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

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_<i>x</i> 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_<i>x</i> emissions could lessen PM_2.5 loadings in Chinese megacities during fog events