Ozone uptake on glassy, semi-solid and liquid organic matter and the role of reactive oxygen intermediates in atmospheric aerosol chemistry

Abstract

Heterogeneous and multiphase reactions of ozone are important pathways for chemical ageing of atmospheric organic aerosols. To demonstrate and quantify how moisture-induced phase changes can affect the gas uptake and chemical transformation of organic matter, we apply a kinetic multi-layer model to a comprehensive experimental data set of ozone uptake by shikimic acid. The bulk diffusion coefficients were determined to be 10(-12) cm(2) s(-1) for ozone and 10(-20) cm(2) s(-1) for shikimic acid under dry conditions, increasing by several orders of magnitude with increasing relative humidity (RH) due to phase changes from amorphous solid over semisolid to liquid. Consequently, the reactive uptake of ozone progresses through different kinetic regimes characterised by specific limiting processes and parameters. At high RH, ozone uptake is driven by reaction throughout the particle bulk; at low RH it is restricted to reaction near the particle surface and kinetically limited by slow diffusion and replenishment of unreacted organic molecules. Our results suggest that the chemical reaction mechanism involves long-lived reactive oxygen intermediates, likely primary ozonides or O atoms, which may provide a pathway for self-reaction and catalytic destruction of ozone at the surface. Slow diffusion and ozone destruction can effectively shield reactive organic molecules in the particle bulk from degradation. We discuss the potential non-orthogonality of kinetic parameters, and show how this problem can be solved by using comprehensive experimental data sets to constrain the kinetic model, providing mechanistic insights into the coupling of transport, phase changes, and chemical reactions of multiple species in complex systems.

Global long-range transport and lung cancer risk from polycyclic aromatic hydrocarbons shielded by coatings of organic aerosol

“…S4B). This result suggests that more complex and aged atmospheric SOA, which is highly viscous at cool/dry conditions (24,26,29), is expected to be highly effective in shielding BaP from oxidation, as is assumed in our new modeling formulation.…”

“…Other measurements have demonstrated that heterogeneous oxidation of particle-surface-bound PAHs in the atmosphere decreases substantially due to accumulation of SOA species over particles during their atmospheric transport and chemical aging (23). Heterogeneous reactions of PAHs with ozone are limited mostly to the particle surface, where ozone can be catalytically destroyed (24). Highly viscous SOA coatings limit the diffusion of PAH molecules from particle bulk to particle surface (24,25), effectively shielding PAHs from chemical degradation (24).…”

“…Highly viscous SOA coatings limit the diffusion of PAH molecules from particle bulk to particle surface (24,25), effectively shielding PAHs from chemical degradation (24). In addition, OA is more viscous in cool/dry conditions (24,26), so the effectiveness of shielding by OA is likely higher at cool locations in the atmosphere compared with laboratory measurements, which are made at room temperature [i.e., around 296°K (18,19,25)]. Thus, temperature-and RH-dependent changes in the viscosity of OA coatings can strongly affect heterogeneous oxidation kinetics of BaP.…”

The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles

“…Note however that for a relevant surface reaction in kinetic flux models, it is necessary to use an effective desorption lifetime, τ d , in the millisecond to second time range Shiraiwa et al, 2010). This is many orders of magnitude longer than would be expected due to pure physisorption as estimated by molecular dynamic simulations (Vieceli et al, 2005), indicating that the adsorption process should involve chemisorption or formation of long-lived intermediates that would have the potential to extend these effective desorption lifetimes (Shiraiwa et al, 2011a;Berkemeier et al, 2016). Surface reactions in viscous aqueous organics are consistent with previous work by Gržinić et al (2015), , and Berkemeier et al (2016) for the uptake of N 2 O 5 to citric acid and the uptake of O 3 to shikimic acid over a range of relative humidities.…”

Quantification of environmentally persistent free radicals and reactive oxygen species in atmospheric aerosol particles

“…The formation of stable radicals can also be induced by heterogeneous and multiphase chemistry of organic aerosols. Heterogeneous ozonolysis of aerosol particles such as polycyclic aromatic hydrocarbons (PAH) and pollen proteins can lead to the formation of long-lived reactive oxygen intermediates (ROI) (Shiraiwa et al, 2011Reinmuth-Selzle et al, 2014;Borrowman et al, 2015;Kampf et al, 2015;Berkemeier et al, 2016).…”