OH and halogen atom influence on the variability of non-methane hydrocarbons in the Antarctic Boundary Layer

Measurements of C₂–C₈ non-methane hydrocarbons (NMHCs) have been made in situ at Halley Base, Antarctica (75◦35°S, 26◦19°W) from February 2004 to February 2005 as part of the Chemistry of the Antarctic Boundary Layer and the Interface with Snow (CHABLIS) experiment. The data show long- and short-term variabilities in NMHCs controlled by the seasonal and geographic dependence of emissions and variation in atmospheric removal rates and pathways. Ethane, propane, iso-butane, n-butane and acetylene abundances followed a general OH-dependent sinusoidal seasonal cycle. The yearly averages were 186, 31, 3.2, 4.9 and 19 pptV, respectively, lower than those which were reported in some previous studies. Superimposed on a seasonal cycle was shorter-term variability that could be attributed to both synoptic airmass variability and localized loss processes due to other radical species. Hydrocarbon variability during periods of hour-to-day-long surface O₃ depletion in late winter/early spring indicated active halogen atom chemistry estimated to be in the range 1.7 × 103–3.4 × 10⁴ atom cm⁻³ for Cl and 4.8 × 10⁶–9.6 × 10⁷ atom cm⁻³ for Br. Longer-term negative deviations from sinusoidal behaviour in the late August were indicative of NMHC reaction with a persistent [Cl] of 2.3×10³ atom cm⁻³.Maximum ethene and propene of 157 and 179 pptV, respectively, were observed in the late February/early March, consistent with increased oceanic biogenic emissions; however, their presence was significant year-round (June–August concentrations of 17.1 ± 18.3 and 7.9 ± 20.0 pptV, respectively).