Variability of SO₂ in an intensive fog in North China Plain: Evidence of high solubility of SO₂

A field experiment was conducted in an intensive fog event between November 5 and November 8, 2009, in a heavily SO₂-polluted area in North China Plain (NCP), to measure SO₂ and other air pollutants, liquid water content (LWC) of fog droplets, and other basic meteorological parameters. During the fog period, the concentrations of SO₂ showed large variability, which was closely related to the LWC in the fog droplets. The averaged concentration of SO₂ during non-fog periods was about 25 ppbv, while during the fog period, it rapidly reduced to about 4-7 ppbv. Such large reduction of SO₂ suggested that a majority of SO₂ (about 70%-80%) had reverted from gas to aqueous phase on account of the high solubility of SO₂ in water in the fog droplets. However, the calculated gas to aqueous phase conversion was largely underestimated by merely using the Henry's Law constant of SO₂, thus suggesting that aqueous reaction of SO₂ in fog droplets might play some important role in enhancing the solubility of SO₂. To simplify the phenomenon, an "effective solubility coefficient" is proposed in this study. This variability of SO₂ measurement during the extensive fog event provides direct evidence of oxidation of SO₂ in fog droplets, thus providing important implications for better understanding of the acidity in clouds, precipitation, and fogs in NCP, now a central environmental focus in China due to its rapid economic development.