| Summary: | We present preliminary results of shipboard measurements using the Queensland University of Technology Neutral Cluster and Air Ion Spectrometer (NAIS) carried on-board the research vessel Investigator on the Southern Ocean in February 2015. The NAIS detects both charged and neutral particles below 42 nm in diameter. During its voyage from Hobart to the Antarctic ice edge, the Investigator made two passes across the southern polar front - southbound on 6/7 Feb and northbound on 9/10 Feb. During each of the two passes, we observed significant increases in particle number concentration as the vessel entered the polar front. The particle data was ‘cleaned’ by removing the data at those times when the sampled air was affected by emissions from the ship’s exhaust, as identified by the corresponding black carbon data and obvious sharp spikes in concentration. After excluding such data, we determined that the number concentrations of the larger particle in the size range 7.5 to 42 nm outside and inside the frontal region were 618 ± 92 cm-3 and 1006 ± 348 cm-3, respectively. This difference was statistically significant at a confidence level of 99%. The particle size distribution was bimodal with peaks around 3 nm and 25 nm. A large fraction of the number of particles, approximately 50% of the total number in the NAIS range, was smaller than 5 nm both inside and outside the frontal band. We do not see this mode in land-based measurements. While accepting that it is unlikely to be a consequence of sea salt owing to the small particle sizes, we have no viable explanation for this observation at present.
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