Photochemical production of gas phase NO_x from ice crystal NO₃⁻

Abstract

Abstract. Recent measurements have demonstrated that sunlight irradiation of snow results in the release of significant amounts of gas phase NO• (NO + NO2). We report here the results of a series of experiments designed to test the hypothesis that the observed NOz production is the result of nitrate photolysis. Snow produced from deionized water with and without the addition of nitrate was exposed to natural sunlight in an outdoor flow chamber. While NOx release from snow produced without added NO• was minimal, the addition of 100 /•M NO• resulted in the release of >500 pptv NOx in a 9 standard liter per minute (sLpm) flow of synthetic air exposed to the snow for 10-20 s; the rate of release was highly correlated with solar radiation. Further addition of radical trap reagents resulted in greatly increased NOx production (to >8 ppbv in a flow of 20 sLpm). In snow produced from deionized water plus sodium nitrate, production of NO2 dominated that of NO. The reverse was true in the presence of radical trap reagents; this suggests sensitivity of the NOx release mechanism to pH, as a basic compound was added, or to the presence of free radical scavengers. A mechanism for NO• release from NO• l)hotolysis consistent with these observations is presented. These results support lvrevious suggestions that surface NOx release may have a significant impact on boundary layer photochemistry in snow-covered regions and that nitrate photolysis on cirrus cloud particles may result in the release of gas phase NO•. A potential for pH-dependent impacts on ice core records of oxidants and oxidized compounds is also suggested.

Surface ozone and its precursors at Summit, Greenland: comparison between observations and model simulations

“…4a). Experiments at Summit by Honrath et al (1999Honrath et al ( , 2000a showed upward fluxes of NO x (2.52 × 10 8 molecules cm −2 s −1 ) from the photolysis of nitrate in snowpack during the summertime, leading to an enhancement of NO x levels in the surface layer by approximately 20 pptv, which was comparable to surface NO x mixing ratios in the Arctic from other sources. Similar results were found over the East Antarctic Plateau snow and ice sheet (Frey et al, 2013;Legrand et al, 2014).…”

Using Singlet Molecular Oxygen to Probe the Solute and Temperature Dependence of Liquid-Like Regions in/on Ice

“…For example, liquid-phase kinetics and mechanisms can explain the direct photolysis of nitrate, nitrite, and hydrogen peroxide on/in ice made from slowly frozen solutions, conditions where we expect solutes to be in LLRs. [19][20][21][22][23] However, in many cases liquid-phase chemistry cannot be extrapolated to ice…”

The role of ozone atmosphere-snow gas exchange on polar, boundary-layer tropospheric ozone – a review and sensitivity analysis

“…These processes result in the formation of gases, including oxidized nitrogen, halogen species, organic compounds and hydrogen peroxide, which subsequently are released into the atmosphere and perturb the gas-phase HO x budget and cycling (e.g. Dibb et al, 1998Honrath et al, 1999Honrath et al, , 2000aSumner and Shepson, 1999;Zhou et al, 2001;Swanson et al, 2002Swanson et al, , 2003.…”

“…Recent research in Greenland, Antarctica and at a midlatitude site in Michigan has shown that photochemical processes in the sunlit snowpack can also lead to production of substantial amounts of nitrogen oxides (NO x =NO+NO 2 ) by photodenitrification (Honrath et al, 1999(Honrath et al, , 2000aJones et al, 2000;Cotter et al, 2003). In a study at Summit, Greenland, Peterson and Honrath (2001) measured diurnal cycles of UV radiation, NO x and ozone in interstitial air (the air between snow grains in the snowpack; also referred to as firn air) at 30 cm depth and compared those measurements with data from above the surface.…”