Release of NO x from sunlight‐irradiated midlatitude snow (original) (raw)

2000, Geophysical Research …

Photochemical production and release of gasphase NOx (NO + NO•.) from the natural snowpack at a remote site in northern Michigan were investigated during the Snow Nitrogen and Oxidants in Winter study in January 1999. Snow was collected in an open 34 L chamber, which was then sealed with a transparent Teflon cover and used as an outdoor flow and reaction chamber. Significant increases in N Ox mixing ratio were observed in synthetic and ambient air pulled through the sunlit chamber. [NO•] enhancements were correlated to ultraviolet sunlight intensity, reaching-300 pptv under partially overcast midday, midwinter conditions. These findings are consistent with NO• production from photolysis of snowpack N O•-; the observed NO• release implies production of significant amounts of OH within the snow. Snowpack NO•-photolysis may therefore significantly alter boundary layer levels of both NO• and oxidized compounds over wide regions of the atmosphere. NOx (NO q-NO•.) in the interstitial air of the snowpack at Summit, central Greenland, and suggested that the NO• was photochemically produced from nitrate [Honrath et al., 1999]; confirming observations have been made in a study of Antarctic snowpack .[Jones et al., 2000]. Through wind pumping, snowpack interstitial air is mixed with the overlying atmosphere [Albert, 1996]. Recent measurements indicate that resulting impacts on ambient atmospheric composition are significant: a diurnal cycle in NO• with amplitude as large as 30-40 pptv was observed at Alert, Nunavut, Canada, following polar sunrise [Ridley et al., 2000], and NO levels in the range of 100 pptv were observed at South Pole, Antarctica [Davis et al., 1999]; release from the snowpack was identified as the most likely cause in both cases. Snow surfaces are present over large areas of midlatitude regions as well: as much as one-half of the land surface Copyright 2000 by the American Geophysical Union. Paper number 1999GL011286. 0094-8276/00/1999GL011286505.00 north of 20øN is snow-covered during winter [Frei and Robinson, 1998]. Much of this region is remote from large anthropogenic emissions of NO•. However, the photochemical release of NO• from snowpack NO•-may effect the recycling of photochemically active nitrogen oxides (NOx), extending the influence of NO• emissions to more remote regions.