Climatology of aerosol pH and its controlling factors at the Melpitz continental background site in Central Europe

<p>Aerosol acidity has importance for the chemical and physical properties of atmospheric aerosol particles and for many processes that affect their transformation and fate. Here, we characterize trends in PM<span class="inline-formula">₁₀</span> pH and its controlling factors during the period of 2010–2019 at the Melpitz research station in eastern Germany, a continental background site in Central Europe. Aerosol liquid water content associated with inorganic species decreased by 3.4 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><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><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">a</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="53pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="f80f1623485927b12994377af17bfc41"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-8871-2025-ie00001.svg" width="53pt" height="15pt" src="acp-25-8871-2025-ie00001.png"/></svg:svg></span></span>, corresponding to a 50 % decrease during the analysed time period, in response to decreasing sulfate and nitrate. Aerosol pH exhibited an increase of 0.06 units per year, a trend that was distinct from other regions. The factors controlling aerosol pH varied by season. Temperature, the most important factor driving pH variability overall, was most important in summer (responsible for 51 % of pH variability) and less important during spring and fall (22 % and 27 %, respectively). <span class="inline-formula">NH₃</span>, the second-most important factor contributing to pH variability overall (29 %), was most important during winter (38 %) and far less important during summer (15 %). Aerosol chemistry in Melpitz is influenced by the high buffering capacity contributed by <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup><mo>/</mo><msub><mi mathvariant="normal">NH</mi><mn mathvariant="normal">3</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="727a976ac89bd2aea53a86b29d185a83"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-8871-2025-ie00002.svg" width="50pt" height="15pt" src="acp-25-8871-2025-ie00002.png"/></svg:svg></span></span> and, to a lesser degree, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup><mo>/</mo><msub><mi mathvariant="normal">HNO</mi><mn mathvariant="normal">3</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="59pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a0e446ae319fab66de56c83999903a56"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-8871-2025-ie00003.svg" width="59pt" height="16pt" src="acp-25-8871-2025-ie00003.png"/></svg:svg></span></span>. Thermodynamic analysis of the aerosol system shows that secondary inorganic aerosol formation is most frequently <span class="inline-formula">HNO₃</span>-limited, suggesting that factors that control <span class="inline-formula">NO_<i>x</i></span> would be more effective than <span class="inline-formula">NH₃</span> controls in reducing PM mass concentrations. However, the non-linear response of gas-phase <span class="inline-formula">HNO₃</span> and aerosol <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="dd23f13eb24280cbe650be4567ce8571"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-8871-2025-ie00004.svg" width="25pt" height="16pt" src="acp-25-8871-2025-ie00004.png"/></svg:svg></span></span> to <span class="inline-formula">NO_<i>x</i></span> emissions in the region, likely due to VOC controls on oxidant formation and subsequent impacts on <span class="inline-formula">NO_<i>x</i></span> conversion to <span class="inline-formula">HNO₃</span>, highlights the challenge associated with the PM reductions needed to attain new air quality standards in this region.</p>