Changes in ionic constituents of free tropospheric aerosol particles obtained at Mt. Norikura (2770 m a.s.l.), central Japan, during the Shurin period in 2000

Free tropospheric aerosols and O3 concentrations with meteorological parameters were observed at the Norikura Cosmic-Ray Observatory (2770 m a.s.l.) near the top of Mt. Norikura, central Japan from 1 September to 8 October 2000. The study period represents the Shurin period in Japan, which is a transition period of dominant air masses over Japan. Automated air sampling was conducted during nighttime: from 00 midnight to 06 a.m. every day to collect free tropospheric aerosols particles under subsidence air flow condition over the mountain. Results of ionic concentrations suggested that the observation period was divided into 2 periods: one was higher equivalent ratios (>1) of non-sea salt (nss) SO42−/NH4+ until 17 September and the other was slightly above unity of the ratio after that. O3 concentrations also shifted simultaneously to higher and stable conditions. Concentrations of oxalate, nssK+, and nssCa2+ were also higher in the later period. Molar ratios of nssSr/nssCa and nssMg/nssCa in the samples were about 0.0024 and 0.172, respectively. These values were very close to ratios found in the desert area in China. Backward air trajectory analysis for the period of high nssCa2+ suggested that Asian dust with pollutants were not only transported in spring but also in fall from the Asian continent to the free troposphere over Japan.