Agricultural waste burning is performed after harvest periods in June and November in Taiwan typically, farmers use open burning to dispose of excess rice straw. PM2.5 and PM2.5-10 measurements were conducted at National Chung Hsing University in Taichung City using a dichotomous sampler. The sampling times were during straw burning periods after rice harvest during 2002-2005. Ionic species including SO42-, NO3-, NH4+. K+, Ca-2(+), Cl- and Na+ and carbonaceous species (EC and OC) in PM2.5 and PM2.5-10 were analyzed. The results showed that the average PM2.5 and PM2.5-10 concentrations were 123.6 and 31.5 mu g m(-3) during agricultural waste burning periods and 32.6 and 21.4 mu g m(-3) during non-waste burning periods, respectively. The fine aerosol ionic species including Cl-, K+ and NO3- increased 11.0, 6.7 and 5.5 times during agricultural burning periods compared with periods when agricultural waste burning is not performed. K+ was found mainly in the fine mode during agricultural burning. High nitrogen oxidation ratio was found during agricultural waste burning periods which might be caused by the conversion of Nitrogen dioxide (NO2) to NO3-. It is concluded that agricultural waste burning with low dispersion often causes high PM2.5 and gases pollutant events. (C) 2008 Elsevier B.V. All rights reserved.