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 Chemistry and the Environment Division (VI)

Commission on Atmospheric Chemistry
Commission on Soil and Water Chemistry


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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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


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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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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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]

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