Estimation of anthropogenic and volcanic SO2 emissions from satellite data in the presence of snow/ice on the ground

<p>Early versions of satellite nadir-viewing UV SO₂ data products assumed snow-free surface conditions. Snow covered terrain, with its high reflectance in the UV, typically enhances satellite sensitivity to boundary layer pollution. However, a significant fraction of high-quality cloud-free measurements over snow is currently excluded from analyses.&#160; This leads to increasing the uncertainties of the satellite emissions estimates and introducing potential seasonal biases due to the lack of data in winter months for some high-latitudinal sources. In this study, we investigated how OMI and TROPOMI satellite SO₂ measurements over snow-covered surfaces could be used to improve the annual emissions reported in our SO₂ emissions catalogue (version 2, Fioletov et al., 2023). Although only 100 out of 759 sources listed in the catalogue have 10% or more of the observations over snow, for 40 high-latitude sources more than 30% of measurements suitable for emission calculations were made over snow-covered surfaces. For example, in the case of Norilsk, the world&#8217;s largest SO₂ emissions point source, annual emissions estimates in the SO₂ catalogue were based only on 3-4 summer months, while addition of data for snow conditions extends that period to 7 months.</p> <p>&#160;</p> <p>Emissions in the SO₂ catalogue were based on satellite measurements of SO₂ slant column densities (SCDs) that were converted to vertical column densities (VCDs) using site-specific clear-sky air mass factors (AMFs), calculated for snow-free conditions. The same approach was applied to measurements with snow on the ground whereby a new set of constant, site-specific, clear-sky with snow AMFs was created, and these were applied to the measured SCDs. Annual emissions were then estimated for each source considering (i) only snow-free days, (ii) only clear-sky with snow days and (iii) a merged dataset (snow and no snow conditions). For individual sources, the difference between emissions estimated for snow and snow-free conditions is within &#177;20% for three quarters of smelters and oil and gas sources, and with practically no systematic bias. This is excellent consistency given that there is typically a 3-5 times difference between AMFs for snow and snow-free conditions. For coal-fired power plants, however, emissions estimated for snow conditions are on average 25% higher than for no snow conditions; this difference is likely real and due to larger production (consumption of coal) and emissions in wintertime.</p> <p>&#160;</p> <p>Reference:</p> <p>Fioletov, V. E., McLinden, C. A., Griffin, D., Abboud, I., Krotkov, N., Leonard, P. J. T., Li, C., Joiner, J., Theys, N., and Carn, S.: Version 2 of the global catalogue of large anthropogenic and volcanic SO₂ sources and emissions derived from satellite measurements, Earth Syst. Sci. Data, 15, 75&#8211;93, https://doi.org/10.5194/essd-15-75-2023, 2023.</p>