High-resolution airborne imaging DOAS measurements of NO<sub>2</sub> above Bucharest during AROMAT

Abstract

Abstract. In this study we report on airborne imaging DOAS measurements of NO 2 from two flights performed in Bucharest during the AROMAT campaign (Airborne ROmanian Measurements of Aerosols and Trace gases) in September 2014. These measurements were performed with the Airborne imaging Differential Optical Absorption Spectroscopy (DOAS) instrument for Measurements of Atmospheric Pollution (AirMAP) and provide nearly gapless maps of column densities of NO 2 below the aircraft with a high spatial resolution of better than 100 m. The air mass factors, which are needed to convert the measured differential slant column densities (dSCDs) to vertical column densities (VCDs), have a strong dependence on the surface reflectance, which has to be accounted for in the retrieval. This is especially important for measurements above urban areas, where the surface properties vary strongly. As the instrument is not radiometrically calibrated, we have developed a method to derive the surface reflectance from intensities measured by AirMAP. This method is based on radiative transfer calculation with SCIATRAN and a reference area for which the surface reflectance is known. While surface properties are clearly apparent in the NO 2 dSCD results, this effect is successfully corrected for in the VCD results. Furthermore, we investigate the influence of aerosols on the retrieval for a variety of aerosol profiles that were measured in the context of the AROMAT campaigns. The results of two research flights are presented, which reveal distinct horizontal distribution patterns and strong spatial gradients of NO 2 across the city. Pollution levels range from background values in the outskirts located upwind of the city to about 4 × 10 16 molec cm −2 in the polluted city center. Validation against two co-located mobile car-DOAS measurements yields good agreement between the datasets, with correlation coefficients of R = 0.94 and R = 0.85, respectively. Estimations on the NO x emission rate of Bucharest for the two flights yield emission rates of 15.1 ± 9.4 and 13.6 ± 8.4 mol s −1 , respectively.

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

“…Temperature and pressure profiles from the NAM-CMAQ in the troposphere and the Realtime Air Quality Modeling System (RAQMS; Pierce et al, 2009) up to 60 km are extracted and merged, and ozone profiles are extracted from the NAM-CMAQ analysis for the troposphere and OMI gridded monthly climatology in the stratosphere for May and June 2017. Noguchi et al (2014) evaluated the influence of surface anisotropy on NO 2 AMFs for geostationary-scale measurements using a 1 km MODIS BRDF product and found that not accounting for BRDF in the AMF calculation, especially in areas with high NO 2 near the surface, can lead to large errors. For this reason, this work uses the BRDF isometric, volumetric, and geometric kernels retrieved in band 3 by the MODIS MCD43A1 daily L3 500 m v006 product (Lucht et al, 2000;Schaaf and Wang, 2015).…”

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

“…NO x is involved in the formation of photochemical ozone and fine aerosol particles, with implications for both surface air quality and climate. Both short-and long-term enhanced NO 2 concentrations are associated with increased mortality (Hoek et al, 2013;Mills et al, 2015). NO x emissions can also lead to excess nitrogen deposition (Fowler et al, 2013;Nowlan et al, 2014).…”

Studies of the horizontal inhomogeneities in NO<sub>2</sub> concentrations above a shipping lane using ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements and validation with airborne imaging DOAS measurements

“…With this setup, 35 across-track pixels are measured simultaneously with an exposure time of 0.5 s, leading to a spatial resolution better than 50 m when the aircraft is flying at 1600 m altitude. For more detailed information on the instrument see Schönhardt et al (2015) and Meier et al (2017).…”