Measurement report: Crustal materials play an increasing role in elevating particle pH – insights from 12-year records in a typical inland city of China

<p>Particle acidity serves as a key determinant in atmospheric chemical processes. Emerging concerns regarding aerosol acidity trends have been highlighted amid China's sustained initiatives to mitigate emissions of both acidic and alkaline precursors, especially in North China, which is significantly affected by dust aerosol. 12-year observational data in Zhengzhou reveal that the annual average <span class="inline-formula">PM_2.5</span> concentration decreased from <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">162</mn><mo>±</mo><mn mathvariant="normal">81</mn><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><mi mathvariant="normal">µ</mi><mi mathvariant="normal">g</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="78pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="1a4aa4fbe0af84bc430f08a7efccb361"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-6943-2025-ie00001.svg" width="78pt" height="15pt" src="acp-25-6943-2025-ie00001.png"/></svg:svg></span></span> in 2011 to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">60</mn><mo>±</mo><mn mathvariant="normal">41</mn><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><mi mathvariant="normal">µ</mi><mi mathvariant="normal">g</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="72pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="35a55c93e066c64a97a6a33966b6bee8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-6943-2025-ie00002.svg" width="72pt" height="15pt" src="acp-25-6943-2025-ie00002.png"/></svg:svg></span></span> in 2022, with the largest reduction in sulfate (73 %). Correspondingly, the annual particle pH increased by 0.10 units from 2011–2019. In addition, the elevated particle pH in 2015 and 2018 was notably influenced by the increase in <span class="inline-formula">TNH_<i>x</i></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msub><mi mathvariant="normal">NH</mi><mn mathvariant="normal">3</mn></msub></mrow><mo>+</mo><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="56pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="fd141197a09f622e01fcf44e044acac8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-6943-2025-ie00003.svg" width="56pt" height="15pt" src="acp-25-6943-2025-ie00003.png"/></svg:svg></span></span>). Note that the crustal material concentrations and their proportions increased significantly during 2019–2022, which might be responsible for the resuspension of surrounding soil dust. Even though the <span class="inline-formula">TNH_<i>x</i></span> concentration was decreasing, the annual average growth rate of pH values increased to 0.21 units from 2019–2022. This phenomenon is not unique to Zhengzhou, as major cities in the North China Plain have also experienced a pronounced upward trend in coarse particles after 2019. Therefore, the long-term evolution of particle acidity in North China will require comprehensive consideration of synergistic effects involving acidic precursors, ammonia, and crustal materials.</p>