Cavity ring down: Construction, validation and experiments measuring the optical extinction of pure clays and their mixtures with salts and carboxylic acids under a range of atmospherically relevant conditions

Publication Year:
2012
Usage 5
Downloads 5
Repository URL:
https://scholars.unh.edu/dissertation/647
Author(s):
Attwood, Alexis Rae
Tags:
Chemistry; Physical; Atmospheric Chemistry
thesis / dissertation description
Aerosols interact directly with solar radiation by scattering and absorbing incident light to affect the Earth's climate system. Models indicate that aerosols cool the Earth counterbalancing greenhouse gases, but contribute the largest uncertainty to anthropogenic effects on climate. To address these uncertainties, a cavity ring down spectrometer (CRD) was built and performance tests were conducted to validate its capabilities for measuring optical properties of laboratory generated, atmospherically relevant aerosols over a range of conditions. The CRD provides an absolute and unbiased in situ measurement of total extinction and can reduce uncertainties associated with aerosols having unusual chemical and physical properties.Mineral dust particles are a large fraction of the total aerosol mass and thus contribute extensively to the climate balance. Assessing this impact is complicated by the irregular shape and sheet-like structures of these species making them difficult to variability in measurements of optical properties can result from water uptake and changes in chemical composition due to interactions with the immediate environment. Three different clay proxies for mineral dust and relevant mixtures were selected for study. First, pure kaolinite, montmorillonite and illite were characterized as a function of relative humidity (RH) revealing differences when compared to Mie theory generated from literature values for size or mass change. Second, internal mixtures of largely insoluble montmorillonite with sodium chloride or ammonium sulfate were studied and a lowering of the optically observed deliquescence RH was observed in comparison to the pure salts. Further, experimental results were compared with Mie theory generated from volume weighted mixing rules to reveal deviations near deliquescence. Lastly, optical properties of montmorillonite internally mixed with atmospherically relevant dicarboxylic acids were used to simulate atmospheric processing and compared to freshly emitted, pure montmorillonite aerosols under dry and humid environments. In all cases, the optical properties measured by cavity ring down have been used to obtain a better estimate of the magnitude of the climate forcing caused by mineral dust aerosols under a range of conditions.