Unraveling the Meaning of Effective Uptake Coefficients in Multiphase and Aerosol Chemistry

ConspectusReactions of gas phase molecules with surfaces play key roles in atmospheric and environmental chemistry. Reactive uptake coefficients (γ), the fraction of gas-surface collisions that yield a reaction, are used to quantify the kinetics in these heterogeneous and multiphase systems. Unlike rate coefficients for homogeneous gas- or liquid-phase reactions, uptake coefficients are system- and observation-dependent quantities that depend upon a multitude of underlying elementary steps. As such, uptake coefficients adopt complex scaling behavior with reactant concentrations and other physicochemical properties of the interface, making predictions of γ particularly challenging.Typically, γ_gas is obtained by measuring the loss rate of a gas-phase molecule above a liquid or solid surface, relative to its collision frequency. By definition, γ_gas ≤ 1. Highly efficient reactions proceed at or near the gas-surface collision frequency and exhibit values of γ_gas near unity. Alternatively, heterogeneous kinetics are often measured using the consumption rate of a condensed phase reactant normalized to the gas-surface collision frequency, yielding instead an effective uptake coefficient (γ_eff). In many cases, γ_eff and γ_gas are not equal, yielding additional insights into the nature of the reaction. For example, substantial diffusive limitations in the condensed phase may inhibit reactivity, yielding γ_eff ≪ γ_gas. In contrast, γ_eff > γ_gas in the presence of condensed phase secondary reactions, with values of γ_eff often exceeding 1 for the case of radical chain reactions.In this Account, we will discuss how measurements of γ_eff in aerosol can uniquely reveal the origins of complex physical and chemical behavior in multiphase reactions under laboratory conditions. For example, measurements of the scaling of γ_eff with shell thickness during the ozonolysis of core-shell aerosol yields insight into the relative transport and reaction time scales of gaseous oxidants, while measurements of γ_eff, as a function of relative humidity (RH) during OH radical oxidation of hygroscopic citric acid aerosol, highlight the role that mixing times of particle-phase reactants play in determining aerosol reaction kinetics. Additionally, the scaling of γ_eff with oxidant and trace gas concentrations during the oxidation of long-chain hydrocarbons in aerosols by OH radicals and chlorine atoms provides distinctive signatures of the underlying competition between free radical propagation and termination mechanisms. Finally, it is shown how changes in γ_eff that are induced by careful selection of the molecular structure of the particle-phase reactant help to identify new reaction pathways, such as alternative mechanisms for lipid peroxidation, and to report on the reaction kinetics of short-lived reactive species, including Criegee Intermediates. Together, these examples will demonstrate how proper experimental design and accurate measurements of an effective uptake coefficient can be used to interrogate the fundamental steps governing complex multiphase reaction mechanisms.