Physical and chemical uptakes of organic vapors by aerosol particles

Aerosols are complex mixtures of inorganic and organic constituents that exert pronounced effects in global climate, regional visibility and human health. Currently, atmospheric models have greatly underestimated the burden of secondary organic aerosols (SOA) partly because of inadequate understanding of the partitioning and heterogeneous reactions of organic species. This work employed a tailor-made Raman spectroscopy system coupled with an electrodynamic balance (EDB) to examine various physical and chemical uptake processes of organic vapors relevant to SOA formation. The masses, chemical compositions and phase states of particles under investigation were simultaneously monitored for an extended period of time (~24 hours) to capture the crucial processes that result in new mass formation. The measured partition coefficients ( Kp) of alcohols in oleic acid droplets matched with the Pankow's absorptive partitioning model and verified the technique. Further, the roles of particle chemical nature and phase states in the heterogeneous reactive uptake were extensively investigated. Particles of sulfuric acid and their mixtures with hydrophobic (oleic acid) or hydrophilic (levoglucosan) organics were allowed to react with nonanal vapor. The initial uptake coefficient of oleic acid/sulfuric acid mixture was much larger than that of pure sulfuric acid showing that the presence of hydrophobic organic materials in particles enhanced the reactive uptake of nonanal. The ozonolysis of maleic acid and the displacement of ammonium by amines in aqueous droplets or in crystalline solids were investigated. The compact solids suffered from substantial mass transfer limitation and the reaction rates were remarkably lower than those of aqueous droplets. The results of the active displacement of ammonium in aqueous particles by various short-chain amines may be related to the field-observed aminium ions in samples during high humidity or fog/rain events.