Comparing surface NO₂ mixing ratios with total column observations for potential application to air quality

Abstract

The total column density 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 concentrations measured by a ground-based gas-analyser at Welgegund, NWU's atmospheric monitoring station (Potchefstroom, South Africa) to study the comparability between columnar and surface NO₂ measurements. The concurrent ground measurements performed over January- March, 2011, were averaged over one hour to correspond to the closest OMI overpasses (~12:00 UTC). A novel method for estimating surface mixing ratios from total-column retrievals, via an atmospheric boundary layer (ABL) height correction factor as tested in the USA was applied (Knepp et al., 2013). This ABL correction factor largely corrects for boundary-layer variability throughout the day, and allows conversion into mixing ratios.Applicability of this technique was briefly explored for Welgegund site that is frequently exposed to background and polluted air masses. The coincidental quantitative and qualitative comparisons are presented.