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

Reactions 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 emergent 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, γgas 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, γgas ≤ 1. Highly efficient reactions proceed at or near the gas-surface collision frequency and exhibit values of γgas 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 (γeff). In many cases, γeff and γgas 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 γeff << γgas. In contrast, γeff > γgas in the presence of condensed phase secondary reactions, with values of γeff often exceeding 1 for the case of radical chain reactions. In this account, we will discuss how measurements of γeff in aerosol reveal the origins of complex physical and chemical behavior in multiphase reactions that can be uniquely observed and understood through the lens of effective uptake coefficients. For example, the scaling of γeff with water vapor in aqueous systems, or shell thickness in core-shell aerosol, yields insight into relative transport and reaction timescales of gaseous oxidants, while the scaling of γeff with oxidant and trace gas concentrations provides a distinctive signature of the underlying competition between free radical propagation and termination mechanisms. Further, changes in γeff induced by careful selection of the molecular structure of the condensed-phase reactants help identify new reaction pathways and indirectly report on the reaction kinetics of short-lived species, including Criegee Intermediates. Through these examples we will show how proper experimental design and accurate measurements of an effective uptake coefficient can be used to interrogate complex multiphase reaction mechanisms.