High solubility of SO₂: evidence in an intensive fog event measured in the NCP region, China

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.