Reduced light absorption of black carbon (BC) and its influence on BC-boundary-layer interactions during “APEC Blue”

• Main Authors: M. Gao, Y. Yang, H. Liao, B. Zhu, Y. Zhang, Z. Liu, X. Lu, C. Wang, Q. Zhou, Y. Wang, Q. Zhang, G. R. Carmichael, J. Hu
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/11405/2021/acp-21-11405-2021.pdf

<p>Light absorption and radiative forcing of black carbon (BC) is influenced by both BC itself and its interactions with other aerosol chemical compositions. Although the changes in BC concentrations in response to emission reduction measures have been well documented, the influence of emission reductions on the light absorption properties of BC and its influence on BC-boundary-layer interactions has been less explored. In this study, we used the online coupled WRF-Chem model to examine how emission control measures during the Asia-Pacific Economic Cooperation (APEC) summit affect the mixing state and light absorption of BC, and the associated implications for BC-PBL interactions. We found that both the mass concentration of BC and the BC coating materials declined during the APEC week, which reduced the light absorption and light absorption enhancement (<span class="inline-formula"><i>E</i>_ab</span>) of BC. The reduced absorption aerosol optical depth (AAOD) during APEC was caused by both the decline in the mass concentration of BC itself (52.0 %), and the lensing effect of BC (48.0 %). The reduction in coating materials (39.4 %) contributed the most to the influence of the lensing effect, and the reduced light absorption capability (<span class="inline-formula"><i>E</i>_ab</span>) contributed 3.2 % to the total reduction in AAOD. Reduced light absorption of BC due to emission control during APEC enhanced planetary boundary layer height (PBLH) by 8.2 m. PM<span class="inline-formula">_2.5</span> and O<span class="inline-formula">₃</span> were found to have different responses to the changes in the light absorption of BC. Reduced light absorption of BC due to emission reductions decreased near-surface PM<span class="inline-formula">_2.5</span> concentrations but near-surface O<span class="inline-formula">₃</span> concentrations were enhanced in the North China Plain. These results suggest that current measures to control SO<span class="inline-formula">₂</span>, NO<span class="inline-formula">_<i>x</i></span>, etc. would be effective in reducing the absorption enhancement of BC and in inhibiting the feedback of BC on the boundary layer. However, enhanced ground O<span class="inline-formula">₃</span> might be a side effect of current emission control strategies. How to control emissions<span id="page11406"/> to offset this side effect of current emission control measures on O<span class="inline-formula">₃</span> should be an area of further focus.</p>