Sulphur dioxide loadings over megacity Lahore Pakistan and adjoining region of Indo-Gangetic Basin

This article presents spatial and temporal variations of planetary boundary layer PBL sulphur dioxide SO2 over megacity Lahore and adjoining region, a typical representative area in the Indo-Gangetic Basin IGB largely influenced by transported volcanic SO2 from Africa, Middle East, and southern Europe, by using data retrieved from satellite-based Ozone Monitoring Instrument OMI during October 2004–September 2015. We find a positive trend of 2.4% per year slope 0.01 ± 0.005 with y-intercept 0.35 ± 0.03 Dobson Unit DU, correlation coefficient r = 0.55 and 2-tailed p-value at 0.1 of OMI-SO2 column with the average value of 0.4 ± 0.05 DU. Strong seasonality of OMI-SO2 column is observed over the region linked with local meteorology, patterns of anthropogenic emissions, crop residue burning, and vegetation cover. There exists a seasonal high value in winter 0.56 ± 0.24 DU with a peak in December 0.67 ± 0.26 DU. The seasonal lowest value is observed to be 0.29 ± 0.11 DU in wet summer with minimum value in July 0.25 ± 0.06 DU. High growth rates of OMI-SO2 column over the study region have been observed in January, June, October, and December ranging from 5.7% to 11.6% per year. Satellite data show elevated OMI-SO2 columns in 2007, 2008, 2011, and 2012 largely contributed by trans-boundary volcanic SO2. A detailed analysis of volcanic SO2 transported from Africa and Middle East Jabal Al-Tair, Dalaffilla, and Nabro volcanoes over the study area is presented. Air mass trajectories suggest the presence of long-range transported volcanic SO2 at high altitude levels over Lahore and IGB region during the volcanic episodes. The SO2 enhancements in PBL during winter season are generally due to significant vertical downdraft of high-altitude volcanic SO2. For the first time, we present significant influence of volcanic SO2 from southern Europe Mt. Etna volcano reaching over the study area. Daily mean OMI-SO2 levels up to 21.4, 10.0, 5.6, and 2.4 DU have been noticed due to the eruptions from Dalaffilla, Mt. Etna, Nabro, and Jabal Al-Tair volcanoes, respectively.