Role of Water and Phase in the Heterogeneous Oxidation of Solid and Aqueous Succinic Acid Aerosol by Hydroxyl Radicals

The effect of the aerosol phase (solid versus aqueous) on the heterogeneous OH oxidation of succinic acid (C4H6O4) 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 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.