Abstract

Abstract. Two consecutive cruises in the Weddell Sea, Antarctica, in winter 2013 provided the first direct observations of sea salt aerosol (SSA) production from blowing snow above sea ice, thereby validating a model hypothesis to account for winter time SSA maxima in the Antarctic. Blowing or drifting snow often leads to increases in SSA during and after storms. For the first time it is shown that snow on sea ice is depleted in sulfate relative to sodium with respect to seawater. Similar depletion in bulk aerosol sized ∼0.3–6 µm above sea ice provided the evidence that most sea salt originated from snow on sea ice and not the open ocean or leads, e.g. >90 % during the 8 June to 12 August 2013 period. A temporally very close association of snow and aerosol particle dynamics together with the long distance to the nearest open ocean further supports SSA originating from a local source. A mass budget estimate shows that snow on sea ice contains even at low salinity (<0.1 psu) more than enough sea salt to account for observed increases in atmospheric SSA during storms if released by sublimation. Furthermore, snow on sea ice and blowing snow showed no or small depletion of bromide relative to sodium with respect to seawater, whereas aerosol was enriched at 2 m and depleted at 29 m, suggesting that significant bromine loss takes place in the aerosol phase further aloft and that SSA from blowing snow is a source of atmospheric reactive bromine, an important ozone sink, even during winter darkness. The relative increase in aerosol concentrations with wind speed was much larger above sea ice than above the open ocean, highlighting the importance of a sea ice source in winter and early spring for the aerosol burden above sea ice. Comparison of absolute increases in aerosol concentrations during storms suggests that to a first order corresponding aerosol fluxes above sea ice can rival those above the open ocean depending on particle size. Evaluation of the current model for SSA production from blowing snow showed that the parameterizations used can generally be applied to snow on sea ice. Snow salinity, a sensitive model parameter, depends to a first order on snowpack depth and therefore was higher above first-year sea ice (FYI) than above multi-year sea ice (MYI). Shifts in the ratio of FYI and MYI over time are therefore expected to change the seasonal SSA source flux and contribute to the variability of SSA in ice cores, which represents both an opportunity and a challenge for the quantitative interpretation of sea salt in ice cores as a proxy for sea ice.

Highlights

  • Atmospheric aerosol represents the largest source of uncertainty in global climate predictions (Boucher et al, 2013) and includes sea salt aerosol (SSA), which is the main background aerosol above the oceans

  • SSA is measured in polar ice cores, but its use as a quantitative proxy of past sea ice conditions is complicated by uncertainties related to SSA source contributions and processes as well as transport meteorology (Abram et al, 2013; Levine et al, 2014; Rhodes et al, 2017), and more recently in the case of bromide Br− (e.g. Spolaor et al, 2013) to post-depositional processing associated with the bromine explosion chemistry of ozone depletion events (ODEs) (Simpson et al, 2005; Pratt et al, 2013)

  • Near-zero or positive ambient temperatures Ta in degrees Celsius occurred when RV Polarstern was in the open ocean at the start and end of ANT-XXIX/6, as well as from the 22 July 2013 onwards, when the ship had moved into the marginal sea ice zone (MIZ) closer to open water

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Summary

Introduction

Atmospheric aerosol represents the largest source of uncertainty in global climate predictions (Boucher et al, 2013) and includes sea salt aerosol (SSA), which is the main background aerosol above the oceans. SSA plays an important role in polar tropospheric ozone and halogen chemistry through the release of active bromine in polar spring contributing to ozone depletion events (ODEs) SSA is measured in polar ice cores, but its use as a quantitative proxy of past sea ice conditions is complicated by uncertainties related to SSA source contributions and processes as well as transport meteorology (Abram et al, 2013; Levine et al, 2014; Rhodes et al, 2017), and more recently in the case of bromide Br− SSA is measured in polar ice cores, but its use as a quantitative proxy of past sea ice conditions is complicated by uncertainties related to SSA source contributions and processes as well as transport meteorology (Abram et al, 2013; Levine et al, 2014; Rhodes et al, 2017), and more recently in the case of bromide Br− (e.g. Spolaor et al, 2013) to post-depositional processing associated with the bromine explosion chemistry of ODEs (Simpson et al, 2005; Pratt et al, 2013)

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