scholarly journals Validation of TROPOMI tropospheric NO<sub>2</sub> columns using dual-scan MAX-DOAS measurements in Uccle, Brussels

2020 ◽  
Author(s):  
Ermioni Dimitropoulou ◽  
François Hendrick ◽  
Gaia Pinardi ◽  
Martina M. Friedrich ◽  
Alexis Merlaud ◽  
...  
Keyword(s):  
Vertical Profile ◽  
A Priori ◽  
Elevation Angle ◽  
Horizontal Distribution ◽  
Optical Absorption Spectroscopy ◽  
Azimuthal Direction ◽  
Inversion Algorithm ◽  
Vertical Column ◽  
Near Surface ◽  
One Year

Abstract. Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of aerosols and tropospheric nitrogen dioxide (NO2) were carried out in Uccle (50.8° N, 4.35° E) Brussels, during one year from March 2018 until March 2019. The instrument was operated in both UV and visible (Vis) wavelength ranges in a dual-scan configuration consisting of two sub-modes: (1) an elevation scan in a fixed viewing azimuthal direction (the so-called main azimuthal direction) pointing to the Northeast and (2) an azimuthal scan in a fixed low elevation angle (2°). By applying a vertical profile inversion algorithm in the main azimuthal direction and a parameterization technique in the other azimuthal directions, near-surface NO2 concentrations (VMRs) and vertical column densities (VCDs) were retrieved in ten different azimuthal directions. The dual-scan MAX-DOAS dataset allows partly resolving the horizontal distribution of NO2 around the measurement site and studying its seasonal variations. Furthermore, we show that measuring the tropospheric NO2 VCDs in different azimuthal directions improves the spatial colocation with measurements from the Sentinel-5 Precursor (S5P), leading to a reduction of the spread in validation results. By using NO2 vertical profile information derived from the MAX-DOAS measurements, we also resolve a systematic underestimation in S5P NO2 data due to the use of inadequate a-priori NO2 profile shape data in the satellite retrieval.

Tropospheric NO2 and HCHO derived from dual-scan MAX-DOAS measurements in Uccle (Belgium) and application to S5P/TROPOMI validation

2020 ◽  
Author(s):  
Ermioni Dimitropoulou ◽  
Francois Hendrick ◽  
Martine M. Friedrich ◽  
Gaia Pinardi ◽  
Frederik Tack ◽  
...  
Keyword(s):  
Vertical Profile ◽  
A Priori ◽  
Elevation Angle ◽  
Optical Absorption Spectroscopy ◽  
Azimuthal Direction ◽  
Inversion Algorithm ◽  
Vertical Column ◽  
Near Surface ◽  
Mixing Ratios ◽  
Horizontal Variation

&lt;p&gt;Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of aerosols, tropospheric nitrogen dioxide (NO&lt;sub&gt;2&lt;/sub&gt;) and formaldehyde (HCHO) have been carried out in Uccle, Brussels, during two years (March 2018 &amp;#8211; March 2020). The MAX-DOAS instrument has been operating in both UV and visible (Vis) wavelength ranges in a dual-scan configuration consisting of two sub-modes: (1) an elevation scan in a fixed viewing azimuthal direction (the so-called main azimuthal direction) pointing and (2) an azimuthal scan in a fixed low elevation angle (2&lt;sup&gt;o&lt;/sup&gt;). By applying a vertical profile inversion algorithm in the main azimuthal direction and an adapted version of the parameterization technique proposed by Sinreich et al. (2013) in the other azimuthal directions, near-surface &amp;#160;concentrations (VMRs) and vertical column densities (VCDs) are retrieved in ten different azimuthal directions.&lt;/p&gt;&lt;p&gt;The present work focuses on the seasonal horizontal variation of NO&lt;sub&gt;2 &lt;/sub&gt;and HCHO around the measurement site. The observations show a clear seasonal cycle of these trace gases. An important application of the dual-scan MAX-DOAS measurements is the validation of satellite missions with high spatial resolution, such as TROPOMI/S5P. Measuring the tropospheric &amp;#160;VCDs in different azimuthal directions is shown to improve the spatial colocation with satellite measurements leading to a better agreement between both datasets. By using &amp;#160;vertical profile information derived from the MAX-DOAS measurements, we show that a persistent systematic underestimation of the TROPOMI &amp;#160;data can be explained by uncertainties in the a-priori NO&lt;sub&gt;2&lt;/sub&gt; profile shape in the satellite retrieval. A similar validation study for TROPOMI HCHO is currently under progress and preliminary results will be presented.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Sinreich, R., Merten, A., Molina, L., and Volkamer, R.: Parameterizing radiative transfer to convert MAX-DOAS dSCDs into near-surface box-averaged mixing ratios, Atmos. Meas. Tech., 6, 1521&amp;#8211;1532, https://doi.org/10.5194/amt-6-1521-2013, 2013.&lt;/p&gt;

scholarly journals Horizontal distribution of tropospheric NO<sub>2</sub> and aerosols derived by dual-scan multi-wavelength MAX-DOAS measurements in Uccle, Belgium

2021 ◽  
Author(s):  
Ermioni Dimitropoulou ◽  
Francois Hendrick ◽  
Martina Michaela Friedrich ◽  
Frederik Tack ◽  
Gaia Pinardi ◽  
...  
Keyword(s):  
A Priori ◽  
Elevation Angle ◽  
Optimal Estimation ◽  
Horizontal Distribution ◽  
Optical Absorption Spectroscopy ◽  
Azimuthal Direction ◽  
Vertical Column ◽  
Near Surface ◽  
Multi Wavelength

Abstract. Dual-scan ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of tropospheric nitrogen dioxide (NO2) and aerosols have been carried out in Uccle (50.8° N, 4.35° E; Brussels region, Belgium) for two years, from March 2018 to February 2020. The MAX-DOAS instrument has been operating in both UV and Visible wavelength ranges in a dual-scan configuration consisting of two sub-modes: (1) an elevation scan in a fixed viewing azimuthal direction and (2) an azimuthal scan in a fixed low elevation angle (2°). By analyzing the O4 and NO2 dSCDs at six different wavelength intervals along every azimuthal direction and by applying a new Optimal-Estimation-based inversion approach, the horizontal distribution of the NO2 near-surface concentrations and vertical column densities (VCDs) and the aerosols near-surface extinction coefficient are retrieved along ten azimuthal directions. The retrieved horizontal NO2 concentration profiles allow the identification of the main NO2 hotspots in the Brussels area. Correlative comparisons of the retrieved horizontal NO2 distribution have been conducted with airborne, mobile, and satellite datasets, and overall a good agreement is found. The comparison with TROPOMI observations reveals that the characterization of the horizontal distribution of tropospheric NO2 VCDs by ground-based measurements, the appropriate sampling of TROPOMI pixels, and an adequate a priori NO2 profile shape in TROPOMI retrievals lead to a better consistency between satellite and ground-based datasets.

scholarly journals Validation of TROPOMI tropospheric NO<sub>2</sub> columns using dual-scan multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements in Uccle, Brussels

2020 ◽  
Vol 13 (10) ◽  
pp. 5165-5191 ◽  
Author(s):  
Ermioni Dimitropoulou ◽  
François Hendrick ◽  
Gaia Pinardi ◽  
Martina M. Friedrich ◽  
Alexis Merlaud ◽  
...  
Keyword(s):  
Optical Absorption ◽  
Absorption Spectroscopy ◽  
Vertical Profile ◽  
Elevation Angle ◽  
Differential Optical Absorption Spectroscopy ◽  
Optical Absorption Spectroscopy ◽  
Azimuthal Direction ◽  
Inversion Algorithm ◽  
Vertical Column ◽  
Near Surface

Abstract. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of aerosols and tropospheric nitrogen dioxide (NO2) were carried out in Uccle (50.8∘ N, 4.35∘ E), Brussels, during 1 year from March 2018 until March 2019. The instrument was operated in both the UV and visible wavelength ranges in a dual-scan configuration consisting of two sub-modes: (1) an elevation scan in a fixed viewing azimuthal direction (the so-called main azimuthal direction) pointing to the northeast and (2) an azimuthal scan in a fixed low elevation angle (2∘). By applying a vertical profile inversion algorithm in the main azimuthal direction and a parameterization technique in the other azimuthal directions, near-surface NO2 volume mixing ratios (VMRs) and vertical column densities (VCDs) were retrieved in 10 different azimuthal directions. The dual-scan MAX-DOAS dataset allows for partly resolving the horizontal distribution of NO2 around the measurement site and studying its seasonal variations. Furthermore, we show that measuring the tropospheric NO2 VCDs in different azimuthal directions improves the spatial colocation with measurements from the Sentinel-5 Precursor (S5P), leading to a reduction of the spread in validation results. By using NO2 vertical profile information derived from the MAX-DOAS measurements, we also resolve a systematic underestimation in S5P NO2 data due to the use of inadequate a priori NO2 profile shape data in the satellite retrieval.

scholarly journals Technical note: Evaluation of profile retrievals of aerosols and trace gases for MAX-DOAS measurements under different aerosol scenarios based on radiative transfer simulations

2021 ◽  
Vol 21 (17) ◽  
pp. 12867-12894
Author(s):  
Xin Tian ◽  
Yang Wang ◽  
Steffen Beirle ◽  
Pinhua Xie ◽  
Thomas Wagner ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Trace Gases ◽  
Optimal Estimation ◽  
Estimation Algorithm ◽  
Optical Absorption Spectroscopy ◽  
Limited Information ◽  
Inversion Algorithm ◽  
Vertical Column ◽  
Near Surface ◽  
Aerosol Pollution

Abstract. Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a state-of-the-art remote sensing technique for deriving vertical profiles of trace gases and aerosols. However, MAX-DOAS profile inversions under aerosol pollution scenarios are challenging because of the complex radiative transfer and limited information content of the measurements. In this study, the performances of two inversion algorithms were evaluated for various aerosol pollution scenarios based on synthetic slant column densities (SCDs) derived from radiative transfer simulations. Compared to previous studies, in our study, much larger ranges of aerosol optical depth (AOD) and NO2 vertical column densities (VCDs) are covered. One inversion algorithm is based on optimal estimation; the other uses a parameterized approach. In this analysis, three types of profile shapes for aerosols and NO2 were considered: exponential, Boltzmann, and Gaussian. First, the systematic deviations of the retrieved aerosol profiles from the input profiles were investigated. For most cases, the AODs of the retrieved profiles were found to be systematically lower than the input values, and the deviations increased with increasing AOD. In particular for the optimal estimation algorithm and for high AOD, these findings are consistent with the results in previous studies. The assumed single scattering albedo (SSA) and asymmetry parameter (AP) have a systematic influence on the aerosol retrieval. However, for most cases the influence of the assumed SSA and AP on the retrieval results are rather small (compared to other uncertainties). For the optimal estimation algorithm, the agreement with the input values can be improved by optimizing the covariance matrix of the a priori uncertainties. Second, the aerosol effects on the NO2 profile retrieval were tested. Here, especially for the optimal estimation algorithm, a systematic dependence on the NO2 VCD was found, with a strong relative overestimation of the retrieved results for low NO2 VCDs and an underestimation for high NO2 VCDs. In contrast, the dependence on the aerosol profiles was found to be rather low. Interestingly, the results for both investigated wavelengths (360 and 477 nm) were found to be rather similar, indicating that the differences in the radiative transfer between both wavelengths have no strong effect. In general, both inversion schemes can retrieve the near-surface values of aerosol extinction and trace gas concentrations well.

scholarly journals Ground-based MAX-DOAS observations of tropospheric aerosols, NO<sub>2</sub>, SO<sub>2</sub> and HCHO in Wuxi, China, from 2011 to 2014

2017 ◽  
Vol 17 (3) ◽  
pp. 2189-2215 ◽  
Author(s):  
Yang Wang ◽  
Johannes Lampel ◽  
Pinhua Xie ◽  
Steffen Beirle ◽  
Ang Li ◽  
...  
Keyword(s):  
Trace Gases ◽  
Optical Absorption Spectroscopy ◽  
Control Procedure ◽  
Systematic Bias ◽  
Aerosol Extinction ◽  
Inversion Algorithm ◽  
Long Distance ◽  
Vertical Column ◽  
Near Surface ◽  
Geometric Approximation

Abstract. We characterize the temporal variation and vertical distribution of nitrogen dioxide (NO2), sulfur dioxide (SO2), formaldehyde (HCHO) and aerosol extinction based on long-term multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations from May 2011 to November 2014 in Wuxi, China. A new inversion algorithm (PriAM) is implemented to retrieve profiles of the trace gases (TGs) and aerosol extinction (AE) from the UV spectra of scattered sunlight recorded by the MAX-DOAS instrument. We investigated two important aspects of the retrieval process. We found that the systematic seasonal variation of temperature and pressure (which is regularly observed in Wuxi) can lead to a systematic bias of the retrieved aerosol profiles (e.g. up to 20 % for the AOD) if it is not explicitly considered. In this study we take this effect into account for the first time. We also investigated in detail the reason for the differences of tropospheric vertical column densities derived from either the geometric approximation or by the integration of the retrieved profiles, which were reported by earlier studies. We found that these differences are almost entirely caused by the limitations of the geometric approximation (especially for high aerosol loads). The results retrieved from the MAX-DOAS observations are compared with independent techniques not only under cloud-free sky conditions, but also under various cloud scenarios. Under most cloudy conditions (except fog and optically thick clouds), the trace gas results still show good agreements. In contrast, for the aerosol results, only near-surface AE could be still well retrieved under cloudy situations. After applying a quality control procedure, the MAX-DOAS data are used to characterize the seasonal, diurnal and weekly variations of NO2, SO2, HCHO and aerosols. A regular seasonality of the three trace gases is found, but not for aerosols. Similar annual variations of the profiles of the trace gases appear in different years. Only NO2 shows a significant seasonality of the diurnal variations. Considerable amplitudes of weekly cycles occur for NO2 and SO2, but not for HCHO and aerosols. The TGs and aerosols show good correlations, especially for HCHO in winter. More pronounced wind direction dependencies, especially for the near-surface concentrations, are found for the trace gases than for the aerosols, which implies that the local emissions from nearby industrial areas (including traffic emissions) dominate the local pollution, while long-distance transport might also considerably contribute to the local aerosol levels.

scholarly journals Technical note：Evaluation of profile retrievals of aerosols and trace gases for MAX-DOAS measurements under different aerosol scenarios based on radiative transfer simulations

2021 ◽  
Author(s):  
Xin Tian ◽  
Yang Wang ◽  
Steffen Beirle ◽  
Pinhua Xie ◽  
Thomas Wagner ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Trace Gases ◽  
A Priori ◽  
Optimal Estimation ◽  
Estimation Algorithm ◽  
Optical Absorption Spectroscopy ◽  
Limited Information ◽  
Inversion Algorithm ◽  
Near Surface ◽  
Aerosol Pollution

Abstract. Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a state of the art remote sensing technique for deriving vertical profiles of trace gases and aerosols. However, MAX-DOAS profile inversions under aerosol pollution scenarios are challenging because of the complex radiative transfer and limited information content of the measurements. In this study, the performances of two inversion algorithms were evaluated for various aerosol pollution scenarios based on synthetic slant column densities (SCDs) derived from radiative transfer simulations. One inversion algorithm is based on optimal estimation, the other uses a parameterized approach. In this analysis, 3 types of profile shapes for aerosols and NO2 were considered: exponential, Boltzmann, and Gaussian. First, the systematic deviations of the retrieved aerosol profiles from the input profiles were investigated. For most cases, the AODs of the retrieved profiles were found to be systematically lower than the input values, and the deviations increased with increasing AOD. Especially for the optimal estimation algorithm and for high AOD, these findings might explain part of the deviations between the AOD retrieved from MAX-DOAS and sun photometers in previous studies. For the optimal estimation algorithm the agreement with the input values can be improved by optimizing the covariance matrix of the a priori uncertainties. Second, the aerosol effects on the NO2 profile retrieval were tested. Here, especially for the optimal estimation algorithm, a systematic dependence on the NO2 VCD was found with a strong relative overestimation of the retrieved results for low NO2 VCDs and an underestimation for high NO2 VCDs. In contrast, the dependence on the aerosol profiles was found to be rather low. In general, both inversion schemes can well retrieve the near-surface values of aerosol extinction and trace gases concentrations.

Validation of Sentinel-5P TROPOMI tropospheric NO2 with airborne imaging, ground-based stationary, and mobile DOAS measurements from the S5P-VAL-DE-Ruhr campaign

2021 ◽  
Author(s):  
Kezia Lange ◽  
Andreas C. Meier ◽  
Michel Van Roozendael ◽  
Thomas Wagner ◽  
Thomas Ruhtz ◽  
...  
Keyword(s):  
A Priori ◽  
Data Sets ◽  
Vertical Column ◽  
Data Set ◽  
Ruhr Area ◽  
Spatial And Temporal Heterogeneity ◽  
Western Germany ◽  
Airborne Imaging ◽  
Urban Character ◽  
One Year

&lt;p&gt;Airborne imaging DOAS and ground-based stationary and mobile DOAS measurements were conducted during the ESA funded S5P-VAL-DE-Ruhr campaign in September 2020 in the Ruhr area. The Ruhr area is located in Western Germany and is a pollution hotspot in Europe with urban character as well as large industrial emitters. The measurements are used to validate data from the Sentinel-5P TROPOspheric Monitoring Instrument (TROPOMI) with focus on the NO&lt;sub&gt;2&lt;/sub&gt; tropospheric vertical column product.&lt;/p&gt;&lt;p&gt;Seven flights were performed with the airborne imaging DOAS instrument, AirMAP, providing continuous maps of NO&lt;sub&gt;2&lt;/sub&gt; in the layers below the aircraft. These flights cover many S5P ground pixels within an area of about 40 km side length and were accompanied by ground-based stationary measurements and three mobile car DOAS instruments. Stationary measurements were conducted by two Pandora, two zenith-sky and two MAX-DOAS instruments distributed over three target areas, partly as long-term measurements over a one-year period.&lt;/p&gt;&lt;p&gt;Airborne and ground-based measurements were compared to evaluate the representativeness of the measurements in time and space. With a resolution of about 100 x 30 m&lt;sup&gt;2&lt;/sup&gt;, the AirMAP data creates a link between the ground-based and the TROPOMI measurements with a resolution of 3.5 x 5.5 km&lt;sup&gt;2&lt;/sup&gt; and is therefore well suited to validate TROPOMI's tropospheric NO&lt;sub&gt;2&lt;/sub&gt; vertical column.&lt;/p&gt;&lt;p&gt;The measurements on the seven flight days show strong variability depending on the different target areas, the weekday and meteorological conditions. We found an overall low bias of the TROPOMI operational NO&lt;sub&gt;2&lt;/sub&gt; data for all three target areas but with varying magnitude for different days. The campaign data set is compared to custom TROPOMI NO&lt;sub&gt;2&lt;/sub&gt; products, using different auxiliary data, such as albedo or a priori vertical profiles to evaluate the influence on the TROPOMI data product. Analyzing and comparing the different data sets provides more insight into the high spatial and temporal heterogeneity in NO&lt;sub&gt;2&lt;/sub&gt; and its impact on satellite observations and their validation.&lt;/p&gt;

scholarly journals Sulfur dioxide retrievals from TROPOMI onboard Sentinel-5 Precursor: algorithm theoretical basis

2017 ◽  
Vol 10 (1) ◽  
pp. 119-153 ◽  
Author(s):  
Nicolas Theys ◽  
Isabelle De Smedt ◽  
Huan Yu ◽  
Thomas Danckaert ◽  
Jeroen van Gent ◽  
...  
Keyword(s):  
Sulfur Dioxide ◽  
Theoretical Basis ◽  
Large Range ◽  
A Priori ◽  
Optical Absorption Spectroscopy ◽  
Retrieval Algorithm ◽  
Pixel Size ◽  
Vertical Column ◽  
Spectral Fitting ◽  
Correction Scheme

Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor (S-5P) platform will measure ultraviolet earthshine radiances at high spectral and improved spatial resolution (pixel size of 7 km  ×  3.5 km at nadir) compared to its predecessors OMI and GOME-2. This paper presents the sulfur dioxide (SO2) vertical column retrieval algorithm implemented in the S-5P operational processor UPAS (Universal Processor for UV/VIS Atmospheric Spectrometers) and comprehensively describes its various retrieval steps. The spectral fitting is performed using the differential optical absorption spectroscopy (DOAS) method including multiple fitting windows to cope with the large range of atmospheric SO2 columns encountered. It is followed by a slant column background correction scheme to reduce possible biases or across-track-dependent artifacts in the data. The SO2 vertical columns are obtained by applying air mass factors (AMFs) calculated for a set of representative a priori profiles and accounting for various parameters influencing the retrieval sensitivity to SO2. Finally, the algorithm includes an error analysis module which is fully described here. We also discuss verification results (as part of the algorithm development) and future validation needs of the TROPOMI SO2 algorithm.

scholarly journals Intercomparison of four airborne imaging DOAS systems for tropospheric NO<sub>2</sub> mapping – the AROMAPEX campaign

2019 ◽  
Vol 12 (1) ◽  
pp. 211-236 ◽  
Author(s):  
Frederik Tack ◽  
Alexis Merlaud ◽  
Andreas C. Meier ◽  
Tim Vlemmix ◽  
Thomas Ruhtz ◽  
...  
Keyword(s):  
Pearson Correlation ◽  
Wavelength Region ◽  
Correlation Coefficients ◽  
Horizontal Distribution ◽  
Primary Objective ◽  
Optical Absorption Spectroscopy ◽  
Data Sets ◽  
Vertical Column ◽  
Airborne Imaging ◽  
The City

Abstract. We present an intercomparison study of four airborne imaging DOAS instruments, dedicated to the retrieval and high-resolution mapping of tropospheric nitrogen dioxide (NO2) vertical column densities (VCDs). The AROMAPEX campaign took place in Berlin, Germany, in April 2016 with the primary objective to test and intercompare the performance of experimental airborne imagers. The imaging DOAS instruments were operated simultaneously from two manned aircraft, performing synchronised flights: APEX (VITO–BIRA-IASB) was operated from DLR's DO-228 D-CFFU aircraft at 6.2 km in altitude, while AirMAP (IUP-Bremen), SWING (BIRA-IASB), and SBI (TNO–TU Delft–KNMI) were operated from the FUB Cessna 207T D-EAFU at 3.1 km. Two synchronised flights took place on 21 April 2016. NO2 slant columns were retrieved by applying differential optical absorption spectroscopy (DOAS) in the visible wavelength region and converted to VCDs by the computation of appropriate air mass factors (AMFs). Finally, the NO2 VCDs were georeferenced and mapped at high spatial resolution. For the sake of harmonising the different data sets, efforts were made to agree on a common set of parameter settings, AMF look-up table, and gridding algorithm. The NO2 horizontal distribution, observed by the different DOAS imagers, shows very similar spatial patterns. The NO2 field is dominated by two large plumes related to industrial compounds, crossing the city from west to east. The major highways A100 and A113 are also identified as line sources of NO2. Retrieved NO2 VCDs range between 1×1015 molec cm−2 upwind of the city and 20×1015 molec cm−2 in the dominant plume, with a mean of 7.3±1.8×1015 molec cm−2 for the morning flight and between 1 and 23×1015 molec cm−2 with a mean of 6.0±1.4×1015 molec cm−2 for the afternoon flight. The mean NO2 VCD retrieval errors are in the range of 22 % to 36 % for all sensors. The four data sets are in good agreement with Pearson correlation coefficients better than 0.9, while the linear regression analyses show slopes close to unity and generally small intercepts.

scholarly journals High-resolution airborne imaging DOAS measurements of NO<sub>2</sub> above Bucharest during AROMAT

2017 ◽  
Vol 10 (5) ◽  
pp. 1831-1857 ◽  
Author(s):  
Andreas Carlos Meier ◽  
Anja Schönhardt ◽  
Tim Bösch ◽  
Andreas Richter ◽  
André Seyler ◽  
...  
Keyword(s):  
Surface Properties ◽  
Urban Areas ◽  
Strong Dependence ◽  
Horizontal Distribution ◽  
Optical Absorption Spectroscopy ◽  
Surface Reflectance ◽  
Emission Rates ◽  
Vertical Column ◽  
Airborne Imaging ◽  
The City

Abstract. In this study we report on airborne imaging DOAS measurements of NO2 from two flights performed in Bucharest during the AROMAT campaign (Airborne ROmanian Measurements of Aerosols and Trace gases) in September 2014. These measurements were performed with the Airborne imaging Differential Optical Absorption Spectroscopy (DOAS) instrument for Measurements of Atmospheric Pollution (AirMAP) and provide nearly gapless maps of column densities of NO2 below the aircraft with a high spatial resolution of better than 100 m. The air mass factors, which are needed to convert the measured differential slant column densities (dSCDs) to vertical column densities (VCDs), have a strong dependence on the surface reflectance, which has to be accounted for in the retrieval. This is especially important for measurements above urban areas, where the surface properties vary strongly. As the instrument is not radiometrically calibrated, we have developed a method to derive the surface reflectance from intensities measured by AirMAP. This method is based on radiative transfer calculation with SCIATRAN and a reference area for which the surface reflectance is known. While surface properties are clearly apparent in the NO2 dSCD results, this effect is successfully corrected for in the VCD results. Furthermore, we investigate the influence of aerosols on the retrieval for a variety of aerosol profiles that were measured in the context of the AROMAT campaigns. The results of two research flights are presented, which reveal distinct horizontal distribution patterns and strong spatial gradients of NO2 across the city. Pollution levels range from background values in the outskirts located upwind of the city to about 4  ×  1016 molec cm−2 in the polluted city center. Validation against two co-located mobile car-DOAS measurements yields good agreement between the datasets, with correlation coefficients of R =  0.94 and R =  0.85, respectively. Estimations on the NOx emission rate of Bucharest for the two flights yield emission rates of 15.1 ± 9.4 and 13.6 ± 8.4 mol s−1, respectively.

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