Preferential settling of elongated mineral dust particles in the atmosphere

Positions of particles' centers of gravity and folding centers were analyzed for individual dust particles in snow on a high mountain in Japan. Bias of dust particles' centers of gravity was observed: L1 ( the longest distance from the center of gravity to the boundary of particles) is 5% ( of L1, on average) longer than L2 ( 1/ 2 of the longest axis of particles), suggesting that a preferential orientation exists for particles settling heavy side down. Applying that preferential orientation of settling particles to Ginoux's model, the settling velocity for ellipsoids with Reynolds numbers lower than 2 was estimated. The results show that, for long- range transport particles, settling velocity of spherical particles is lower than that of ellipsoids with equal surface area. Our results also indicate that, away from the source regions, dust particles are essentially spherical, which considerably simplify the calculation of settling velocity in transport and of radiative transfer models.