Multi-source SO₂ emission retrievals and consistency of satellite and surface measurements with reported emissions

Abstract. Reported sulfur dioxide (SO₂) emissions from US and Canadian sources have declined dramatically since the 1990s as a result of emission control measures. Observations from the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite and ground-based in situ measurements are examined to verify whether the observed changes from SO₂ abundance measurements are quantitatively consistent with the reported changes in emissions. To make this connection, a new method to link SO₂ emissions and satellite SO₂ measurements was developed. The method is based on fitting satellite SO₂ vertical column densities (VCDs) to a set of functions of OMI pixel coordinates and wind speeds, where each function represents a statistical model of a plume from a single point source. The concept is first demonstrated using sources in North America and then applied to Europe. The correlation coefficient between OMI-measured VCDs (with a local bias removed) and SO₂ VCDs derived here using reported emissions for 1° by 1° gridded data is 0.91 and the best-fit line has a slope near unity, confirming a very good agreement between observed SO₂ VCDs and reported emissions. Having demonstrated their consistency, seasonal and annual mean SO₂ VCD distributions are calculated, based on reported point-source emissions for the period 1980–2015, as would have been seen by OMI. This consistency is further substantiated as the emission-derived VCDs also show a high correlation with annual mean SO₂ surface concentrations at 50 regional monitoring stations.