Articles | Volume 15, issue 23
Atmos. Chem. Phys., 15, 13339–13364, 2015
Atmos. Chem. Phys., 15, 13339–13364, 2015

Research article 02 Dec 2015

Research article | 02 Dec 2015

Boundary layer new particle formation over East Antarctic sea ice – possible Hg-driven nucleation?

R. S. Humphries1,a, R. Schofield2,3, M. D. Keywood4, J. Ward4, J. R. Pierce5, C. M. Gionfriddo2, M. T. Tate6, D. P. Krabbenhoft6, I. E. Galbally4, S. B. Molloy4, A. R. Klekociuk7, P. V. Johnston8, K. Kreher8,b, A. J. Thomas8, A. D. Robinson9, N. R. P. Harris9, R. Johnson10, and S. R. Wilson1 R. S. Humphries et al.
  • 1Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, Australia
  • 2School of Earth Sciences, University of Melbourne, Melbourne, Australia
  • 3ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, Australia
  • 4CSIRO Ocean and Atmosphere Business Unit, Aspendale, Australia
  • 5Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA
  • 6United States Geological Survey, Wisconsin, USA
  • 7Australian Antarctic Division, Hobart, Australia
  • 8National Institute of Water and Atmospheric Research, Lauder, New Zealand
  • 9Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, England
  • 10Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia
  • anow at: CSIRO Ocean and Atmosphere Business Unit, Aspendale, Australia
  • bnow at: Bodeker Scientific, Alexandra, New Zealand

Abstract. Aerosol observations above the Southern Ocean and Antarctic sea ice are scarce. Measurements of aerosols and atmospheric composition were made in East Antarctic pack ice on board the Australian icebreaker Aurora Australis during the spring of 2012. One particle formation event was observed during the 32 days of observations. This event occurred on the only day to exhibit extended periods of global irradiance in excess of 600 W m−2. Within the single air mass influencing the measurements, number concentrations of particles larger than 3 nm (CN3) reached almost 7700 cm−3 within a few hours of clouds clearing, and grew at rates of 5.6 nm h−1. Formation rates of 3 nm particles were in the range of those measured at other Antarctic locations at 0.2–1.1 ± 0.1 cm−3 s−1. Our investigations into the nucleation chemistry found that there were insufficient precursor concentrations for known halogen or organic chemistry to explain the nucleation event. Modelling studies utilising known sulfuric acid nucleation schemes could not simultaneously reproduce both particle formation or growth rates. Surprising correlations with total gaseous mercury (TGM) were found that, together with other data, suggest a mercury-driven photochemical nucleation mechanism may be responsible for aerosol nucleation. Given the very low vapour pressures of the mercury species involved, this nucleation chemistry is likely only possible where pre-existing aerosol concentrations are low and both TGM concentrations and solar radiation levels are relatively high (∼ 1.5 ng m−3 and ≥ 600 W m−2, respectively), such as those observed in the Antarctic sea ice boundary layer in this study or in the global free troposphere, particularly in the Northern Hemisphere.

Short summary
An atmospheric new particle formation event that was observed in the pristine East Antarctic pack ice during a springtime voyage in 2012 is characterised in terms of formation and growth rates. Known nucleation mechanisms (e.g. those involving sulfate, iodine and organics) were unable to explain observations; however, correlations with total gaseous mercury were found, leading to the suggestion of a possible mercury-driven nucleation mechanism not previously described.
Final-revised paper