Relation between total vertical column density and near-surface NO2 based on in situ and Pandora ground-based remote sensing observations

Nitrogen dioxide (NO2) is a major pollutant that at high concentrations may affect human health. It
is also a photochemically reactive gas that is important for the oxidation potential of the atmosphere and acts
as a precursor for the formation of aerosol particles and ozone. However, monitoring of near-surface (NS) NO2
faces the challenge of spatial discontinuity due to large distances between ground-based monitoring stations,
whereas satellite remote sensing provides total vertical column density (VCD) that is related to near-surface (NS)
concentrations in a complicated manner. In this study, the relation between total VCD and NS concentrations
of NO2 was analyzed based on total VCD from remote sensing observations using a ground-based Pandora
spectrometer and NS NO2 concentrations from in situ observations. Both instruments were located at the Beijing-
RADI site (Beijing, China) during January 2022. The ratio between total VCD and NS NO2 concentrations
varies throughout the day with substantially different relations in the morning and afternoon. During the night
and morning, the atmosphere was vertically stratified, with disconnected layers that prevented vertical mixing
of atmospheric constituents. In the afternoon, these layers connected, allowing for vertical mixing and transport
between the surface and the top of the boundary layer. Thus, the prohibition of vertical transport in the morning
and the mixing in the afternoon resulted in different relations between the total VCD and NS NO2 concentrations.
These different relationships have consequences for the use of satellite remote sensing to estimate NS NO2
concentrations.