Effects of Organic Phase States on the Amine-ammonium Exchange Reactions of Ammonium Sulfate Particles

To understand the impacts of atmospheric aerosol particles on the environment, much recent research has focused on the secondary organic aerosol (SOA) formation. Current models over-simplify SOA formation by assuming that (1) gas-to-particle conversion is instantaneous since particles are assumed to be in a liquid phase; (2) the conversion is purely physical partitioning without involving any chemical reaction once the organic vapors are in the particle phase. However, there is growing evidence that atmospheric particles are complex mixtures of inorganics and organics in different phase states, and that chemical reactions in the particle phase can be a significant contributor to SOA formation. In this study, we propose to investigate the effects of particulate organics at different phase states (liquid, solid, and semi-solid) mixed with ammonium sulfate on the reactive uptake of gas phase amines by ammonium sulfate particles. Amines, as an important class of atmospheric organic nitrogen species, were found to efficiently replace ammonium ions which are commonly found in atmospheric particles, and lead to the incorporation of amine vapors into particle phase. Previous studies have often used pure ammonium salts to study this reaction. The difference in phase states (hence viscosity and diffusivity) may affect the reaction kinetics of amine-ammonia exchange reactions. The objective of the current study is to understand whether, and to what extent, organics will suppress (e.g., for solid and semi-solid organics) or enhance (e.g., for liquid organics) amine-ammonium exchange reactions. An electrodynamic balance (EDB) coupled with Raman spectroscopic and light-scattering measurements will be employed to measure mass and compositional changes of the particles. Glycerol, sucrose, and oleic acid (without and with ozone exposure) are chosen as the organics to mix with ammonium sulfate. They represent a wide range of phase state, viscosity, and hydrophilicity of organics. Methylamine and dimethylamine, commonly observed in atmosphere, are chosen as the organic vapors to react with ammonium sulfate. Uptake kinetics (or coefficients) in these systems will be investigated under different relative humidity (RH) conditions. The results of the current study will be helpful in understanding how the phase, viscosity (thus diffusivity), and hydrophobic interactions between organic vapors and particulate organics affect the reactive uptake of amines into ammonium sulfate particles. Model representation of SOA formation will be improved with more information of the effects of phase states on multi-phase processes.