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

In order to characterize the long-term trend of remote marine aerosols, a 12-year observation was conducted for water-soluble ions in TSP (total suspended particulate) aerosols collected from 2001 to 2012 in the Asian outflow region at 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 non-sea salt (nss-) SO42-, NO3-, NH4+, nss-K+ and nss-Ca2+ are high in winter and spring and low in summer. On the other hand, MSA(-) (methanesulfonate) 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 to 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 to 2012 during winter and spring seasons, demonstrating that biomass burning and/or terrestrial biological emissions in East Asia are being increasingly transported from the Asian continent to the western North Pacific. This study also demonstrates that Asian dusts can act as an important source of nutrients for phytoplankton and thus sea-to-air emission of dimethyl sulfide over the western North Pacific.