A model for heterogeneous formation of HONO on aqueous surfaces at night based on measurements in the marine boundary layer

The importance of nitrous acid (HONO) in the polluted troposphere has been known for some time. Its photolysis is a major source term in the total daily budget of the hydroxyl radical, OH. Highest concentrations of HONO are seen in the boundary layer at night, leading to a burst of OH production in the morning. Despite significant research activity and its importance, a complete understanding of the atmospheric sources and sinks of HONO, both night and day, has remained elusive. Literature consensus does suggest though, that a major source of HONO at night is the heterogeneous hydrolysis of NO2 on surfaces (R1) and that water on the surface is especially important in the reaction: NO2 + NO2 + H2O → HONO + HNO3 (R1). Here we describe our measurements of ambient HONO over the surface of the ocean in the polluted marine boundary layer, a truly aqueous environment. Our observations suggest that a pseudo steady state (PSS) of HONO exists at night in this environment, a balance between heterogeneous hydrolysis of NO2 on the surface of the ocean, and reversible deposition of atmospheric HONO back to the ocean surface. The levels of HONO during the nocturnal PSS period are positively correlated with temperature, consistent with a partitioning of HONO from the surface to the gas phase with an apparent enthalpy of vaporization of 55.5 ± 5.4 kJ mol-1. A conceptual model for HONO formation on ambient aqueous surfaces is presented, with the main elements being the presence of a thin surface layer, perhaps a monolayer, highly acidic and saturated with N (IV) precursors, production of HNO3, which diffuses to underlying water layers, and HONO, which partitions reversibly between the surface and the gas phase. Implications of the conceptual model will be discussed.