Atmospheric SO2 oxidation efficiency over a semi-arid region: Seasonal patterns from observations and GEOS-Chem model

• Assess molar ratio of SO42− to total SOx (SOx = SO2 + SO42−) at a high altitude site.
• Observed patterns are validated against GEOS-Chem model simulations.
• The ratios from field measurements as well as simulations exhibit a seasonality.
• Assess contributions from PBL, OH, RH, dust load, transport and dry deposition.

The oxidation efficiency of atmospheric SO2, measured as a molar ratio of SO42− to total SOx (SOx = SO2 + SO42−), referred as S-ratio, have been studied from a high altitude site (Gurushikhar, Mt. Abu: 24.6° N, 72.7° E, 1680 m ASL) in a semi-arid region of western India. A global 3-dimensional Chemical Transport Model (CTM), GEOS-Chem (v8-03-01), is employed to interpret the observed patterns. The S-ratios derived from time series SO2 and SO42− measurements exhibited a pronounced seasonality, with relatively low ratios in Feb–Mar 2010, high ratios in Nov–Dec 2009 and intermediate values in Sep–Oct 2009. The lower S-ratios for Feb ‘10 and Mar ‘10 (median values 0.10 and 0.08 respectively) have been attributed to the relatively high planetary boundary layer (PBL) heights – to reduce the SO2 loss from the atmosphere via dry deposition – as well as the lower OH radical levels and low ‘aged air mass influx’ during these months. On the other hand, low PBL heights and significant long range transport contributions are projected to be the possible causes for the higher S-ratios during Nov ‘09 and Dec ‘09 (median values 0.30 and 0.28 respectively). The seasonal patterns for the S-ratios predicted by the CTM for the GEOS-Chem 4° × 5° grid cell containing the sampling site showed highest ratios in Jul–Aug, and the lowest in Apr. The model has been employed further to study the contributions from various parameters to the S-ratios such as PBL, OH, RH, dust load, transport pattern and dry deposition. Sensitivity simulations showed the S-ratios enhancing with dust load with the peak in May (∼4.7% (median)). Similarly, the ‘dry deposition’ is seen to boost the S-ratios with the peak in August (∼66.3% (median)). Also, model simulations to assess the ‘altitudinal dependence of S-ratios’ have revealed a pronounced seasonal behaviour.