Atmospheric
Deposition and its Impact on Ecosystems, with Reference to the Mid-East
Region:
Overview of International Symposium
Tel Aviv, Israel, June 4-5, 2000
R. Van Grieken and Y. Shevach
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Introduction
Symposium Issues
Topics of Discussion and Main Sessions:
- Wet and Dry Atmospheric Deposition - Long
Range Transport - Short Range Transport - Ozone
Depletion - Trace Metals - Organic
Pollutants - N and S Compounds - Saharan
Dust
Conclusions
Acknowledgment
Introduction
Atmospheric supplies of nitrogen and phosphorus, heavy metals, PCBs,
chlorinated pesticides and other persistent organic pollutants (POPs)
may play a major role in ecosystem dynamics, particularly in oligotrophic
marine areas like the South-East Mediterranean, in terrestrial ecosystems
and inland freshwater bodies. The interface between the atmosphere and
the sea plays a central role in the exchange of matter. Trace elements,
such as lead, cadmium and mercury, enter the sea to a considerable extent
via the atmospheric pathway. Over 50% of the nitrogen input to the North
Sea may be supplied by the atmosphere, with most of this being derived
from anthropogenic sources. In estuaries and seas, extra nutrients can
cause eutrophication, with enhanced growth of algae populations, and
subsequent oxygen deficiency when the dead algae material decomposes.
Hundreds of lakes and streams can no longer sustain life,
while the threat to forests and watersheds in many parts of the world
is growing. In Israel, the fresh water Sea of Galilee, providing 35%
of the water supply, is now showing a very unstable quality and it is
likely that the dustfall which can amount to 60% of the total solid
input into the lake, may exert a profound influence on the properties
and behaviour of the lake in general and on the water quality in particular.
In the Dead Sea area, ozone depletion was reported to coincide with
an interaction of atmospheric oxidants with bromide at the Dead Sea
salt pans.
In view of the growing threat of atmospheric deposition
and the increasing need to expand our knowledge in the field, an International
Symposium on Atmospheric Deposition and Impact on Ecosystems was convened
in Tel Aviv, Israel, on June 4-5, 2000. The symposium, organized by
IUPAC's Division of Environmental Chemistry and by the Israel Chemical
Society, brought together about 70 internationally recognized experts
from USA, Europe, the Mediterranean Region and Israel.
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Symposium Issues
- Atmospheric processes related to the Mid-East Region as a zone in
which air mass trajectories can trace both local pollution and the
influence of Europe and the Sahara as sources.
- Effects of the large efforts which have been made to reduce or control,
the emissions to air of SO2 and NOx, volatile
organic compounds, and more recently, of metals and POPs.
- Cycling of pollutants between the atmosphere and the ecosystem compartments
and new developments in experimental techniques for flux measurements.
- Review of atmospheric deposition studies covering the work of EMEP,
EUROTRAC, ASE, BIATEX and interaction with regional and local studies
in the Middle East.
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Topics of Discussion and Main Sessions
- Backward Trajectories of Air Masses over the Mediterranean
- Effects of Atmospheric Pollutants on the South-East Mediterranean
Region
- Saharan Dust
- Impact on Ecosystems
- Research Co-operation
Wet
and Dry Atmospheric Deposition
Atmospheric removal occurs by dry deposition of aerosol particles and
gases, or by wet deposition in rain, fog, hail and snow. The relative
importance of these two processes varies between locations and is primarily
a function of the rainfall intensity. E.g. at the North and West European
temperate latitudes, wet deposition amounts to about half of the total
atmospheric input of nitrogen to natural ecosystems.
The processes of dry deposition were described by C. Davidson
to include three major steps. The first step, aerodynamic transport,
carries contaminants from the free atmosphere into the relatively quiescent
layer close to the surface, which can be described using the friction
velocity, stability class, and other parameters pertinent to turbulent
flow. The second step is boundary layer transport of contaminants across
the viscous layer of air, immediately adjacent to surface. Analogies
with momentum and heat transport across boundary layers are often used
to describe contaminant mass transport in the viscous layer. The third
step refers to interactions of the contaminants with the surface. For
gases, this step describes adsorption and absorption interactions. For
particles, it is important whether they adhere to the surface or bounce
off.
The processes which govern the formation of rain, the scavenging
of aerosol particles both within and below clouds and the effects of
aerosol size on these processes were discussed by L. Spokes, including
the problems associated with the collection and determination of wet
fluxes, the chemical reactions occurring in rainwater and controlling
the pH and the aerosol solubility, and the importance of episodic high
concentration deposition events and their effect on surface water biogeochemistry.
Long
Range Transport
The East Mediterranean region is influenced by "European" air masses
with high anthropogenic pollutants. This import is compounded by the
scavenging of alkaline Saharan desert dust, which has a magnified effect
on the cloud physics and chemistry and subsequently on the natural precipitation,
cloud seeding and deposition onto terrestrial and aquatic ecosystems.
Recent investigations on long-range transport of European
pollutants to Israel were discussed by P. Alpert, who calculated monthly
cyclonic tracks over the Mediterranean based on ECMWF data. Summer back
trajectories from Tel Aviv pointed to sources spreading from South-East
Russia to South Europe, Spain and North Africa. Z. Levine speculated
that transported particles that pass through clouds are affected by
wet chemical reactions and by physical processes leading to the formation
of dust particles coated with soluble salts such as sulphates from industrial
origin or from dimethyl sulphide naturally emitted by the Mediterranean
Sea itself. Gas scavenging and subsequent liquid phase oxidation add
additional sulphate. The soluble coating of the mineral dust particles
could significantly change their ability to serve as cloud condensation
nuclei (CCN), causing the cloud water to redistribute into larger concentrations
of smaller droplets.
Sulphur-coated particles were reported D. Rosenfeld as preventing
rain drops from forming in low clouds and as a possible cause for lower
rainfall. "Polluted" clouds are composed of much smaller droplets, without
any precipitation echoes detectable by a Precipitation Radar.
Regional Atmospheric Modeling System (RAMS) for meteorological
simulations and the Hybrid Particle and Concentration Transport package
(HYPACT) for dispersion modelling were used by M. Luria to explain the
transport of polluted air masses towards the coast of Israel. Specific
synoptic and wind conditions over the East Mediterranean govern the
movement of polluted air masses towards Israel. In one case study involving
aircraft measurements , prevailing Noth-West winds forced the pollution
from South Europe and the Balkans into the E Mediterranean coast along
the180 km flight path, significantly increasing the averaged measured
concentrations of SO2, NOy, O3 and particulate sulphate.
Short
Range Transport
Aerial short-range-transport was described for pesticides by W. Kordel;
direct spray drift occurred during application and by volatilisation
from the target area in the post-application phase. Pesticide concentrations
in non-target plants exceeding 10 % of the actual pesticide concentration
in the target plants were observed in neighbouring non-target ecotones,
which may cause unintended effects on fauna and flora. Particularly
for semi-volatile pesticides with vapour pressures between 5*10-3 and
10-6 Pa, this is important. The observed shelter effects of downwind
hedges were low.
Ozone
Depletion
Photochemistry of NO2 is the main source of O3
in the ground and boundary layer. L. Klasinc showed that, in Croatia,
the O3 concentration has more than doubled recently and is
still rising. Vertical fluxes of O3 and energy over a plant
growth cycle in a large coniferous forest of South-West France were
reported by Lopez et al.; they indicated the effects of dry, wet and
dew conditions on the O3 deposition velocity and the effect
of the stomatal conductance on the increased deposition velocity during
the day.
Atmospheric reactive BrO was measured and its effect on
O3 ozone chemistry was assessed over the Dead Sea in Israel
by Peleg et al., who reported a negative diurnal repeating cycle of
O3 and BrO variations, correlated with solar radiation and wind direction.
During the elevated BrO events, O3 regularly decreased from
noon-time levels of 50-80 ppb or higher down to 10-30 ppb and occasionally
to levels below the detection limit of 5 ppb. Interaction of atmospheric
oxidants with bromide at the salt pans of the Dead Sea were stipulated
to be the source of BrO. The only other places where this kind of chemistry
occurs is over the Arctic and Antarctic.
Trace
Metals
A literature survey of published concentrations of atmospheric trace
metals above the North Sea and the English Channel over the period 1971-1994
was conducted by Van Grieken et al. Of the six trace metals, Cd, Cu,
Pb, Zn, Ni and Cr, that were evaluated, Pb, Zn and Cd showed a very
strong decreasing trend with time. Similarly, a seasonal variability
of atmospheric Pb concentrations over the English Channel was reported
by Puskaric et al.; they indicated a decrease by about one order of
magnitude over the last fifteen years and a different isotopic signature
for Pb aerosols from E Europe versus those originating in W Europe.
Cd, Cu, Pb, Zn and natural elements (Al, Fe, Mn, Cr) were also measured
in dry atmospheric inputs at the coast of Israel, where mixing and dilution
effects of European emissions with local emissions are taking place
(Herut et al.).
Fossil fuel combustion sources and incinerators are the
major sources of reactive gas phase Hg (RGM). Compared to elemental
Hg° which has a slow removal rate from the atmosphere (low solubility
in water), RGM is extremely water-soluble and efficiently removed from
the atmosphere during rain events. Hg deposition studies are being conducted
by Mamane et al. for Hg, RGM and total particulate mercury (TPM) to
model the Hg emission in Europe, its transport and deposition on the
S Baltic Sea and the Mediterranean Sea.
Organic
Pollutants
Gas-phase POPs over Lake Michigan have been studied by Hornbuckle et
al., who employed the Lake Michigan Mass Balance Project to identify
the large summertime 'plume' of gas-phase PCBs from Chicago, differentiating
between short range (higher molecular weight congeners) and long range
atmospheric transport (lower molecular weight congeners).
N
and S Compounds
Due to the discontinued used of soft coal, emissions of SO2
over Europe have been reduced by 55% since 1980, resulting in lower
concentrations of S-components in air and precipitation. In the meantime,
the emissions of NOx, contributing to acidification and to
photochemistry, remain very much the same today as in 1980, although
long term series of nitrate concentrations in precipitation between
the early 80s to present seem to indicate that concentrations have been
falling (Schaug et al. & N.O. Jensen). It was shown that large inputs
of alkaline species and a relatively high abundance of Ca and NH4+
cause neutralisation of the acid rain. In this context, a rather neutral
value of pH 6.4 in the precipitation of Ankara, Turkey, was reported
by Incecik.
Saharan
Dust
The Saharan dust is an important component in the wet deposition, accounting
for approximately half of the annual deposition of Al and Fe in the
East Mediterranean region. Tuncel et al. showed that the dust particles
are potential CCNs, an atmospheric sink for trace gases and a major
factor in the neutralisation of rain acidity by CaCO3. Rudich
et al. further suggested that mineral particles are coated with organic
compounds, which can potentially influence the hydrophilic behaviour
of the particles and their optical properties. It was also speculated
by Danin that airborne dust trapped in vegetation is an important factor
in the amelioration of growth conditions and functions as a trap of
eolian dust, avoiding soil erosion. Ganor et al. showed that dustfall
generated locally and dust storms originating in North Africa are deposited
over the Sea of Galilee in Israel. Calcite, quartz, feldspar, dolomite
and to a lesser extent, gypsum and halite, and clay minerals, like kaolinite,
illite and palygorskite, can be as much as 60% of the total solid input
into the lake.
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Conclusions
The symposium and the associated discussions focussed on the issue of
atmospheric deposition and their impact on ecosystems and highlighted
ongoing research and problems that still need monitoring. The symposium
provided the scientific basis for effective emission, mitigation and
adaptation policies, emphasising:
- the numerous sources of polluting emissions,
- the chemical transformation processes of pollutants in the atmosphere,
- the relationship between sources and effects which is not always
easy to establish,
- the deficiency of point measurements which are not sufficient to
characterise air quality in a particular region since the information
about transport or atmospheric chemistry is missing,
- the need for integration of atmospheric pollutant concentrations
and deposition which are intricately related and need to be studied
together.
The symposium also emphasised the need for:
- improving the knowledge on cause-effect relationships between air
pollution and factors affecting conditions and health of ecosystems,
- improving prediction and detection,
- better ways of monitoring the environment,
- international standards for measuring and reporting emissions and
- the necessary regional and international co-operation.
These issues are of a global dimension and regional and
international co-ordination is highly essential. Therefore, the role
of conventions, e.g. the Helsinki Commission (HELCOM) for the Baltic
Sea, the Oslo and Paris Commission (OSPAR) for the North Sea and North-East
Atlantic, the Barcelona Convention for the Mediterranean and the UNDP
Regional Seas programme should be strengthened, as well as the institutions
involved in measurement of air and precipitation quality and data base
formation, such as the Norwegian Institute for Air Research (NILU).
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Description
Acknowledgment
The authors are grateful to IUPAC and all the contributors who made
this symposium a successful event.
[also published in Chem. Int. 22(6), 2000]
