Indirect evidence for elevated SO2 layers in the North China Plain

Abstract:

Sulphur dioxide (SO₂) is the precursor of sulphate aerosols and has crucial impacts on atmospheric chemistry, cloud physics, air quality, ecological environment and global climate change. The vertical distribution of SO₂ can directly influence atmospheric chemistry and cloud processes. Information on the SO₂ vertical profile, however, scarcely exist. Indirect evidence for the frequent occurrence of elevated SO₂ layers in the North China Plain (NCP) was found in the case study of three consecutive fog cases and in the detailed analysis of long-term SO₂ surface concentration measurements at four different sites. During fog episodes, high concentrations of SO₂ over the fog layer was believed responsible for the rapid increases of surface SO₂ after the dissipation of the fog. Different from the more common SO₂ diurnal patterns with high nighttime concentrations, a frequent SO₂ noon-time peak phenomenon was found in the NCP with an occurrence frequency of 50 to 72% at four stations, amongst which 30-52% of the cases were attributed to down-mixing of elevated SO₂ layers. The formation of the elevated SO₂ layer was attributed to emissions of high stacks accompanied with shallow temperature inversion layers. Elevated SO₂ layers will promote the oxidative transformation of SO2 in cloud water. The SO₂ noontime peaks caused partly by the elevated SO₂ layer will lead to a 13 to 35% increase in sulphur dry deposition, 9 to 23% increase in gas phase oxidation and 8 to 33% increase in aqueous phase conversions, which will increase the hygroscopicity and the light scattering of aerosols, thus having important impacts on atmospheric chemistry, cloud physics and climate.