Heterogeneous oxidation shapes inorganic aerosol composition and acidity in East Asia

Sulfate–nitrate–ammonium aerosols are the main secondary inorganic components of particulate matter, particularly PM_2.5 (particulate matter with an aerodynamic diameter of 2.5 µm or less), contributing more than 40% of PM_2.5 mass (Tao et al., 2017), and even up to 60% during polluted events in East Asia (Geng et al., 2017). Sulfate and nitrate are formed through the chemical oxidation of precursor gases (i.e., NO_x and SO₂). These oxidation pathways include homogeneous processes (i.e., in the gas and aqueous phases) and heterogeneous processes (i.e., on particle surfaces). Previous studies have emphasized the importance of heterogeneous oxidation in secondary inorganic aerosol formation during severe haze events. However, heterogeneous oxidation mechanisms are either neglected in current models or represented using empirical and oversimplified parameterizations, leading to substantial discrepancies between field observations and model simulations, particularly the underestimation of sulfate concentrations. In addition, the size distribution of sulfate–nitrate–ammonium mass fractions and aerosol acidity is influenced by heterogeneous chemistry. However, most studies focus on the fine mode (PM_2.5), and the size-resolved responses of aerosol chemical composition and acidity remain poorly understood. In this study, we incorporate an updated heterogeneous oxidation scheme into the formation mechanism of sulfate–nitrate–ammonium aerosols to improve simulations over East Asia using the EMAC atmospheric chemistry–climate model. Compared with observational datasets, the inclusion of the updated heterogeneous oxidation scheme improves model performance, with the normalized mean bias of sulfate, nitrate, and ammonium decreasing from −50%, 61%, and −51% to −45%, 60%, and −35%, respectively. We find that enhanced sulfate formation promotes the partitioning of ammonium into the aerosol phase, especially in the coarse size mode (PM ≥ 2.5 µm), while increased sulfate in the coarse mode suppresses coarse nitrate formation. In addition, aerosol acidity in the fine mode shows a negligible response, whereas acidity in the coarse mode increases by approximately 0.1 pH units. These findings highlight the importance of heterogeneous oxidation mechanisms, particularly in the coarse size mode.

References

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