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Volume 17, issue 12 | Copyright
Atmos. Chem. Phys., 17, 7459-7479, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 21 Jun 2017

Research article | 21 Jun 2017

Effects of the Wegener–Bergeron–Findeisen process on global black carbon distribution

Ling Qi1,2, Qinbin Li1,2, Cenlin He1,2, Xin Wang3, and Jianping Huang3 Ling Qi et al.
  • 1Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA
  • 2Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, CA, USA
  • 3Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, College of Atmospheric Sciences, Lanzhou University, Lanzhou, China

Abstract. We systematically investigate the effects of Wegener–Bergeron–Findeisen process (hereafter WBF) on black carbon (BC) scavenging efficiency, surface BCair, deposition flux, concentration in snow (BCsnow, ng g−1), and washout ratio using a global 3-D chemical transport model (GEOS-Chem). We differentiate riming- versus WBF-dominated in-cloud scavenging based on liquid water content (LWC) and temperature. Specifically, we implement an implied WBF parameterization using either temperature or ice mass fraction (IMF) in mixed-phase clouds based on field measurements. We find that at Jungfraujoch, Switzerland, and Abisko, Sweden, where WBF dominates in-cloud scavenging, including the WBF effect strongly reduces the discrepancies of simulated BC scavenging efficiency and washout ratio against observations (from a factor of 3 to 10% and from a factor of 4–5 to a factor of 2). However, at Zeppelin, Norway, where riming dominates, simulation of BC scavenging efficiency, BCair, and washout ratio become worse (relative to observations) when WBF is included. There is thus an urgent need for extensive observations to distinguish and characterize riming- versus WBF-dominated aerosol scavenging in mixed-phase clouds and the associated BC scavenging efficiency. Our model results show that including the WBF effect lowers global BC scavenging efficiency, with a higher reduction at higher latitudes (8% in the tropics and up to 76% in the Arctic). The resulting annual mean BCair increases by up to 156% at high altitudes and at northern high latitudes because of lower temperature and higher IMF. Overall, WBF halves the model–observation discrepancy (from −65 to −30%) of BCair across North America, Europe, China and the Arctic. Globally WBF increases BC burden from 0.22 to 0.29–0.35mg m−2 yr−1, which partially explains the gap between observed and previous model-simulated BC burdens over land. In addition, WBF significantly increases BC lifetime from 5.7 to  ∼ 8 days. Additionally, WBF results in a significant redistribution of BC deposition in source and remote regions. Specifically, it lowers BC wet deposition (by 37–63% at northern mid-latitudes and by 21–29% in the Arctic), while it increases dry deposition (by 3–16% at mid-latitudes and by 81–159% in the Arctic). The resulting total BC deposition is lower at mid-latitudes (by 12–34%) but higher in the Arctic (by 2–29%). We find that WBF decreases BCsnow at mid-latitudes (by  ∼ 15%) but increases it in the Arctic (by 26%) while improving model comparisons with observations. In addition, WBF dramatically reduces the model–observation discrepancy of washout ratios in winter (from a factor of 16 to 4). The remaining discrepancies in BCair, BCsnow and BC washout ratios suggest that in-cloud removal in mixed-phased clouds is likely still excessive over land.

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Short summary
Black carbon (BC) is the second only to CO2 in heating the planet, but the simulation of BC is associated with large uncertainties. BC burden is largely underestimated over land and overestimated over ocean. Our study finds that a missing process in current Wegener–Bergeron–Findeisen models largely explains the discrepancy in BC simulation over land. We call for more observations of BC in mixed-phase clouds to understand this process and improve the simulation of global BC.
Black carbon (BC) is the second only to CO2 in heating the planet, but the simulation of BC is...