Heterogeneous reactivity of nitrous acid on submicron sulfuric acid aerosol

Abstract

An aerosol flow reactor has been employed to study nitrous acid decomposition (2HONO→NO+NO₂+H₂O) on submicron sulfuric acid aerosol. The reactive uptake coefficient was obtained for 60 wt.% sulfuric acid at room temperature and pressure. It was found to be linearly dependent upon the nitrous acid partial pressure with γ_r≈p_HONO(ppb)×10^-8. The bulk-liquid phase reaction rate constant was measured in a separate experiment and a value of k^II=(19.5±2.0) d mol^-1 s^-1 was obtained for 60 wt.% sulfuric acid at room temperature. This value is too small to explain the observed loss of HONO to the aerosol particles implying that the heterogeneous decay of nitrous acid occurs on the surface of the aerosol. A second-order surface rate constant of k_surf=(4.6±1.5)×10^-17 cm³ m molecule^-1 s^-1 was determined that was a factor of a thousand greater than that previously reported for other surfaces. Despite this relatively large decay constant, it is still too small to have any significance upon nitrous acid concentrations in the night-time boundary layer. This work adds to the increasing number of heterogeneous reactions that are found to be significantly enhanced at the surface of a liquid.