SO₂ emissions and lifetimes derived from TROPOMI observations over India using a flux-divergence method

Abstract. The rapid development of the economy and the implementation of environmental policies adapted in India has led to fast changes of regional SO2 emissions. We present a monthly SO2 emission inventory for India covering December 2018 to November 2023 based on the TROPOMI Level-2 COBRA SO2 dataset, by using an improved flux-divergence method and estimated local SO2 lifetime which includes both its chemical loss and dry deposition. We update the methodology to use the daily CAMS model output estimates of the hydroxyl-radical distribution as well as the measured dry deposition velocity to account for the variability in the tropospheric SO2 lifetime. The results show the application of the local SO2 lifetime improves the accuracy of SO2 emissions estimation when compared to calculations using a constant lifetime. Our improved flux-divergence method reduced the spreading of the point source emissions compared to the standard flux-divergence method. The averaged SO2 emissions covering the recent 5 years are about 5.2 Tg year-1, which is lower than the bottom-up emissions of 11.0 Tg year-1 from CAMS-GLOB-ANT v5.3. The total emissions from the 92 largest point source emissions are estimated to be 2.9 Tg year-1, lower than the estimation of 5.2 Tg year-1 from the global SO2 catalog MSAQSO2LV4. We argue that for other important regions that have high SO2 emissions, the variability in the SO2 lifetime becomes more important to account for estimating top-down SO2 emissions.