Assessing the Effectiveness of SO₂, NOx, and NH₃ Emission Reductions in Mitigating Winter PM_2.5 in Taiwan Using CMAQ Model

Abstract. 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 NH₃, NOx, and SO₂ emissions on PM_2.5 secondary inorganic species (i.e., SO­₄^2-, NO₃^-, and NH₄⁺). For sulfate, ~ 43.7 % is derived from the chemical reactions of local SO₂ emission, emphasizing the substantial contribution of regional transported sulfate. In contrast, local NOx and NH₃ emissions predominantly influence nitrate and ammonium. Reducing SO₂ emissions decreases sulfate, thereby influencing NH₃ partitioning and resulting in a decreased ammonium. Similarly, reducing NOx emissions lowers HNO₃, impacting nitrate and ammonium concentrations due to changes in HNO₃ and NH₃ partitioning. A particularly significant finding is that NH₃ emissions reduction decreases not only nitrate and ammonium but also sulfate by altering cloud droplet pH and SO₂ oxidation processes. While SO₂ reduction's PM_2.5 impact is less than NOx and NH₃, 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 NH₃ in Taiwan, NH₃ has a more significant consequence in mitigating PM_2.5 per unit mass emission reduction. The cost-effectiveness analysis suggests that NH₃ reduction outperforms SO₂ and NOx. Nevertheless, cost estimates vary due to methodological differences and regional emission sources. Overall, this study considers both efficiency and costs, highlighting NH₃ emissions reduction as a promising strategy for PM_2.5 mitigation in the studied Taiwan's environment.