In this study we report on airborne imaging DOAS measurements of NO2 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 NO2
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 NO2 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 NO2 across the city. Pollution levels range from background
values in the outskirts located upwind of the city to about 4 × 1016 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 NOx 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