Retrieval of total column and surface NO₂ from Pandora zenith-sky measurements

Pandora spectrometers can retrieve nitrogen dioxide (NO₂) vertical column densities (VCDs) via two viewing geometries: direct Sun and zenith sky. The direct-Sun NO₂ VCD measurements have high quality (0.1 DU accuracy in clear-sky conditions) and do not rely on any radiative transfer model to calculate air mass factors (AMFs); however, they are not available when the Sun is obscured by clouds. To perform NO₂ measurements in cloudy conditions, a simple but robust NO₂ retrieval algorithm is developed for Pandora zenith-sky measurements. This algorithm derives empirical zenith-sky NO₂ AMFs from coincident high-quality direct-Sun NO₂ observations. Moreover, the retrieved Pandora zenith-sky NO₂ VCD data are converted to surface NO₂ concentrations with a scaling algorithm that uses chemical-transport-model predictions and satellite measurements as inputs. NO₂ VCDs and surface concentrations are retrieved from Pandora zenith-sky measurements made in Toronto, Canada, from 2015 to 2017. The retrieved Pandora zenith-sky NO₂ data (VCD and surface concentration) show good agreement with both satellite and in situ measurements. The diurnal and seasonal variations of derived Pandora zenith-sky surface NO₂ data also agree well with in situ measurements (diurnal difference within ±2 ppbv). Overall, this work shows that the new Pandora zenith-sky NO₂ products have the potential to be used in various applications such as future satellite validation in moderate cloudy scenes and air quality monitoring.