When mineral dust, which is especially abundant in arid regions, exists in conjunction with sulfates and possibly other pollutants, the dust can become coated with sulfate, making them more active as CCN. Such coated dust particles, if in large aerosol particles, could enhance the precipitation efficiency of clouds, and if they are in small particles they may produce cloud growth. An important aspect that needs study in field experiments is the effect of sulfate-coated desert dust on the precipitation process in clouds. To assess the impact of the desert dust we will collect filters in the Middle East that will be analyzed using transmission electron microscopy (TEM). This method provides the unique advantage of providing all the necessary size, shape, composition, crystallographic structure and thus speciation information for phase identification and determination of extent of aggregation. We can determine precise species characterizations and are able to detect and image particles as small as a few nanometers in diameter. Moreover, this information can be obtained on an individual-particle basis.

We propose to assess if local mineral dust in the atmosphere does in fact get coated with sulfate and thereby explore the role of sulfates and dust together. We plan to collect parallel sets of particles on (a) the standard types of “holey-carbon” TEM grids that we normally use for our high-resolution and analytical measurements and (b) grids coated with Ca that react when sulfate or sulfate-coated particles land on them. We propose to use parallel samplers to collect aerosols simultaneously, which will result in two sets of three size-separated uncoated and coated TEM grids. In this way we will assess the abundance of both anhydrous, inorganic particles and those consisting of acid droplets. The result will be greatly increased knowledge of the role of mineral pollution particles in producing environmental and climate effects.