The reactive uptake of N2O5 to aqueous aerosol is a major loss channel for nitrogen oxides in the troposphere. Despite its importance, a quantitative picture of the uptake mechanism is missing. In particular, the experimentally measured reactive uptake of N2O5 shows a large variation that cannot be explained with the currently widely employed resistor model. Of particular relevance to this puzzle is the behavior of molecules at liquid interfaces, in this case the water/vapor interface. Here we employ molecular dynamics simulations with a data-driven many-body model of coupled-cluster accuracy to quantify both thermodynamics and kinetics of solvation and adsorption of N2O5 in water. The free energy profile shown in the accompanying figure highlights that N2O5 is selectively adsorbed to the liquid-vapor interface and weakly solvated. Accommodation into bulk water occurs slowly, competing with evaporation upon adsorption from gas phase. Leveraging the quantitative accuracy of the model, we parameterize and solve a reaction-diffusion equation to determine hydrolysis rates consistent with experimental observations. Importantly, we find a short reaction-diffusion length and thus only weak aerosol particle size dependence, indicating that the uptake is dominated by interfacial features. The framework and parameters determined here, including solubility, accommodation coefficient, and hydrolysis rate, afford a foundation for which to consider the reactive loss of N2O5 in more complex solutions. This will eventually enable us to provide a complete picture of reactive uptake of N2O5 with the full complexity of field measurements.

Uptake of N2O5 by aqueous aerosol unveiled using molecular simulations with chemically accurate many-body potentials

ACS Spring 2022

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