Summertime NOx measurements during the CHABLIS campaign: can source and sink estimates unravel observed diurnal cycles?
- 1British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK
- 2School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
- 3School of Earth and the Environment, University of Leeds, LS2 9JT, UK
- 4School of Environmental Sciences, UEA, Norwich NR4 7TJ, UK
- *now at: Facility for Airborne Atmospheric Measurements, Natural Environment Research Council, Cranfield, Bedfordshire, MK43 0AL, UK
- **now at: School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, B15 2TT, UK
- ***now at: University of York/National Centre for Atmospheric Science, Department of Chemistry, University of York, York, YO10 5DD, UK
- ****now at: Laboratorio de Ciencias de la Atmósfera y el Clima, Consejo Superior de Investigaciones Científicas (CSIC), c/Rio Cabriel S/N, 45007 Toledo, Spain
- *****now at: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
Abstract. NOx measurements were conducted at the Halley Research Station, coastal Antarctica, during the austral summer period 1 January–10 February 2005. A clear NOx diurnal cycle was observed with minimum concentrations close to instrumental detection limit (5 pptv) measured between 04:00–05:00 GMT. NOx concentrations peaked (24 pptv) between 19:00–20:00 GMT, approximately 5 h after local solar noon. An optimised box model of NOx 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 NOx 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 HOx and halogen oxidation processes show that the lifetime of NOx 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 NOx concentrations by decreasing the NO/NO2 ratio. We conclude that in coastal Antarctica, the potential ozone production efficiency of NOx emitted from the snowpack is mitigated by the more rapid NOx loss due to halogen nitrate hydrolysis.