NOx during background and ozone depletion periods at Alert:: Fluxes above the snow surface -: art. no. 4584

Measurements of nitric oxide (NO) and nitrogen dioxide (NO2) at Alert, Nunavut, Canada showed median background mixing ratios of 0.2 and 1.3 pmol mol(-1), respectively, during darkness in late winter 2000, and 2.8 and 10.8 pmol mol 1 during spring in 24-hour light. Both NO and NO2 showed clear diurnal cycles with noontime maxima during spring. In darkness, no NOx exchange between the snow surface and the overlying atmosphere was detected. During the period of 24-hour sunlight, the snow surface constituted a source of NOx, whose noon-time flux reached approximately 40 nmol m(-2) h(-1). Measured NOx fluxes were roughly equal to HONO fluxes reported during the Alert campaign. The fluxes were correlated to ultraviolet light intensity, but anticorrelated to wind speeds. During 2 days of high wind speeds under O-3 depletion conditions, the fluxes were not significantly different from zero. However, under low wind speeds during the O-3 depletion event, the snowpack continued to emit a detectable NOx flux. The observed release of NOx and HONO during the sunlit period was small relative to the observed decrease in the snowpack surface-layer nitrate inventory. Finally, as part of this study, the nitrous acid (HONO) interference in the Xe-lamp-based photolytic NO2 measurements was determined; it amounted to 24% of the HONO mixing ratio.