The objective of this project is to determine the heterogeneous reactivity of natural mineral aerosol components, representative of chemically-processed and freshly-emitted mineral aerosol, with single atmospheric gases and with gaseous mixtures over atmospherically relevant temperature and humidity ranges. A low-pressure, low-temperature Diffuse Reflectance Infrared Fourier Transform Spectrometer (DRIFTS) equipped reaction chamber will be used to carry out laboratory studies. The chamber system allows for in situ, simultaneous gas- and condensed-phase measurements of the heterogeneous uptake of reactive gases and gaseous mixtures on thin films of mineral components representative of those observed in the atmosphere. Heterogeneous uptake will be studied experimentally as a function of temperature, relative humidity and reactive gas concentration. Associated theoretical studies will (a) complement experiments by interpreting infrared spectra and identifying intermediate and terminal adsorbed reaction products, providing additional mechanistic detail, (b) model electronic properties of the mineral dust components studied and determine the effects of metal impurities on surface reactivity, and (c) determine how heterogeneous reactions on mineral dust surfaces alter these electronic properties.
Results of the studies will provide important mechanistic, kinetic and thermodynamic details needed for incorporation into current atmospheric chemistry models in order to more accurately model the effects of heterogeneous reactivity (impacts on tropospheric chemistry), surface composition (impacts on health, climate and further reactivity) and water adsorption (impacts on visibility and climate) in the Earth's atmosphere. The work will extend beyond previous studies of mineral dust aerosol heterogeneous chemistry that were limited to heterogeneous uptake under dry conditions, room temperature and in the presence of only a single reactive gas. The project will foster a unique partnership between a large university and a small liberal arts college. This relationship will cultivate experiential learning via active educational research opportunities for undergraduate students and aid in the recruitment of well-trained, science-oriented students at the undergraduate and graduate levels. Beyond traditional dissemination, research results will be incorporated into undergraduate teaching curricula at Hendrix College. Additionally, hands-on workshops and laboratory experiments will be conducted at Hendrix College using the DRIFTS system.
Mineral dust aerosol emitted from arid and semiarid regions of the world can be transported globally and thus have significant impacts on global climate, atmospheric chemistry, biogeochemical cycles, visibility and human health. Many of these effects are altered by the presence of surface water and atmospheric trace gases reacting on the mineral surfaces. The awarded project aimed to further our current understanding of gas/surface interactions on mineral dust aerosol and their effects on the environment. Specifically, experimental and theoretical methods were used to study gas/surface interactions on macroscopic and microscopic scales. Experimental measurements to study gas/surface interactions were conducted on a newly designed and validated infrared reaction chamber capable of simultaneously monitoring the condensed (surface) and gas phases. Reactive gas and water vapor adsorption measurements were conducted on common mineral dust components, including silica and clay minerals. Theoretical studies were conducted in order to identify surface reaction products of the experimentally studied reactions and determine the effect of natural impurities on the reactivity of the mineral surfaces with various acidic atmospheric gases as natural mineral dust components are rarely found in their pure states. Experimental studies of water adsorption using combined experimental and theoretical techniques illustrated that water layers on the surface of mineral dust particles enable crustal aerosols to take up significant amounts of water and efficiently grow to large enough sizes to form cloud droplets under atmospherically relevant conditions and thus contribute to atmospheric cooling effects on climate as well as impact precipitation patterns. Experimental studies of reactive gas uptake, including organic acids, on mineral surfaces with and without water vapor or in the presence of other reactive surface species were conducted to mimic aged or chemically-reacted mineral surfaces. These studies showed that the mechanism of surface adsorption, either physically or chemically bound, depends on the presence of other adsorbing species (ex: water vapor or nitric acid vapor). Gas/surface reactions of this kind alter the chemical composition and reactivity of the mineral particles, and thus the atmospheric impacts. Computational methods aided in identification of surface bound products on a molecular scale. Computational studies were performed to elucidate the mechanisms of atmospheric heterogeneous surface interactions on common mineral components, including aluminum and magnesium oxides, containing natural impurities. The surface reactivity and properties of these surfaces were found to be altered with the impurities present near the surface layer. The results showed that natural impurities within mineral dust particles can alter gas adsorption capacity, photochemical processing and reactive uptake of atmospheric trace gases, therefore affecting atmospheric chemistry and climate. In general, the findings of this work contributes to the field of atmospheric chemistry by providing a better understanding of the role of mineral dust aerosol on the chemistry and climate of the Earth's atmosphere and expands our current knowledge of mineral dust water adsorption, surface chemistry and structure, and changes in chemical composition due to heterogeneous reactivity.
