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The role of mineral aerosols in climate and biogeochemistry

Much of my research focuses on understanding the role of mineral aerosols in climate and biogeochemistry. Mineral aerosols, or desert dust, are small soil particles suspended in the air. These aerosols have as sources, unvegetated, dry regions with easily erodible soils and strong winds. Ice core records of mineral aerosols show that at high latitudes, mineral aerosols were 2-20 fold higher during cold time periods (e.g. last glacial maximum or ice age) than warm periods (like today). Marine sediment cores suggest global changes of 3-4 fold between cold and warm periods. Thus mineral aerosols fluctuate greatly in time. These observations suggest stronger winds, less vegetated surfaces and/or decreases in precipitation, and indicate that mineral aerosols are very sensitive to climate. Over the past 35 years, we have seen large fluctuations in mineral aerosols coming off of North Africa, due to natural variability in precipitation, and/or due to human activities. Part of my work focuses on understanding the causes of variability in the current climate, and the role of humans in changing mineral aerosols.

Mineral aerosols impact the climate directly through changes in radiation. Mineral aerosols both scatter and absorb incoming solar radiation and outgoing planetary radiation. Also, mineral aerosols may act as cloud condensation nuclei or ice nuclei, and thus change the properties of clouds. Changing properties of cloud change both precipitation patterns as well as change the radiative balance of the atmosphere. Using NCAR's community climate system model, we look at the impacts of mineral aerosols on climate including the direct radiative effect. One study, by my graduate student, Masaru Yoshoika, shows that desert dust may be playing an important role in maintaining the 1970s-current Sahel drought, which has impacted millions of people's lives.

The deposition of mineral aerosols onto oceans and land surfaces changse the nutrients available to ecosystems. In the oceans, iron inputs from the atmosphere to the ocean are thought to be important to ocean biota, leading to regions with high nutrients and low chlorophyll (HNLC) in the eastern tropical Atlantic and Southern Oceans. Studies have shown that adding iron to these regions leads to increases in productivity. Additionally, nitrogen fixing organisms in the ocean capable of converting ubiquitous molecular nitrogen into biological useful fixed nitrogen compounds are thought to have iron requirements which are much larger than most ocean biota. This is thought to be the reason that nitrogen fixation appears to occur predominately downwind of the largest dust source in North Africa. In collaboration with oceanographer colleagues, we have shown that desert dust changes may cause large changes in ocean uptake of carbon dioxide. We have also worked on modeling the bioavailability of the iron deposited to the ocean surfaces, which appears to increase as the mineral aerosols move away from the source areas. On the land surface, inputs of phosphorus and other nutrients within the mineral nutrients are hypothesized to maintain biological productivity in regions with well weathered soils such as tropical forests.

For additional information, visit my homepage at: http://www.cgd.ucar.edu/tss/staff/mahowald/index.html.