Aerosols produce a direct climate forcing by scattering and absorbing solar and infrared radiation, and an indirect forcing by altering cloud processes. Since human activity causes significant increases in the concentration of atmospheric aerosols, it is important to understand the effects of aerosols on climate. Despite considerable progress in our understanding of aerosols-climate interactions, climate forcing by mineral dust aerosols is still one of the most uncertain processes in current climate models (IPCC, 2007). Therefore, it is important to study the lifting and transport of dust aerosols so that aerosol climate forcing can be assessed accurately.

Intellectual merits: Mineral dust aerosols are lifted (ejected into the atmosphere) by saltation and transported away from the surface by convective plumes (in particular, their updrafts), convective vortices (dust devils), dust storms (dust lifting by gust fronts), and large-scale winds (wind-blown dust). So far most research has focused on studies of dust lifting by large-scale processes such as dust storms and wind-blown dust because they are the most important contributors to the global aerosol budget. However, recent studies suggest that small-scale processes such as convective plumes and vortices also play an important role on the global mineral dust budget. This suggestion is consistent with evidence that variations in the annual dust cycle of West African dust "hot spots" are strongly correlated with variations in small-scale wind gusts produced by dry boundary layer convection, but not with variations in the mean surface wind.

In order to fully understand aerosols-climate interactions we must understand the basic physical processes involved in saltation and their variability with soil and weather conditions. This project will achieve this goal by studying saltation and dust lifting in Nevada, an area where small-scale dust lifting processes are ubiquitous, the Owens Dry Lake, one of the most important dust source areas in the U.S.

Broader impacts: The research has the potential to have a broad impact on science and society. Besides studying an important scientific problem key to our understanding of global climate change, we will make our data and models available to the scientific community. We have an educational website that provides simple physical explanations for the formation of convective plumes, dust devils, and dust storms. The website will include descriptions of the weather and climate of Nevada, the Owens Dry Lake. Finally, this project will contribute to the training of graduate students.

Project Report

Mineral dust emissions are second only to sea salt emissions in their contribution to the global aerosol budget. Dust aerosols are primarily lifted from the surface by saltation, a process by which sand particles are forced to move by the wind and bounce on the surface, ejecting the smaller, harder to lift, dust particles into the air. Therefore, a good understanding of saltation is essential for improving our physical understanding of the aerosol budget and its variability. The Aerosols-Climate Interaction (ACI) project studied saltation and dust lifting in weather systems ranging from small to large-scales, to test a physically based model of saltation (COMSALT) developed under a previous NSF project, and to study the effects of electric fields on dust lifting. The ACI Project conducted simultaneous measurements of the processes that force saltation and eject dust into the air, obtaining all data necessary to test models of saltation and dust lifting in one of the largest sources of dust in US, the Owens Lake salty playa in California. The ACI project developed an autonomous instrument suite to measure saltation, dust lifting and dust electrification. A cell phone modem was used to transmit the data in real time to a server at the University of Michigan. The instrument suite includes a patent protected electric field sensor developed with funding from NSF. This sensor is unique because it is capable of measuring both DC and AC electric fields accurately even when the sensor is subject to the impact of charged dust particles. The development of this technology lead to the creation of Electric Field Solutions Inc., a startup company that is currently being acquired by a public company. A key scientific outcome of this project was the discovery that the vertical fluxes of dust are higher when the soil of the Owens Lake salty playa is moister. The fact that the dust flux is higher when the soil is moister is surprising because classical theories for saltation predict increases in the threshold friction velocity with increases in soil moisture. However, this unexpected dependence is consistent with qualitative observations in other salty playas. The detailed measurements made during the ACI project provide motivation for two hypotheses capable of explaining our unexpected result.

National Science Foundation (NSF)
Division of Atmospheric and Geospace Sciences (AGS)
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A. Gannet Hallar
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University of Michigan Ann Arbor
Ann Arbor
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