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<p>The reduction of fine particles (PM<span class="inline-formula">_2.5</span>) and reactive N (N<span class="inline-formula">_r</span>) and S (S<span class="inline-formula">_r</span>) species is a key objective for air pollution control policies because of their major adverse effects on human health, ecosystem diversity, and climate. The sensitivity of global and regional N<span class="inline-formula">_r</span>, S<span class="inline-formula">_r</span>, and PM<span class="inline-formula">_2.5</span> to 20 % and 40 % individual and collective reductions in anthropogenic emissions of NH<span class="inline-formula">₃</span>, NO<span class="inline-formula">_<i>x</i></span>, and SO<span class="inline-formula">_<i>x</i></span> (with respect to a 2015 baseline) is investigated using the EMEP MSC-W (European Monitoring and Evaluation Programme Meteorological Synthesizing Centre – West) atmospheric chemistry transport model with WRF (Weather Research and Forecasting) meteorology. Regional comparisons reveal that the individual emissions reduction has multiple co-benefits and small disbenefits on different species, and those effects are highly geographically variable. A 40 % NH<span class="inline-formula">₃</span> emission reduction decreases regional average NH<span class="inline-formula">₃</span> concentrations by 47 %–49 % but only decreases NH<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">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="9ae7496f73369cba691a3bcfb358d1a5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-6083-2023-ie00001.svg" width="8pt" height="15pt" src="acp-23-6083-2023-ie00001.png"/></svg:svg></span></span> by 18 % in Euro_Medi, 15 % in East Asia, 12 % in North America, and 4 % in South Asia. This order follows the regional ammonia richness. A disbenefit is the increased SO<span class="inline-formula">₂</span> concentrations in these regions (10 %–16 % for 40 % reductions) because reduced NH<span class="inline-formula">₃</span> levels decrease SO<span class="inline-formula">₂</span> deposition through altering atmospheric acidity. A 40 % NO<span class="inline-formula">_<i>x</i></span> emission reduction reduces NO<span class="inline-formula">_<i>x</i></span> concentrations in East Asia by 45 %, Euro_Medi and North America by <span class="inline-formula">∼</span> 38 %, and South Asia by 22 %, whilst the regional order is reversed for fine NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" 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="01db1dfc152077444e810c7eff7103ec"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-6083-2023-ie00002.svg" width="9pt" height="16pt" src="acp-23-6083-2023-ie00002.png"/></svg:svg></span></span>, which is related to enhanced O<span class="inline-formula">₃</span> levels in East Asia (and also, but by less, in Euro_Medi) and decreased O<span class="inline-formula">₃</span> levels in South Asia (and also, but by less, in North America). Consequently, the oxidation of NO<span class="inline-formula">_<i>x</i></span> to NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" 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="d4917cb251612ae03efebb0a66479930"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-6083-2023-ie00003.svg" width="9pt" height="16pt" src="acp-23-6083-2023-ie00003.png"/></svg:svg></span></span> and of SO<span class="inline-formula">₂</span> to SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" 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="820c2a883eb6831c3f3dfbf9fe5018ac"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-6083-2023-ie00004.svg" width="13pt" height="17pt" src="acp-23-6083-2023-ie00004.png"/></svg:svg></span></span> is enhanced in East Asia but decreased in South Asia, which causes a less effective decrease in NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" 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="d96e0e0e6a6172a7d34ac185b1d0a8a7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-6083-2023-ie00005.svg" width="9pt" height="16pt" src="acp-23-6083-2023-ie00005.png"/></svg:svg></span></span> and even an increase in SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" 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="400cdf836a92a8210fecc81dc876b9d6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-6083-2023-ie00006.svg" width="13pt" height="17pt" src="acp-23-6083-2023-ie00006.png"/></svg:svg></span></span> in East Asia but quite the opposite in South Asia. For regional policy making, it is thus vital to reduce three precursors together to minimize such adverse effects. A 40 % SO<span class="inline-formula">_<i>x</i></span> emission reduction is slightly more effective in reducing SO<span class="inline-formula">₂</span> (42 %–45 %) than SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" 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="5c032620c8f70b9e15908a9c5486e9a8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-6083-2023-ie00007.svg" width="13pt" height="17pt" src="acp-23-6083-2023-ie00007.png"/></svg:svg></span></span> (34 %–38 %), whilst the disbenefit is that it yields a <span class="inline-formula">∼</span> 12 % increase in total NH<span class="inline-formula">₃</span> deposition in the four regions, which further threatens ecosystem diversity. This work also highlights important messages for policy makers concerning the mitigation of PM<span class="inline-formula">_2.5</span>. More emissions controls focusing on NH<span class="inline-formula">₃</span> and NO<span class="inline-formula">_<i>x</i></span> are necessary for regions with better air quality, such as northern Europe and eastern North America. In East Asia, the three individual reductions are equally effective, whilst in South Asia only SO<span class="inline-formula">_<i>x</i></span> reduction is currently effective. The geographically varying non-one-to-one proportionality of chemical responses of N<span class="inline-formula">_r</span>, S<span class="inline-formula">_r</span>, and PM<span class="inline-formula">_2.5</span> to emissions reductions revealed by this work show the importance of both prioritizing emissions strategies in different regions and combining several precursor reductions together to maximize the policy effectiveness.</p>