AS3.22

Nitrogen dioxide is an atmospheric gas of major impact on climate change and air quality and its monitoring through its detection and quantification is essential to control its impact on the environment and health.

The detection and estimation of trace gases are limited by cost of the acquisitions devices and spectro-temporal resolution of the acquisitions. Conventional imaging systems are the result of a trade-off in terms of size, and spectral and spatial resolutions.

In order to overcome these technological limitations, a new device known under the patent of Imaging Spectrometer on Chip (ImSPOC) allows for real-time acquisition and a significant spatial resolution. As its volume about the size of a matchbox, it has the potential to become a base brick for nano-satellites, drones, or ground-based measurement platforms. The ImSpoC device is based on an array of Fabry-Perot interferometers of diffrent thickness mounted over a high-sensitivity CCD imaging detector (a matrix of photodiodes). As ImSPOC performs a division of the field of view (with a matrix of
micro lenses coupled with the interferometers), a typical acquisition consists in a matrix of sub-images which can be recombined in order to form a single hyperspectral image of the observed scene in which each pixel yields an interferogram.

When ImSPCO is used as a spectrometer, a common processing involves reconstructing spectra from interferograms as an inverse problem. This operation is importatnt since the commonly used techniques, such as Differential Optical Absorption Spectroscopy (DOAS) rely on light spectra instead of interferograms.

This work explores a way to adapt these techniques directly on the interferograms captured by ImSPOC is explored by relying on an optical model of the instrument. As spectra and interferograms are linked by a Fourier-like transform, the interferograms exhibit the periodicity of the incident light spectrum. In this work we propose to use an optical filter to isolate a wavelength range where the absorption cross-section of NO₂ is strongly periodic and not correlated with that of other trace gases. We expect that the correlation between a difference of interferograms and the Fourier transform of the (filtered) absorption cross-section of the target gas is proportional to the difference of slant column densities of the targeted gas, here NO₂. 

Some experimental results were obtained by processing ImSPOC acquisitions over several hours at sunrise, noon, and sunset in two configurations: zenith light (the sensor being oriented towards the zenith), and direct light (the sensor being directed towards a surface of material with high diffuse reflectance). In order to validate the results obtained by processing the ImSPOC acquisitions, data from a conventional diffraction grating based spectrometer were used, providing reference measures for the air-mass factors and allowing for a comparison with a regular DOAS method.

How to cite: Bourdin, Y., Dolet, A., Gousset, S., Dalla Mura, M., Picone, D., Voisin, D., and le Coarer, E.: NO2 vertical column density estimation from interferograms captured by a snapshot interferometric imaging spectrometer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2926, https://doi.org/10.5194/egusphere-egu22-2926, 2022.

An instrument capable of imaging the field of NO₂ in various open-air situations has been designed, manufactured, and tested. It is an improved version of the NO₂ camera relying on an AOTF (acousto-optical tunable filter) which has demonstrated, amongst other things, its capability to quantify the NO₂ released by power plant smokestacks. The improved version which is presented has a larger field of view, a higher frame rate, and better spectral registration performance.

The working principle of the instrument has been preserved: by driving the AOTF with the appropriate acoustic frequency, a spectral image of the scene captured by the camera is recorded at a particular wavelength. The recording of a number of spectral images allows to form an hypercube: two spatial dimensions, and a spectral one.

While the earlier instrument was relying on a handful of wavelengths to quantify the slant column density of NO₂ observed in each pixel line of sight, the new instrument can now record "continuous" portions of the visible-light spectrum, typically between 440, and 460nm, where the NO₂ exhibits some of its largest absorption lines.

When the target is stable, like the air observed above a city skyline, the NO₂ camera has enough time to build a large hypercube, and the spectrum measured in each pixel can be processed by the DOAS (differential optical absorption spectroscopy) method. This approach is better suited when NO₂ is expected across the entire scene, not just in the plume of a smokestack for instance.

The new instrument will be presented, and results of measurements performed in an urban context will be shown. The performance of the NO₂ camera will be discussed based on the results of an intercomparison with the MAX-DOAS of Uccle, Brussels, and other air quality stations.

How to cite: Dekemper, E., Vanhamel, J., and Van Roozendael, M.: Performance of the AOTF-based NO2 camera for urban pollution imaging, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3572, https://doi.org/10.5194/egusphere-egu22-3572, 2022.

Carbon Dioxide (CO₂) is the most important anthropogenic greenhouse gas driving global climate change. Strong point sources like coal-fired power plants contribute roughly 30% to global CO₂ emissions. Precise knowledge about the distribution and strength of these sources is the target of many ongoing and planned research missions, e.g., the Orbiting Carbon Observatory-2 (OCO2, Wu et al., 2018), the Copernicus CO2 Monitoring (CO2M, Sierk et al., 2019), and high-resolution missions like CO2Image (Strandgren et al., 2020). Spatially resolving CO₂ exhaust plumes with imaging spectroscopy allows an estimate of the source’s emission. CO₂ imaging efforts in the shortwave-infrared spectral range have been exclusively in top-down viewing geometry from satellites (e.g., Cusworth et al., 2021) or airplanes (e.g., Foote et al., 2021).
We present first results of CO₂ emission estimation from hyperspectral imaging in ground-based viewing geometry. We deploy a NEO HySpex SWIR-384 camera stationary in the vicinity of a strong emission source. Thus, the camera can repeatedly take images of shortwave-infrared spectra (1−2.5 μm) from sky-scattered sunlight. This allows us to retrieve atmospheric CO₂ enhancements with an adapted matched filter algorithm (Foote et al., 2020, 2021) in the 2 μm absorption band. Imaging in a horizontal viewing geometry enables observing the time-averaged vertical profile of the exhaust plume. First case studies at a local power plant (7 MtCO2/yr) in Mannheim demonstrate our ability to reliably detect CO₂ exhaust plumes above chimneys. Our ongoing efforts focus on modeling the temporal evolution of the plume rise (Janicke and Janicke, 2001) and use it with the integrated mass enhancement of the observed plume to estimate the instantaneous emissions of the source. Such estimates can complement bottom-up inventories and state-of-the-art top-down measurements in the future. Furthermore, this technique may readily apply to greenhouse gases like methane, which we plan to examine in an upcoming field campaign in the Upper Silesian Coal Basin.

References
Cusworth et al., 2021: Quantifying Global Power Plant Carbon Dioxide Emissions With Imaging Spectroscopy, https://doi.org/10.1029/2020AV000350
Foote et al., 2020: Fast and Accurate Retrieval of Methane Concentration from Imaging Spectrometer Data Using Sparsity Prior, http://arxiv.org/abs/2003.02978
Foote et al., 2021: Impact of Scene-Specific Enhancement Spectra on Matched Filter Greenhouse Gas Retrievals from Imaging Spectroscopy, https://doi.org/10.1016/j.rse.2021.112574
Janicke, U. and Janicke, L., 2001: A Three-Dimensional Plume Rise Model for Dry and Wet Plumes, https://doi.org/10.1016/S1352-2310(00)00372-1
Sierk et al., 2019: The European CO2 Monitoring Mission: Observing Anthropogenic Greenhouse Gas Emissions from Space, https://doi.org/10.1117/12.2535941
Strandgren et al., 2020, Towards Spaceborne Monitoring of Localized CO2 Emissions: An Instrument Concept and First Performance Assessment, https://doi.org/10.5194/amt-13-2887-2020
Wu et al., 2018: Carbon Dioxide Retrieval from OCO-2 Satellite Observations Using the RemoTeC Algorithm and Validation with TCCON Measurements, https://doi.org/10.5194/amt-11-3111-2018

How to cite: Knapp, M., Hemmer, B., Kleinschek, R., Sindram, M., Schmitt, T., Pilz, L., Burger, B., and Butz, A.: Towards Carbon Dioxide emission estimation with a stationary hyperspectral camera, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3924, https://doi.org/10.5194/egusphere-egu22-3924, 2022.

Satellite observations of tropospheric nitrogen dioxide (NO2) form an important basis for estimating the environmental impact of nitrogen oxide emissions and for assessing the impact of atmospheric pollution on human health. There is a great need to evaluate the accuracy of satellite tropospheric NO2 vertical columns by validating these data products against other measurements, for example ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations. Moreover, the conversion of tropospheric NO2 vertical columns to surface NO2 concentrations, which is of great interest for exposure studies, requires new scientific approaches to reduce existing uncertainties.

From 2016 to 2021, the VINDOBONA (VIenna horizontal aNd vertical Distribution OBservations Of Nitrogen dioxide and Aerosols) project was carried out in Vienna, Austria. One major goal of the VINDOBONA project was to improve the spatial representativeness of ground-based MAX-DOAS observations in urban environments by making use of measurements taken simultaneously at three locations in Vienna, each covering a range of azimuth directions. By comparing MAX-DOAS integrated NO2 concentrations along horizontal columns with each other as well as with in-situ NO2 data from local air quality measuring stations, interesting insights into the spatial distribution of NO2 in Vienna was gained. Even more insights and in fact, a higher spatial variability of NO2 on the scale of the city was found from case study-based DOAS horizontal measurements taken on the rotating Café of the Danube Tower. These results highlight the need to refine the colocation of ground-based MAX-DOAS with satellite pixels in future validation activities.

The impact of lockdowns on ambient NO2 pollution during the COVID-19 pandemic as well as the value of MAX-DOAS measurements of other species (formaldehyde, glyoxal, and aerosols), which form the basis of past and ongoing research activities, will also be highlighted.

Acknowledgements: This research has been financially supported by the Austrian Science Fund (grant no. I 2296-N29), the German Research Foundation (grant no. Ri 1800/6-1), and A1 Telekom Austria.

How to cite: Schreier, S., Richter, A., Weihs, P., Schmalwieser, A., and Burrows, J. and the VINDOBONA Contributors: Five years of ground-based MAX-DOAS measurements in Vienna, Austria: Overview and highlights of the VINDOBONA project, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3935, https://doi.org/10.5194/egusphere-egu22-3935, 2022.

The Trace Gas Orbiter (TGO) was launched in 2016 to probe the atmosphere of Mars. It has on board the Atmospheric Chemistry Suite (ACS), which is a set of observation instruments in the infrared domain, in particular, the NIR (Near Infrared) and the MIR (Mid-Infrared) photometers. These photometers detect the atmospheric components of Mars when they are pointed at the Sun and the Line Of Sight (LOS) crosses the atmosphere. The solar spectrum is directly measured when the LOS is above the atmosphere of Mars and serves as a spectral reference for calibration. We consider here particular observations made with the NIR photometer to construct the solar spectrum allowed by its spectral range, i.e. 0.7 to 1.7 nm. This is motivated by the need to have a solar spectrum with high spectral resolution for NIR calibrations but also useful for models (Sun, climate…) or for other remote sensing experiments (Earth or other planets). We have acquired all the diffraction orders of the NIR by continuously varying the frequency of its AOTF (Acousto-Optic Tunable Filters, a component used to separate the orders). They were then successively imaged on the CCD and series of its useful part were recorded for each order. We will present how we process this data to extract the solar spectrum, in particular how we calculate the flat field useful for image correction. We will then present how to overcome the contamination of successive spectral orders using a geometric method. We will then show how to correct the order intensity variations to obtain the solar spectrum. We will end by discussing our results, in particular by comparing them with other existing spectra on the same band.

How to cite: Irbah, A., Bertaux, J.-L., Montmessin, F., Scheveiler, L., Lacombe, G., Trokhimovskiy, A., Korablev, O., Fedorova, A., Patrakeev, A., and Shakun, A.: The 0.7-1.7 µm spectral range of the solar spectrum obtained from Mars thanks to TGO-ACS observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4103, https://doi.org/10.5194/egusphere-egu22-4103, 2022.

Differential optical absorption spectroscopy (DOAS) has become a standard method for analysing the amount of various molecular species in the Earth's atmosphere. For student education we have developed a small device aiming to be used for practising this technique. It is based on three 50mm aperture sized telescopes, which are aligned on a SkywatcherAZ-EQ5 astronomical mount enabling a multi-axis movement. Each of the three telescopes feeds an individual spectrograph to cover the entire wavelength range between 300 and 980nm in three arms: Ultraviolett/blue (UVB) arm, ranging from 300-507nm (Stellarnet BLUE-Wave UV2-14 spectrometer, 0.2nm resolution, 14µm slit); Visual (VIS) arm covering 500-680nm (Stellarnet BLUE-Wave NIR4-14, 0.2nm resolution, 14µm slit); red/near-infrared (NIR) arm: 600-980nm (Stellarnet  BLUE-Wave NIR2-14, 0.4nm resolution, 14µm slit). All three spectrometers are equipped with a detector amplification lens upgrade. The main lens of the telescope feeding the UVB arm was replaced by one with a fused silica glass for a better UV transparency. In addition, we use wavelength-optimised 600µm fibres in each spectral arm.

The software to take the spectra is SpectraWiz v5.33. The data calibration is done with MIDAS, a software specifically developed for astronomical purposes. The final DOAS measurements are done with molecfit, a software originally developed to remove absorption features from astronomical spectra by fitting model-based synthetic transmission spectrum to absorption features in scientific data.

In this presentation we show a technical overview of this instrument, the concept of the experiment and some results. 

How to cite: Kausch, W., Kimeswenger, S., Przybilla, N., and Noll, S.: A small DOAS device for student training, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4662, https://doi.org/10.5194/egusphere-egu22-4662, 2022.

Imaging of trace gases by optical remote sensing provides insight in the dynamics of physical and chemical processes within the atmosphere. Among the various sources for atmospheric trace gases, volcanoes pose additional challenges, as their highly variable emissions necessitate a high spatio-temporal resolution and their sometimes remote and inaccessible locations call for a robust and also portable measuring device.

We applied Fabry-Perot interferometer (FPI) correlation spectroscopy (IFPICS) that fulfils all of the above criteria. The periodic transmission of an FPI is matched to the periodicity of the vibronic narrowband absorption structure of the target trace gas absorption. The apparent absorptivity is then calculated from the difference of optical densities in two measurement settings whereas the FPI transmission coincides with the maxima of trace gas absorption in one setting and with the minima of the absorption in the second setting. Since the difference in wavelength between these two settings is only about 1nm, it is theorised that measurements with cloudy backgrounds become possible as their scattering properties aren't expected to differ much between the measurement settings and thus allow for cancellation. This is not the case for a conventional SO₂-Cameras, as they rely on band-pass filters with transmission spectra that are about 20 nm apart.

We will show results of a first study on the influence of cloudy backgrounds on measurement results by determining the amount of SO₂ caused by meteorological clouds in the field of view.

We also present measurements from July 2021 of SO₂ fluxes at Mt. Etna with an IFPICS instrument with a detection limit of ≈ 5e17 molec/cm² at 4 Megapixel spatial resolution and 1 s temporal resolution and discuss uncertainties and challenges of the technique. 

How to cite: Heimann, J., Nies, A., Fuchs, C., Kuhn, J., Bobrowski, N., and Platt, U.: Imaging Fabry-Perot interferometer correlation spectroscopy - improving the accuracy of SO2 flux measurements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6494, https://doi.org/10.5194/egusphere-egu22-6494, 2022.

The MethaneSAT satellite mission aims at quantifying anthropogenic methane emissions by measuring reflected solar radiation in two spectral windows in the short wave infrared range. In the 1,249 nm - 1,305 nm spectral range the sensor will measure the oxygen (O2) singlet Delta band with a full width at half maximum (FWHM) of 0.17 nm. Additionally, the instrument will observe most of the 2ν₃ absorption band of methane (CH4) and the P-branch of a 3ν₁+ν₃ carbon dioxide (CO2) band in the spectral range 1,605 nm - 1,683 nm at a spectral resolution of 0.23 nm at FWHM. Clouds and aerosols can introduce biases into the inversion of methane column concentrations from solar backscatter measurements and we therefore develop retrieval processors to filter out contaminated scenes.

One processor makes use of surface pressure retrievals to infer the presence of scattering particles in the field of view of the sensor. These retrievals specifically take into account O2 airglow emission following the approach of Sun et al. (2018). Surface pressure is retrieved by fitting spectra in the O2 band, assuming a non-scattering atmosphere.  We study thresholds in the variations between retrieved surface pressure and a priori meteorological databases which are suitable to screen for clouds and aerosols. A complementary algorithm takes advantage of differences in the atmospheric light path between the two spectral windows of MethaneSAT in the presence of aerosols and clouds. Here, we retrieve water vapor (H2O) column concentrations from the 1.3 µm and 1.6 µm windows under the assumption of the geometric lightpath. The ratio of the H2O retrievals from the two windows is used to construct a filter for aerosol and cloud contaminated scenes. We simulate MethaneSAT measurements with various cloud and aerosol loads to derive the retrieval configurations with highest sensitivity to scattering events.

Both filtering approaches are applied to measurements of the MethaneAIR instrument (Staebell et al., 2021) to demonstrate their capacity in screening for clear scenes. Finally, we discuss our on-going efforts in developing a filter for observations affected by cloud shadows.

References

Staebell, C., Sun, K., Samra, J., Franklin, J., Chan Miller, C., Liu, X., Conway, E., Chance, K., Milligan, S., and Wofsy, S.: Spectral calibration of the MethaneAIR instrument, Atmospheric Measurement Techniques, 14, 3737–3753, https://doi.org/10.5194/amt-14-3737-2021, 2021.

Sun, K., Gordon, I. E., Sioris, C. E., Liu, X., Chance, K., and Wofsy, S. C.: Reevaluating the use of O2 a1Δg band in spaceborne remote sensing of greenhouse gases, Geophysical Research Letters, 45, 5779–5787, https://doi.org/10.1029/2018GL077823, 2018.

How to cite: Wilzewski, J. S., Roche, S., Chan Miller, C., Souri, A. H., Conway, E., Franklin, J., Samra, J., Hohl, J., Sun, K., Liu, X., Chance, K., and Wofsy, S.: Development of the MethaneSAT cloud and aerosol filter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6625, https://doi.org/10.5194/egusphere-egu22-6625, 2022.

Turbulence in the planetary boundary layer controls the exchange fluxes of passive and active tracers between the Earth’s surface and the atmosphere. In climate and meteorological models, such effects of turbulence need to be parameterized, ultimately based on experimental data. A modeling/experimental approach was developed within the COMTESSA project to study turbulence statistics. Using controlled tracer releases, UV camera images and estimates of the background radiation, different tomographic algorithms were applied to obtain time series of 3D representations of the scalar dispersion. We used initially synthetic data to investigate different reconstruction algorithms with emphasis on algebraic iterative methods, studying the dependence of the reconstruction quality on the discretization resolution and the geometry of the experimental device in both 2D and 3D cases. For the iterative methods we assessed the computational aspects of the iterative algorithms focusing of the phenomenon of semi-convergence applying a variety of stopping rules. In addition to the synthetic studies, we present actual tomographic 3D reconstructions of artificial SO2 puffs using multiple camera measurements from the experimental campaigns in Norway.

How to cite: Pisso, I., Cassiani, M., Stebel, K., Kylling, A., Dinger, A. S., Ardeshiri, H., Park, S.-Y., Schmidbauer, N., and Stohl, A.: Tomographic 3D reconstructions of artificial releases of SO2 in the atmospheric boundary layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7625, https://doi.org/10.5194/egusphere-egu22-7625, 2022.

We measured the four trace species (NO₂, SO₂, HCHO, and CHOCHO) during two extensive car MAX-DOAS measurement campaigns in and around the megacity of Lahore. To our knowledge, CHOCHO was successfully measured from car-MAX-DOAS observations for the first time. The car-MAX-DOAS measurements were performed in summer 2017 and spring 2018. We retrieved the vertically integrated concentration (the so-called tropospheric vertical column density, VCD) of the trace gases along the driving routes from the measured spectra by using the so-called geometric approximation method. We present the results of the correlation analyses performed for all possible trace gas pairs. Strong correlations were found between the NO₂-SO₂ and HCHO-CHOCHO pairs.  The results indicate that NO₂ and SO₂ have common sources, most probably emissions from fossil fuel burning. Also, HCHO and CHOCHO have common sources, most probably secondary formation from common precursors like volatile organic compounds (VOCs).

How to cite: Razi, M., Dörner, S., Donner, S., Ahmad, N., Khokhar, F., and Wagner, T.: Study of the interrelationships between NO2, SO2, HCHO and CHOCHO vertical column densities derived by car MAX-DOAS observations in and around the megacity of Lahore, Pakistan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10254, https://doi.org/10.5194/egusphere-egu22-10254, 2022.