UC Irvine

Atmospheric aerosols reduce visibility, adversely affect human health, and influence the climate. However, modeling the evolution of these systems is complicated by the large number of species involved and their complex interactions in both the gas and particle phase. To reduce this complexity, key species and generalized mechanisms of particle formation, growth and evolution are sought to aid development of computationally-feasible regional and global atmospheric models. These models, alongside simple, accurate measurement techniques for particles and key gas-phase precursors, will improve predictions of the effects of changing emissions on visibility, climate and human health.

This dissertation reports work identifying a new source of particles in the atmosphere, those formed from methanesulfonic acid (MSA), amines and water. Experiments performed in an aerosol flow reactor with MSA, water vapor, and dimethylamine or trimethylamine demonstrate that this system rapidly forms particles, and that all three species (MSA, water and amine) are required for significant particle formation. A simplified kinetics mechanism for particle formation from this system was developed based on these experiments and theoretical calculations of small clusters of these species, performed by the Gerber group. Predictions of particle formation from this mechanism agree well with our measurements, making this computationally inexpensive calculation useful for large-scale atmospheric models. A more comprehensive aerosol model is under development to test this mechanism while incorporating the effects of wall losses, coagulation between particles, and growth based on particle-phase activity.

Predicting particle formation in the laboratory and the real atmosphere requires accurate measurement techniques for the gas-phase precursors. A major limitation to many existing techniques for measuring gas-phase amines is rapid loss of the analyte to instrument surfaces, such as inlet tubing. A new technique for measuring gas-phase ammonia and amines was developed that involves collection of the sample on weak ion-exchange resin in a cartridge designed to minimize the exposure of the incoming sample to surfaces prior to uptake on the resin. These cartridges are simultaneously extracted and analyzed by ion chromatography, using a novel instrument configuration designed to lower the detection limit of this method to the parts-per-trillion level in air for a 60 min sample. This technique using inexpensive, reusable cartridges is shown to efficiently measure ammonia and three aliphatic amines.

Unlike MSA, ammonia and amines have high vapor pressures and must be neutralized to remain in the particle phase. This opens the possibility of complex displacement reactions between gas- and particle-phase ammonia and amine species. Experiments were performed in collaboration with scientists at PNNL's EMSL User Facility using the SPLAT-II single-particle mass spectrometer to investigate the displacement of ammonia and amines in ammonium- and aminium-methanesulfonate particles on addition of a different gas-phase amine. The results suggest a complex system where the effects of particle-phase ammonium or aminium on hygroscopicity and particle phase result in different degrees of displacement by gas-phase amines.

The work presented here will aid in the development and evaluation of regional and global atmospheric models, by identifying and quantifying new sources of particles, understanding complex reactions affecting particle growth, and facilitating simple, accurate measurements of gas-phase particle precursors in the field.