Retrieval of surface NO 2 from Pandora direct sun measurements

Abstract. Nitrogen dioxide (NO2), a criteria pollutant used for the Air Quality Health Index (AQHI) reporting, can have important environmental and human respiratory health effects when present in the near-surface atmosphere. For this study, urban surface volume mixing ratios (VMRs) are derived from Pandora total column measurements. Pandora remote sensing instruments can be used to evaluate surface air quality and complement in situ instruments to support long-term monitoring of ambient air quality. At four sites in the Greater Toronto Area and Detroit-Windsor Area, Pandora direct sun measurements are converted to surface VMRs following a method utilizing a high-resolution air quality forecasting model. The resulting VMRs agree with coincident in situ measurements to within 2 ppbv from 2020 through 2024. This method is shown to be most effective when the Planetary Boundary Layer (PBL) is above 750 m as estimated by ERA5 reanalysis, while dependence on PBL height below 750 m is evaluated. The surface VMR derived from Pandora direct sun measurements when the PBL is below 750 m has a median difference of -1.7 ppbv (parts per billion by volume) (10th percentile: -8.2 ppbv, 90th percentile: 3.2 ppbv) with respect to coincident in situ measurements. When a logarithmic correction is applied as a function of PBL height for these shallow PBL conditions, the median difference is reduced to 0.5 ppbv (-4.8 ppbv, 4.1 ppbv).