Evidence of haze-driven secondary production of supermicrometer aerosol nitrate and sulfate in size distribution data in South Korea

<p>This study reports measurements of size-resolved aerosol composition at a site in Incheon along with other aerosol characteristics for contrast between Incheon (coastal) and Seoul (inland), South Korea, during a transboundary pollution event during the early part of an intensive sampling period between 4 and 11 March 2019. Anthropogenic emissions were dominant in the boundary layer over the study region between 4 and 6 March, with much smaller contributions from dust, smoke, and sea salt. The meteorology of this period (shallow boundary layer, enhanced humidity, and low temperature) promoted local heterogeneous formation of secondary inorganic and organic species, including high nitrate (NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="12pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="454fcbc2827c64c4045c795e8a493d8f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-7505-2022-ie00001.svg" width="12pt" height="16pt" src="acp-22-7505-2022-ie00001.png"/></svg:svg></span></span> relative to sulfate (SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="16pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="a5156908e62a029a5ba93d516e3fe9f2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-7505-2022-ie00002.svg" width="16pt" height="17pt" src="acp-22-7505-2022-ie00002.png"/></svg:svg></span></span>. Seoul exhibited higher PM<span class="inline-formula">_2.5</span> levels than Incheon, likely due to local emissions. The following findings point to secondary aerosol formation and growth sensitivity to water vapor during this pollution event: (i) significant concentrations of individual inorganic and organic acids in the supermicrometer range relative to their full size range (<span class="inline-formula">∼40</span> %) at higher humidity; (ii) high correlation (<span class="inline-formula"><i>r</i>=0.95</span>) between oxalate and SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" 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="4a53e7d1f00f4334c934356877052515"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-7505-2022-ie00003.svg" width="13pt" height="17pt" src="acp-22-7505-2022-ie00003.png"/></svg:svg></span></span>, a marker of secondary aqueous production of oxalate; (iii) increased sulfur and nitrogen oxidation ratios as a function of humidity; and (iv) matching composition apportionment (for soluble ions) between the PM<span class="inline-formula">₁</span> and PM<span class="inline-formula">_2.5−1</span> size fractions. The last finding confirms that PM<span class="inline-formula">₁</span> aerosol composition measurements fully capture PM<span class="inline-formula">_2.5</span> composition apportionment (for soluble ions) during haze events and may therefore be reliably applied in modeling studies of such events over the full PM<span class="inline-formula">_2.5</span> size range. However, the differences evident in the periods following the haze event imply that under other atmospheric conditions PM<span class="inline-formula">₁</span> composition measurements will not fully reflect the apportionment of PM<span class="inline-formula">_2.5</span> aerosols. The study period was marked by relatively low temperatures that made NO<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">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="ad57fe4a8dcf7ebabf2d1e48d90b5292"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-7505-2022-ie00004.svg" width="9pt" height="16pt" src="acp-22-7505-2022-ie00004.png"/></svg:svg></span></span> the most abundant species detected, pointing to the sensitivity of PM<span class="inline-formula">_2.5</span> levels and composition as a function of season during such transboundary events. For instance, other such events in previous studies exhibited more comparable levels between SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" 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="7f75eaded4497f2452ab2c17fc6da474"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-7505-2022-ie00005.svg" width="13pt" height="17pt" src="acp-22-7505-2022-ie00005.png"/></svg:svg></span></span> and NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" 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="5a2143864edd3f7cf8f1639018917994"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-7505-2022-ie00006.svg" width="9pt" height="16pt" src="acp-22-7505-2022-ie00006.png"/></svg:svg></span></span> coincident with higher temperatures than the current study. This dataset can contribute to future evaluation of model PM<span class="inline-formula">_2.5</span> composition to better support regulatory efforts to control PM<span class="inline-formula">_2.5</span> precursors.</p>

Abstract licence: CC BY

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