title: Effects of Relative Humidity and Particle Phase Water on the Heterogeneous OH Oxidation of 2-Methylglutaric Acid Aqueous Droplets.
abstract: Organic aerosols can exist as aqueous droplets, with variable water content depending on their composition and environmental conditions (e.g., relative humidity (RH)). Recent laboratory studies have revealed that oxidation kinetics in highly concentrated droplets can be much slower than those in dilute solutions. However, it remains unclear whether aerosol phase water affects the formation of reaction products physically and/or chemically. In this work, we investigate the role of aerosol phase water on the heterogeneous chemistry of aqueous organic droplets consisting of 2-methylglutaric acid (2-MGA), measuring the reaction kinetics and the reaction products upon heterogeneous OH oxidation over a range of RH. An atmospheric pressure soft ionization source (direct analysis in real time, DART) coupled with a high-resolution mass spectrometer is used to obtain real-time molecular information on the reaction products. Aerosol mass spectra show that the same reaction products are formed at all measured RH. At a given reaction extent of the parent 2-MGA, the aerosol composition is independent of RH. These results suggest the aerosol phase water does not alter reaction mechanisms significantly. Kinetic measurements find that the effective OH uptake coefficient, γeff, decreases with decreasing RH below 72%. Isotopic exchange measurements performed using aerosol optical tweezers reveal water diffusion coefficients in the 2-MGA droplets to be 3.0 × 10-13to 8.0 × 10-13m2s-1over the RH range of 47-58%. These values are comparable to those of other viscous organic aerosols (e.g., citric acid), indicating that 2-MGA droplets are likely to be viscous at low humidity. Smaller γeffat low RH is likely attributed to the slower diffusion of reactants within the droplets. Taken together, the observed relationship between the γeffand RH is likely attributed to changes in aerosol viscosity rather than changes in reaction mechanisms.
authors.affiliation: Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong , Hong Kong, China.
authors.forename: Man Mei
authors.initials: MM
authors.lastname: Chim
authors.affiliation: Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong , Hong Kong, China.
authors.forename: Chun Yin
authors.initials: CY
authors.lastname: Chow
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720-8176, United States.
authors.forename: James F
authors.initials: JF
authors.lastname: Davies
authors.affiliation: Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong , Hong Kong, China.|The Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong , Hong Kong, China.
authors.forename: Man Nin
authors.identifier: http://orcid.org/0000-0002-2384-2695
authors.initials: MN
authors.lastname: Chan