Abstract. Trace gases play a key role in the chemistry of urban atmospheres.
Therefore, knowledge about their spatial distribution is needed to fully
characterize air quality in urban areas. Using a new Multi-AXis Differential
Optical Absorption Spectroscopy two-dimensional (MAXDOAS-2D) instrument, along with an
inversion algorithm (bePRO), we report the first two-dimensional maps of
nitrogen dioxide (NO2) and nitrous acid (HONO) concentrations in the
city of Madrid, Spain. Measurements were made during 2 months (6 May–5 July 2019), and peak mixing ratios of 12 and 0.7 ppbv (parts per billion by volume) for NO2
and HONO, respectively, were observed in the early morning in the southern
part of the downtown area. We found good general agreement between the
MAXDOAS-2D mesoscale observations – which provide a typical spatial range of
a few kilometers – and the in situ measurements provided by
Madrid's air quality monitoring stations. In addition to
vertical profiles, we studied the horizontal gradients of NO2 in the
surface layer by applying the different horizontal light path lengths in the
two spectral regions included in the NO2 spectral analysis: ultraviolet
(UV, at 360 nm) and visible (VIS, 477 nm). We also investigate the
sensitivity of the instrument to infer vertically distributed information on
aerosol extinction coefficients and discuss possible future ways to improve
the retrievals. The retrieval of two-dimensional distributions of trace gas
concentrations reported here provides valuable spatial information for the
study of air quality in the city of Madrid.
Spatially resolved measurements of nitrogen dioxide in an urban environment using concurrent multi-axis differential optical absorption spectroscopy