Role of Water and Phase in the Heterogeneous Oxidation of Solid and Aqueous Succinic Acid Aerosol by Hydroxyl Radicals Man Nin Chan, † Haofei Zhang, †,‡ Allen H. Goldstein, ‡,§,∥ and Kevin R. Wilson* ,† † Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡ Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720, United States § Environmental and Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∥ Department of Civil and Environmental Engineering, University of California, Berkeley, California 94720, United States ABSTRACT: The effect of the aerosol phase (solid versus aqueous) on the heterogeneous OH oxidation of succinic acid (C 4 H 6 O 4 ) is investigated using an aerosol flow tube reactor. The molecular and elemental transformation of the aerosol is quantified using Direct Analysis in Real Time (DART), a soft atmospheric pressure ionization source, coupled to a high-resolution mass spectrometer. The aerosol phase, controlled by liquid water content in the particle, is observed to have a pronounced effect on the reaction kinetics, the distribution of the oxidation products, and the average aerosol carbon oxidation state. In highly concentrated aqueous droplets (∼28 M), succinic acid within the aerosol reacts 41 times faster than in solid aerosol, producing a larger quantity of both functionalization and fragmentation reaction products. These observations are consistent with the more rapid diffusion of succinic acid to the surface of aqueous droplets than solid particles. For aqueous droplets at an OH exposure of 2.5 × 10 12 molecules cm −3 s, the average aerosol carbon oxidation state is +2, with higher molecular weight functionalization products accounting for ∼5% and lower carbon number (C < 4) fragmentation products comprising 70% of the aerosol mass. The remaining 25% of the aqueous aerosol is unreacted succinic acid. This is in contrast with solid aerosol, at an equivalent oxidation level, where unreacted succinic acid is the largest aerosol constituent with functionalization products accounting for <1% and fragmentation products ∼8% of the aerosol mass, yielding an average aerosol carbon oxidation of only +0.62. On the basis of exact mass measurements of the oxidation products and a proposed reaction mechanism, succinic acid in both phases preferentially reacts with OH to form smaller carbon number monoacids and diacids (e.g., oxalic acid). These results illustrate the importance of water in controlling the rate at which the average aerosol carbon oxidation state evolves through the formation and evolution of C−C bond scission products with high carbon oxidation states and small carbon numbers. These results also point more generally to a potential complexity in aerosol oxidation, whose chemistry may ultimately depend upon the “exposure history” of particles to relative humidity. 1. INTRODUCTION Hydrocarbons are a significant fraction of ambient aerosol mass 1 and can continuously undergo oxidation by colliding with gas-phase oxidants such as hydroxyl radicals (OH), ozone (O 3 ), and nitrate radicals (NO 3 ) in the atmosphere. The impact of heterogeneous oxidation on the composition and properties of organic aerosol has been reviewed in the recent literature. 2 For example, chemical tracers such as levoglucosan (a tracer for biomass burning) can be efficiently oxidized by OH, 3,4 which may complicate the use of these tracers for source apportion- ment studies. 5,6 Furthermore, chemically reduced organic aerosol (e.g., long chain alkanes) can become efficient cloud condensation nuclei upon only a few generations of heterogeneous oxidation by OH. 7,8 A number of previous studies have focused on the oxidation of chemically reduced organic compounds (i.e., linear, branched, or cyclic large hydrocarbons). 5,9−13 These results indicate that the observed OH reaction probability or uptake coefficient (i.e., the inferred fraction of OH−particle collisions that yield a reaction) is larger than ∼0.1. In broad terms these heterogeneous reactions occurring on liquid hydrocarbon aerosols proceed statistically 14 both by the formation of new functional groups (termed functionalization) and by the production of more volatile products formed via C−C bond scission reactions (termed fragmentation). 15 Together these reaction pathways can yield complex oxidation trajecto- ries, 8,12,14 for example, when represented in the average aerosol carbon oxidation state vs carbon number space as shown by Kroll et al. 16 For chemically reduced hydrocarbons, the initial stages of oxidation form new alcohol and ketone oxygenated functional groups, while smaller carbon number fragmentation Special Issue: John C. Hemminger Festschrift Received: February 3, 2014 Revised: March 3, 2014 Published: March 4, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 28978 dx.doi.org/10.1021/jp5012022 | J. Phys. Chem. C 2014, 118, 28978−28992