Nitrate formation from heterogeneous uptake of dinitrogen pentoxide during a severe winter haze in southern China

• Main Authors: H. Yun, W. Wang, T. Wang, M. Xia, C. Yu, Z. Wang, S. C. N. Poon, D. Yue, Y. Zhou
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-12-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/17515/2018/acp-18-17515-2018.pdf

<p>Nitrate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" 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="57a4663cbf0d11bf294d99bb32c9ae29"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-17515-2018-ie00001.svg" width="25pt" height="16pt" src="acp-18-17515-2018-ie00001.png"/></svg:svg></span></span>) has become a major component of fine particulate matter (PM<span class="inline-formula">_2.5</span>) during hazy days in China. However, the role of the heterogeneous reactions of dinitrogen pentoxide (<span class="inline-formula">N₂O₅</span>) in nitrate formation is not well constrained. In January 2017, a severe haze event occurred in the Pearl River Delta (PRD) of southern China during which high levels of PM<span class="inline-formula">_2.5</span> (<span class="inline-formula">∼400</span>&thinsp;<span class="inline-formula">µ</span>g&thinsp;m<span class="inline-formula">⁻³</span>) and <span class="inline-formula">O₃</span> (<span class="inline-formula">∼160</span>&thinsp;ppbv) were observed at a semi-rural site (Heshan) in the western PRD. Nitrate concentrations reached 108&thinsp;<span class="inline-formula">µ</span>g&thinsp;m<span class="inline-formula">⁻³</span> (1&thinsp;h time resolution), and the contribution of nitrate to PM<span class="inline-formula">_2.5</span> was nearly 40&thinsp;%. Concurrent increases in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" 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="9712381780fcc4de6c4d72f703a8771c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-17515-2018-ie00002.svg" width="25pt" height="16pt" src="acp-18-17515-2018-ie00002.png"/></svg:svg></span></span> and <span class="inline-formula">ClNO₂</span> (with a maximum value of 8.3&thinsp;ppbv at a 1&thinsp;min time resolution) were observed in the first several hours after sunset, indicating an intense <span class="inline-formula">N₂O₅</span> heterogeneous uptake by aerosols. The formation potential of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" 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="48a6d5724cc017ced9c974ab9a81c03a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-17515-2018-ie00003.svg" width="25pt" height="16pt" src="acp-18-17515-2018-ie00003.png"/></svg:svg></span></span> via <span class="inline-formula">N₂O₅</span> heterogeneous reactions was estimated to be between 29.0 and 77.3&thinsp;<span class="inline-formula">µ</span>g&thinsp;m<span class="inline-formula">⁻³</span> in the early hours (2 to 6&thinsp;h) after sunset based on the measurement data, which could completely explain the measured increase in the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" 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="827b0fe0e97f70953101fc9e20cd0031"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-17515-2018-ie00004.svg" width="25pt" height="16pt" src="acp-18-17515-2018-ie00004.png"/></svg:svg></span></span> concentration during the same time period. Daytime production of nitric acid from the gas-phase reaction of <span class="inline-formula">OH+NO₂</span> was calculated with a chemical box model built using the Master Chemical Mechanism (MCM v3.3.1) and constrained by the measurement data. The integrated nocturnal nitrate formed via <span class="inline-formula">N₂O₅</span> chemistry was comparable to or even higher than the nitric acid formed during the day. This study confirms that <span class="inline-formula">N₂O₅</span> heterogeneous chemistry was a significant source of aerosol nitrate during hazy days in southern China.</p>