Summertime NO_x measurements during the CHABLIS campaign: can source and sink estimates unravel observed diurnal cycles?

Abstract. NO_x measurements were conducted at the Halley Research Station, coastal Antarctica, during the austral summer period 1 January–10 February 2005. A clear NO_x diurnal cycle was observed with minimum concentrations close to instrumental detection limit (5 pptv) measured between 04:00–05:00 GMT. NO_x concentrations peaked (24 pptv) between 19:00–20:00 GMT, approximately 5 h after local solar noon. An optimised box model of NO_x concentrations based on production from in-snow nitrate photolysis and chemical loss derives a mean noon emission rate of 3.48 × 10⁸ molec cm⁻² s⁻¹, assuming a 100 m boundary layer mixing height, and a relatively short NO_x lifetime of ~6.4 h. This emission rate compares to directly measured values ranging from 2.1 to 12.6 × 10⁸ molec cm⁻² s⁻¹ made on 3 days at the end of the study period. Calculations of the maximum rate of NO₂ loss via a variety of conventional HO_x and halogen oxidation processes show that the lifetime of NO_x is predominantly controlled by halogen processing, namely BrNO₃ and INO₃ gas-phase formation and their subsequent heterogeneous uptake. Furthermore the presence of halogen oxides is shown to significantly perturb NO_x concentrations by decreasing the NO/NO₂ ratio. We conclude that in coastal Antarctica, the potential ozone production efficiency of NO_x emitted from the snowpack is mitigated by the more rapid NO_x loss due to halogen nitrate hydrolysis.