The role of coarse aerosol particles as a sink of HNO<sub>3</sub> in wintertime pollution events in the Salt Lake Valley
oleh: A. Hrdina, A. Hrdina, J. G. Murphy, A. G. Hallar, J. C. Lin, A. Moravek, A. Moravek, R. Bares, R. C. Petersen, R. C. Petersen, A. Franchin, A. Franchin, A. M. Middlebrook, L. Goldberger, L. Goldberger, B. H. Lee, M. Baasandorj, S. S. Brown
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2021-05-01 |
Deskripsi
<p>Wintertime ammonium nitrate (NH<span class="inline-formula"><sub>4</sub></span>NO<span class="inline-formula"><sub>3</sub></span>) pollution events burden urban mountain basins around the globe. In the Salt Lake Valley of Utah in the United States, such pollution events are often driven by the formation of persistent cold-air pools (PCAPs) that trap emissions near the surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 <span class="inline-formula">µ</span>m in diameter (PM<span class="inline-formula"><sub>2.5</sub></span>), largely in the form of NH<span class="inline-formula"><sub>4</sub></span>NO<span class="inline-formula"><sub>3</sub></span>, build up during these events and lead to severe haze. As part of an extensive measurement campaign to understand the chemical processes underlying PM<span class="inline-formula"><sub>2.5</sub></span> formation, the 2017 Utah Winter Fine Particulate Study, water-soluble trace gases and PM<span class="inline-formula"><sub>2.5</sub></span> constituents were continuously monitored using the ambient ion monitoring ion chromatograph (AIM-IC) system at the University of Utah campus. Gas-phase NH<span class="inline-formula"><sub>3</sub></span>, HNO<span class="inline-formula"><sub>3</sub></span>, HCl, and SO<span class="inline-formula"><sub>2</sub></span> along with particulate NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" 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="d0f05de3ef9fdb7d2948354df3a21cd8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00001.svg" width="8pt" height="15pt" src="acp-21-8111-2021-ie00001.png"/></svg:svg></span></span>, Na<span class="inline-formula"><sup>+</sup></span>, K<span class="inline-formula"><sup>+</sup></span>, Mg<span class="inline-formula"><sup>2+</sup></span>, Ca<span class="inline-formula"><sup>2+</sup></span>, 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-21-8111-2021-ie00002.svg" width="9pt" height="16pt" src="acp-21-8111-2021-ie00002.png"/></svg:svg></span></span>, Cl<span class="inline-formula"><sup>−</sup></span>, and SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" 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="68b04fae408a0dd5dd675e0ef9121343"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00003.svg" width="13pt" height="17pt" src="acp-21-8111-2021-ie00003.png"/></svg:svg></span></span> were measured from 21 January to 21 February 2017. During the two PCAP events captured, the fine particulate matter was dominated by secondary NH<span class="inline-formula"><sub>4</sub></span>NO<span class="inline-formula"><sub>3</sub></span>. The comparison of total nitrate (HNO<span class="inline-formula"><sub>3</sub></span> <span class="inline-formula">+</span> PM<span class="inline-formula"><sub>2.5</sub></span> NO<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">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="d4f68d92324ab64740c52d46a6e06853"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00004.svg" width="9pt" height="16pt" src="acp-21-8111-2021-ie00004.png"/></svg:svg></span></span>) and total NH<span class="inline-formula"><sub><i>x</i></sub></span> (NH<span class="inline-formula"><sub>3</sub></span> <span class="inline-formula">+</span> PM<span class="inline-formula"><sub>2.5</sub></span> NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M32" 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="05b70fd8394bf4db7d404440a905bbf8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00005.svg" width="8pt" height="15pt" src="acp-21-8111-2021-ie00005.png"/></svg:svg></span></span>) showed NH<span class="inline-formula"><sub><i>x</i></sub></span> was in excess during both pollution events. However, chemical composition analysis of the snowpack during the first PCAP event revealed that the total concentration of deposited NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M34" 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="4455bd3af612d9fcf5c166994cd0cebb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00006.svg" width="9pt" height="16pt" src="acp-21-8111-2021-ie00006.png"/></svg:svg></span></span> was nearly 3 times greater than that of deposited NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M35" 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="0418808ca2c60509de54ede1d4870f8a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00007.svg" width="8pt" height="15pt" src="acp-21-8111-2021-ie00007.png"/></svg:svg></span></span>. Daily snow composition measurements showed a strong correlation between NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M36" 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="bf5cd1886372f37758e950f0330107db"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00008.svg" width="9pt" height="16pt" src="acp-21-8111-2021-ie00008.png"/></svg:svg></span></span> and Ca<span class="inline-formula"><sup>2+</sup></span> in the snowpack. The presence of non-volatile salts (Na<span class="inline-formula"><sup>+</sup></span>, Ca<span class="inline-formula"><sup>2+</sup></span>, and Mg<span class="inline-formula"><sup>2+</sup></span>), which are frequently associated with coarse-mode dust, was also detected in PM<span class="inline-formula"><sub>2.5</sub></span> by the AIM-IC during the two PCAP events, accounting for roughly 5 % of total mass loading. The presence of a significant particle mass and surface area in the coarse mode during the first PCAP event was indicated by size-resolved particle measurements from an aerodynamic particle sizer. Taken together, these observations imply that atmospheric measurements of the gas-phase and fine-mode particle nitrate may not represent the total burden of nitrate in the atmosphere, implying a potentially significant role for uptake by coarse-mode dust. Using the NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M42" 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="80ce405e3e818ae1ef7a963d511bdf3e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00009.svg" width="9pt" height="16pt" src="acp-21-8111-2021-ie00009.png"/></svg:svg></span></span> : NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M43" 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="56b705f3dd2e3fdbb9fcd9e1a30289d8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8111-2021-ie00010.svg" width="8pt" height="15pt" src="acp-21-8111-2021-ie00010.png"/></svg:svg></span></span> ratio observed in the snowpack to estimate the proportion of atmospheric nitrate present in the coarse mode, we estimate that the amount of secondary NH<span class="inline-formula"><sub>4</sub></span>NO<span class="inline-formula"><sub>3</sub></span> could double in the absence of the coarse-mode sink. The underestimation of total nitrate indicates an incomplete account of the total oxidant<span id="page8112"/> production during PCAP events. The ability of coarse particles to permanently remove HNO<span class="inline-formula"><sub>3</sub></span> and influence PM<span class="inline-formula"><sub>2.5</sub></span> formation is discussed using information about particle composition and size distribution.</p>