Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou

• Main Authors: J. Y. Sun, C. Wu, D. Wu, C. Cheng, M. Li, L. Li, T. Deng, J. Z. Yu, Y. J. Li, Q. Zhou, Y. Liang, T. Sun, L. Song, P. Cheng, W. Yang, C. Pei, Y. Chen, Y. Cen, H. Nian, Z. Zhou
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-02-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/2445/2020/acp-20-2445-2020.pdf

<p>Black carbon (BC) aerosols have been widely recognized as a vital climate forcer in the atmosphere. Amplification of light absorption can occur due to coatings on BC during atmospheric aging, an effect that remains uncertain in accessing the radiative forcing of BC. Existing studies on the absorption enhancement factor (<span class="inline-formula"><i>E</i>_abs</span>) have poor coverage on both seasonal and diurnal scales. In this study, we applied a recently developed minimum <span class="inline-formula"><i>R</i></span> squared (MRS) method, which can cover both seasonal and diurnal scales, for <span class="inline-formula"><i>E</i>_abs</span> quantification. Using field measurement data in Guangzhou, the aims of this study are to explore (1) the temporal dynamics of BC optical properties at seasonal (wet season, 31 July–10 September; dry season, 15 November 2017–15 January 2018) and diel scales (1&thinsp;h time resolution) in the typical urban environment and (2) the influencing factors on <span class="inline-formula"><i>E</i>_abs</span> temporal variability. Mass absorption efficiency at 520&thinsp;nm by primary aerosols (MAE<span class="inline-formula">_p520</span>) determined by the MRS method exhibited a strong seasonality (8.6&thinsp;m<span class="inline-formula">²</span>&thinsp;g<span class="inline-formula">⁻¹</span> in the wet season and 16.8&thinsp;m<span class="inline-formula">²</span>&thinsp;g<span class="inline-formula">⁻¹</span> in the dry season). <span class="inline-formula"><i>E</i>_abs520</span> was higher in the wet season (<span class="inline-formula">1.51±0.50</span>) and lower in the dry season (<span class="inline-formula">1.29±0.28</span>). Absorption Ångström exponent (AAE<span class="inline-formula">_470–660</span>) in the dry season (<span class="inline-formula">1.46±0.12</span>) was higher than that in the wet season (<span class="inline-formula">1.37±0.10</span>). Collective evidence showed that the active biomass burning (BB) in the dry season effectively altered the optical properties of BC, leading to elevated MAE, MAE<span class="inline-formula">_p</span> and AAE in the dry season compared to those in the wet season. Diurnal <span class="inline-formula"><i>E</i>_abs520</span> was positively correlated with AAE<span class="inline-formula">_470–660</span> (<span class="inline-formula"><i>R</i>²=0.71</span>) and negatively correlated with the AE33 aerosol loading compensation parameter (<span class="inline-formula"><i>k</i></span>) (<span class="inline-formula"><i>R</i>²=0.74</span>) in the wet season, but these correlations were significantly weaker in the dry season, which may be related to the impact of BB. This result suggests that during<span id="page2446"/> the wet season, the lensing effect was more likely dominating the AAE diurnal variability rather than the contribution from brown carbon (BrC). Secondary processing can affect <span class="inline-formula"><i>E</i>_abs</span> diurnal dynamics. The <span class="inline-formula"><i>E</i>_abs520</span> exhibited a clear dependency on the ratio of secondary organic carbon to organic carbon (<span class="inline-formula">SOC∕OC</span>), confirming the contribution of secondary organic aerosols to <span class="inline-formula"><i>E</i>_abs</span>; <span class="inline-formula"><i>E</i>_abs520</span> correlated well with nitrate and showed a clear dependence on temperature. This new finding implies that gas–particle partitioning of semivolatile compounds may potentially play an important role in steering the diurnal fluctuation of <span class="inline-formula"><i>E</i>_abs520</span>. In the dry season, the diurnal variability in <span class="inline-formula"><i>E</i>_abs520</span> was associated with photochemical aging as evidenced by the good correlation (<span class="inline-formula"><i>R</i>²=</span>0.69) between oxidant concentrations (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mi>x</mi></msub></mrow><mo>=</mo><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">3</mn></msub></mrow><mo>+</mo><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="74pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="28a7a81edb8f463e7f73110978604198"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-2445-2020-ie00001.svg" width="74pt" height="13pt" src="acp-20-2445-2020-ie00001.png"/></svg:svg></span></span>) and <span class="inline-formula"><i>E</i>_abs520</span>.</p>