Abstract
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.
Highlights
Nitrogen dioxide (NO2) is an atmospheric trace gas and a key pollutant that attracts considerable attention due to several reasons: (1) NO2 is a proxy for air pollution in general, as its abundance mostly coincides with a range of other pollutants; (2) recent studies by the World Health Organization (WHO, 2013) have shown that NO2 exposure can have a direct health impact; (3) it is a precursor in the formation of aerosols (Chan et al, 2010) and tropospheric ozone (Crutzen, 1970) and it contributes locally to radiative forcing (Solomon et al, 1999), through which it indirectly affects the climate system
The ground sample distance (GSD) is 3 by 4 m2 assuming a typical altitude of 6.1 km a.g.l., ground speed of 72 m s−1 and integration time of 58 ms, the latter being a good balance between the obtained signal-to-noise ratio and the occurrence of saturated scans
The NO2 vertical column densities (VCDs) diurnal variation retrieved at the Uccle MAX-differential optical absorption spectroscopy technique (DOAS) station (Fig. 14) shows overall larger columns for the flight on 15 April when compared to the flight on 30 June, with mean NO2 VCDs of respectively 10 and 5 × 1015 molec cm−2 between 80◦ solar zenith angle (SZA) sunrise and sunset
Summary
Nitrogen dioxide (NO2) is an atmospheric trace gas and a key pollutant that attracts considerable attention due to several reasons: (1) NO2 is a proxy for air pollution in general, as its abundance mostly coincides with a range of other pollutants; (2) recent studies by the World Health Organization (WHO, 2013) have shown that NO2 exposure can have a direct health impact; (3) it is a precursor in the formation of aerosols (Chan et al, 2010) and tropospheric ozone (Crutzen, 1970) and it contributes locally to radiative forcing (Solomon et al, 1999), through which it indirectly affects the climate system. Richter and Burrows, 2002; Beirle et al, 2010; Boersma et al, 2011; Hilboll et al, 2013; Valks et al, 2011; Bucsela et al, 2013) Such observations are well suited for global monitoring,; the coarse spatial resolution of typically several tens of kilometres make them inadequate for detecting city-scale NO2 variability and to resolve individual emission sources Several studies discuss instruments and experiments for atmospheric trace gas retrieval from airborne platforms Most of these works focus on the retrieval of the vertical distribution of trace gases, based on along-track Multi-AXis (MAX-) DOAS observations
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