Recent Advances in Quantifying Wet Scavenging Efficiency of Black Carbon Aerosol

Abstract

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.

Direct Quantification of Droplet Activation of Ambient Black Carbon Under Water Supersaturation

“…A mean number fraction of 24% ± 8% (60% ± 10%, 70% ± 9%) was observed to be activated at SS = 0.1% (0.2%, 0.3%), corresponding to a rBC mass activation of 47% ± 9% (74% ± 10%, 79% ± 9%). These values are higher than most previous reports on the BC activation fraction by liquid clouds (Yang et al., 2019). For example, a mass scavenging fraction of 6%–71% was obtained for BC in urban environments (Hallberg et al., 1992; Wang et al., 2014) and 5%–39% in rural environments (Collett et al., 2008; Gilardoni et al., 2014).…”

Amazonian Biomass Burning Enhances Tropical Andean Glaciers Melting

“…BC from wildfires was assumed to be 60% soluble, meaning that in the presence of a cloud comprising 60% aerosols was captured as water in the cloud 32 . For high mountains regions, in-cloud mass scavenging efficiencies (MSEs) of BC were reported to be approximately 0.6 57,58 . In the HYSPLIT model, wet removal was defined as a scavenging coefficient expressed directly as a rate constant and modified by the precipitation rate (P in mm/h).…”

Lifecycle of light-absorbing carbonaceous aerosols in the atmosphere

“…The total (in-cloud and below-cloud) wet removal of aerosols has been long studied by collecting precipitation water and measuring the concentration of particulate species in it, as reviewed by Herckes et al 179 . However, such measurements typically are unable to separate rainout and washout contributions 180 or to identify the location where the scavenging was initiated. Offline analysis on the composition in rainwater was performed to characterize OC/EC concentration 181 , rBC 182 , and to isolate the water soluble and insoluble OC 162 .…”

“…A few studies also investigated the brownness of cloud water or rainwater 183,184 . By comparing BC concentrations in the water and in the ambient air, some studies obtained the wet scavenging efficiency of BC, showing a general consensus that freshly emitted BC has a lower scavenging efficiency than aged BC at remote sites 180 and references therein.…”