Airborne mineral particulates provide unique surfaces on which atmospheric gases, including natural emissions and anthropogenic pollution, may react, thereby affecting air quality and climate-relevant processes. Laboratory experiments are used to quantify the extent and products of such chemistry on these ubiquitous mineral surfaces, providing parameters for inclusion in computational atmospheric models. Experiments supported by this grant systematically increased the complexity of laboratory systems to more accurately replicate the chemistry of ambient mineral dust. In particular, we found that monoterpenes, which are abundant biogenic volatile organic compounds (BVOCs) responsible for a significant fraction of secondary organic aerosol (SOA), readily react with mineral dust surfaces forming surface-bound products that alter the chemical and physical properties of these airborne particulates.  Specific outcomes related to the intellectual merit of this project are:

1. Mineral dust surfaces readily react with gaseous limonene forming less-volatile surface-adsorbed products, such as terpineol, that contain carbon-carbon double bonds making them susceptible to further reactions with common pollutants. Surface-bound organics formed as a result of these reactions do not desorb on the time scale of weeks and therefore can permanently alter mineral surface chemistry. Several atmospheric variables influence the rate of limonene uptake on mineral surfaces. We found that (1) increasing relative humidity correlates with a decrease in limonene uptake consistent with adsorbed water blocking reactive surface sites, and (2) “chemical weathering” of mineral aerosol by exposure to gaseous nitric acid, a prevalent pollutant, resulted in a three-fold increase in uptake of limonene on mineral dust. 

2. The abundant monoterpene pinene and oxidation product pinene oxide readily react with mineral dust surfaces. Our analysis indicated that the formation of surface-adsorbed products decreased with increasing relative humidity and increased in reactivity from pinene < pinene oxide. In contrast to the limonene reaction products, both pinene and pinene oxide resulted in very low volatile high molecular weight dimers that can significantly impact SOA quantity and mass. For pinene, dimer products include C20-hydrocarbons, while high molecular weight products from pinene oxide reactions were more diverse as a result of aldol condensation and hemiacetal reactions.

3. Surface-bound organics are a ubiquitous component of airborne mineral particles. To quantify the impact of such organics on mineral dust chemistry, we systematically studied the decomposition of ozone on mineral dust pre-exposed to limonene and pinene. Surface-bound organic products, as described above, increased ozone decomposition by nearly two orders of magnitude, which is attributed to organics that contain alkene functionalities susceptible to ozonolysis. These results demonstrate the significant impact adsorbed organics can have on ozone reactivity on mineral aerosol, which should be accounted for in atmospheric modeling studies, especially of polluted urban regions.

4. The oxidation of aqueous organics in atmospheric droplets by gaseous ozone presents a potential pathway for the formation of secondary organic aerosol (SOA) in clouds and fogs. We investigated the oxidation of aqueous terpineol, an important BVOC, with gaseous ozone and found that the reaction products and kinetics differed from the previously reported gas phase chemistry. Our kinetic results indicate that aqueous ozonolysis will be competitive with aqueous oxidation by hydroxyl radicals (OH) and ozonolysis of gaseous terpineol. An important product of the aqueous ozone reaction was hydrogen peroxide, which can further impact droplet chemistry. 

5. Nitrated aromatic compounds strongly absorb visible radiation, and therefore, when found in the atmosphere, can influence earth’s energy balance. The absorption spectra of nitrophenols in clouds depend on droplet pH. We determined that aqueous dinitrophenol aerosols at pH = 5.5 will absorb three times of solar energy than at pH = 3.5. Nitrophenols bound to mineral dust surface also exhibit red-shifted absorption spectra and will absorb twice as much solar energy on a per molecule basis than aqueous or gaseous nitrophenols. These results indicate that in order to accurately account for the impact of nitrophenols on atmospheric radiative processes, it is important to characterize the exact matrix in which they are found.

Broader impact outcomes of this project supported the training and professional development of twelve undergraduate students, one high school science teacher, and two high school students. Research training for the twelve undergraduate students also supported ten student presentations at national and regional American Chemical Society meetings and resulted in seven students published as co-authors on peer-reviewed articles. Six undergraduate students wrote and defended bachelor’s honors theses. Professional development for the high school teacher included three summers of full-time research and two articles published as co-author. The research laboratory infrastructure supported by this project also facilitated research experience for two high school students, including an ACS Project SEED fellow.

Last Modified: 10/18/2018
Modified by: Ryan Z Hinrichs