Abstract:

Aerosol particles are an important component of the atmosphere, where they influence climate, impact health, and participate in chemical processing of trace species. Recent work has suggested that reactions occurring in droplets with diameters of 10s of μm are accelerated over bulk by factors as large as 10⁶. This phenomenon may be of significance in the atmosphere, where aqueous aerosol particles containing mixtures of organic and inorganic compounds can potentially act as microreactors. An apparatus for optical levitation, single-particle Raman spectroscopy, and brightfield microscopy has been developed to investigate the in situ chemical and physical aging of individual aqueous aerosol particles. I will present two case studies to demonstrate the viability of the technique to explore multiphase aerosol chemistry. First, the kinetics of imidazole formation in aqueous glyoxal/ammonium sulfate microdroplets was investigated to explore a model system for so-called “brown carbon” aerosol formation. Formation rates were found to increase with solution pH and were greater in microdroplets than in the bulk solution. Second, the rates of evaporation of methylglyoxal from microdroplets containing sodium sulfate, chloride, and nitrate were measured. While the presence of salts causes methylglyoxal to “salt out”, the evaporation rates do not follow the expected Hofmeister trend (SO₄^2– > Cl^– > NO₃^–) but do depend on microdroplet ionic strength and viscosity.