Enhanced ClNO${}_2$ formation at the interface of sea-salt aerosol

Abstract

The reactive uptake of ${\mathrm{N}}_2{\mathrm{O}}_5$ on sea-spray aerosol plays a key role in regulating NO${}_{\mathrm{x}}$ concentration in the troposphere. Despite numerous field and laboratory studies, a microscopic understanding of its heterogeneous reactivity remains unclear. Here, we use molecular simulation and theory to elucidate the chlorination of ${\mathrm{N}}_2{\mathrm{O}}_5$ to form ClNO${}_2$, the primary reactive channel within sea-spray aerosol. We find the formation of ClNO${}_2$ is markedly enhanced at the air-water interface due to the stabilization of the charge-delocalized transition state, as evident from the formulation of bimolecular rate theory in heterogeneous environments. We explore the consequences of the enhanced interfacial reactivity in the uptake of ${\mathrm{N}}_2{\mathrm{O}}_5$ using numerical solutions of molecular reaction-diffusion equations as well as their analytical approximations. Our results suggest that the current interpretation of aerosol branching ratios needs to be revisited.