NICE
Measurements of atmospheric and snow nitrates and nitrites and their
fluxes above the snow surface were made during two campaigns during
spring 2001 at Ny-Ålesund, Svalbard as part of the EU project "The
NItrogen Cycle and Effects on the oxidation of atmospheric trace species
at high latitudes" (NICE).
Recent findings of NOx and HONO production in snow interstitial air
showed that photochemical production of NOx in snow surfaces is
sufficient to alter the composition of the overlying atmosphere. The
possible effects of this reaction cycle are fivefold. 1) reactivation of
nitrate extends the influence of NOx emissions. 2) the resulting
emissions of NOx may alter the rate of tropospheric O3 production. 3) if
some of the released NOx is exported, nitrate deposition to snowpacks may
be less than previously believed. 4) in remote regions, the production of
OH radicals within the snowpack or from photolysis of released HONO may
surpass OH production from O3 photolysis. 5) the reactivation of NO3- is
apparently coupled with oxidation of snowpack organic matter, resulting
in release of oxidized compounds such as aldehydes.
At Ny-Ålesund only HNO3 showed a significant flux above the snow surface,
a mean deposition of -8.7 nmol h-1m-2 was observed in late April/early
May 2001. The main source for N is wet deposition in falling snow. Other
measured nitrogen species showed no significant fluxes which may be in
part due to the measurement uncertainties and long integration times of
the used denuder technique, and atmospheric turbulence and mixing. These
HNO3 fluxes may be due to the reaction of HNO3 with NaCl, or may be
simply uptake of HNO3 by ice, which is alkaline because of the sea salt
in our marine environment.
The surface snow at Ny-Ålesund showed very complex stratigraphy; the NO3-
mixing ratio in snow varied between 65 and 520 ng g-1, the total NO3-
content of the snowpack was on the order of 2700 ng cm-2. In comparison
the atmospheric boundary layer column showed a NO3- content of only 8 ng
cm-2. The limited exchange, between the snow and the atmosphere was
attributed to low mobility of NO3- in the observed snow. NO3- was mostly
connected to Ca2+ in alkaline snow.
HNO3 is not a major contributor to the NO3- signal in snow. This is not
surprising, as NO3- arrives to the snow surface as Ca(NO3)2, and is
immobile. Thus exchanges are limited. Also photolysis will also have only
limited effects, since reaction products may be immobilized inside a
solid. This is unlike continental Arctic sites, where NO3- was in the
absence of NaCl, (in acidic ice) in more mobile forms.
The results of NICE show that the reactivation of NOx from snow nitrate
is an important source of NOx over snow surfaces. These results are very
important in formulating possible scenarios in more polluted mid latitude
snow-covered European environments.
NICE investigated a new fundamental chemical mechanism; a property of
snow interstitial air, that was formerly not recognized.
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