Atmospheric aerosols can be categorized into primary particles, which are directly emitted by their sources, and secondary particles, generated in the atmosphere from gaseous inorganic and organic precursors. For example, atmospheric oxidation of sulfur containing compounds leads to sulfuric acid and its salts, which represent a major secondary inorganic component of atmospheric aerosols. Likewise, oxidation of nitrogen oxides leads to nitric acid or its salts, which are also abundant in aerosols. The ocean surface, which covers three-quarters of the planet, offers a remarkably dynamic and chemically complex surface for interfacial reactions in the marine boundary layer. The porous nature of permanent or perennial snowpacks adds a tremendous amount of surface area, with which the atmosphere interacts. In short, solar radiation can provide the energy to initiate reactions while atmospherically available surfaces or condensed phases may act to reduce the required energy for a given photochemical pathway, for instance, by allowing a longer wavelength for reaction of species associated with a surface or bulk phase environment.

Heterogeneous photochemistry in the atmosphere.

Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air-ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions. Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed.

/pdf/a-review-of-air-ice-chemical-and-physical-interactions-aici-1ppnq367no.pdf

A review of air-ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow

Photolysis of nitric acid on the surface has been found recently to be greatly enhanced from that in the gas phase. Yet, photolysis of particulate nitrate (pNO3) associated with atmospheric aerosols is still relatively unknown. Here, aerosol filter samples were collected both near the ground surface and throughout the troposphere on board the NSF/NACR C-130 aircraft. The photolysis rate constants of pNO3 were determined from these samples by directly monitoring the production rates of nitrous acid (HONO) and nitrogen dioxide (NO2) under UV light (>290 nm) irradiation. Scaled to the tropical noontime condition on the ground level (solar zenith angle = 0°), the normalized photolysis rate constants (jpNO3N) are in the range from 6.2 × 10–6 s–1 to 5.0 × 10–4 s–1 with a median of 8.3 × 10–5 s–1 and a mean (±1 SD) of (1.3 ± 1.2) × 10–4 s–1. Chemical compositions, specifically nitrate loading and organic matter, affect the rate of photolysis. Extrapolated to ambient pNO3 loading conditions, e.g. ≤ 10 nmol m–3, t...

Photolysis of Particulate Nitrate as a Source of HONO and NOx

Photolysis of nitrous acid generates hydroxyl radicals — a key atmospheric oxidant. Measurements at a forest in Michigan suggest that sunlight-induced breakdown of nitric acid on the canopy surface serves as a significant source of nitrous acid to the overlying atmosphere.

Nitric acid photolysis on forest canopy surface as a source for tropospheric nitrous acid

Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed. A 'renoxification' process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth's surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.

/pdf/rapid-cycling-of-reactive-nitrogen-in-the-marine-boundary-3umw7cor9y.pdf

Rapid cycling of reactive nitrogen in the marine boundary layer

We have studied the photolysis of nitric acid (HNO(3)) in the gas phase at 253 and 295 K, on aluminum surfaces at 253 and 295 K, and on ice films at 253 K, by using 308 nm excimer laser photolysis combined with cavity ring-down spectroscopy. We monitored both the ground-state NO(2) and the electronically excited NO(2), NO(2)*, produced from the HNO(3) photolysis. NO(2)* + OH is a predominant photolysis pathway (if not the only photolysis pathway) from the gas-phase photolysis of HNO(3) at 308 nm. The NO(2)* quantum yields from the HNO(3) photolysis on aluminum surfaces are 0.80 +/- 0.15 at 295 K and 0.92 +/- 0.26 at 253 K, where errors quoted represent 2sigma measurement uncertainty. The corresponding NO(2)* quantum yield from the HNO(3) photolysis on ice films is 0.60 +/- 0.34 at 253 K. The 308 nm absorption cross sections of HNO(3) on Al surfaces and on ice films have been directly measured. Absorption cross sections of HNO(3) on Al surface at 308 nm are (4.19 +/- 0.17) x 10(-18) and (4.23 +/- 0.45) x 10(-18) cm(2)/molecule at 253 and at 295 K, whereas the corresponding absorption cross section of HNO(3) on ice films is (1.21 +/- 0.31) x 10(-18) cm(2)/molecule at 253 K (errors quoted represent 2sigma measurement uncertainty). Atmospheric implications of the results are discussed.

308 nm photolysis of nitric acid in the gas phase, on aluminum surfaces, and on ice films.

Gas-phase absorption cross sections of 2-nitrobenzaldehyde and benzaldehyde in the 285-400 nm region, and photolysis of 2-nitrobenzaldehyde vapor at 308 and 351 nm

Determination of absorption cross sections of surface-adsorbed HNO3 in the 290–330 nm region by Brewster angle cavity ring-down spectroscopy

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308 nm photolysis of nitric acid in the gas phase, on aluminum surfaces, and on ice films.

Gas-phase absorption cross sections of 2-nitrobenzaldehyde and benzaldehyde in the 285-400 nm region, and photolysis of 2-nitrobenzaldehyde vapor at 308 and 351 nm

Determination of absorption cross sections of surface-adsorbed HNO3 in the 290–330 nm region by Brewster angle cavity ring-down spectroscopy