Aerosol pH and its driving factors in Beijing

• Main Authors: J. Ding, P. Zhao, J. Su, Q. Dong, X. Du, Y. Zhang
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-06-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/7939/2019/acp-19-7939-2019.pdf

<p>Aerosol acidity plays a key role in secondary aerosol formation. The high-temporal-resolution PM<span class="inline-formula">_2.5</span> pH and size-resolved aerosol pH in Beijing were calculated with ISORROPIA II. In 2016–2017, the mean PM<span class="inline-formula">_2.5</span> pH (at relative humidity (RH)&thinsp;&gt;&thinsp;30&thinsp;%) over four seasons was <span class="inline-formula">4.5±0.7</span> (winter)&thinsp;&gt;&thinsp;<span class="inline-formula">4.4±1.2</span> (spring)&thinsp;&gt;&thinsp;<span class="inline-formula">4.3±0.8</span> (autumn)&thinsp;&gt;&thinsp;<span class="inline-formula">3.8±1.2</span> (summer), showing moderate acidity. In coarse-mode aerosols, <span class="inline-formula">Ca²⁺</span> played an important role in aerosol pH. Under heavily polluted conditions, more secondary ions accumulated in the coarse mode, leading to the acidity of the coarse-mode aerosols shifting from neutral to weakly acidic. Sensitivity tests also demonstrated the significant contribution of crustal ions to PM<span class="inline-formula">_2.5</span> pH. In the North China Plain (NCP), the common driving factors affecting PM<span class="inline-formula">_2.5</span> pH variation in all four seasons were <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">SO</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="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="8c898138530c760447165fe6cdc920bb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-7939-2019-ie00001.svg" width="29pt" height="17pt" src="acp-19-7939-2019-ie00001.png"/></svg:svg></span></span>, <span class="inline-formula">TNH₃</span> (total ammonium (gas&thinsp;<span class="inline-formula">+</span>&thinsp;aerosol)), and temperature, while unique factors were <span class="inline-formula">Ca²⁺</span> in spring and RH in summer. The decreasing <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</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="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="14485914c781d9e26f0da54782d5723d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-7939-2019-ie00002.svg" width="29pt" height="17pt" src="acp-19-7939-2019-ie00002.png"/></svg:svg></span></span> and increasing <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" 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="ecc3e6dd5af0ffb1da8bfbfcb16b8e8b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-7939-2019-ie00003.svg" width="25pt" height="16pt" src="acp-19-7939-2019-ie00003.png"/></svg:svg></span></span> mass fractions in PM<span class="inline-formula">_2.5</span> as well as excessive <span class="inline-formula">NH₃</span> in the atmosphere in the NCP in recent years are the reasons why aerosol acidity in China is lower than that in Europe and the United States. The nonlinear relationship between PM<span class="inline-formula">_2.5</span> pH and <span class="inline-formula">TNH₃</span> indicated that although <span class="inline-formula">NH₃</span> in the NCP was abundant, the PM<span class="inline-formula">_2.5</span> pH was still acidic because of the thermodynamic equilibrium between <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="44642da34e3da1fffc83c720c465c894"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-7939-2019-ie00004.svg" width="24pt" height="15pt" src="acp-19-7939-2019-ie00004.png"/></svg:svg></span></span> and <span class="inline-formula">NH₃</span>. To reduce nitrate by controlling ammonia, the amount of ammonia must be greatly reduced below excessive quantities.</p>