Fine particulate matters (PM 2.5 ) have been identified as one of the major air pollutants in urban areas, which are responsible for the deterioration of the atmospheric air quality as well as adverse effects on public health. In this study, the mass concentration, water-soluble ionic component, trace metal component, carbon component and modeling the contribution source for PM 2.5 was characterized for Chiayi City, which has high population density and surrounded by agricultural area. The lowest PM 2.5 mass concentrations were registered in the summer (9-22 µg m -3 ), while for the spring, autumn, and winter were well above the healthy level suggested by World Health Organization (WHO). For all seasons, the dominants were the sulfate (SO 4 2-), nitrate (NO 3 -) and followed closely by ammonium (NH 4 + ). Those secondary aerosols were transformed from SO 2 and NO 2 into particulate NO 3 -and SO 4 2-during spring, autumn and winter. Lower carbon mass concentrations were observed for summer (2.03-2.49 µg m -3 ) corresponding to the highest carbon content in PM 2.5 mass concentrations in terms of percentages (average 18.1%). Using the Chemical Mass Balance receptor model, the secondary nitrate (NO 3 -), primary traffic source, secondary sulfate (SO 4 2-), re-suspending soil particle, and petrochemical industry were identified as the major sources of PM 2.5 in Chiayi City. Consequently, the PM 2.5 contributions were complicated in a small but various seasons and geological distributing area. The air quality control strategies were thus seasonal and periodical dependent.

In order to analyze the characteristics and long-term trend levels of PM 2.5 in Tainan City, Taiwan, Mesoscale Modeling System Generation 5 (MM5) and Community Multiscale Air Quality Model (MM5-CMAQ) modelling as well as box plots and time series analysis, were utilized in this study. The long-term trend analysis shows that the levels of PM 2.5 (averaged at 38.3 µg m -3 and ranged between 33.1 and 41.9 µg m -3 ) in Tainan City for ten years (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014) were above the yearly average standards of 15 µg m -3 , showing non-attainment status. Overall, the results show a decreasing trend (from 41.9 µg m -3 in 2005 to 35.0 µg m -3 in 2014) in the levels of PM 2.5 in Tainan atmosphere in the ten year period. The results of the MM5-CMAQ air quality modeling, indicate that the highest contribution on PM 2.5 in Tainan City was from trans-boundary pollution from neighbouring cities (34.2%), while long-range transport and local emissions from Tainan each contributed a fraction of approximately 32.9%. In terms of local sources, the highest influence is from area sources (18.6%), followed by line sources (7.7%) and point sources (6.6%). Thus, to control PM 2.5 in Tainan City, the focus should be on construction, road dust, and residential activities.