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EURASAP Workshop

Particles in the size range 2.5 to 10 microns in urban areas
4-6 November 2002, Berlin

organised by
The Free University of Berlin and the Brandenburg Technical University, Cottbus;
under the auspices of the European Association for the Science of Air Pollution, EURASAP, and the Society for Atmospheric Environmental Research, GeFUe V.

Summary

Measurements have indicted that in urban areas the concentration by mass of particles in the size range 2.5 to 10 microns is frequently comparable in magnitude with that in the finer fraction below 2.5 microns. This contributes to exceedance of air quality objectives defined in terms of total PM10, which many cities in Europe are finding difficult to meet. Whereas the finer fraction below 2.5 microns is fairly well understood and quantified, there are very large uncertainties about the origin and composition of this coarse fraction, and how it might be reduced. This workshop was therefore convened in Berlin to pool information and experience on this coarser fraction.

The meeting brought together scientists from thirteen countries and drew on measurements from several instrumented roads and city locations, as well as background sites. Some of these sites involved extensive sampling close to roads, encompassing major highways and street canyon situations. Analysis of chemical composition was also explored to investigate the origins and source apportionment of collected samples. The different transport scales of the coarser and fine fractions was discussed in reviewing modelling capabilities for fine particles from continental to urban scales.

The data confirmed that the coarser fraction of PM10 is often as large as, and can sometimes dominate the mass of the finer fraction. However the characteristics and temporal variation differ between different cities and situations. For example measurements in the Netherlands and the UK illustrated episodes with large contributions from sea salt, and work from Spain the contribution in southern Europe from natural dust and soil particles. Industrial combustion and power plants can also contribute in some countries, though in Europe there is already extensive technology to control particle emissions.

There was clear evidence for enhanced concentrations of the coarser fraction close to roads. In Norway a marked seasonal increase has been observed when studded tyres are used in the winter, while UK measurements illustrated local areas with enhanced PM10 concentrations due to coarser fraction particles close to certain dirtier sites involving heavy lorry access..

There was discussion on the difference between emissions resulting from road abrasion, and suspension of dusts deposited on roads. The latter is more complex, as is illustrated by complementary research on run-off from road surfaces. In any case it is clear that there are large non-exhaust sources of particles in the coarser size fraction of PM10 along roads, which are still transport related, but will not be reduced by technological measures to abate exhaust emissions.

Concerning the status of the modelling of particulate matter it was felt that by now a reasonable agreement exist between regional scale model results and observations. Further improvements could be expected if better size dependent data on PM emissions become available. Mainly in southern Europe and during summer PM emissions in the coarse fraction are often incomplete, which might be due to missing data on windblown dust and biological material. The gap between modelled and measured PM10 concentrations becomes even larger in the urban and local scale, which is likely due to lacking data of emissions from construction work and inappropriate parameterisation of resuspension.

The workshop finished with 3 working groups addressing modelling, emissions and abatement, and measurements. It was concluded that there are very large questions to be resolved, and much research needed, on the origin, concentrations, and abatement potential for the coarser fraction of PM10;. The meeting had provided a valuable forum in bringing together those involved in relevant research, and it is hoped that the attendance can provide the basis of a network to coordinate future studies and strengthen the information available. This includes a good selection of well instrumented road sites in different countries, where parallel experimental studies would be extremely valuable. This is highly relevant to attainment of air quality standards in just those areas close to roads where exceedance of the objectives set in the EU directive is most likely to occur.

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Discussion group 2 on emission and abatement

This group addressed emissions of particles in the size range 2.5 to 10 microns from different source sectors, reviewing the current information available to quantify them, and gaps where research is needed. Evidence from the workshop had indicated that in urban areas emissions in this size range are comparable with those of particles below 2.5 microns, but are not accounted for in current inventories. However as we know little about many of the sources it is generally difficult to quantify their individual contribution, or to define efficient abatement strategies and estimate their effectiveness. Technological measures aimed at reducing exhaust emissions will not generally be effective in reducing this coarser fraction.

A particular problem arose in discussing suspension of road dust by traffic, which seems to be a major contribution. It is important to distinguish between the direct emissions from such processes as grinding and erosion of the road surface, and suspension of dust particles already on the road surface. The latter is complex and needs to address the origin and build up of dust in urban areas and along roads, and modelling of the processes involved. This is certainly an area where further research is needed.

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Road transport

Exhaust emissions are mainly below 2.5 microns, and account for a large proportion of urban emission inventories for European cities. (However two stroke engines may possibly be a significant source of coarser particles, especially in southern Europe. Evidence for this needs to be checked. Also some new technologies like direct injection gasoline cars may bear the risk of higher PM emission).

There is strong evidence for a significant contribution to PM10 from non-exhaust emissions caused by road traffic, most of it in the coarse fraction. The workshop had illustrated how these coarser fraction emissions vary from city to city and may exceed the total exhaust emissions, depending on the seasonal features and composition of the fleet (e.g. heavy vehicles as opposed to cars, and factors like studded tyres). There is a need to build on what has been achieved in pooling data and information on simultaneous measurements of both the coarser and fine fractions together with relevant meteorological and traffic parameters in different situations.

Direct emission may arise from processes like road abrasion, where the characteristics of the road surface and its condition, the speed and weight of the vehicles, the type of tyre, and weather conditions are all likely to play a role. At present there is insufficient information to quantify such emissions, and further research is urgently required. It may be helpful to seek cooperation with road engineers on rates of wear of different roads. There is already clear evidence that control of studded tyres is a potential abatement measure where these are used. But further information is required before other measures such as preferred road surfaces can be identified. Possible side effects such as safety implications also need to be considered.

Tyre and brake wear may also contribute to the coarse fraction, and are already included in some inventories. Trace elements from the tyres and brakes are a useful tool here; but there is a lack of representative studies on measurements of these compounds, and the rate, composition and size distribution of particulate samples to distinguish tyre and brake-wear contributions. It was noted that new information may emerge from current studies underway at EU laboratories (JRC, VELO), and in national programmes.

Suspension of road dust

This appears to be a major source of particles in the coarse fraction, and is also traffic related. There is a huge lack of understanding of the processes involved, and the origin of dusts on roads. Sources may include construction sites and various commercial and industrial activities, waste and refuse sites, soil erosion, material imported from rural areas, biological particles, and road salting and sanding. It can be difficult to differentiate particles from different sources, and hence the relative proportion from disparate origins. Specific studies are required to quantify such contributions to road dust; and processes of suspension, and removal by other means such as periodic wash-off in rain. There are complementary interests with research on polluted run-off into drains. Potential reduction of suspension by road cleaning was discussed, but experience in techniques for sampling of particles below 10 microns in road dusts imply that normal street cleaning methods may be very ineffective. This needs to be investigated.

Other transport sources

Information is lacking on non-road sources such as emission rates from brakes and rail wear for trains, and emissions from two-stroke engines in lawn mowers, snowmobiles etc.

Industrial emissions

There is generally good technology available to control emissions of particles in the coarser range from industrial combustion sources. However the “real-life” efficiency may be less than that potentially achievable if equipment is not properly deployed and maintained. There are large uncertainties about fugitive sources, and a lack of data on emission rates, size fractions and composition. Such emissions may arise from stockpiles, handling of bulk materials and wastes, landfills, quarrying, and construction work. In view of the paucity of information there is a poor understanding of potential abatement measures and their efficiency, although good housekeeping practices to reduce general dust nuisance may help. These sources need to be investigated, and may also contribute to road dust and recycling by suspension.

Domestic sources

Domestic heating contributions, mainly from coal, will depend on how much is burned and the type of grate or burner. Information on the size distribution for different types of installation is required to assess the contribution to the coarser PM10 fraction. Other domestic sources of fine dust also need to be investigated.

Agriculture

This may be a significant source in some circumstances, but there is a paucity of reliable data on emission rates, size fractions and composition from different agricultural activities- and hence the understanding of the efficiency and potential for abatement is weak.

Natural sources.

Sea salt and windblown dust are an important contribution in several countries- for example sea salt in coastal areas and wind blow dust in southern Europe. There are systematic variations with season and wind strength and direction. Further data and analysis is needed to quantify emission rates and size fractions. Another potential contribution is in the form of biological particles, but again there is a lack of information to quantify and characterise the emissions.

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