Sulfate Formation Enhanced by a Cocktail of High NOx, SO2, Particulate Matter, and Droplet pH during Haze-Fog Events in Megacities in China: An Observation-Based Modeling Investigation

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• 作者：Jian Xue ; Zibing Yuan ; Stephen M. Griffith ; Xin Yu ; Alexis K. H. Lau ; Jian Zhen Yu
• 刊名：Environmental Science & Technology
• 出版年：2016
• 出版时间：July 19, 2016
• 年：2016
• 卷：50
• 期：14
• 页码：7325-7334
• 全文大小：614K
• 年卷期：0
• ISSN：1520-5851

文摘

In recent years in a few Chinese megacities, fog events lasting one to a few days have been frequently associated with high levels of aerosol loading characterized by high sulfate (as high as 30 μg m^–3), therefore termed as haze-fog events. The concomitant pollution characteristics include high gas-phase mixing ratios of SO₂ (up to 71 ppbv) and NO₂ (up to 69 ppbv), high aqueous phase pH (5–6), and smaller fog droplets (as low as 2 μm), resulting from intense emissions from fossil fuel combustion and construction activities supplying abundant Ca²⁺. In this work, we use an observation-based model for secondary inorganic aerosols (OBM-SIA) to simulate sulfate formation pathways under conditions of haze-fog events encountered in Chinese megacities. The OBM analysis has identified, at a typical haze-fogwater pH of 5.6, the most important pathway to be oxidation of S_(IV) by dissolved NO₂, followed by the heterogeneous reaction of SO₂ on the aerosol surface. The aqueous phase oxidation of S_(IV) by H₂O₂ is a very minor formation pathway as a result of the high NO_x conditions suppressing H₂O₂ formation. The model results indicate that the unique cocktail of high fogwater pH, high concentrations of NO₂, SO₂, and PM, and small fog droplets are capable of greatly enhancing sulfate formation. Such haze-fog conditions could lead to rapid sulfate production at night and subsequently high PM_2.5 in the morning when the fog evaporates. Sulfate formation is simulated to be highly sensitive to fogwater pH, PM, and precursor gases NO₂ and SO₂. Such insights on major contributing factors imply that reduction of road dust and NO_x emissions could lessen PM_2.5 loadings in Chinese megacities during fog events.