Long-range taransport of air pollution studied by Bark Pocket Method

点击翻译按钮获取中文摘要

Long-range transport of air pollution from the continental Asia is currently an important issue concerning the Japanese environment, especially in regions susceptible to acidification due to low buffering capacity, such as Murakami, Niigata prefecture located in on the west coast of central Japan. Evidence for long range transport was obtained through lead and lead isotopic analysis of 84 archived precipitation filters, showing seasonal changes in lead deposition from May 1999 to May 2002. Lead deposition was highest in winter and spring (November through May) each year and lowest in summer. Computed 72-hour back trajectories showed that in winter air masses were predominantly transported from the northwest, passing over northern China and eastern Russia, whilst in summer air masses predominantly originated from the southeast passing over Japan. Lead isotopie analysis showed higher ^<208>Pb/^<206>Pb during winter, indicating that lead originated from a different source. A plot of ^<207>Pb/^<206>Pb vs.^<208>Pb/^<206>Pb identified a thorogenic component, which is excess ^<208>Pb compared to a standard lead growth carve, indicative of certain lead ores and coals in continental Asia. The data provided evidence of long-range transport of lead from continental Asia to Japan. Bark pockets included within the trunks of two Japanese cedar trees harvested near Murakami, dating between 1972 and 1982,exhibited lead isotope ratios indicative of Japanese sourced lead. In contrast, current (2003) bark showed thorogenic ratios, consistent with a relative decline in Japanese sourced and increase in continental soureed lead. The result is consistent with the scenario of decreasing Japanese sourced lead emission and increasing long range transport of pollution from continental Asia, signifying the threat of transboundary air pollution to the region. Thorogenic lead isotope rations were also measured in tree bark collected at high altitude in the area.