Recent Advances in Quantifying Wet Scavenging Efficiency of Black Carbon Aerosol

Black carbon (BC) aerosol is of great importance not only for its strong potential in heating air and impacts on cloud, but also because of its hazards to human health. Wet deposition is regarded as the main sink of BC, constraining its lifetime and thus its impact on the environment and climate. However, substantial controversial and ambiguous issues in the wet scavenging processes of BC are apparent in current studies. Despite of its significance, there are only a small number of field studies that have investigated the incorporation of BC-containing particles into cloud droplets and influencing factors, in particular, the in-cloud scavenging, because it was simplicitly considered in many studies (as part of total wet scavenging). The mass scavenging efficiencies (MSEs) of BC were observed to be varied over the world, and the influencing factors were attributed to physical and chemical properties (e.g., size and chemical compositions) and meteorological conditions (cloud water content, temperature, etc.). In this review, we summarized the MSEs and potential factors that influence the in-cloud and below-cloud scavenging of BC. In general, MSEs of BC are lower at low-altitude regions (urban, suburban, and rural sites) and increase with the rising altitude, which serves as additional evidence that atmospheric aging plays an important role in the chemical modification of BC. Herein, higher altitude sites are more representative of free-tropospheric conditions, where BC is usually more aged. Despite of increasing knowledge of BC–cloud interaction, there are still challenges that need to be addressed to gain a better understanding of the wet scavenging of BC. We recommend that more comprehensive methods should be further estimated to obtain high time-resolved scavenging efficiency (SE) of BC, and to distinguish the impact of in-cloud and below-cloud scavenging on BC mass concentration, which is expected to be useful for constraining the gap between field observation and modeling simulation results.