Implications of droplet nucleation to mineral dust aerosol deposition and transport

Abstract

Calculations from a microphysics model are shown which indicate the factors that control droplet nucleation scavenging of hydrophilic mineral dust particles over a large range of conditions including the size, chemical composition, and number density of particles in both cumulus and stratus clouds. We focus specifically on the activation threshold radius (ATR) for droplet nucleation which determines the particles that are activated and those available for further transport and subsequent iron deposition to the remote ocean. Results suggest: the ATR is typically found in the range of clay‐sized particles (radius = .1 to 1. μm), a spectrum over which the amount of dust removed declines ∼60% both in surface area and particle number; nucleation of silt‐sized particles (1.–10. μm) occurs under most conditions; larger fractions of mineral aerosols are removed in cumulus clouds than in stratus; and while acid coating of dust particles in polluted environments acts to decrease the ATR, the effect is reduced by competition with soluble aerosols. Regional mineral dust environments exhibit potentially diverse aerosol wet removal impacts. The ATR representative of the tropical Atlantic ocean basin (<.2 μm) indicates ∼80% removal of the total dust surface area, while in the pristine southern hemisphere mid latitudes an ATR ∼.5 μm implies ∼60%. In contrast, varying conditions in the polluted region of East Asia suggest a large ATR spectrum (.2 to 3. μm) with dust surface area removal ranging from >80% to <10%.