Retrieval of total column and surface NO ₂ from Pandora zenith-sky measurements

Pandora spectrometers can retrieve nitrogen dioxide (inline-formulaNO₂) vertical column densities (VCDs) via two viewing geometries: direct Sun and zenith sky. The direct-Sun inline-formulaNO₂ 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 inline-formulaNO₂ measurements in cloudy conditions, a simple but robust inline-formulaNO₂ retrieval algorithm is developed for Pandora zenith-sky measurements. This algorithm derives empirical zenith-sky inline-formulaNO₂ AMFs from coincident high-quality direct-Sun inline-formulaNO₂ observations. Moreover, the retrieved Pandora zenith-sky inline-formulaNO₂ VCD data are converted to surface inline-formulaNO₂ concentrations with a scaling algorithm that uses chemical-transport-model predictions and satellite measurements as inputs. inline-formulaNO₂ VCDs and surface concentrations are retrieved from Pandora zenith-sky measurements made in Toronto, Canada, from 2015 to 2017. The retrieved Pandora zenith-sky inline-formulaNO₂ 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 inline-formulaNO₂ data also agree well with in situ measurements (diurnal difference within inline-formula±2 ppbv). Overall, this work shows that the new Pandora zenith-sky inline-formulaNO₂ products have the potential to be used in various applications such as future satellite validation in moderate cloudy scenes and air quality monitoring.