Particles and iodine compounds in coastal Antarctica

Aerosol particle number concentrations have been measured at Halley and Neumayer on the Antarctic coast, since 2004 and 1984 respectively. Sulphur compounds known to be implicated in particle formation and growth were independently measured: sulphate ions and methane sulphonic acid in filtered aerosol samples; and gas-phase di-methyl sulphide for limited periods. Iodine oxide, IO, was determined by a satellite sensor from 2003 to 2009, and by different ground-based sensors at Halley in 2004 and in 2007. Previous model results and mid-latitude observations show that iodine compounds consistent with the large values of IO observed may be responsible for an incresase in number concentrations of small particles. Coastal Antarctica is useful for investigating correlations between particles, sulphur and iodine compounds, because of their large annual cycles and the source of iodine compounds in sea ice. After smoothing all the measured data by several days, the shapes of the annual cycles in particle concentration at Halley and at Neumayer are approximated by linear combinations of the shapes of sulphur compounds and IO, but not by sulphur compounds alone. However, there is no short-term correlation between IO and particle concentration. The apparent correlation by eye after smoothing but not in the short term suggests that iodine compounds and particles are sourced some distance offshore. This suggests that new particles formed from iodine compounds are viable, i.e. they can last long enough to grow to the larger particles that contribute to Cloud Condensation Nuclei, rather than being simply collected by existing particles. If so there is significant potential for climate feedback near the sea ice zone via the aerosol indirect effect.