Mixing state of refractory black carbon in fog and haze at rural sites in winter on the North China Plain
<p>The variability of the mixing state of refractory black carbon aerosol (<span class="inline-formula"><i>r</i></span>BC) and the corresponding complicated light absorption capacity imposes great uncertainty for its climate forcing assessment. In this study,...
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| Autores principales: | , , , , , , , , , , , , , , , |
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| Formato: | article |
| Lenguaje: | EN |
| Publicado: |
Copernicus Publications
2021
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| Materias: | |
| Acceso en línea: | https://doaj.org/article/339b1de701884e65878789d4754b413c |
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| Sumario: | <p>The variability of the mixing state of refractory black carbon aerosol (<span class="inline-formula"><i>r</i></span>BC) and the corresponding complicated light absorption capacity imposes great uncertainty for its climate forcing assessment. In this study, field
observations using a single-particle soot photometer (SP2) were conducted to investigate the mixing state of <span class="inline-formula"><i>r</i></span>BC under different meteorological
conditions at a rural site on the North China Plain. The results showed that the hourly mass concentration of <span class="inline-formula"><i>r</i></span>BC during the observation periods was <span class="inline-formula">2.6±1.5</span> <span class="inline-formula">µg m<sup>−3</sup></span> on average, with a moderate increase (<span class="inline-formula">3.1±0.9</span>) during fog episodes. The mass-equivalent size distribution of <span class="inline-formula"><i>r</i></span>BC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 213 <span class="inline-formula">nm</span>. We found that the count median diameter (CMD) of <span class="inline-formula"><i>r</i></span>BC particles during snowfall episodes was larger than that before snowfall, and the number of <span class="inline-formula"><i>r</i></span>BC particles with <span class="inline-formula"><i>D</i><sub>c</sub><121</span> <span class="inline-formula">nm</span> were reduced by 28.4 <span class="inline-formula">%</span> after snow. This may indicate that <span class="inline-formula"><i>r</i></span>BC-containing particles with small core sizes (<span class="inline-formula"><i>D</i><sub>c</sub></span>) were much more effectively removed by snow with light snow intensity (0.23 <span class="inline-formula">mm h<sup>−1</sup></span>). Based on the Mie scattering theory simulation, the relative and absolute coating thicknesses of <span class="inline-formula"><i>r</i></span>BC-containing particles were estimated to be <span class="inline-formula">∼1.6</span> and <span class="inline-formula">∼52</span> <span class="inline-formula">nm</span> for the <span class="inline-formula"><i>r</i></span>BC core with a mass-equivalent diameter (<span class="inline-formula"><i>D</i><sub>c</sub></span>) of 170 to 190 <span class="inline-formula">nm</span>, respectively, which indicates that most of the <span class="inline-formula"><i>r</i></span>BC-containing particles were thinly coated. Furthermore, a moderate light absorption enhancement (<span class="inline-formula"><i>E</i><sub>abs</sub>=1.3</span>) and relatively low absorption cross section (MAC <span class="inline-formula">=</span> 5.5 <span class="inline-formula">m<sup>2</sup> g<sup>−1</sup></span>) at 880 <span class="inline-formula">nm</span> were observed at the Gucheng (GC) site in winter compared with other typical rural sites.</p>
<p><span id="page17632"/>The relationship between the microphysical properties of <span class="inline-formula"><i>r</i></span>BC and
meteorological conditions was also studied. Relatively warm and high-RH
environments (<span class="inline-formula">RH>50 <i>%</i></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M31" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">4</mn><mspace width="0.125em" linebreak="nobreak"/><mrow class="unit"><msup><mi/><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow><mo><</mo><mi>T</mi><mo><</mo><mn mathvariant="normal">4</mn><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><msup><mi/><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="80pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="5813c5afe23174f6f9e14a25ade99371"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-17631-2021-ie00001.svg" width="80pt" height="11pt" src="acp-21-17631-2021-ie00001.png"/></svg:svg></span></span>) were more favorable to <span class="inline-formula"><i>r</i></span>BC aging than dry and cold environments (<span class="inline-formula">RH<60 <i>%</i></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M34" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>T</mi><mo><</mo><mo>-</mo><mn mathvariant="normal">8</mn><mspace width="0.125em" linebreak="nobreak"/><mrow class="unit"><msup><mi/><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="48pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="2f27c72cdc42b2b7f3e9a547bbeb00c6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-17631-2021-ie00002.svg" width="48pt" height="11pt" src="acp-21-17631-2021-ie00002.png"/></svg:svg></span></span>). And the increase in ambient RH at the same temperature favors <span class="inline-formula"><i>r</i></span>BC aging. An increasing mass fraction of secondary inorganic aerosols (SIAs; especially sulfate and nitrate) and a decreasing mass fraction of organic aerosols in the environment support the formation of thick coatings by <span class="inline-formula"><i>r</i></span>BC. The RH dependence of absorption enhancement (<span class="inline-formula"><i>E</i><sub>abs</sub></span>) was likely caused by the relative
coating thickness (RCT) as supported by the gradual increase in the mass concentration and mass fraction of secondary components as a function of RH in the ambient air. The mass fractions of aqueous-phase formation of secondary components had a limited effect on <span class="inline-formula"><i>E</i><sub>abs</sub></span> under a high-RH environment. The measured <span class="inline-formula"><i>r</i></span>BC concentrations and the mixing state of <span class="inline-formula"><i>r</i></span>BC in different meteorological environments will be useful for evaluating the radiative forcing of <span class="inline-formula"><i>r</i></span>BC in regional climate models.</p> |
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