| الوصف: |
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. |