The study proposes an approach to quantify necessary emission reductions of various related pollutants to meet national PM 2.5 air quality standard (AQS). Many scenarios of emission reductions are simulated to establish the relationships between the reduction ratios of air pollutants and improvements of atmospheric PM 2.5 concentration for specific air basins in Taiwan. The air pollutants include primary PM 2.5 and secondary PM 2.5 precursor species, such as nitrogen oxides (NO x ), sulfur oxides (SO x ), volatile organic compounds (VOCs), and ammonia (NH 3 ). These relationships were used to estimate the required amounts of emission reduction for various pollutants to achieve the national PM 2.5 AQS of annual average with 15 μg m − 3 . Three air basins of KPAB, YCNAB and CTAB with severe PM 2.5 pollution in Taiwan were selected as case studies. The results follow: (1) The primary PM 2.5 emission reductions and the PM 2.5 concentration improvements exhibit linear relationships, and its effects are relatively significant. (2) Among the secondary PM 2.5 precursor species, emission reductions of NO x and NH 3 will have greater effectiveness, but higher proportions (> 50%) of emission reductions are needed. (3) In terms of mitigating PM 2.5 pollution, the priorities for pollutant emission control should be in the order of primary PM 2.5 , NO x , and SO x , while NMHC and NH 3 should not currently be a focus. (4) If three air basins of KPAB, YCNAB and CTAB with high PM 2.5 levels are required to meet the AQS, it is necessary to reduce primary PM 2.5 , NO x and SO x emissions simultaneously in case that LRT effect remains constant. Under the premise of meeting the AQS for the three air basins, the reduction of primary PM 2.5 , NO x and SO x emission in Taiwan should be up to 76%, 93% and 20% respectively. Fortunately, many efforts have been and will be done to improve the PM 2.5 concentration in China. Those efforts will mitigate significantly the loading of emission reduction and AQS should be more possible to achieve for all air basins in Taiwan. [ABSTRACT FROM AUTHOR]