Assessing the Effectiveness of SO ₂, NOx, and NH ₃ Emission Reductions in Mitigating Winter PM _2.5 in Taiwan Using CMAQ Model

Taiwan experiences higher air pollution in winter when particulate matter (PM _2.5) levels frequently surpass national standards. This study employs the Community Multiscale Air Quality model to assess the effectiveness of reducing NH ₃, NOx, and SO ₂ emissions on PM _2.5 secondary inorganic species (i.e., SO­ ₄^2-, NO ₃^-, and NH ₄⁺). For sulfate, ~ 43.7 % is derived from the chemical reactions of local SO ₂ emission, emphasizing the substantial contribution of regional transported sulfate. In contrast, local NOx and NH ₃ emissions predominantly influence nitrate and ammonium. Reducing SO ₂ emissions decreases sulfate, thereby influencing NH ₃ partitioning and resulting in a decreased ammonium. Similarly, reducing NOx emissions lowers HNO ₃, impacting nitrate and ammonium concentrations due to changes in HNO ₃ and NH ₃ partitioning. A particularly significant finding is that NH ₃ emissions reduction decreases not only nitrate and ammonium but also sulfate by altering cloud droplet pH and SO ₂ oxidation processes. While SO ₂ reduction's PM _2.5 impact is less than NOx and NH ₃, it emphasizes the complexity of regional sensitivities. Most of western Taiwan is NOx-sensitive, so reducing NOx emissions has a more substantial impact on lowering PM _2.5. However, given the higher mass emissions of NOx than NH ₃ in Taiwan, NH ₃ has a more significant consequence in mitigating PM _2.5 per unit mass emission reduction. The cost-effectiveness analysis suggests that NH ₃ reduction outperforms SO ₂ and NOx. Nevertheless, cost estimates vary due to methodological differences and regional emission sources. Overall, this study considers both efficiency and costs, highlighting NH ₃ emissions reduction as a promising strategy for PM _2.5 mitigation in the studied Taiwan's environment.