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

Meng Gao,Yuesi Wang,Qiang Zhang,Xiao Lu,Zirui Liu,Qiming Zhou,Yang Yang,Yuxuan Zhang,Gregory R Carmichael,Chen Wang,Bin Zhu,Jianlin Hu,Hong Liao

doi:10.5194/acp-21-11405-2021

Abstract

Abstract. 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 (Eab) 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 (Eab) 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. PM2.5 and O3 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 PM2.5 concentrations but near-surface O3 concentrations were enhanced in the North China Plain. These results suggest that current measures to control SO2, NOx, 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 O3 might be a side effect of current emission control strategies. How to control emissions to offset this side effect of current emission control measures on O3 should be an area of further focus.

Highlights

Black carbon (BC) in the atmosphere is produced both naturally and by human activities, attributable to the incomplete combustion of hydrocarbons (Bond et al, 2013; Ramanathan and Carmichael, 2008)
We address the following questions using the Asia-Pacific Economic Cooperation (APEC) event as a case study: (1) how did emission reductions affect the aging processes and light absorption of black carbon (BC) during APEC; (2) what were the relative contributions of reduced mass concentrations of BC, aging processes of BC, and the reshaped mixing state of BC to the changes in light absorption of BC during APEC; and (3) how did these processes affect BC-PBL interactions and the formation of air pollution? In Sect. 2, we describe the WRF-Chem model configurations and the observational data sets used in this study
absorption aerosol optical depth (AAOD) is still underestimated by the model, which might be caused by missing sources of absorbing particles in the model

Summary

Introduction

Black carbon (BC) in the atmosphere is produced both naturally and by human activities, attributable to the incomplete combustion of hydrocarbons (Bond et al, 2013; Ramanathan and Carmichael, 2008). In addition to contributing to particulate matter and degraded air quality, it is the dominant absorber of visible solar radiation, playing a unique and pivotal role in the Earth’s climate system (Bond et al, 2013; Menon et al, 2002; Ramanathan and Carmichael, 2008; Yang et al, 2019). The direct radiative forcing of atmospheric black carbon was estimated to be 0.4 W m−2 (0.05–0.8 W m−2) (IPCC, 2014), and BC has been targeted in emission control policies to mitigate both air pollution and global warming (Grieshop et al, 2009). Mean BC direct radiative forcing in China is ∼ 1.22 W m−2, more than three times the global mean forcing (Li et al, 2016), two-thirds to threefourths of which were contributed by local emissions of BC in China, and the rest by emissions in other countries (Li et al, 2016; Yang et al, 2017)

Objectives

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Results

Conclusion