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First direct observation of sea salt aerosol production from blowing snow above sea ice
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.
This research has been supported by the Natural
Environment Research Council (UK) through the BLOWSEA
project (grant nos. NE/J023051/1 and NE/J020303/1
Airborne mercury species at the Råö background monitoring site in Sweden: distribution of mercury as an effect of long-range transport
Within the EU-funded project, Global Mercury Observation System (GMOS)
airborne mercury has been monitored at the background Råö measurement
site on the western coast of Sweden from mid-May 2012 to the beginning of
July 2013 and from the beginning of February 2014 to the end of May 2015. The
following mercury species/fractions were measured: gaseous elemental mercury
(GEM), particulate bound mercury (PBM) and gaseous oxidised mercury (GOM)
using the Tekran measurement system. The mercury concentrations measured at
the Råö site were found to be low in comparison to other, comparable,
European measurement sites. A back-trajectory analysis to study the origin of
air masses reaching the Råö site was performed. Due to the remote
location of the Råö measurement station it receives background air
about 60 % of the time. However, elevated mercury concentrations arriving
with air masses coming from the south-east are noticeable. GEM and PBM
concentrations show a clear annual variation with the highest values
occurring during winter, whereas the highest concentrations of GOM were
obtained in spring and summer. An evaluation of the diurnal pattern of GOM,
with peak concentrations at midday or in the early afternoon, which often is
observed at remote places, shows that it is likely to be driven by local
meteorology in a similar way to ozone. Evidence that a significant part of
the GOM measured at the Råö site has been formed in free tropospheric
air is presented