS with broad band light sources, the broad-band CEAS (BB-CEAS). BB-CEAS lends itself to the application of the DOAS technique to analyse the derived absorption spectra. While the DOAS approach has enormous advantages in terms of sensitivity and specificity of the measurement, an important implication is the reduction of the light path by the trace gas absorption, since cavity losses due to absorption by gases reduce the quality (Q) of the cavity. In fact, at wavelength, where the quality of the BB-CEAS cavity is dominated by the trace gas absorption (esp. at very high mirror reflectivity), the light path will vary inversely with the trace gas concentration and the strength of the band will become nearly independent of the trace gas concentration c in the cavity, rendering the CEAS Method useless for trace gas measurements. Only in the limiting case where the mirror reflectivity determines Q at all wavelength, the strength of the band as seen by the BB-CEAS instrument becomes proportional to the concentration c. We investigate these relationships in detail and present methods to correct for the cases between the two above extremes, which are of course the important ones in practice.

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Improved Sensitivity of Infrared Spectroscopy by the Application of Least Squares Methods

Applied Spectroscopy ◽

10.1366/0003702804731258 ◽

1980 ◽

Vol 34 (5) ◽

pp. 539-548 ◽

Cited By ~ 129

Author(s):

David M. Haaland ◽

Robert G. Easterling

Keyword(s):

Infrared Spectroscopy ◽

Least Squares ◽

Trace Gases ◽

Region Of Interest ◽

Detection Limits ◽

Trace Gas ◽

Reference Spectrum ◽

Baseline Shift ◽

Data Points ◽

The Individual

Improved sensitivity and precision in the quantitative analysis of trace gases by Fourier transform infrared spectroscopy have been achieved by the application of new spectral least squares methods. By relating all of the spectral information present in the reference spectrum of a trace gas to that of the unknown sample and by appropriately fitting the baseline, detections of trace gases can be obtained even though the individual spectral features may lie well below the noise level. Four least squares methods incorporating different baseline assumptions were investigated and compared using calibrated gases of CO, N2O, and CO2 in dry air. These methods include: (I) baseline known, (II) baseline linear over the spectral region of interest, (III) baseline linear over each spectral peak, and (IV) negligible baseline shift between successive data points. Methods III and IV were found to be most reliable for the gases studied. When method III is applied to the spectra of these trace gases, detection limits improved by factors of 5 to 7 over conventional methods applied to the same data. “Three sigma” detection limits are equal to 0.6, 0.2, and 0.08 ppm for CO, N2O, and CO2, respectively, when a 10-cm pathlength at a total pressure of 640 Torr is used with a ∼35 min measurement time at 0.06 cm−1 resolution.

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Seasonal cycles and variability of O3 and H2O in the UT/LMS during SPURT

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-5-7247-2005 ◽

2005 ◽

Vol 5 (4) ◽

pp. 7247-7282

Author(s):

M. Krebsbach ◽

C. Schiller ◽

D. Brunner ◽

G. Günther ◽

M. I. Hegglin ◽

...

Keyword(s):

Trace Gases ◽

Distribution Functions ◽

Trace Gas ◽

Large Set ◽

Transport Barrier ◽

Mixing Ratios ◽

Tropical Tropopause ◽

Time Period ◽

Data Coverage

Abstract. Airborne high resolution in situ measurements of a large set of trace gases including ozone (O3) and total water (H2O) in the upper troposphere and the lowermost stratosphere (UT/LMS) have been performed above Europe within the SPURT project. With its innovative campaign concept, SPURT provides an extensive data coverage of the UT/LMS in each season within the time period between November 2001 and July 2003. Ozone volume mixing ratios in the LMS show a distinct spring maximum and autumn minimum, whereas the O3 seasonal cycle in the UT is shifted by 2 to 3 month later towards the end of the year. The more variable H2O measurements reveal a maximum during spring/summer and a minimum during autumn/winter with no phase shift between the two atmospheric compartments. For a comprehensive insight into trace gas composition and variability in the UT/LMS several statistical methods are applied using chemical, thermal and dynamical vertical coordinates. In particular, 2-dimensional probability distribution functions serve as a tool to transform localised aircraft data to a more comprehensive view of the probed atmospheric region. It appears that both trace gases, O3 and H2O, reveal the most compact arrangement and are best correlated in the view of potential vorticity (PV) and distance to the local tropopause, indicating an advanced mixing state on these surfaces. Thus, strong gradients of PV seem to act as a transport barrier both in the vertical and the horizontal direction. The alignment of trace gas isopleths reflects the existence of a year-round extra-tropical tropopause transition layer. The SPURT measurements reveal that this layer is mainly affected by stratospheric air during winter/spring and by tropospheric air during autumn/summer. Mixing entropy values for O3 and H2O in the LMS appear to be maximal during spring and summer, respectively, indicating highest variability of these trace gases during the respective seasons.

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Airborne DOAS limb measurements of tropospheric trace gas profiles: case studies on the profile retrieval of O4 and BrO

Atmospheric Measurement Techniques ◽

10.5194/amt-4-1241-2011 ◽

2011 ◽

Vol 4 (6) ◽

pp. 1241-1260 ◽

Cited By ~ 38

Author(s):

C. Prados-Roman ◽

A. Butz ◽

T. Deutschmann ◽

M. Dorf ◽

L. Kritten ◽

...

Keyword(s):

Boundary Layer ◽

Vertical Distribution ◽

Vertical Profile ◽

Trace Gases ◽

Trace Gas ◽

Vertical Profiles ◽

Cloud Particle ◽

Upper Troposphere ◽

Mixing Ratios ◽

Tropospheric Aerosol

Abstract. A novel limb scanning mini-DOAS spectrometer for the detection of UV/vis absorbing radicals (e.g., O3, BrO, IO, HONO) was deployed on the DLR-Falcon (Deutsches Zentrum für Luft- und Raumfahrt) aircraft and tested during the ASTAR 2007 campaign (Arctic Study of Tropospheric Aerosol, Clouds and Radiation) that took place at Svalbard (78° N) in spring 2007. Our main objectives during this campaign were to test the instrument, and to perform spectral and profile retrievals of tropospheric trace gases, with particular interest on investigating the distribution of halogen compounds (e.g., BrO) during the so-called ozone depletion events (ODEs). In the present work, a new method for the retrieval of vertical profiles of tropospheric trace gases from tropospheric DOAS limb observations is presented. Major challenges arise from modeling the radiative transfer in an aerosol and cloud particle loaded atmosphere, and from overcoming the lack of a priori knowledge of the targeted trace gas vertical distribution (e.g., unknown tropospheric BrO vertical distribution). Here, those challenges are tackled by a mathematical inversion of tropospheric trace gas profiles using a regularization approach constrained by a retrieved vertical profile of the aerosols extinction coefficient EM. The validity and limitations of the algorithm are tested with in situ measured EM, and with an absorber of known vertical profile (O4). The method is then used for retrieving vertical profiles of tropospheric BrO. Results indicate that, for aircraft ascent/descent observations, the limit for the BrO detection is roughly 1.5 pptv (pmol mol−1), and the BrO profiles inferred from the boundary layer up to the upper troposphere and lower stratosphere have around 10 degrees of freedom. For the ASTAR 2007 deployments during ODEs, the retrieved BrO vertical profiles consistently indicate high BrO mixing ratios (∼15 pptv) within the boundary layer, low BrO mixing ratios (&leq;1.5 pptv) in the free troposphere, occasionally enhanced BrO mixing ratios (∼1.5 pptv) in the upper troposphere, and increasing BrO mixing ratios with altitude in the lowermost stratosphere. These findings agree reasonably well with satellite and balloon-borne soundings of total and partial BrO atmospheric column densities.

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

Atmospheric Measurement Techniques ◽

10.5194/amt-10-4209-2017 ◽

2017 ◽

Vol 10 (11) ◽

pp. 4209-4234 ◽

Cited By ~ 4

Author(s):

Tilman Hüneke ◽

Oliver-Alex Aderhold ◽

Jannik Bounin ◽

Marcel Dorf ◽

Eric Gentry ◽

...

Keyword(s):

Trace Gases ◽

Trace Gas ◽

The Novel ◽

Scaling Method ◽

Mixing Ratios ◽

Research Mission ◽

Model Predictions ◽

First Results ◽

Research Aircraft ◽

Spectral Ranges

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.

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Online technique for isotope and mixing ratios of CH4, N2O, Xe and mixing ratios of organic trace gases on a single ice core sample

Atmospheric Measurement Techniques ◽

10.5194/amt-7-2645-2014 ◽

2014 ◽

Vol 7 (8) ◽

pp. 2645-2665 ◽

Cited By ~ 18

Author(s):

J. Schmitt ◽

B. Seth ◽

M. Bock ◽

H. Fischer

Keyword(s):

Nitrous Oxide ◽

Trace Gases ◽

Ice Core ◽

Ice Cores ◽

Isotope Ratios ◽

Trace Gas ◽

Vacuum Extraction ◽

Data Set ◽

Mixing Ratios ◽

Organic Trace

Abstract. Firn and polar ice cores enclosing trace gas species offer a unique archive to study changes in the past atmosphere and in terrestrial/marine source regions. Here we present a new online technique for ice core and air samples to measure a suite of isotope ratios and mixing ratios of trace gas species on a single sample. Isotope ratios are determined on methane, nitrous oxide and xenon with reproducibilities for ice core samples of 0.15‰ for δ13C–CH4, 0.22‰ for δ15N–N2O, 0.34‰ for δ18O–N2O, and 0.05‰ per mass difference for δ136Xe for typical concentrations of glacial ice. Mixing ratios are determined on methane, nitrous oxide, xenon, ethane, propane, methyl chloride and dichlorodifluoromethane with reproducibilities of 7 ppb for CH4, 3 ppb for N2O, 70 ppt for C2H6, 70 ppt for C3H8, 20 ppt for CH3Cl, and 2 ppt for CCl2F2. However, the blank contribution for C2H6 and C3H8 is large in view of the measured values for Antarctic ice samples. The system consists of a vacuum extraction device, a preconcentration unit and a gas chromatograph coupled to an isotope ratio mass spectrometer. CH4 is combusted to CO2 prior to detection while we bypass the oven for all other species. The highly automated system uses only ~ 160 g of ice, equivalent to ~ 16 mL air, which is less than previous methods. The measurement of this large suite of parameters on a single ice sample is new and key to understanding phase relationships of parameters which are usually not measured together. A multi-parameter data set is also key to understand in situ production processes of organic species in the ice, a critical issue observed in many organic trace gases. Novel is the determination of xenon isotope ratios using doubly charged Xe ions. The attained precision for δ136Xe is suitable to correct the isotopic ratios and mixing ratios for gravitational firn diffusion effects, with the benefit that this information is derived from the same sample. Lastly, anomalies in the Xe mixing ratio, δXe/air, can be used to detect melt layers.

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Online technique for isotope and mixing ratios of CH4, N2O, Xe and mixing ratios of organic trace gases on a single ice core sample

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-7-2017-2014 ◽

2014 ◽

Vol 7 (3) ◽

pp. 2017-2069 ◽

Cited By ~ 2

Author(s):

J. Schmitt ◽

B. Seth ◽

M. Bock ◽

H. Fischer

Keyword(s):

Nitrous Oxide ◽

Trace Gases ◽

Ice Core ◽

Ice Cores ◽

Isotope Ratios ◽

Small Sample ◽

Trace Gas ◽

Vacuum Extraction ◽

Mixing Ratios ◽

Organic Trace

Abstract. Polar ice cores enclosing trace gas species offer a unique archive to study changes in the past atmosphere and in terrestrial/marine source regions. Here we present a new online technique for ice core and air samples to measure a suite of isotope ratios and mixing ratios of trace gas species on a single small sample. Isotope ratios are determined on methane, nitrous oxide and xenon with reproducibilities for ice core samples of 0.15‰ for δ13C-CH4, 0.22‰ for δ15N-N2O, 0.34 ‰ for δ18O-N2O, and 0.05‰ for δ136Xe. Mixing ratios are determined on methane, nitrous oxide, xenon, ethane, propane, methyl chloride and dichloro-difluoromethane with reproducibilities of 7 ppb for CH4, 3 ppb for N2O, 50 ppt for 136Xe, 70 ppt for C2H6, 70 ppt for C3H8, 20 ppt for CH3Cl, and 2 ppt for CCl2F2. The system consists of a vacuum extraction device, a preconcentration unit and a gas chromatograph coupled to an isotope ratio mass spectrometer. CH4 is combusted to CO2 prior to detection while we bypassed the oven for all other species. The highly automated system uses only ~160 g ice, equivalent to ~16 mL air, which is less than previous methods. This large suite of parameters on a single ice sample is new and helpful to study phase relationships of parameters which are usually not measured together. A multi-parameter dataset is also key to understand in situ production processes of organic species in the ice, a critical issue observable in many organic trace gases. Novel is the determination of xenon isotope ratios using doubly charged Xe ions. The attained precision for δ136Xe is suitable to correct the isotopic ratios and mixing ratios for gravitational firn effects, with the benefit that this information is derived from the same sample. Lastly, anomalies in the Xe mixing ratio, δXe/air, can be used to detect melt layers.

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Recent improvements of long-path DOAS measurements: impact on accuracy and stability of short-term and automated long-term observations