High solubility of SO 2 : evidence in an intensive fog event measured in the NCP region, China

Abstract

Abstract. A field experiment was conducted in a heavy SO2 pollution area located in north China plain (NCP). During the experiment, SO2 and other air pollutants, liquid water content (LWC) of fog droplets, and basic meteorological parameters were measured. During the experiment, an intensive fog event occurred between 5 and 8 November 2009. During the fog period, the concentrations of SO2 showed a strong variability, and the variability was closely correlated to the appearances of fogs and LWC. For example, the averaged concentration of SO2 during the non-fog period was about 25 ppbv. By contrast, during the fog period, the concentration of SO2 reduced to about 4–7 ppbv. The large reduction of SO2 suggests that a majority of SO2 (about 70–80%) had been converted from gas-phase to aqueous-phase, showing a high solubility of SO2. However, according to the value of Henry Law constant, the solubility of SO2 is modest, which cannot explain the measured large reduction of SO2. This study highlights that the aqueous reactions of SO2 in the droplets of fogs play important roles to enhance the solubility of SO2. To account for the effect of aqueous reactions on the solubility of SO2, an "effective" Henry Law constant of SO2 is proposed in this study. The study shows that without considering aqueous reactions of SO2 in fog droplets, the estimate of the partitioning of SO2 in droplets is significantly lower than the measured values. By contrast, when the "effective" Henry Law constant is applied in the calculation, the calculated SO2 concentrations are significantly improved, showing that the aqueous reactions of SO2 play important roles in controlling the solubility of SO2, and should be considered in model calculations.