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

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.

Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols

“…In the study of OH reaction with 2,2-dimethylsuccinic acid and 2,3-dimethylsuccinic acid, at approximately one oxidation lifetime, the alcohol-to-ketone functionalization product ratio is reported to be 3.03 and 77.06, respectively (Cheng et al, 2015). However, a ratio close to 1 at the same oxidation lifetime has been found for succinic acid, which is the less branched counterpart (Chan et al, 2014). These results suggest that the presence of branched methyl group(s) in the backbone of succinic acid greatly alters the heterogeneous OH chemistry relative to succinic acid.…”

“…Experimental details have been described elsewhere (Chan et al, 2014). In brief, the aerosol was first generated by atomizing an aqueous methylsuccinic acid solution using a constant output atomizer (TSI Inc. model 3076).…”

“…Organic compounds dissolved within liquid aqueous droplets have been found to be oxidized by gas-phase OH radicals at a much faster rate than within amorphous (semi)solid aerosol particles (Chan et al, 2014;Slade and Knopf, 2014;Davies and Wilson, 2015;Arangio et al, 2015). This is attributed to shorter mixing times in liquid submicron-sized droplets, allowing the condensed-phase material to diffuse rapidly within the droplet bulk and access the surface on the timescale of a reaction Berkemeier et al, 2013Berkemeier et al, , 2016Houle et al, 2015).…”

“…In addition to the bulk elemental composition, knowledge of molecular composition of the particles before and after oxidation is needed to better understand how the aerosol hygroscopicity evolves during oxidation. Recently, atmospheric pressure ionization techniques such as extractive electrospray ionization (Doezema et al, 2012;Gallimore and Kalberer, 2013), Direct Analysis in Real Time (DART) Chan et al, 2013Chan et al, , 2014Cheng et al, 2015Cheng et al, , 2016Zhao et al, 2017), and Aerosol Flowing Atmospheric-Pressure Afterglow (AeroFAPA) (Brüggemann et al, 2015) coupled with high-resolution mass spectrometers have been used to characterize the composition of organic aerosols at a molecular level in real time. These techniques are of particular interest for investigating the chemical transformation of aqueous organic droplets through heterogeneous oxidation because the composition of the droplets can be directly analyzed in their native states.…”

Influence of Relative Humidity on the Heterogeneous Oxidation of Secondary Organic Aerosol

“…George et al, 2007;McNeill et al, 2008;Hennigan et al, 2010;Nah et al, 2013;Slade and Knopf, 30 2013;Chan et al, 2014;Davies and Wilson, 2015;Fan et al, 2015;Kroll et al, 2015; Atmos. Chem.…”