Jökull - 01.12.1990, Page 102

units, from the average pH of streams and lakes, is
common during these episodes. Laboratory and field
experiments indicate that 50-80% of the pollutants in
snow are released with the first 30% of the meltwa-
ter and that the very first fraction of meltwater may
contain more than 5 times the average snowpack con-
centrations (Johannessen and Henriksen, 1978). Al-
though it is known that small amounts of salts and
pollutants may be included in solid solutions in ice
crystals (Fletcher, 1970) there are reasons to believe
that most of the salts and pollutants are found on the
surface of the snow crystals, readily available to the
early meltwaters (Johannessen and Henriksen, 1978).
A large portion of the yearly precipitation in Ice-
land falls as snow. The frequent alternation of freeze
and thaw periods in the Lowlands of Iceland in the
winter time, will probably hinder the accumulation of
sea-salts and plausible pollutants into a massive snow-
pack to be released during a short melting period in
spring. However, the reverse might be true in the
Highlands and therefore on the glaciers of Iceland.
The presence of particles and ions in meltwater
from the polar regions and other remote parts of the
world, demonstrates that the transport of gases and
aerosols is a regular occurrence, world wide. The ma-
joraerosol sources are: oceanic aerosol (lOOOmillion
tonnes/year), soil or crust derived aerosol, sometimes
referred to as continental aerosol (100 to 1000 mil-
lion tonnes/year), extraterrestrial dust ( 0.1-1 million
tonnes/year), and anthropogenic and volcanic mate-
rials (Shaw, 1989). Today’s yearly deposit of sulfu-
ric and nitric acids in S.E. Greenland, attributed to
anthropogenic activities, is 4 and 2 times the yearly
amount deposited prior to the years 1900 and 1950
respectively. At present, the yearly sulfate deposit
appears to have levelled off but the yearly nitrate de-
posit continues to increase (Clausen and Langway,
1989). Oceanic aerosol is abundant on land close to
the oceans but continental and anthropogenic aerosols
often dominate in the center of the continents (Junge
and Werby, 1958). The quantity of volcanic aerosol
can be enormous in the vicinity of erupting volcanoes
and even worldwide during major eruptions.
Evaporated sea water droplets that form as jet
drops from rising bubbles or result from the disruption
of surface films in breaking ocean waves constitutethe
source of sea salt particles, often referred to as oceanic
or marine aerosol. These particles range from a few
tenths of a micrometer to more than 100 microme-
ters in diameter. There are about 0.5 to 2 sea salt
particles with radii greater than a micrometer (/im) in
each cubic centimeter of air in the marine environ-
ment (Shaw, 1989). This translates to a concentration
of 10-20 /íg/m3 close to the ocean surface. The con-
centration decreases rapidly with altitude and is less
than 1 % of the near ocean surface concentration at 2-
3 km altitude. Along coastlines, the sea salt aerosols
reach greater altitude than over the ocean because of
increased wind turbulence over land (Shaw, 1989).
The transport of aerosols from the atmosphere to
the surfaces of glaciers may occur by wet or dry depo-
sition. Wet deposition refers to the removal of gases
and particles from the atmosphere by precipitation.
Wet deposition generally dominates (Davidson, 1989)
and proceeds in three main steps: (1) nucleation, (2)
in-cloud scavenging by existing cloud droplets and ice
crystals, and (3) below-cloud scavenging.
The snow in Greenland and Antarctica contains a
mixture of small amounts of sea salt and two mineral
acids, H2SO4 and HNO3 which are attributed to an-
thropogenic activities. The continental contribution
is very weak. However, during the ice ages, the in-
put of crustal material played a very significant role,
while the sea salt contribution is also increased, but
to a lesser extent. The major cations in snow from
Greenland and Antarctica are H+, Na+, K+, NH4+,
Mg2+ and Ca2+ and the major anions are Cl_, S042-
and N03_(Delmas and Legrand, 1989).
The purpose of this study is, to determine the
chemistry of the 1987-1988 precipitation on the
Vatnajökull glacier in SE-Iceland, to look for spatial
changes in the snow chemistry, to study the preferen-
tial release of salts and pollutants caused by the partial
melting of snow, and finally to develop a model to
analyse the effect of the degree of partial melting of
snow upon the pH of meltwater.
98 JÖKULL, No. 40, 1990