Cooperative Institute for Research in Environmental Sciences

a few recent graduates
Chemistry/Biochemistry
Environmental Studies
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Elizabeth K. Frinak, Ph.D.

Advisor: Margaret A. Tolbert, Chemistry and Bio-chemistry
Funding: National Science Foundation

Laboratory Studies of Surfaces Representative of Mineral Dust in the Troposphere

Mineral dust can be lofted into the Earth's atmosphere and transported on a global scale. While in the atmosphere, these particles may be exposed to varying conditions of relative humidity and may serve as reactive surfaces for trace gases such as nitric acid. Thus the dust particles age as they are transported, and may change from relatively inert particles to particles of higher toxicity. Additionally, the initial dry particles may have little impact on atmospheric chemistry or climate, while the aged particles could promote heterogeneous chemistry or be better able to nucleate cloud droplets. This dissertation examines the aging process for two components of mineral dust aerosol. Specifically, laboratory studies were performed on the mineral gamma-iron oxide and on the clay sodium-montmorillonite.

The laboratory studies used a high vacuum chamber equipped with transmission Fourier transform infrared spectroscopy to probe the dust samples and mass spectrometry to simultaneously probe the gaseous species such as water and nitric acid. Prior to these studies, there was a disagreement of three orders of magnitude in the literature about the rate at which nitric acid was taken up by clay surfaces. To improve upon the past work, initial efforts focused on measuring the specific surface area of the dust samples in situ. This measurement eliminated an assumption about the specific surface area and resolved the existing controversy in the literature.

Experiments showed little uptake of nitric acid on mineral aerosol surfaces at low relative humidity. For example, at 0% relative humidity, studies suggest that the lifetime of nitric acid in the troposphere is 65 days due to heterogeneous loss on iron oxide. This lifetime is too long to represent a significant loss mechanism. However, the uptake can change dramatically as water vapor increases. For example, at 44% relative humidity, work suggests that the lifetime of nitric acid is only two days with respect to heterogeneous loss to sodium-montmorillonite. This timescale is short enough to be of significance in the troposphere. The strong dependence on relative humidity is likely due to increased water in the clay. In fact, it was found that at 50% relative humidity, the clay contains 10% water by mass. The results determined in this work suggest mineral dust as a potentially significant reactive surface in the troposphere. The magnitude of the impact is dependent on factors such as the composition of the dust and the relative humidity.