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, Leeds, LS2 9JT, UK
4School of Environmental Sciences, UEA, Norwich, NR4 7TJ, UK
*now at: Facility for Airborne Atmospheric Measurements, NERC, Cranfield, Bedfordshire, MK43 0AL, UK
**now at: School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, B15 2TT, UK
***now at: Department of Chemistry, University of York, York, YO10 5DD, UK
****now at: Atomic and Molecular Physics Division, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
*****now at: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
Abstract. NOx measurements were conducted at the Halley Research Station, 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 molecules 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 1.7 to 3.4×108 molecules 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, with a potential smaller contribution from HNO4 formation and 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. These results suggest that the role of halogen oxides need to be considered when interpreting the isotopic signature of nitrate impurities held within snow and ice.