Coupled Interfacial and Bulk Kinetics Govern the Timescales of Multiphase Ozonolysis Reactions

Abstract

Chemical transformations in aerosols impact the lifetime of particle phase species, the fate of atmospheric pollutants, and both climate- and health-relevant aerosol properties. Timescales for multiphase reactions of ozone in atmospheric aqueous phases are governed by coupled kinetic processes between the gas phase, the particle interface, and its bulk, which respond dynamically to reactive consumption of O3. However, models of atmospheric aerosol reactivity often do not account for the coupled nature of multiphase processes. To examine these dynamics, we use new and prior experimental observations of aqueous droplet reaction kinetics, including three systems with a range of surface affinities and ozonolysis rate coefficients (trans-aconitic acid (C6H6O6), maleic acid (C4H4O4), and sodium nitrite (NaNO2)). Using literature rate coefficients and thermodynamic properties, we constrain a simple two-compartment stochastic kinetic model which resolves the interface from the particle bulk and represents O3 partitioning, diffusion, and reaction as a coupled kinetic system. Our kinetic model accurately predicts decay kinetics across all three systems, demonstrating that both the thermodynamic properties of O3 and the coupled kinetic and diffusion processes are key to making accurate predictions. An enhanced concentration of adsorbed O3, compared to gas and bulk phases is rapidly maintained and remains constant even as O3 is consumed by reaction. Multiphase systems dynamically seek to achieve equilibrium in response to reactive O3 loss, but this is hampered at solute concentrations relevant to aqueous aerosol by the rate of O3 arrival in the bulk by diffusion. As a result, bulk-phase O3 becomes depleted from its Henry’s law solubility. This bulk-phase O3 depletion limits reaction timescales for relatively slow-reacting organic solutes with low interfacial affinity (i.e., trans-aconitic and maleic acids, with k rxn ≈ 103–104 M–1 s–1), which is in contrast to fast-reacting solutes with higher surface affinity (i.e., nitrite, with k rxn ≈ 105 M–1 s–1) where surface reactions strongly impact the observed decay kinetics.

Accelerated Sulfur Oxidation by Ozone on Surfaces of Single Optically Trapped Aerosol Particles

“…smaller, and such difference may also imply that the effective ionic strength at the interface layers where the surface reactions occurs is smaller than that in the aerosol bulks. Indeed, the fitted values of partial coverage θ ss suggest that the surface STS concentration remains around 1.3 × 10 14 cm −2 (or around 2.5 M), such as [STS] ss,0 plotted in Figure 7, when the bulk STS concentration is larger than ∼4 M. Such kind of dependence has also been found in the cases of nitrites and nitrates, and it was rationalized in terms of Langmuir adsorption model, 39 where the surface sites for these ions are also limited, leading to a constant plateau of surface concentrations at higher bulk concentrations. However, it should note that θ ss employed in the present work is assumed to be independent of time, and thus in principle it takes into account the surface depletions of all ions (Na + , S 2 O 2-3 , SO 2-4 and HSO -4 ) appeared at different reaction time scales.…”

“…[35][36][37][38] A very recent kinetics study of nitrite ozonolysis in aerosols investigated the significant dependence of interfacial nitrite concentrations on bulk nitrite concentrations and its potential role in the interfacial kinetics. 39 The pH at surfaces of aerosols can also be different from those in aerosol bulks, 40 and a recent aerosol kinetics study observed the enhanced rates of catalyzed oxidations in aerosols and attributed this enhancement to the pH gradient at the air-water interfaces of aerosols. 41 In the present work, we choose the oxidation reaction of sodium thiosulfate (STS), Na 2 S 2 O 3 , with O 3 as a model system of sulfur oxidation in aerosols, as such model reaction has been utilized in several studies associated with atmospheric aerosol chemistry.…”

A Kinetic Model for Predicting Trace Gas Uptake and Reaction

“…This framework does not resolve the formation of chemical gradients that might emerge in either the interfacial or bulk volumes, but instead considers [X] and [Y] in each region as average quantities. For the derivations presented below, the same view of the interface and bulk regions of the droplet is used, which although simplistic in molecular terms, appears to be realistic enough to correctly predict the experimentally measured kinetics and reactive uptake coefficients observed during multiphase ozonolysis.…”

“…H cc gs can be expressed kinetically using the rate constants for the forward adsorption and backward desorption elementary steps (Figure ), where A is surface area, c̅ is the mean speed of X (g) and σ is the sticking probability per adsorption site, with the assumption that each X (g) may occupy a single site to become X (ads) . H cc sb can be similarly expressed using the rate constants for the forward solvation and backward desolvation elementary steps illustrated in Figure , Γ ∞( X ) is the maximum surface concentration (molec cm –2 ) of X and δ is the interfacial thickness. We assume an interfacial thickness of 1 nm, which is consistent with the estimated solvation energy profile for trace gas adsorption in MD simulations. , δ is used to compute volumetric surface concentrations.…”

“…This paper is organized as follows. In section , we summarize the model framework and its assumptions, with a brief summary of how it was implemented in previous reaction–diffusion simulations . In section , prior simulation and experimental results are analyzed to derive an expression for γ and the associated closed-form kinetic equations, which include both surface and bulk reactions.…”