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High winds that blow sand from the desert regions of central Asia to points east are commonly known as Asian dust storms (ADS). In this paper we study the impact of an ADS on the PM2.5 aerosol extant in Tainan, a city in southern Taiwan. PM2.5 aerosol was collected at an urban and a coastal site in Tainan before the ADS (4-12 January 2001), during the ADS (13-16 January 2001), and the following summer (3-12 June 2001). Total PM2.5 mass was highest at both sites during the ADS despite already moderately high levels of PM2.5 beforehand, demonstrating that the ADS brought with it a significant amount of PM2.5 mass. The mass percentage of Ca2+ in PM2.5 increased noticeably during the ADS and there was a higher non-sea-salt SO42-/elemental carbon ratio. This latter change was due to a heightened level of non-sea-salt SO42- which has a serious impact on air quality in southern Taiwan, and was likely sourced from sulfur integrated into the ADS aerosol as it passed major cities and industrial districts in China. The percentage water content in PM2.5 was at its lowest during the ADS. This was despite high levels of hygroscopic sulfate in the ADS aerosol and indicates that sulfate in the ADS may combine with Ca2+ to form CaSO4, a major component of ADS dust and one that does not have strong hygroscopic characteristics. Water content was at its highest in summer when winds arrive in Taiwan with elevated sea salt concentrations, having spent several days traversing the South China Sea. Non-ADS urban site aerosols were weakly acidic while coastal site aerosols were weakly alkaline. Urban site acidity was always higher than equivalent to coastal site acidity, due to locally produced acidic aerosols. Acidity peaked during the ADS, however, as a result of additional acidic aerosols transported by the ADS from urban and industrial regions in China. (c) 2006 Elsevier Ltd. All rights reserved.