Novel application of satellite and in-situ measurements to map surface-level NO₂ in the Great Lakes region

Abstract. Ozone Monitoring Instrument (OMI) tropospheric NO₂ vertical column density data were used in conjunction with in-situ NO₂ concentrations collected by permanently installed monitoring stations to infer 24 h surface-level NO₂ concentrations at 0.1° (~11 km) resolution. The region examined included rural and suburban areas, and the highly industrialised area of Windsor, Ontario, which is situated directly across the US-Canada border from Detroit, MI. Photolytic NO₂ monitors were collocated with standard NO₂ monitors to provide qualitative data regarding NO_z interference during the campaign. The accuracy of the OMI-inferred concentrations was tested using two-week integrative NO₂ measurements collected with passive monitors at 18 locations, approximating a 15 km grid across the region, for 7 consecutive two-week periods. When compared with these passive results, satellite-inferred concentrations showed an 18% positive bias. The correlation of the passive monitor and OMI-inferred concentrations (R=0.69, n=115) was stronger than that for the passive monitor concentrations and OMI column densities (R=0.52), indicating that using a sparse network of monitoring sites to estimate concentrations improves the direct utility of the OMI observations. OMI-inferred concentrations were then calculated for four years to show an overall declining trend in surface NO₂ concentrations in the region. Additionally, by separating OMI-inferred surface concentrations by wind direction, clear patterns in emissions and affected down-wind regions, in particular around the US-Canada border, were revealed.