Mineral dust aerosol size distribution change during atmospheric transport

Airborne mineral dust can impact visibility, climate, biogeochemical processes, and possibly human health. The magnitude of the impact of dust depends on particle size. We measured the size distributions of airborne mineral dust over the Canary Islands during July 1995 and Puerto Rico during July 2000. Dust size distributions do not appear lognormal. Stokes gravitational settling overestimates losses of large dust particles during atmospheric transport from North Africa over the tropical North Atlantic and Caribbean. Normalized mineral dust size distributions of particles smaller than 7.3 μm over the Canary Islands and Puerto Rico were indistinguishable, indicating these particles were not preferentially removed during atmospheric transport. However, mineral dust aerosols larger than 7.3 μm were preferentially removed during atmospheric transport. Larger particles were more efficiently removed. A simple empirical model setting the vertical velocity of dust particles equal to the Stokes gravitational settling velocity minus an upward velocity of ∼0.33 cm s−1 accurately predicts changes in dust size distribution during atmospheric transport. Thus it appears some atmospheric process(es) partially counteracts gravitational settling.