Precipitation removal of ultrafine aerosol particles from the atmospheric boundary layer

Ultrafine particles (UFP) formed in the boundary layer (BL) by nucleation processes need to grow up to a diameter d(p) similar to 100 nm to become activated as cloud droplets ( CD). The time required to reach d(p) = 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 (t(05)) during rain events and results show that t(05) similar to 1 hour for R similar to 1 mm h(-1) for newly created particles (d(p)< 10 nm) and t(05) similar to 1 day for larger UFP (d(p) similar to 10-100 nm). To infer the likelihood of UFP removal at a given location, the average time interval (Delta(t)) 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 (Delta t similar to 1 day), removed to a significant extent in eastern U. S. regions (Delta t similar to 3 days), and are less likely to be removed in southwest U. S. regions (Delta t similar to 6-8 days), where the frequency of dry periods is high and UFP have sufficient time to grow.