Quantifying Scavenging Efficiencies of Different Aerosol Species and Size-Resolved Volume Concentrations in Tropical Convective Clouds over the West Pacific

Abstract In-cloud aerosol scavenging remains a large source of model uncertainty, affecting capabilities to capture the aerosol lifetime and impacts on air quality and climate. While past work quantified aerosol scavenging efficiencies (SEs) in midlatitude mixed-phase deep convection, SEs are less well known for shallower convection. We used aircraft data over the tropical west Pacific to calculate SEs for three marine cumuli of different top heights (3–7 km MSL) using a simple entrainment model and measurements of the cloud outflow and nearby clear air. Across cases, efficient scavenging was observed for sulfate (>86%) and black carbon (70%–80%), while organic aerosols (53%–60%) and nitrate (61.5%) were moderately scavenged. Ammonium had a wide SE range (53%–87%). SEs of aerosol volume concentration showed near-total removal of aerosols with diameters greater than 100 nm (>92%) and inefficient removal for aerosols with diameters less than 100 nm (30%–50%), associated with the preferential activation and removal of larger particles. Mass-based SEs did not differ substantially between tropical cumuli and midlatitude deep convection, attributed to the negligible mass activated at higher supersaturations. The efficient scavenging of black carbon (BC) can be explained by an enhanced hygroscopic fraction of BC based on model results from the Community Earth System Model, version 2 (CESM2), Community Atmosphere Model with Chemistry, suggesting the internal mixing of BC with more soluble species during long-range transport through the marine atmosphere. The estimates of BC SEs provide direct evidence of substantial BC removal in convection as inferred by previous work and should motivate improvements in chemical transport models. Significance Statement The scavenging (removal) of aerosols is critical for estimating the lifetime of aerosols and their impact on air quality and climate; however, scavenging efficiency remains uncertain in global models especially for shallow convection. To address this uncertainty, we calculated aerosol scavenging efficiencies in tropical marine cumuli for different aerosol species, including sulfate and black carbon. We find that scavenging efficiencies depend on aerosol composition, size, and to a lesser extent, cloud-top height. The efficient removal of black carbon is explained by high hygroscopic fractions based on a model, related to the mixing of black carbon with more soluble species during transport. The reported scavenging efficiencies can be implemented into chemical transport models to improve model estimates of aerosol removal.