NO_x during background and ozone depletion periods at Alert: Fluxes above the snow surface

Abstract

Measurements of nitric oxide (NO) and nitrogen dioxide (NO 2 ) 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 NO 2 showed clear diurnal cycles with noontime maxima during spring. In darkness, no NO x 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 NO x , whose noon‐time flux reached approximately 40 nmol m −2 h −1 . Measured NO x 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 NO x flux. The observed release of NO x 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 NO 2 measurements was determined; it amounted to 24% of the HONO mixing ratio.