Heterogeneous and multiphase reactions involving atmospheric gases and aerosol particles play a controlling role in regulating the production and loss of air pollutants and greenhouse gases in the troposphere. A reaction of particular interest is the heterogeneous hydrolysis of N2O5, a nocturnal reservoir for reactive nitrogen oxides. At night, N2O5 has been shown to react at enormously diverse rates at the surfaces of aerosol particles and other atmospheric interfaces. Dissolution leads to either hydrolysis of N2O5 to form HNO3 or reaction with Cl– ions, present at the liquid surface, to generate ClNO2. The interfacial reactivity of N2O5 is difficult to predict due to the chemical complexity of the relevant interfacial regions which are often populated by a wide array of surface-active organic compounds. Here, we explore the potential of macroscopic properties, such as the dielectric constant, to explain the wide range in measured N2O5 reactive uptake coefficients, γ(N2O5). A review of the existing literature on γ(N2O5) reveals a correlation between the dielectric constant ε of the medium and γ(N2O5). We suspect this may be due to enhanced solubility and reactivity enabled by an increase in the induced charge separation into NO2+ and NO3– at higher ε. We discuss quantitative measurements of the reactive uptake of N2O5 into a wide array of liquid organic films with varying dielectric constants and viscosities to contrast the role of near-surface solubility and diffusion. Preliminary experimental studies focus on evaluating the reaction of N2O5 in hexadecane and formamide (ε = 2 and 111, and viscosity = ~3 and ~4 cP, respectively). Initial modeling using a continuum treatment of the solvent qualitatively supports the correlation between ε and γ but also points to the importance of examining these reactions at the molecular scale.

Is the dielectric constant a macroscopic predictor of N2O5 reactivity at complex atmospheric interfaces?

2019 Fall National Meeting

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