Assessing the effectiveness of SO₂, NO_<i>x</i>, and NH₃ emission reductions in mitigating winter PM_2.5 in Taiwan using CMAQ

<p>Taiwan experiences higher air pollution in winter when fine particulate matter (PM<span class="inline-formula">_2.5</span>) levels frequently surpass national standards. This study employs the Community Multiscale Air Quality model to assess the effectiveness of reducing SO<span class="inline-formula">₂</span>, NO<span class="inline-formula">_<i>x</i></span>, and NH<span class="inline-formula">₃</span> emissions on PM<span class="inline-formula">_2.5</span> secondary inorganic species (i.e., SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="6181d6711c50a62cc80d469e7dc67eab"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-10759-2024-ie00001.svg" width="13pt" height="17pt" src="acp-24-10759-2024-ie00001.png"/></svg:svg></span></span>, NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="ee54bb0fff66afdafaf51bed1fde360d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-10759-2024-ie00002.svg" width="9pt" height="16pt" src="acp-24-10759-2024-ie00002.png"/></svg:svg></span></span>, and NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="f1ca5762abf079d28af10bf21d382d4c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-10759-2024-ie00003.svg" width="8pt" height="15pt" src="acp-24-10759-2024-ie00003.png"/></svg:svg></span></span>). For sulfate, <span class="inline-formula">∼</span> 43.7 % is derived from the chemical reactions of local SO<span class="inline-formula">₂</span> emission, emphasizing the substantial contribution of regionally transported sulfate. In contrast, nitrate and ammonium are predominantly influenced by local NO<span class="inline-formula">_<i>x</i></span> and NH<span class="inline-formula">₃</span> emissions. Reducing SO<span class="inline-formula">₂</span> emissions decreases sulfate levels, which in turn leads to more NH<span class="inline-formula">₃</span> remaining in the gas phase, resulting in lower ammonium concentrations. Similarly, reducing NO<span class="inline-formula">_<i>x</i></span> emissions lowers HNO<span class="inline-formula">₃</span> formation, impacting nitrate and ammonium concentrations by decreasing the available HNO<span class="inline-formula">₃</span> and leaving more NH<span class="inline-formula">₃</span> in the gas phase. A significant finding is that reducing NH<span class="inline-formula">₃</span> emissions decreases not only ammonium and nitrate but also sulfate by altering cloud droplet pH and SO<span class="inline-formula">₂</span> oxidation processes. While the impact of SO<span class="inline-formula">₂</span> reduction on PM<span class="inline-formula">_2.5</span> is less than that of NO<span class="inline-formula">_<i>x</i></span> and NH<span class="inline-formula">₃</span>, it emphasizes the complexity of regional sensitivities. Most of western Taiwan is NO<span class="inline-formula">_<i>x</i></span>-sensitive, so reducing NO<span class="inline-formula">_<i>x</i></span> emissions has a more substantial impact on lowering PM<span class="inline-formula">_2.5</span> levels. However, given the higher mass emissions of NO<span class="inline-formula">_<i>x</i></span> than NH<span class="inline-formula">₃</span> in Taiwan, NH<span class="inline-formula">₃</span> has a more significant consequence in mitigating PM<span class="inline-formula">_2.5</span> per unit mass emission reduction (i.e., 2.43 <span class="inline-formula">×</span> 10<span class="inline-formula">⁻⁵</span> and 0.85 <span class="inline-formula">×</span> 10<span class="inline-formula">⁻⁵</span> <span class="inline-formula">µ</span>g m<span class="inline-formula">⁻³</span> (t yr<span class="inline-formula">⁻¹</span>)<span class="inline-formula">⁻¹</span>​​​​​​​ for NH<span class="inline-formula">₃</span> and NO<span class="inline-formula">_<i>x</i></span>, respectively, under current emission reduction). The cost-effectiveness analysis suggests that NH<span class="inline-formula">₃</span> reduction outperforms SO<span class="inline-formula">₂</span> and NO<span class="inline-formula">_<i>x</i></span> reduction (i.e., USD 0.06 billion yr<span class="inline-formula">⁻¹</span> <span class="inline-formula">µ</span>g<span class="inline-formula">⁻¹</span> m<span class="inline-formula">³</span>, USD 0.1 billion yr<span class="inline-formula">⁻¹</span> <span class="inline-formula">µ</span>g<span class="inline-formula">⁻¹</span> m<span class="inline-formula">³</span>, and USD 1 billion yr<span class="inline-formula">⁻¹</span> <span class="inline-formula">µ</span>g<span class="inline-formula">⁻¹</span> m<span class="inline-formula">³</span> for NH<span class="inline-formula">₃</span>, SO<span class="inline-formula">₂</span>, and NO<span class="inline-formula">_<i>x</i></span>, respectively, under the current emission reduction). Nevertheless, the costs of emission reduction vary due to differences in methodology and regional emission sources. Overall, this study considers both the efficiency and costs, highlighting NH<span class="inline-formula">₃</span> emissions reduction as a promising strategy for PM<span class="inline-formula">_2.5</span> mitigation in the studied environment in Taiwan.</p>