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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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doi:10.5194/amt-2016-400
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
02 Jan 2017
Review status
This discussion paper is under review for the journal Atmospheric Measurement Techniques (AMT).
High resolution mapping of the NO2 spatial distribution over Belgian urban areas based on airborne APEX remote sensing
Frederik Tack1, Alexis Merlaud1, Marian-Daniel Iordache2, Thomas Danckaert1, Huan Yu1, Caroline Fayt1, Koen Meuleman2, Felix Deutsch2, Frans Fierens3, and Michel Van Roozendael1 1BIRA-IASB, Royal Belgian Institute for Sp ace Aeronomy, Brussels, 1180, Belgium
2VITO, Flemish Institute for Technological Research, Mol, 2400, Belgium
3IRCEL-CELINE, Belgian Interregional Environment Agency, Brussels, 1210, Belgium
Abstract. We present retrieval results of tropospheric nitrogen dioxide (NO2) vertical column densities (VCDs), mapped at high spatial resolution over three Belgian cities, based on the DOAS analysis of Airborne Prism EXperiment (APEX) observations. APEX, developed by a Swiss-Belgian consortium on behalf of ESA (European Space Agency), is a pushbroom hyperspectral imager characterised by a high spatial resolution and high spectral performance. APEX data have been acquired under clear sky conditions over the three largest and most heavily polluted Belgian cities, i.e. Brussels, Antwerp and Liège on 15 April and 30 June 2015. Additionally, a number of background sites have been covered for the reference spectra. The APEX instrument was mounted in a Dornier DO-228 airplane, operated by Deutsches Zentrum für Luft- und Raumfahrt (DLR). NO2 VCDs were retrieved from spatially aggregated radiance spectra allowing to resolve urban plumes at the resolution of 60 x 80 m2. Main sources in the Antwerp area appear to be related to (petro)chemical industry while traffic-related emissions dominate in Brussels. The NO2 levels observed in Antwerp range between 3 and 35 x 1015 molec m−2, with a mean VCD of 17.4 ± 3.7 x 1015 molec cm−2. In the Brussels area, smaller levels are found, ranging between 1 and 20 x 1015 molec cm−2 and a mean VCD of 7.7 ± 2.1 x 1015 molec cm−2. The overall error on the retrieved N2 VCDs is on average 21 % and 28 % for the Antwerp and Brussels data set, respectively. Low VCD retrievals are mainly limited by noise (1-sigma slant error), while high retrievals are mainly limited by systematic errors. Compared to coincident car mobile-DOAS measurements performed in Antwerp and Brussels, both data sets are in good agreement with correlation coefficients around 0.85 and slopes close to unity. APEX retrievals tend to be on average 12 % and 6 % higher for Antwerp and Brussels, respectively. Results demonstrate that the NO2 distribution in an urban environment, and its fine scale variability, can be mapped accurately with high spatial resolution and in a relatively short time frame, and the contributing emission sources can be resolved. High resolution quantitative information about the atmospheric NO2 horizontal variability is currently rare, but can be very valuable for (air quality) studies at the urban scale.

Citation: Tack, F., Merlaud, A., Iordache, M.-D., Danckaert, T., Yu, H., Fayt, C., Meuleman, K., Deutsch, F., Fierens, F., and Van Roozendael, M.: High resolution mapping of the NO2 spatial distribution over Belgian urban areas based on airborne APEX remote sensing, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2016-400, in review, 2017.
Frederik Tack et al.
Frederik Tack et al.
Frederik Tack et al.

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Short summary
The manuscript presents retrieval results of NO2 vertical column densities, mapped at high spatial resolution over three Belgian cities, based on the DOAS analysis of Airborne APEX observations. A major objective of the study is to assess the technical and operational capabilities of the APEX hyperspectral pushbroom imager to map the NO2 horizontal distribution at high spatial resolution. The retrieval algorithm is fully characterised and retrieved NO2 distribution maps are discussed.
The manuscript presents retrieval results of NO2 vertical column densities, mapped at high...
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