Trans-boundary air pollution over remote islands in Japan: observed data and estimates from a numerical model

Abstract

Observations of air pollutants were conducted in remote Japanese islands (Oki Island and Okinawa Island) in early spring to clarify the extent of trans-boundary air pollution from the Asian continent. A three-dimensional Eulerian model calculation, which included parameters on emission, transport and transformation of sulfur oxides, nitrogen oxides and ammonia, was performed to compile sulfate isosurface concentrations over the observational sites. Concentrations of non-sea-salt sulfate (nss-SO 4 2−) of greater than 10 μg m −3 were observed at Oki after the northeastward passage of low-pressure systems in the Sea of Japan. At these times, the weather showed a typical winter pattern and air pollutants over China were transported southeastward to Japan with the northwesterly wind. The model calculation reproduced the observed variations of nss-SO 4 2− concentration well, except for one case in which the model calculation could not reproduce the extremely low nss-SO 4 2− concentration observed on 8 March. In Hedo (Okinawa Island), we observed long-lasting (3 days) medium concentrations of nss-SO 4 2− (approximately 5 μg m −3). Although the model reproduced these observed medium concentrations well, in general the observed results were reproduced better for Oki than for Hedo. Under the synoptic weather conditions of early spring, high concentrations of nss-sulfate were sometimes transported to these remote Japanese islands from areas of continental Asia with a strong outflow of air pollutants.