Dust, Aerosol Ions and Their Interactions with Gaseous Species in East Asia During Spring 2001: A three-dimensional model Study

A comprehensive regional chemical transport model is developed to study the aerosol-related issues for TRACE-P and ACE-ASIA experiments, which includes on-line thermodynamic module SCAPE II and on-line photolysis-rate calculation TUV, and explicitly considers dust heterogeneous reactions and chemical-aging process. The Asian outflow during March and April of 2001 is heavy polluted with high aerosol loading. Under cation-limited condition, SO₂ oxidation and ammonium availability determined the nitrate size and gas-aerosol distributions. Dust was one of most important aerosol outflow during this period, which brought significant influences on other aerosols and gaseous species. A main role of dust in the equilibrium process is through the enhancement of the aerosol calcium concentration, which shifts the equilibrium balance to an anion-limited status. This status benefits the uptake of sulfate and nitrate, but repels ammonium. Dust influence on secondary aerosols and their size distributions is also determined by dust mass, size distribution and fresh ratio. The impacts of heterogeneous reactions on fresh dust involving O₃, NO₂, SO₂ and HNO₃ are studied by incorporating these reactions into the analysis. These reactions have significant influence on regional chemistry. For examples, the low O₃ concentrations in the C-130 flight 6 can be explained only by the influence of heterogeneous reactions. Dust appearance significantly increased optical depth, and the radiative influence of dust can also affect the photochemical system. For example, OH levels can decrease by 20% near surface. All these dust impacts is sensitive to the dust mass, its size distribution, assumptions about its mixing state (internal vs. external), and the fraction of the aerosol mass available for heterogeneous reactions and equilibrium process.