Chemical characteristics of size-resolved aerosols from Asian dust and haze episode in Seoul Metropolitan City

We collected aerosol particles in Seoul using a 10 stage Micro-Orifice Uniform Deposit Impactor (MOUDI) to investigate the size distributions of aerosol mass and water-soluble inorganic ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl^-, NO₃^-, and SO₄^2-) for the two high-mass episodes taking place in February and April, 2009. The former was a heavy Asian dust (AD) event and the latter was a haze episode associated with stagnant condition that prevailed over the Yellow Sea region. In AD plume, the mass peak was noticeable at coarse mode between 1.0 and 1.8μm but SO₄^2- and NH₄⁺ were enriched in condensation mode between 0.056 and 0.1μm. There was little chance for the heavy AD plume to pick up SO₂ and water vapor, which are in good accordance with its transport paths and the chemical characteristics of aerosols and gaseous species. These results imply that the heterogeneous reaction of SO₂ on dust particles would not be substantial in determining sulfate concentrations for this particular type of dust plume, considering the possibility of loss of large soil particles in MOUDI. During the haze episode, both total aerosol mass and water-soluble inorganic ions showed bimodal size distributions with the droplet (0.32-0.56μm) and coarse (1.0-1.8μm) mode peaks. In this haze event, acidic gases tend to be dissolved more efficiently in larger particles, shifting the peaks of SO₄^2- and NO₃^- to larger droplet particles. For NH₄⁺, however, the mode change was not observed, which was probably due to the depleted source and high solubility of NH₃. These results demonstrated that the availability of precursor gases such as SO₂, NO₂, and NH₃, and the water-vapor contents were important factor to determine the formation of droplet-mode particles and their sizes.