High-resolution mapping of the NO<sub>2</sub> spatial distribution over Belgian urban areas based on airborne APEX remote sensing

Abstract

Abstract. We present retrieval results of tropospheric nitrogen dioxide (NO 2 ) 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). NO 2 VCDs were retrieved from spatially aggregated radiance spectra allowing urban plumes to be resolved at the resolution of 60 × 80 m 2 . The main sources in the Antwerp area appear to be related to the (petro)chemical industry while traffic-related emissions dominate in Brussels. The NO 2 levels observed in Antwerp range between 3 and 35 × 10 15 molec cm −2 , with a mean VCD of 17.4 ± 3.7 × 10 15 molec cm −2 . In the Brussels area, smaller levels are found, ranging between 1 and 20 × 10 15 molec cm −2 and a mean VCD of 7.7 ± 2.1 × 10 15 molec cm −2 . The overall errors on the retrieved NO 2 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 NO 2 distribution in an urban environment, and its finescale 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 NO 2 horizontal variability is currently rare, but can be very valuable for (air quality) studies at the urban scale.

The Small Whiskbroom Imager for atmospheric compositioN monitorinG (SWING) and its operations from an unmanned aerial vehicle (UAV) during the AROMAT campaign

“…Previous studies indicate that the two main uncertainty sources for nadir observations of the NO 2 column are the ground albedo and the relative position of the NO 2 and aerosol layer (Leitão et al, 2010;Meier et al, 2017;Tack et al, 2017). In our case, the lidar and sonde indicate that the NO 2 and aerosols have a similar profile, as can be expected considering the large source in a rural area.…”

“…Regarding the atmospheric horizontal distribution, a growing interest appeared in the last decade for nadir-looking airborne DOAS instruments able to map trace gases below the aircraft. Maps of tropospheric NO 2 were acquired around power plants in South Africa (Heue et al, 2008) and in Germany (Schönhardt et al, 2015), or above urban areas like Houston (Kowalewski and Janz, 2009;Nowlan et al, 2016), Zurich (Popp et al, 2012), Indianapolis (General et al, 2014), Brussels and Antwerp (Tack et al, 2017), Leicester (Lawrence et al, 2015), and Bucharest (Meier et al, 2017). General et al (2014) also quantified NO 2 in the vicinity of the Prudhoe Bay oil field in Alaska.…”

Evaluating the impact of spatial resolution on tropospheric NO<sub>2</sub> column comparisons within urban areas using high-resolution airborne data

“…Similar instruments have also been developed by other countries. In the last decade, airborne spectrometers have mapped high-resolution NO 2 in parts of Europe (Popp et al, 2012;Schönhardt et al 2015;Lawrence et al, 2015;Meier et al, 2017;Tack et al, 2017Tack et al, , 2019, the United States (Nowlan et al, 2016Lamsal et al, 2017;Judd et al, 2018), Asia , and Africa (Broccardo et al, 2018). Results from these efforts have illustrated the capability of airborne spectrometers to observe the impact of emission sources and meteorology on the spatial distribution of NO 2 (Popp et al, 2012;Schönhardt et al, 2015;Judd et al, 2018, andTack et al, 2019) and have shown utility for evaluating NO x emissions (Schönhardt et al, 2015;Souri et al, 2018) as these high-resolution measurements can resolve detailed NO 2 spatial patterns that satellites, at their current spatial resolutions, cannot (Broccardo et al, 2018;Lamsal et al, 2017;Judd et al, 2018).…”

Nitrogen dioxide and formaldehyde measurements from the GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator over Houston, Texas

“…NO 2 amounts over industrial regions and urban areas have been mapped at high spatial resolution by several recently developed airborne push-broom sensors (Heue et al, 2008;Popp et al, 2012;Schönhardt et al, 2015;Lawrence et al, 2015;Nowlan et al, 2016;Meier et al, 2017;Tack et al, 2017Tack et al, , 2018Vlemmix et al, 2017;Broccardo et al, 2018). Airborne remote-sensing CH 2 O measurements have previously been made from aircraft using limb-viewing geometry by airborne multi-axis differential optical absorption spectroscopy (AMAX-DOAS) (Baidar et al, 2013) and by the whisk-broom scanning technique (where the cross-track spatial dimension is provided by mechanical scanning) using the Airborne Compact Atmospheric Mapper (ACAM) (Liu et al, 2015b).…”