Sulfate Formation Enhanced by a Cocktail of High NO_x, SO_2, Particulate Matter, and Droplet pH during Haze-Fog Events in Megacities in China: An Observation-Based Modeling Investigation

摘要

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 SO_2 (up to 71 ppbv) and NO_2 (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 Ca~(2+). 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 NO_2, followed by the heterogeneous reaction of SO_2 on the aerosol surface. The aqueous phase oxidation of S_((Ⅳ)) by H_2O_2 is a very minor formation pathway as a result of the high NO_x conditions suppressing H_2O_2 formation. The model results indicate that the unique cocktail of high fogwater pH, high concentrations of NO_x, SO_2, 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 NO_2 and SO_2. 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.