Abstract
AbstractAerosol 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 dimethyl 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 2007. Previous model results and midlatitude observations show that iodine compounds consistent with the large values of IO observed may be responsible for an increase 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 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.
Highlights
Iodine compounds are accepted as a source of new tropospheric particles [O’Dowd et al, 2002; Mahajan et al, 2010]; iodine compounds are found in large concentrations in and above the sea ice of the Weddell Sea in Antarctica in spring and summer [Atkinson et al, 2012; Schönhardt et al, 2008]; and new particles have been observed above the sea ice of the Weddell Sea in spring and summer [Davison et al, 1996; Atkinson et al, 2012]
We conclude that iodine compounds may contribute to the secondary production of significant numbers of aerosol particles measured by Condensation Particle Counters on the Weddell Sea coast during spring, because of the clear correlation we observe between iodine oxide (IO) and particle density
The correlation we observe between IO and particle density seems to be improved if the IO is lagged by about 8 days, which leads us to suggest that many of the iodine compounds and associated particle production occur some distance offshore
Summary
Iodine compounds are accepted as a source of new tropospheric particles [O’Dowd et al, 2002; Mahajan et al, 2010]; iodine compounds are found in large concentrations in and above the sea ice of the Weddell Sea in Antarctica in spring and summer [Atkinson et al, 2012; Schönhardt et al, 2008]; and new particles have been observed above the sea ice of the Weddell Sea in spring and summer [Davison et al, 1996; Atkinson et al, 2012]. Frequent episodes of particle production in the Weddell Sea in spring and summer are expected. If this occurs near coastal stations where particle numbers are routinely measured, we might expect bursts of particles in the data. There is clearly some degree of correlation of the summer maxima in sulphate and MSA with features of the annual cycle in particles. We show that this secondary maximum in particles in spring arises in many years at two separate Antarctic stations and appears to be correlated by eye with observed iodine oxide (IO) near. A variety of measurements were made from the Clean Air Sector Laboratory (CASLab) located about 1 km south-east of the station, between 2004 and 2012
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