Numerical Investigation of the Influence of Mineral Dust on the Tropospheric Chemistry of East Asia

An investigation of the influence of mineral dust ontrace gas cycles in the troposphere is carried out inthis study. A 3D regional scale atmospheric chemistrymodel (STEM-III) which includes aerosol processes isused for the numerical simulations for May 1987.Heterogeneous interactions between gaseous species(SO₂, N₂O₅, HNO₃, HO₂andH₂O₂) and the dust particles are considered.Emissions of dust behind convective cold fronts aremodeled. The transport and distribution of mineraldust predicted from the model is compared withsatellite measurements (aerosol index from TOMS). Themodel is shown to capture the synoptic variability inthe observed aerosol index. Calculations show twomajor dust events in May 1987, during which thedust levels close to the source reach more than500 g/m³. The transport of dust is mostlyrestricted towards the north, with the net continentaloutflow of 6 Tg for the entire month. Results showthat the presence of mineral aerosol can greatlyimpact sulfate and nitrate distributions. Averagedover the month of May, the presence of dust isestimated to increase particulate sulfate and nitratelevels in east Asia by 40%. Furthermore, the sulfateand nitrate on the dust particles are predicted to beassociated with the coarse mode (3–5 m particlediameter), consistent with observations over Japan.The influence of mineral dust on the photochemicaloxidant cycle is also investigated. For the entiremonth, a5–10% decrease in boundary layer ozone ispredicted by the model closer to regions of higherdust levels. The ratio of nitric acid to NO_x overmarine regions is reduced by a factor between 1 and 2in the boundary layer to more than 2 in the freetroposphere as a result of aerosol processes.