Distinct diurnal variation in organic aerosol hygroscopicity and its relationship with oxygenated organic aerosol
<p>The hygroscopicity of organic aerosol (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter <span class="inline-formula"><i>κ</i><sub>OA</sub></span> remains poorly characterized, es...
| Main Authors: | , , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/20/865/2020/acp-20-865-2020.pdf |
| Summary: | <p>The hygroscopicity of organic aerosol (OA) is important for investigation
of its climatic and environmental impacts. However, the hygroscopicity
parameter <span class="inline-formula"><i>κ</i><sub>OA</sub></span> remains poorly characterized, especially in the
relatively polluted environment on the North China Plain (NCP). Here we
conducted simultaneous wintertime measurements of bulk aerosol chemical
compositions of PM<span class="inline-formula"><sub>2.5</sub></span> and PM<span class="inline-formula"><sub>1</sub></span> and bulk aerosol hygroscopicity of
PM<span class="inline-formula"><sub>10</sub></span> and PM<span class="inline-formula"><sub>1</sub></span> on the NCP using a capture-vaporizer time-of-flight
aerosol chemical speciation monitor (CV-ToF-ACSM) and a humidified
nephelometer system which measures the aerosol light-scattering enhancement
factor <span class="inline-formula"><i>f</i>(RH)</span>. A method for calculating <span class="inline-formula"><i>κ</i><sub>OA</sub></span> based on <span class="inline-formula"><i>f</i>(RH)</span> and bulk aerosol chemical-composition measurements was developed. We found that <span class="inline-formula"><i>κ</i><sub>OA</sub></span> varied
in a wide range with significant diurnal variations. The derived <span class="inline-formula"><i>κ</i><sub>OA</sub></span> ranged from almost 0.0 to 0.25, with an average (<span class="inline-formula">±1<i>σ</i></span>)
of 0.08 (<span class="inline-formula">±0.06</span>) for the entire study. The derived <span class="inline-formula"><i>κ</i><sub>OA</sub></span> was
highly correlated with <span class="inline-formula"><i>f</i><sub>44</sub></span> (fraction of <span class="inline-formula"><i>m</i>∕<i>z</i></span> 44 in OA measured by
CV-ToF-ACSM), an indicator of the oxidation degree of OA (<span class="inline-formula"><i>R</i>=0.79</span>), and the
relationship can be parameterized as <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">κ</mi><mi mathvariant="normal">OA</mi></msub><mo>=</mo><mn mathvariant="normal">1.04</mn><mo>×</mo><msub><mi>f</mi><mn mathvariant="normal">44</mn></msub><mo>-</mo><mn mathvariant="normal">0.02</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="111pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="9890895a6671a6bb2a4f1ad14f52e15a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-865-2020-ie00001.svg" width="111pt" height="12pt" src="acp-20-865-2020-ie00001.png"/></svg:svg></span></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">κ</mi><mi mathvariant="normal">OA</mi></msub><mo>=</mo><mn mathvariant="normal">0.3</mn><mo>×</mo><mrow class="chem"><mi mathvariant="normal">O</mi><mo>:</mo><mi mathvariant="normal">C</mi></mrow><mo>-</mo><mn mathvariant="normal">0.02</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="116pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="0c7ee3d913682589678ebfbabba540ac"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-865-2020-ie00002.svg" width="116pt" height="13pt" src="acp-20-865-2020-ie00002.png"/></svg:svg></span></span>, based on the relationship
between the <span class="inline-formula"><i>f</i><sub>44</sub></span> and <span class="inline-formula">O∕C</span> ratio for CV-ToF-ACSM). On average, <span class="inline-formula"><i>κ</i><sub>OA</sub></span>
reached the minimum (0.02) in the morning near 07:30 local time (LT) and then increased
rapidly, reaching the peak value of 0.16 near 14:30 LT. The diurnal variations
in <span class="inline-formula"><i>κ</i><sub>OA</sub></span> were highly and positively correlated with those of mass
fractions of oxygenated OA (<span class="inline-formula"><i>R</i>=0.95</span>), indicating that photochemical
processing played a dominant role in the increase in <span class="inline-formula"><i>κ</i><sub>OA</sub></span> in
winter on the NCP. Results in this study demonstrate the potential wide
applications of a humidified nephelometer system together with aerosol
composition measurements for investigating the hygroscopicity of OA in
various environments and highlight that the parameterization of <span class="inline-formula"><i>κ</i><sub>OA</sub></span> as a function of OA aging processes needs to be considered in
chemical transport models for better evaluating the impacts of OA on cloud
formation, atmospheric chemistry, and radiative forcing.</p> |
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| ISSN: | 1680-7316 1680-7324 |