Observational Indications of Indirect Acidification in Asia: Enhanced Deposition of Nitric Acid Gas Expelled from the Aerosol Phase by Sulfate

Abstract

東アジア酸性雨モニタリングネットワーク(EANET)の国内6地点の観測結果から，間接的酸性化効果が越境大気汚染に伴い生じている事が分かった。SO42-濃度増加に伴いエアロゾル中のNO3- が追い出され，HNO3ガス濃度が増加する。 HNO3ガスの沈着速度は非常に速いので，結果としてトータル(ガス状＋エアロゾル状)のNO3-濃度が変化しなくても，NO3-の沈着量が増加する事になる。西日本の日本海側では，アジア大陸からのアウトフローの影響下で間接的酸性化効果は顕著になり，一方，海塩粒子が豊富にある冬季日本海側ではこの効果はあまり見えなかった。

Indirect acidification, i.e., enhancement of semi-volatile aerosol components deposition associated with increased SO42- concentration has now been shown to occur in general air pollution, analyzing Acid Deposition Monitoring Network in East Asia (EANET) monitoring data collected at six stations in Japan. This effect was firstly detected as a result of volcanic SO2 plumes (Satsumabayashi et al., 2004, Kajino et al., 2005). Results indicate that as SO42- concentration increases, NO3- in aerosol phase is expelled, increasing the HNO3 gas concentration. The deposition of HNO3 gas occurs at a high velocity, consequently accelerating the deposition of NO3- despite a constant total (gas plus aerosol) nitrate concentration. In western Japan, this indirect acidification is most influential from spring to autumn, when the Asian continental outflow carries sulfate-rich contaminated air masses, but is not pronounced in the air masses containing abundant sea-salt particles and related cation components in aerosols.