A 12 year observation of water-soluble inorganic ions in TSP aerosols collected at a remote marine location in the western North Pacific: an outflow region of Asian dust

Abstract

Abstract. In order to characterize the long term trend of remote marine aerosols, a 12 year observation was conducted for water-soluble inorganic ions in TSP aerosols collected from 2001–2012 in the Asian outflow region at a Chichijima Island in the western North Pacific. We found a clear difference in chemical composition between the continentally affected and marine background air masses over the observation site. Asian continental air masses are delivered from late autumn to spring, whereas marine air masses were dominated in summer. Concentrations of nss-SO42-, NO3-, NH4+, nss-K+ and nss-Ca2+ are high in winter and spring and low in summer. On the other hand, MSA- exhibits higher concentrations during spring and winter, probably due to springtime dust bloom or due to the direct continental transport of MSA- to the observation site. We could not find any clear decadal trend for Na+, Cl-, Mg2+ and nss-Ca2+ in all seasons, although there exists a clear seasonal trend. However, concentrations of nss-SO42- continuously decreased from 2007–2012, probably due to the decreased SO2 emissions in East Asia especially in China. In contrast, nss-K+ and MSA- concentrations continuously increased from 2001–2012 during winter and spring seasons, demonstrating that biomass burning and/or terrestrial biological emissions in East Asia are increasingly more transported from the Asian continent to the western North Pacific.