Chemical Characteristics of Nitrate and Mineral Aerosols over the Western North Pacific During the Asian Dust Episode (ACE-Asia Experiment)

Atmospheric deposition of mineral and nitrate particles to the Ocean is regarded as one of important passes of nutrients supply for marine biological activity. It has been reported that the mineral and nitrate particles are transported over the North Pacific from the Asian continent. The purpose of this study is to examine the chemical interaction between atmospheric nitrate and mineral particle during the transport often the Asian continent to the North Pacific. As a part of VMAP project, the chemical composition of marine aerosols was studied during the ACE-Asia experiment that took place in the northwestern Pacific Ocean since March of 2001. Coastal marine aerosols around the Sagami Bay (the N. Pacific side of the Japanese main island) were collected using a 13-stage low pressure impactor on board R/V Tansei-maru and analyzed for major inorganic ions by ion chromatography. Dust storm occurred in the Gobi Desert region on 5 March. The dust plume covered the Korean Peninsula and the Japanese islands on 6 March. High concentrations of anthropogenic and mineral substances were observed on 6 March. When the Asian Dust event was observed on board, nitrate concentration was 4 times higher than sulfate concentration in coarse mode, and about 80% of nitrate in coarse mode was reacted with mineral particles. HNO₃ is thought to be the most active acid gas that reacts with mineral particles. The particle number concentration (2<D<5 μ m) increased due to the contribution of mineral particles during the dust event, and mineral particle concentration (2<D<5 μ m) was approximately 1 cm^-1. Based on considering the mineral particle number concentration and the amount of nitrate that has been reacted with mineral particle, HNO₃ has reacted with 0.1 \mum thickness of mineral particle surface. On the other hand, nitrate showed a bimodal size distribution which peaks consist of NH₄ NO₃ in fine mode and NaNO₃ in coarse mode, when Asian Dust event was not observed.