Distinct diurnal variation in organic aerosol hygroscopicity and its relationship with oxygenated organic aerosol

• Main Authors: Y. Kuang, Y. He, W. Xu, P. Zhao, Y. Cheng, G. Zhao, J. Tao, N. Ma, H. Su, Y. Zhang, J. Sun, P. Cheng, W. Yang, S. Zhang, C. Wu, Y. Sun, C. Zhao
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/865/2020/acp-20-865-2020.pdf
• ISSN: 1680-7316; 1680-7324

<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>_OA</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">_2.5</span> and PM<span class="inline-formula">₁</span> and bulk aerosol hygroscopicity of PM<span class="inline-formula">₁₀</span> and PM<span class="inline-formula">₁</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>_OA</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>_OA</span> varied in a wide range with significant diurnal variations. The derived <span class="inline-formula"><i>κ</i>_OA</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>_OA</span> was highly correlated with <span class="inline-formula"><i>f</i>₄₄</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>₄₄</span> and <span class="inline-formula">O∕C</span> ratio for CV-ToF-ACSM). On average, <span class="inline-formula"><i>κ</i>_OA</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&thinsp;LT. The diurnal variations in <span class="inline-formula"><i>κ</i>_OA</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>_OA</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>_OA</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>