Precipitation removal of ultrafine aerosol particles from the atmospheric boundary layer

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Abstract
Ultrafine particles (UFP) formed in the boundary layer (BL) by nucleation processes need to grow up to a diameter dp ∼ 100 nm to become activated as cloud droplets (CD). The time required to reach dp = 100 nm is about 2–3 days for a typical growth rate of 5 nm h−1. If precipitation occurs, most UFP are too small to become CD, and some particles are removed by scavenging processes. A model to estimate the UFP wet removal from the BL by rainfall and coagulation is presented. The scavenging coefficient that describes the decay of aerosol mass in various size bins is a function of aerosol size (dp), rainfall rate (R), and BL background aerosol. The model is applied to determine the UFP 0.5‐folding time (t05) during rain events and results show that t05 ∼ 1 hour for R ∼ 1 mm h−1 for newly created particles (dp < 10 nm) and t05 ∼ 1 day for larger UFP (dp ∼ 10–100 nm). To infer the likelihood of UFP removal at a given location, the average time interval (Δt) between rain events with rainfall rate R = 1 mm h−1 at stations with different precipitation regimes was determined. Results show that on average, UFP are very effectively removed from the BL by below‐cloud scavenging in tropical regions (Δt ∼ 1 day), removed to a significant extent in eastern U.S. regions (Δt ∼ 3 days), and are less likely to be removed in southwest U.S. regions (Δt ∼ 6–8 days), where the frequency of dry periods is high and UFP have sufficient time to grow.