Release of NOx from sunlight-irradiated midlatitude snow

Photochemical production and release of gasphase NOx (NO + NO2) 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 NOx mixing ratio were observed in synthetic and ambient air pulled through the sunlit chamber. [NOx] enhancements were correlated to ultraviolet sunlight intensity, reaching similar to 300 pptv under partially overcast midday, midwinter conditions. These findings are consistent with NOx production from photolysis of snowpack NO3- the observed NOx release implies production of significant amounts of OH within the snow. Snowpack NO3- photolysis may therefore significantly alter boundary layer levels of both NOx and oxidized compounds over wide regions of the atmosphere.