Photo-oxidants, Acidification and Tools; Policy applications of EUROTRAC results

(Executive Summary)

Peter Borrell, Peter Builtjes, Peringe Grennfelt, Øystein Hov, Roel van Aalst, David Fowler, Gérard Mégie, Nicolas Moussiopoulos, Peter Warneck, Andreas Volz-Thomas, Richard Wayne

The report itself is volume 10 of the final report of EUROTRAC.

In this report, the scientific results from EUROTRAC have been assimilated by the Application Project and the principle findings are presented in a condensed form, suitable for use by those responsible for environmental planning and management in Europe. EUROTRAC and the Application Project

EUROTRAC and the Application Project

In most European countries, air pollution exceeds the acceptable national and international standards but, as many pollutants are transported great distances in the air, only international measures will be successful in control and abatement. The initial measures taken under the Convention on the Long Range Transport of Air Pollution (LRTAP) are bringing some benefits but it is clear that further reductions in emissions will probably be very expensive to implement. Future cost effective abatement strategies will only be successful if they are underpinned by a thorough understanding of the detailed atmospheric processes in which the pollutants are produced, transformed, consumed, transported and deposited from the air. EUROTRAC, the policy relevant findings of which are discussed in this report, was set up in 1985 to help provide the improved scientific understanding necessary for future policy development in the field of air pollution control and abatement. It was recognised that the scientific problems associated with trans-boundary air pollution could only be solved by an international high level state-of-the-art inter-disciplinary project.

EUROTRAC is a co-ordinated environmental research programme, within the EUREKA initiative, studying the transport and chemical transformation of trace substances in the troposphere over Europe. The project consists of more than 250 research groups in 24 European countries and is organised into 14 subprojects. (Section 1.4.1) The EUROTRAC Application Project (AP) was set up

"to assimilate the scientific results from EUROTRAC and present them in a condensed form, together with recommendations where appropriate, so that they are suitable for use by those responsible for environmental planning and management in Europe".

Three themes were addressed by the AP and are presented in this report:

The first two themes bear directly on issues of environmental concern in Europe. The third, "tools", is a recognition that, while some of the work done during the project has found use immediately, much will find its application in the longer term either directly, by serving applications to policy, or indirectly, by incorporation into the general understanding. (Section 1.4.2)

  1. Policy applications of EUROTRAC scientific results The research accomplished in EUROTRAC provides substantial scientific support for the negotiations in the second generation of abatement strategy protocols under the UN-ECE Convention on the Long Range Transport of Air Pollution (LRTAP), in particular the second revised NOx protocol and the revisions of the recently signed sulphur protocol. It also finds application for work within the European Environmental Agency, the EU Framework and ozone directives, and in the development of national strategies. EUROTRAC models and measurements are also used by WMO/UNEP and the IPCC for assessment of the current ozone budget and its sensitivity to changes in precursor concentrations.

    The application of effects-based control strategies by European governments, in which the maximum environmental benefit is being sought for the investment in control technology, places great demands on our knowledge and understanding of the links between sources, deposition and effects. Within EUROTRAC, major developments in the science for developing these links have been made.

    Photo-oxidants in Europe: in the free troposphere, in rural and in urban atmospheres

    The concentration of photo-oxidants in Europe is strongly influenced by photochemical production from man made precursors that are emitted within the region

  2. The concentration of photo-oxidants in Europe is strongly influenced by photochemical production from man-made precursors that are emitted within the region From experimental studies it is concluded that the natural background of ozone over Europe at the turn of the century, within the atmospheric boundary layer, was about 10 to 15 ppb at ground level and 20 to 30 ppb one to two kilometres above the ground. Today, the concentration of ozone near the sea surface is 30 to 35 ppb before air masses move into Europe from the west. On a seasonal basis, photochemical processes over western and central Europe add about 30 to 40 % to this background in summer, and subtract about 10% in winter.

    Within Europe very high concentrations of more than 100 ppb are observed during photochemical episodes under unfavourable meteorological conditions, i.e. high solar radiation combined with stagnant air or circulating wind systems.

    In the free troposphere, that is from the top of the atmospheric boundary layer (1 to 2 km above the ground) to the tropopause which constitutes the boundary with the stratosphere (10 to 12 km above the ground), the background concentration before the air masses pass over Europe is higher than in the atmospheric boundary layer, being about 40 to 50 ppb in winter and autumn and 50 to 70 ppb in spring and summer.

    The concentration of free tropospheric ozone over Europe is influenced not only by European emissions but also by North American and Japanese emissions.

  3. Tropospheric ozone in the northern hemisphere has increased since the 1950s The concentration of ozone in the troposphere north of 20oN has increased since the beginning of modern measurements. This increase was larger at northern mid-latitudes than in the tropics, and larger over Europe and Japan than over North America. Comparison with historical data suggests that ozone in the troposphere over Europe has doubled since the turn of the century and that most of the increase has occurred since the 1950s. Measurements of nitrate in ice cores from Alpine glaciers provide strong circumstantial evidence for man-made emissions being responsible for the observed ozone trend.

  4. Tropospheric ozone increase slowed down in the 1980s Long-term observations show that the increase of ozone in the free troposphere was smaller in the eighties than in the seventies. The average ozone concentrations in the boundary layer near the ground have even decreased at some locations, for example at Garmisch-Partenkirchen in Germany and at Delft, a polluted site in the Netherlands. The concentration of peroxyacetylnitrate (PAN), a photo-oxidant like ozone, increased by a factor of three at Delft in the 1970s and stabilised in the 1980s.

  5. Weekday/weekend differences in the emissions of ozone precursors The understanding of the temporal and spatial resolution of the emissions of NOx and VOC has improved considerably. Results show for example that emissions are approximately 30% lower at weekends than during the week. Such variations provide a regular "natural experiment" for examining the effects of the short term reductions of emissions. The effects of the reductions on the photo-oxidant concentrations in this case appear to be rather small.

  6. Should NOx or VOC be controlled, or both? Recent model simulations have suggested that the effective abatement of elevated ozone concentrations in Europe require the reduction of the emissions of both NOx and VOC, with more emphasis being put on VOCs, especially in north-west and central Europe. However some field experiments in EUROTRAC have identified possible shortcomings in the models that are presently used for quantifying ozone/VOC and ozone/NOx relationships. These experiments emphasise the greater importance of NOx emissions in controlling the photochemical ozone balance. The reasons are:

    Based on today's knowledge, the following picture emerges:

  7. Are photo-oxidants a home made or trans-boundary problem? The pre-industrial concentration of ozone of about 10 to 15 ppb at ground level resulted from the approximate balance between the transfer of ozone from the stratosphere to the troposphere, the destruction in the troposphere by photochemical reactions and by deposition to the ground. The 15 to 20 ppb difference between today's ozone levels, near the sea surface before air masses pass over Europe, and the pre-industrial ozone concentration is probably due to photochemical formation from precursors such as VOC and NOx, emitted in other parts of the northern hemisphere, in particular North America. Reduction in the background tropospheric level will require agreement on a hemispheric scale. In addition, precursors emitted from biomass burning have a large impact on ozone concentrations in the tropics and in the southern hemisphere.

    The very high ozone, NO2 and PAN concentrations that are observed in some urban and suburban areas (photochemical smog) are due to photochemical production in the atmospheric boundary layer from precursors that are mostly emitted within the area.

    On a continental scale, the enhanced photo-oxidant concentrations observed are a consequence of both in-situ chemistry and transport from regions with higher emissions. Studies in EUROTRAC have greatly added to our understanding of the relevant processes for quantifying ozone/precursor relationships and, hence, provide a better basis for determining how reductions in precursor emissions in one region would reduce the photo-oxidant levels in regions downwind, especially in moderately populated and rural areas (scales >50 km). However, source-receptor relationships are still difficult to assign because the chemistry is non-linear and there are large differences in the residence times of photo-oxidants and precursors in the atmospheric boundary layer close to the ground, compared with the residence times in the free troposphere.

    While the highest photo-oxidant levels can be counteracted by local pollution control measures, abatement of enhanced ozone formation on a European scale requires a co-ordinated abatement strategy.

  8. Practical applications of photo-oxidant models on all scales Strategies to abate photochemical air pollution at any relevant scale may be assessed with models developed within EUROTRAC.

    On the local scale, a zooming model (EZM) has already been successfully utilised to optimise the air pollution abatement strategy for Athens, to support the decisions taken with regard to traffic regulations in Barcelona during the 1992 Olympics and to interpret the observations during measuring campaigns in the Upper Rhine Valley.

    Models have also been developed to describe the distribution of ozone in the global troposphere. The results show that the concentrations of ozone throughout large parts of the northern hemisphere have been substantially increased by anthropogenic emissions of nitrogen oxides, VOCs and carbon monoxide. Since ozone is a greenhouse gas, these elevated levels could be making an appreciable contribution to global warming..

    The EURAD model as well as global models have been used to calculate the influence of aircraft emissions on upper tropospheric ozone levels.

    Acidification of soil and water and the atmospheric contribution to nutrient inputs

  9. Sulphur and nitrogen deposition is still causing severe damage to ecosystems in Europe There is clear evidence that deposition of anthropogenic sulphur and nitrogen compounds over central and northern Europe has caused severe changes in the composition and functioning of many ecosystems. The deposition of these compounds has also been an important factor in the deterioration of materials and of our cultural heritage of ancient buildings. These effects have to a large extent occurred during the second half of the twentieth century and are mainly caused by emissions of sulphur dioxide, nitrogen oxides and ammonia within Europe.

    Annual budget calculations on a national scale and for Europe as a whole show that 50 to 70% of the emissions of sulphur and oxidised nitrogen and approximately 80% of the emissions of reduced nitrogen are deposited within Europe. These inputs exceed the critical loads for soils and for freshwater acidification over large areas of Europe. A protocol to the LRTAP for reducing the sulphur deposition over the next decade has recently been agreed. For nitrogen, protocols are being developed.

    SO2 emissions vary markedly with the time of day, the day of the week and the season, the variations being explicable in terms of working hours and the influence of temperature on energy demand.

  10. The natural sources of sulphur are unimportant relative to the anthropogenic sources in Europe.

    The inputs of dimethyl sulphide (DMS) to the atmosphere are important on a global scale for the atmospheric sulphur budget. The emissions from oceans contribute to the background 'natural' sulphur, and sulphur compounds are deposited together with anthropogenically-derived sulphur. However, except in small coastal areas of northern and western Europe, the contributions from natural sources to annual sulphur inputs is negligible.

  11. The contribution of deposited nitrogen to acidification and eutrophication is increasing

    The protocols agreed in 1994 should substantially reduce sulphur emissions and deposition throughout Europe by 2005, and a decrease in sulphur emission and deposition in Europe will reduce the scale of environmental damage due to acidification. Sulphur is however not the only contributor. Deposited nitrogen (both oxidised and reduced) contributes to the acidification problem and in many areas the deposition of nitrogen alone exceeds critical loads to ecosystems. The downward trends in sulphur emission over the last decade, and those expected in the next, are rapidly increasing the relative importance of nitrogen in acidification and eutrophication.

  12. Most of the sulphur dioxide oxidation in Europe occurs in clouds Cloud chemical processes have been shown to be at least as important as gas-phase processes for the oxidation of sulphur dioxide. New pathways for the aqueous-phase oxidation have been discovered and quantified. Incorporation of transfer and reaction mechanisms in simulation models is in progress and it should soon be possible to include more realistic cloud modules in source-receptor models.

  13. Cloud chemical processes cause non-linearity Field experiments performed in recent years have led to a better understanding of the formation of cloud droplets and of how particulate matter is incorporated in the droplets. In these experiments two groups of aerosols were observed with different hygroscopic properties. These findings show that the different chemical species in aerosols are scavenged by clouds and therefore by wet deposition processes at different rates, which will lead to different residence times in the atmosphere.

    The reduction of sulphur emissions in western Europe after 1980 is not reflected in atmospheric concentrations in a simple way. Measurements show that sulphur dioxide concentrations are reduced faster than expected, while concentrations of sulphur in precipitation show a slower decrease than expected. Aqueous phase chemistry in clouds may be a key factor contributing to the observed anomalies.

  14. Effects-based emission controls demand great spatial detail for ecosystem sensitivity and deposition The application of approaches to develop emission controls, based on critical loads, provides a means of maximising the environmental benefits for the investments in controls. These approaches require the ecosystem sensitivity and the actual deposition input to be quantified on the same scale. This scale of variability in ecosystem sensitivity to acidification varies with land use and vegetation, with the landscape scale being mostly between 1 and 10 km.

  15. Fine-scale maps of deposition of the key acidifying species have been provided by EUROTRAC work Models and data bases have been developed to provide the fine scale (1 to 10 km) resolution in the land-use-specific deposition maps that are required to calculate exceedances of critical loads. These methods have been developed from field and laboratory based studies of deposition processes and validated by long term flux measurements. The new equipment developed to make the measurements, the collaborative field campaigns and the long term flux studies have provided the science which underpins the deposition maps. The work has been developed for both national and international approaches.

  16. The elimination of ammonia emissions in Europe would change the transboundary fluxes of sulphur The dry deposition of sulphur dioxide to vegetation has been shown to be influenced by the presence of surface water (rain, dew etc.). These effects lead to larger rates of SO2 deposition in the presence of ammonia. Atmospheric ammonia may therefore reduce the atmospheric lifetime of sulphur dioxide. Similar effects of SO2 on rates of ammonia exchange over vegetation are expected. The quantitative detail has yet to be provided, but it is clear that emission controls of either of these gases will influence the lifetime and the deposition 'footprint' of both gases.

  17. Reduced as well as oxidised nitrogen emissions need to be controlled

    Up to now the control of emissions of nitrogen compounds has been largely on emissions of NOx from vehicles and large combustion plants. However some of the major environmental effects (acidification and eutrophication) are caused by both oxidised and reduced nitrogen. Moreover, deposition and the effects of deposited nitrogen over large areas of Europe are dominated by inputs of reduced nitrogen as NH3 in dry deposition and NH4+ in wet deposition. To reduce the effects or eliminate exceedances of critical loads for nitrogen, it is necessary to control emissions of both NOx and NH3.

  18. Anthropogenic sulphate aerosols in the free troposphere counteract global warming Models have been developed to describe the global transport of sulphur and nitrogen compounds. These models show that sulphate aerosols (originating from anthropogenic sulphur emissions) reflect sunlight back into space and thereby cool the surface. In heavily industrialised regions, where the concentration of sulphate aerosols is high, this cooling effect may mask a major fraction of the warming due to greenhouse gases.

    The contribution of EUROTRAC to the development of tools for the study of tropospheric pollution

  19. Tools now available for the evaluation of abatement strategies Methods and tools, in the form of simulation models, instrumentation and the results of laboratory studies, have been developed which are being used and will be used in the future in the evaluation of abatement strategies on continental and urban scales, for air pollution episodes and long-term averaged situations.

  20. New and improved instrumentation has been developed within this EUREKA project Major improvements and new developments in instrumentation have been made. These include long-path absorption techniques, tunable-diode laser spectrometers, tropospheric lidar techniques, automated VOC measuring systems and a sensitive NOx analyser. Numerous instrument intercomparisons have led to a better assessment of the accuracy of the instruments. Several of these techniques are now in general use and have been commercialised; as expected in a EUREKA project. The EUROTRAC experience indicates however that if the separate development of innovative instrumentation is required, precisely defined objectives and centralised funding are needed.

  21. Laboratory results are improving our knowledge of atmospheric chemistry Laboratory studies of free radical processes, of the oxidation reactions of aromatic and biogenic compounds and of individual reactions of many chemical intermediates have considerably improved the knowledge of tropospheric ozone chemistry. Laboratory studies of chemistry in cloud droplets and aerosol particles have added to the understanding of the elementary reaction steps of the oxidation of sulphate in clouds, of the absorption process of gases by cloud droplets and of aerosol surface reactions. These new findings have been incorporated into simulation models, so improving their reliability.

  22. Simulation models required to address policy-oriented questions Simulation models on urban, continental and global scales have been developed, improved and validated. These models, which integrate the results of process-oriented studies, describe in a quantitative manner the relationship between emissions and atmospheric concentrations or deposition. As such models are based on state-of-the-art knowledge and experience, and they should be viewed as adequate and reliable tools which have been and will be asked to answer policy-oriented questions. However simulation models require continuous improvement to include new findings and understanding, and frequent validation to demonstrate their reliability.

  23. Good emission data essential Emission data bases, including the procedures developed for temporal and spatial disaggregation of the data, are an essential tool since both accurate emission data and optimal meteorological input is required as input for simulation models in order to provide quantitative source/receptor relationships. Only with good and accurate emission data can reliable results be expected from simulation models. Data bases of field observations and field campaigns collected within EUROTRAC are also valuable tools for future studies.

  24. Networks of scientists are sources of expert advice The network of scientists, created by EUROTRAC, has and will lead to more synergistic scientific activities and a coherent environmental policy in Europe. The network provides an efficient way of transferring knowledge and provides a major source of expert advice to the participating countries, to the EU and the UN-ECE.

    Uncertainties in our present knowledge

  25. Uncertainties requiring resolution While much has been achieved within the present project a number of uncertainties remain. These will have to be resolved in the future if the source/receptor relationships, quantitative enough to support the present trends in policy development, are to be produced. The following headings mention the major uncertainties and lists some of the work needed to reduce them.

    For photo-oxidants, there are uncertainties in estimates of precursor emissions and in the detailed chemical and meteorological mechanisms by which photo-oxidants are processed. Studies are necessary to:

    For acidification and deposition of nutrients, the principle uncertainties are in emission estimates, the effects of clouds on the production of acidity and the effects of complex terrain on deposition. Studies are necessary to:

    For the development of tools, improvements are needed in the models themselves, in their validation, in instrumentation for monitoring and research and in the mechanisms of the fundamental reactions involved in the formation of photo-oxidants and acidity. Studies are necessary to