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.