Size-Segregated Chemical Analyses of Particles Transported from East Asia and Their Transformation Mechanisms during the Transport

Abstract

In order to investigate the processes of chemical transformations of aerosol transported from East Asia to Japan the concentrations of ionic species in size-segregated aerosols were measured in Cape Hedo, Okinawa, Japan. By use of a cascade impactor (Nano sampler) we collected aerosols at 5 stages. Throughout the observation periods, species originated from anthropogenic sources such as NH₄^＋ and nss-SO₄^2- were dominant in fine mode particles (0.1–0.5 μm, 0.5–1 μm), whereas species originated from natural sources such as Na^＋ and
Cl^- were dominant in coarse mode particles (2.5–10μm, ＞10μm). The (Cl^-＋NO₃^-)/Na^＋ ratio in coarse mode, 1.14 in equivalent basis, was very close to Cl^-/Na^＋ of sea water (1.17), which indicated that NH₄NO₃ was decomposed to gaseous NH₃ and HNO₃ during the long-range transport, and HNO₃ deposited on sea-salt particles. The concentrations of NH₄^＋, nss-SO₄^2- and nss-K^＋ in fine mode were higher during high pressure episodes; it suggests that the fine particles emitted from anthropogenic sources (e.g. biomass burning) were transported with a migratory anticyclone. Increase of nss-Ca^2＋ and nss-SO₄^2- in coarse mode during Asian dust episode implied that surface reactions with gaseous species on CaCO₃ particles occurred during the transport.