Heterogeneous Oxidation of Atmospheric Organic Aerosol: Kinetics of Changes to the Amount and Oxidation State of Particle-Phase Organic Carbon

Abstract

Atmospheric oxidation reactions are known to affect the chemical composition of organic aerosol (OA) particles over timescales of several days, but the details of such oxidative aging reactions are poorly understood. In this study we examine the rates and products of a key class of aging reaction, the heterogeneous oxidation of particle-phase organic species by the gas-phase hydroxyl radical (OH). We compile and reanalyze a number of previous studies from our laboratories involving the oxidation of single-component organic particles. All kinetic and product data are described on a common basis, enabling a straightforward comparison among different chemical systems and experimental conditions. Oxidation chemistry is described in terms of changes to key ensemble properties of the OA, rather than to its detailed molecular composition, focusing on two quantities in particular, the amount and the oxidation state of the particle-phase carbon. Heterogeneous oxidation increases the oxidation state of particulate carbon, with the rate of increase determined by the detailed chemical mechanism. At the same time, the amount of particle-phase carbon decreases with oxidation, due to fragmentation (C-C scission) reactions that form small, volatile products that escape to the gas phase. In contrast to the oxidation state increase, the rate of carbon loss is nearly uniform among most systems studied. Extrapolation of these results to atmospheric conditions indicates that heterogeneous oxidation can have a substantial effect on the amount and composition of atmospheric OA over timescales of several days, a prediction that is broadly in line with available measurements of OA evolution over such long timescales. In particular, 3-13% of particle-phase carbon is lost to the gas phase after one week of heterogeneous oxidation. Our results indicate that oxidative aging represents an important sink for particulate organic carbon, and more generally that fragmentation reactions play a major role in the lifecycle of atmospheric OA.

Electrodynamic balance–mass spectrometry of single particles as a new platform for atmospheric chemistry research

“…Because of these twin impacts, one long-standing goal of atmospheric research has been to assemble via experiment a detailed fundamental understanding of the coupled chemical-physical processes controlling the prevalence and composition of these particles, such as gas-particle partitioning (reviewed in Bilde et al, 2015), homogeneous and heterogeneous chemistry (e.g., George et al, 2015;Herrmann et al, 2015;Kroll et al, 2015), and kinetic barriers arising from high particle viscosity or phase separation (e.g., Bastelberger et al, 2017;Shiraiwa et al, 2013).…”

Importance of Sulfate Radical Anion Formation and Chemistry in Heterogeneous OH Oxidation of Sodium Methyl Sulfate, the Smallest Organosulfate

“…They can continuously react with gasphase oxidants such as hydroxyl (OH) radicals, ozone (O 3 ), and nitrate (NO 3 ) radicals at or near the particle surface 25 throughout their atmospheric lifetime. These heterogeneous oxidative processes have been found to change the size, composition, and physiochemical properties of both laboratory-generated organic particles and atmospheric particles (Rudich et al, 2007;George and Abbatt, 2010;Kroll et al, 2015). To gain a better understanding of how organosulfates chemically transform through heterogeneous oxidation in the atmosphere, this work investigates the heterogeneous OH radical-initiated oxidation of sodium methyl sulfate (CH 3 SO 4 Na) particles, the smallest organosulfate detected in atmospheric particles, using 30 an aerosol flow tube reactor at a high relative humidity (RH) of 85 %.…”

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

“…Atmospheric organic aerosols can be continuously oxidized by gas-phase oxidants such as hydroxyl (OH) radicals and ozone (O 3 ) throughout their lifetime, with a chemical lifetime of several days to weeks (Rudich et al, 2007;Jimenez et al, 2009;George and Abbatt, 2010;Kroll et al, 2015). This heterogeneous oxidation of organic aerosols is an important aging process that can change the aerosol composition and, therefore, may alter the properties of aerosols, such as their light scattering ability, hygroscopicity, and cloud condensation nuclei (CCN) activity (Broekhuizen et al, 2004;Shilling et al, 2007;Cappa et al, 2011;Wong et al, 2011;Lambe et al, 2011;Dennis-Smither et al, 2012).…”

“…The formation of oxygenated functionalization products of increased water solubility can enhance the hygroscopicity property of the aerosols at a certain RH. Furthermore, the change in the composition can also change RH and water content at which the particles undergo deliquescence or efflorescence-phase transitions (Kroll et al, 2015). Alternatively, through a fragmentation process, organic molecules can decompose to form smaller products upon oxidation with fewer carbons than their parent molecules.…”