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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.