Study of mixing ratios of SO2 in a tropical rural environment in south India

Abstract

Sulphur dioxide is a toxic pollutant in the atmosphere emitted from natural sources and human activities. Normally, SO2 has a life-time of about 2 days in the atmosphere and is not transported to long distances from its source region. However, under favourable circumstances such as low humidity or high wind speed, it can travel intercontinental distances from the point of emission. In this article, we have analysed the surface mixing-ratio of SO2 measured over the time period from January 2010 to April 2012 at a rural region in south India. It is found that SO2 mixing-ratio is very low over this region with an annual mean value in the range of 0.21–0.24 ppbv. OMI satellite estimates an annual mean value of 0.5 Dobson Units (DU) over the same location and period. However, during January to May relatively higher concentrations of SO2 are observed, mainly coming from power plants located in southern and eastern India as indicated by higher SO2/NO2 ratios of greater than 0.5. In one instance, on June 20th, 2011, it is found that the OMI SO2 value was a factor of 13 higher than 2011 annual mean at Gadanki. Using the FLEXible PARTicle dispersion model (FLEXPART) and satellite data, it is found that the observed higher SO2 value on 20th June was due to intercontinental transport of SO2 from Nabro volcanic eruption. Using the FLEXPART model with ECLIPSE-v5 emission inventory, the observed seasonal variation of SO2 could be well reproduced; however, the mixing ratios are found to be overestimated. CAMS (Copernicus Atmosphere Monitoring Service) SO2 reanalysis values available through its implementation in the ECMWF Integrated Forecasting System are a factor of 7.8 higher than observations, possibly due to incorrect vertical profile used in the model