  The reactive uptake of $\mathrm{N_2O_5}$ 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 $\mathrm{N_2O_5}$ 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 $\mathrm{N_2O_5}$ 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.
