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 Goddard Earth Observing System Composition Forecast 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 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. Plain Language Summary Relating satellite-derived observations of nitrogen dioxide (NO2) to ground-level air quality has long been an area of research with important applications to human and environmental welfare. Here, we use new remote sensing observations from ground- and satellite-based instruments, along with ground-level air quality monitoring and meteorological observations, to gain new insight into the relationship of NO2 total column abundance and surface concentrations in a costal urban environment. We show how the total columns (the NO2 between the surface and top of the atmosphere) derived from remote sensing are sensitive to different processes than surface concentrations (the amount of NO2 at a point near the surface), which complicate the interpretation of surface air quality from satellite observations. We also find evidence that the mixing of surface emissions in our urban environment is slow relative to the NOx lifetime even under conditions when the boundary layer is expected to be well-mixed. We find that sea breeze systems common to coastal environments present particularly challenging circumstances, due to higher spatial and temporal variability in pollutant concentrations. Our results should provide some insights into the interpretation of new geostationary air quality observations.