High-resolution mapping of the NO₂ spatial distribution over Belgian urban areas based on airborne APEX remote sensing

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 two largest and most heavily polluted Belgian cities, i.e. Antwerp and Brussels 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 aeroplane, operated by Deutsches Zentrum für Luft- und Raumfahrt (DLR). NO₂ VCDs were retrieved from spatially aggregated radiance spectra allowing urban plumes to be resolved at the resolution of 60  ×  80 m². The main sources in the Antwerp area appear to be related to the (petro)chemical industry while traffic-related emissions dominate in Brussels. The NO₂ levels observed in Antwerp range between 3 and 35  ×  10¹⁵ molec cm⁻², with a mean VCD of 17.4 ± 3.7  ×  10¹⁵ molec cm⁻². In the Brussels area, smaller levels are found, ranging between 1 and 20  ×  10¹⁵ molec cm⁻² and a mean VCD of 7.7 ± 2.1  ×  10¹⁵ molec cm⁻². The overall errors on the retrieved NO₂ VCDs are on average 21 and 28 % for the Antwerp and Brussels data sets. Low VCD retrievals are mainly limited by noise (1σ slant error), while high retrievals are mainly limited by systematic errors. Compared to coincident car mobile-DOAS measurements taken 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.