Abstract

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... Show moreA 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. Show less