Keywords :
Wegener-Bergeron-Findeisen
Global Black Carbon
บทคัดย่อ :
We systematically investigate the effects of Wegener-Bergeron-Findeisen (WBF) on BC scavenging efficiency, surface BCair, deposition flux, concentration in snow (BCsnow, ng g-1 10 ), and washout ratio using a global 3D chemical transport model (GEOS-Chem). We differentiate riming- versus WBF-dominated in-cloud scavenging based on liquid water content
(LWC) and temperature. Specifically, we relate the WBF effect with either temperature or ice mass fraction (IMF) in mixed-
phase clouds. 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
15 observations (from a factor of 3 to 10% and from a factor of 4-5 to a factor of two). 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 20 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 0.29-0.35 mg 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 days to ~8 days. Additionally, WBF results in a significant
25 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 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 30 , BCsnow and BC washout ratios suggest that in-cloud
removal in mixed-phased clouds is likely still excessive over land.
เอกสารอ้างอิง :
Qi, L., Li, Q., He, C., Wang, X., & Huang, J. (2017). Effects of the Wegener-Bergeron-Findeisen process on global black carbon distribution. Atmospheric Chemistry and Physics, 17(12), 7459-7479.
