Impacts of dust on regional tropospheric chemistry during the ACE-Asia experiment: A model study with observations

A comprehensive regional-scale chemical transport model, Sulfur Transport and Emissions Model 2001 (STEM-2K1), is employed to study dust outflows and their influence on regional chemistry in the high-dust Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) period, from 4-14 April 2001. In this period, dust storms are initialized in the Taklamagan and Gobi deserts because of cold air outbreaks, are transported eastward, and are often intensified by dust emitted from exposed soils as the front moves off the continent. Simulated dust agrees well with surface weather observations, satellite images, and the measurements of the C-130 aircraft. The C-130 aircraft observations of chemical constituents of the aerosol are analyzed for dust-rich and low-dust periods. In the submicron aerosol, dust-rich air masses have elevated ratios of ΔCa/ΔMg, ΔNH ₄⁺/ΔSO₄^2-, and ΔNO₃^-/ΔCO (Δ represents the difference between observed and background concentrations). The impacts of heterogeneous reactions on dust involving O₃, NO ₂, SO₂, and HNO₃ are studied by incorporating these reactions into the analysis. These reactions have significant influence on regional chemistry. For example, the low O ₃ concentrations in C-130 flight 6 can be explained only by the influence of heterogeneous reactions. In the near-surface layer, the modeled heterogeneous reactions indicated that O₃, SO₂, NO₂, and HNO₃ are decreased by up to 20%, 55%, 20%, and 95%, respectively, when averaged over this period. In addition, NO, HONO, and daytime OH can increase by 20%, 30%, and 4%. respectively, over polluted regions. When dust encounters fresh pollutants, these heterogeneous reactions can lead to a series of complex responses of the photochemical system. In addition, these reactions can alter the chemical-size distribution of the aerosol. Under heavy dust loadings, these reactions can lead to >20% of the sulfate and >70% of the nitrate being associated with the coarse fraction. The radiative influence of dust can also affect the photochemical system. For example, OH levels can decrease by 20% near surface. The dust radiative influence is shown to be weaker than the heterogeneous influence for most species.