Impact of 3D radiative transfer on airborne NO 2 imaging remote sensing over cities with buildings

Schwaerzel, Marc; Brunner, Dominik; Jakub, Fabian; Emde, Claudia; Buchmann, Brigitte; Berne, Alexis; Kuhlmann, Gerrit

Airborne imaging remote sensing is increasingly used to map the spatial distribution of nitrogen dioxide (inline-formulaNO2) in cities. Despite the small ground-pixel size of the sensors, the measured inline-formulaNO2 distributions are much smoother than one would expect from high-resolution model simulations of inline-formulaNO2 over cities. This could partly be caused by 3D radiative transfer effects due to observation geometry, adjacency effects and effects of buildings. Here, we present a case study of imaging a synthetic inline-formulaNO2 distribution for a district of Zurich using the 3D MYSTIC (Monte carlo code for the phYSically correct Tracing of photons In Cloudy atmospheres) solver of the libRadtran radiative transfer library. We computed inline-formulaNO2 slant column densities (SCDs) using the recently implemented 3D-box air mass factors (3D-box AMFs) and a new urban canopy module to account for the effects of buildings. We found that for a single ground pixel (50 m inline-formula× 50 m) more than 50 % of the sensitivity is located outside of the pixel, primarily in the direction of the main optical path between sun, ground pixel, and instrument. Consequently, inline-formulaNO2 SCDs are spatially smoothed, which results in an increase over roads when they are parallel to the optical path and a decrease otherwise. When buildings are included, inline-formulaNO2 SCDs are reduced on average by 5 % due to the reduced sensitivity to inline-formulaNO2 in the shadows of the buildings. The effects of buildings also introduce a complex pattern of variability in SCDs that would show up in airborne observations as an additional noise component (about 12 inline-formulaµmol m−2) similar to the magnitude of typical measurement uncertainties. The smearing of the SCDs cannot be corrected using 1D-layer AMFs that assume horizontal homogeneity and thus remains in the final inline-formulaNO2 map. The 3D radiative transfer effects by including buildings need to be considered to compute more accurate AMFs and to reduce biases in inline-formulaNO2 vertical columns obtained from high-resolution city-scale inline-formulaNO2 remote sensing.

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Schwaerzel, Marc / Brunner, Dominik / Jakub, Fabian / et al: Impact of 3D radiative transfer on airborne NO2 imaging remote sensing over cities with buildings. 2021. Copernicus Publications.

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