New insights into the role of atmospheric transport and mixing on column and surface concentrations of NO2 at a coastal urban site

We use a multi-year record of Pandora-derived NO2 total column abundance in Boston to examine the influence of atmospheric transport on column NO2 and its surface concentrations during the warm season in a coastal urban environment. We derive tropospheric NO2 estimates from the total column with a measurement-model fusion approach using near-real-time estimates of stratospheric NO2 from NASA’s GEOS-CF model system and find the average influence of stratospheric NO2 at this urban site can be 30-70% depending on season and time of day. Sea breeze days tend to exhibit rapid temporal variability in the column that which can go in the opposite direction of changes in surface NO2 concentrations. By comparing tropospheric NO2 with surface concentrations, we constrain the role of boundary layer entrainment processes in the evolution of surface NO2 concentrations, while highlighting the value of column measurements in identifying sea breeze frontal dynamics. We estimate an apparent equal mixing layer height of NO2 and infer that surface NOx emissions remain concentrated near the surface regardless of atmospheric stability regime. When comparing the Pandora- to TROPOMI-derived column NO2 measurements, we find that sea breeze days present a unique challenge likely due to higher spatial heterogeneity in NO2 and the meteorology involved that is not well represented in operational retrieval inputs. Our observations provide new insights into column and surface variability of NO2 which will be relevant to interpreting geostationary observations, especially in coastal urban locations.