A comparison of surface NO2 mixing ratios and total column observations at a South African site

Abstract

The total column density nitrogen dioxide (NO2) retrievals collated by a ground-based sun-tracking spectrometer (Pandora/GSFC) and the satellite-borne (Aura) Ozone Monitoring Instrument (OMI) were compared to the volume mixing ratios measured by a ground-based gas-analyser at Welgegund, North-West University (NWU) atmospheric monitoring station (Potchefstroom, South Africa). An assessment of the comparability between columnar and surface NO2 measurements was performed. The concurrent ground measurements results over January-March, 2011, were averaged over one hour to correspond to the closest local OMI overpasses (~12:00 UTC). A novel method for estimating surface mixing ratios from total-column retrievals, via a planetary boundary layer (PBL) height correction factor as tested by Knepp et al. (2013) in the USA was applied. This PBL correction factor largely corrects for boundary-layer variability throughout the day, and allows conversion into mixing ratios. The data for the ground instruments were in agreement within the expected uncertainty for each technique and between two remote sensing instruments. However, NO2 between the ground gas analyser and satellite borne instrument were out of the expected uncertainty limits.