Pseudo steady states of HONO measured in the nocturnal marine boundary layer: a conceptual model for HONO formation on aqueous surfaces

Abstract. A complete understanding of the formation mechanism of nitrous acid (HONO) in the ambient atmosphere is complicated by a lack of understanding of processes occurring when aqueous water is present. We report nocturnal measurements of HONO, SO₂ and NO₂ by differential optical absorption spectroscopy over the ocean surface in a polluted marine environment. In this aqueous environment, we observed reproducible pseudo steady states (PSS) of HONO every night, that are fully formed shortly after sunset, much faster than seen in urban environments. During the PSS period, HONO is constant with time, independent of air mass source and independent of the concentration of NO₂. The independence of HONO on the concentration of NO₂ implies a 0° order formation process, likely on a saturated surface, with reversible partitioning of HONO to the gas phase, through vaporization and deposition to the surface. We observed median HONO/NO₂ ratios starting at 0.13 at the beginning of the PSS period (with an apparent lower bound of 0.03), rising to median levels of ~0.30 at the end of the PSS period (with an upper bound >1.0). The implication of these numbers is that they suggest a common surface mechanism of HONO formation on terrestrial and aqueous surfaces, with an increase in the HONO/NO₂ ratio with the amount of water available at the 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 ΔH_SNL (HONO)=55.5±5.4 kJ mol⁻¹. The formation mechanism on aqueous surfaces is independent of relative humidity (RH), despite observation of a negative HONO-RH correlation. A conceptual model for HONO formation on ambient aqueous surfaces is presented, with the main elements being the presence of a surface nanolayer (SNL), highly acidic and saturated with N(IV) precursors, production of HNO₃, that diffuses to underlying water layers, and HONO, which partitions reversibly between the SNL and the gas phase. Implications of the conceptual model are discussed.