Wet deposition of black carbon: A synthesis

Abstract

Black carbon (BC) influences the global radiation budget and adversely affects air quality and human health. Removal of BC from the atmosphere can mitigate these effects. Although wet deposition (rainfall) is thought to be a dominant pathway for BC removal over land and oceans, studies quantifying rainfall BC concentrations and deposition are scarce. Here, we synthesize empirical measurements of BC in rainfall conducted between 1984 and 2016 from sites around the world. Data from 20 peer-reviewed publications and 42 unique sites were compiled. All observations were from the northern hemisphere, with the vast majority located in mid-latitudes with medium to high annual rainfall. Differences in rainfall BC concentrations and deposition among urban, rural, and remote sites were examined. Annual rainfall BC concentrations ranged 120-fold, from 2.8 μg L−1 at a remote site to 333 μg L−1 at an urban site, while annual BC deposition ranged 16-fold from 0.014 mg m−2 d−1 at a remote site to 0.22 mg m−2 d−1 at an urban site. The small number of observations, high measurement variability, and location relative to coast appeared to mask the effects of land use on BC in rainfall. However, characteristic seasonal patterns in BC concentrations and/or deposition were found for geographic regions with similar rainfall patterns, BC emission sources, and air mass trajectories. Annual rainfall BC deposition across sites was comparable to modeled estimates for the global oceans, but BC concentrations were somewhat higher than those measured in surface snow at central North American sites. Neither tropical regions nor urban areas (BC-emission hotspots) were well represented in the dataset, highlighting fundamental gaps in understanding of rainfall BC deposition. More extensive monitoring of rainfall BC deposition is imperative to develop improved global climate and carbon cycling models.