Atmospheric Environment (v.43, #31)
Editorial board (i).
Editorial by Ranjeet S. Sokhi (4669).
Simulations of the dispersion of reactive pollutants in a street canyon, considering different chemical mechanisms and micromixing by A. Garmory; I.S. Kim; R.E. Britter; E. Mastorakos (4670-4680).
The Stochastic Fields (SF) or Field Monte Carlo method has been used to model the dispersion of reactive scalars in a street canyon, using a simple chemistry and the CBM-IV mechanism. SF is a Probability Density Function (PDF) method which allows both means and variances of the scalars to be calculated as well as considering the effect of segregation on reaction rates. It was found that the variance of reactive scalars such as NO2 was very high in the mixing region at roof-top level with rms values of the order of the mean values. The effect of segregation on major species such as O3 was found to be very small using either mechanism, however, some radical species in CBM-IV showed a significant difference. These were found to be the seven species with the fastest chemical timescales. The calculated photostationary state defect was also found to be in error when segregation is neglected.
Keywords: Turbulent reacting flows; Stochastic Fields; Probability Density Function; Pollution; Urban;
Road traffic emission factors for heavy metals by Christer Johansson; Michael Norman; Lars Burman (4681-4688).
Quantifying the emissions and concentrations of heavy metals in urban air is a prerequisite for assessing their health effects. In this paper a combination of measurements and modelling is used to assess the contribution from road traffic emissions. Concentrations of particulate heavy metals in air were measured simultaneously during 1 year at a densely trafficked street and at an urban background site in Stockholm, Sweden. Annual mean concentrations of cadmium were 50 times lower than the EU directive and for nickel and arsenic concentrations were 10 and six times lower, respectively. More than a factor of two higher concentrations was in general observed at the street in comparison to roof levels indicating the strong influence from local road traffic emissions. The only compound with a significantly decreasing trend in the urban background was Pb with 9.1 ng m−3 in 1995/96 compared to 3.4 ng m−3 2003/04. This is likely due to decreased emissions from wear of brake linings and reduced emissions due to oil and coal combustion in central Europe.Total road traffic emission factors for heavy metals were estimated using parallel measurements of NOx concentrations and knowledge of NOx emission factors. In general, the emission factors for the street were higher than reported in road tunnel measurements. This could partly be due to different driving conditions, since especially for metals which are mainly emitted from brake wear, more stop and go driving in the street compared to in road tunnels is likely to increase emissions. Total emissions were compared with exhaust emissions, obtained from the COPERT model and brake wear emissions based on an earlier study in Stockholm. For Cu, Ni and Zn the sum of brake wear and exhaust emissions agreed very well with estimated total emission factors in this study. More than 90% of the road traffic emissions of Cu were due to brake wear. For Ni more than 80% is estimated to be due to exhaust emissions and for Zn around 40% of road traffic emissions are estimated to be due to exhaust emissions. Pb is also mainly due to exhaust emissions (90%); a fuel Pb content of only 0.5 mg L−1 would give similar emission factor as that based on the concentration increment at the street. This is the first study using simultaneous measurements of heavy metals at street and roof enabling calculations of emission factors using a tracer technique.
Keywords: Copper; Lead; Cadmium; Nickel; Chromium; Antimony; PM10; Urban aerosol;
Trend of vehicle emission levels until 2020 – Prognosis based on current vehicle measurements and future emission legislation by Martin Rexeis; Stefan Hausberger (4689-4698).
The paper describes the incorporation of actual emission measurements and future emission standards into the emission model ‘NEMO’ (Network Emission Model). This model is then applied to make predictions on vehicle emission levels on basis of the Austrian fleet composition until 2020. The output is compared to the results based on the most common emission tool for the calculation of vehicle emissions in Central Europe – the recent version (2.1) of the ‘Handbook Emission Factors for Road Transport’. The discussion is focused on NO x and particulate matter (PM), since these pollutants are considered to be the most critical for the local air quality level.The NO x emission levels of recent modern diesel vehicle generations observed in several real world driving conditions were observed to be clearly higher than demanded in the type approval procedure. Due to the growing number of modern diesel engine concepts equipped with coated catalytic exhaust after treatment, the fraction of NO2 of the total tailpipe NO x emissions is predicted to continue to increase in the next few years. Bearing in mind the upcoming tightening of the NO2 air quality limits and the steady increase of traffic volumes, excesses of the NO2 air quality limits at roadside locations have to be expected to an increasing extent for the beginning of the next decade. The issue of particle emissions originated from the diesel engine combustion process can be regarded as being technically solved due to the extensive introduction of diesel particle filters in the vehicle fleet if these systems will prove a high efficiency over the entire vehicle life in real world operation conditions. However, PM emissions from road transport will continue to be in the focus of public attention due to particle emissions caused by dust re-suspension and abrasion processes.
Keywords: NEMO; HBEFA2.1; NO x ; PM emissions; Austria;
Factors influencing PM10 emissions from road pavement wear by Mats Gustafsson; Göran Blomqvist; Anders Gudmundsson; Andreas Dahl; Per Jonsson; Erik Swietlicki (4699-4702).
Accelerated pavement wear is one of the major environmental disadvantages of studded tyres in northern regions and results in increased levels of PM10. Measurements of PM10 in a road simulator hall have been used to study the influence of pavement properties, tyre type and vehicle speed on pavement wear. The test set-up included three different pavements (one granite and two quartzite with different aggregate sizes), three different tyre types (studded, non-studded, and summer tyres) and different speeds (30–70 km h−1). The results show that the granite pavement was more prone to PM10 production compared to the quartzite pavements. Studded winter tyres yield tens of times higher PM10 concentrations compared to non-studded winter tyres. Wear from summer tyres was negligible in comparison. It was also shown that wear is strongly dependent on speed; every 10 km h−1 increase yielded an increase of the PM10 concentration of 680 μg m−3 in one of the simulator experiments.
Keywords: Pavement wear; Studded tyres; Granite; Quartzite; Speed dependence;
Non-exhaust emission measurement system of the mobile laboratory SNIFFER by L. Pirjola; K.J. Kupiainen; P. Perhoniemi; H. Tervahattu; H. Vesala (4703-4713).
In this paper we describe and quality assure the sampling system of a mobile research laboratory SNIFFER which was shown to be a useful tool for studying emission levels of respirable dust from street surfaces. The dust plume had bimodal structure; another mode rising to higher altitudes whereas the other mode remained at lower altitudes. The system was tested on a route in Helsinki, Finland, during spring 2005 and 2006. The PM2.5 and PM10 were positively correlated and the PM levels increased with the vehicle speed. SNIFFER was able to identify the characteristic emission levels on different streets. A clear downward trend in the concentrations was observed in all street locations between April and June. The composition of the street dust collected by SNIFFER was compared with springtime PM10 aerosol samples from the air quality monitoring stations in Helsinki. The results showed similarities in the abundance and composition of the mineral fraction but contained significantly more salt particles.
Keywords: Street dust; Road wear; Mobile measurements; Re-suspension; PM10; PM2.5;
Ultrafine particle fluxes above four major European cities by Claire L. Martin; Ian D. Longley; James R. Dorsey; Rick M. Thomas; Martin W. Gallagher; Eiko Nemitz (4714-4721).
Little information is known about the behaviour of ultrafine particles (UFP) on a citywide scale. Total particle number flux, dominated by UFP, and other meteorological parameters were collected from tower sites in Manchester, London, Edinburgh and Gothenburg between 1999 and 2006 using the eddy covariance technique. Averaged diurnal cycles were produced for particle number flux, concentration, sensible heat flux, emission velocity, friction velocity, wind speed and temperature. UFP flux cycles showed clear diurnal trends which were linked to traffic activity and local sources. Wind sector analysis showed contributions to flux from local heavily urbanised areas. A simple parameterised model linking UFP flux to traffic activity, sensible heat and friction velocity above the city was produced.
Keywords: Urban; Flux; Emission; Ultrafine particles; Sensible heat; Traffic activity;
Wood smoke as a source of particle-phase organic compounds in residential areas by Md. Aynul Bari; Guenter Baumbach; Bertram Kuch; Guenter Scheffknecht (4722-4732).
The objective of this study was to investigate the organic composition of wood smoke emissions and ambient air samples in order to determine the wood smoke contribution to the ambient air pollution in the residential areas. From November 2005 to March 2006 particle-phase PM10 samples were collected in the residential town Dettenhausen surrounded by forests near Stuttgart in southern Germany. Samples collected on pre-baked glass fibre filters were extracted using toluene with ultrasonic bath and analysed by gas chromatography mass spectrometry (GC-MS). 21 polycyclic aromatic hydrocarbons (PAH) including 16 USEPA priority pollutants, different organic wood smoke tracers, primarily 21 species of syringol and guaiacol derivatives, levoglucosan and its isomers mannosan, galactosan and dehydroabietic acid were detected and quantified in this study. The concentrations of these compounds were compared with the fingerprints of emissions from hardwood and softwood combustion carried out in test facilities at Universitaet Stuttgart and field investigations at a wood stove during real operation in Dettenhausen. It was observed that the combustion derived PAH was detected in higher concentrations than other PAH in the ambient air PM10 samples. Syringol and its derivatives were found in large amounts in hardwood burning but were not detected in softwood burning emissions. On the other hand, guaiacol and its derivatives were found in both softwood and hardwood burning emissions, but the concentrations were higher in the softwood smoke compared to hardwood smoke. So, these compounds can be used as typical tracer compounds for the different types of wood burning emissions. In ambient air samples both syringol and guaiacol derivatives were found which indicates the wood combustion contribution to the PM load in such residential areas. Levoglucosan was detected in high concentrations in all ambient PM10 samples. A source apportionment modelling, Positive Matrix Factorization (PMF) was implemented to quantify the wood smoke contribution to the ambient PM10 bound organic compounds in the residential area.
Keywords: PAH; Levoglucosan; Wood combustion; Source fingerprints; Positive Matrix Factorization;
Source apportionment of particulate matter (PM2.5) in an urban area using dispersion, receptor and inverse modelling by Herdis Laupsa; Bruce Denby; Steinar Larssen; Jan Schaug (4733-4744).
Air pollution emission inventories are the basis for air quality assessment and management strategies. The quality of the inventories is of great importance since these data are essential for air pollution impact assessments using dispersion models. In this study, the quality of the emission inventory for fine particulates (PM2.5) is assessed: first, using the calculated source contributions from a receptor model; second, using source apportionment from a dispersion model; and third, by applying a simple inverse modelling technique which utilises multiple linear regression of the dispersion model source contributions together with the observed PM2.5 concentrations. For the receptor modelling the chemical composition of PM2.5 filter samples from a measurement campaign performed between January 2004 and April 2005 are analysed. Positive matrix factorisation is applied as the receptor model to detect and quantify the various source contributions. For the same observational period and site, dispersion model calculations using the Air Quality Management system, AirQUIS, are performed. The results identify significant differences between the dispersion and receptor model source apportionment, particularly for wood burning and traffic induced suspension. For wood burning the receptor model calculations are lower, by a factor of 0.54, but for the traffic induced suspension they are higher, by a factor of 7.1. Inverse modelling, based on regression of the dispersion model source contributions and the PM2.5 concentrations, indicates similar discrepancies in the emissions inventory. In order to assess if the differences found at the one site are generally applicable throughout Oslo, the individual source category emissions are rescaled according to the receptor modelling results. These adjusted PM2.5 concentrations are compared with measurements at four independent stations to evaluate the updated inventory. Statistical analysis shows improvement in the estimated concentrations for PM2.5 at all sites. Similarly, inverse modelling is applied at these independent sites and this confirms the validity of the receptor model results.
Keywords: PM2.5; Source apportionment; Positive matrix factorisation; Dispersion modelling; Multiple linear regression;
Influence of particulate matter on the air quality situation in a mediterranean island by Savvas Kleanthous; Md. Aynul Bari; Guenter Baumbach; Leire Sarachage-Ruiz (4745-4753).
The objective of this study was to describe the ambient levels of particulate matter (PM) and its influence to air quality situation on the dry Mediterranean island of Cyprus. From October 2002 to August 2003 PM10 and PM2.5 samples were collected at 31 different sampling sites in Cyprus. In addition, continuous measurements of PM10 were carried out from 2003 to 2007 at a traffic and a rural site. It can be recognised that at all traffic and at some residential and urban background sites, the actual EU limit values have been exceeded. Special events e.g. long-range transport of Sahara dust storms were recorded over urban as well as rural areas in the order of 6–8 events per year, with a major frequency in summer and spring periods. The comparison of the PM10 concentrations in Cyprus cities with values of other European cities demonstrates the PM10 problem in Cyprus, especially in the dry summer season, when no rain is cleaning the air and the dry surfaces. This underlines the necessity of PM abatement strategies.
Keywords: PM10; PM2.5; Sahara dust; Limit value; Dust resuspension;
Enhancement of source traceability of atmospheric PM by elemental chemical fractionation by Silvia Canepari; Adriana Pietrodangelo; Cinzia Perrino; Maria Luisa Astolfi; Maria Letizia Marzo (4754-4765).
During a 1-year study (“Fine dust” Project) funded by the Lazio regional government (Italy), about 1000 daily PM10 and PM2.5 samples collected from six sites in the region were subjected to chemical fractionation based on differences in elemental solubility. In this way, it was possible to achieve meaningful characterization of the elemental composition of individual samples. For most of the investigated elements, we found significant differences between the extracted and the mineralized residual fraction. In general, fine particulate was best characterized by the composition of the extracted fraction, while coarse particles from traffic-related sources were best characterized using residues. For several metals (Cd, Pb, Sn, Sb and V) having a critical environmental impact, this result was particularly clear.The application of Principal Component Analysis (PCA) and receptor modelling (PCR) to the data set allowed us to evidence the enhancement of selectivity towards different emission sources that is obtained when chemical fractionated data are considered instead of total element content. Chemical fractionation seems to generate very selective markers for specific emission sources and in particular for re-suspended road dusts, one of the main factors responsible for the increase of elemental concentrations in urban areas.
Keywords: Particulate matter; Traffic-related dusts; Elemental solubility; Receptor modelling; Principal Component Analysis;
Influence of natural events on the concentration and composition of atmospheric particulate matter by Cinzia Perrino; Silvia Canepari; Maria Catrambone; Stefano Dalla Torre; Elena Rantica; Tiziana Sargolini (4766-4779).
A ten-month field study aimed to determine the contribution of natural events (i.e. sea-salt and mineral dust events) to urban PM concentration was carried out at six sampling sites in Central Italy (Lazio region). Four indicators have been used to identify natural events during the period of the study. The first one is constituted by the ratio between number of particles in the coarse to the accumulation mode. It is simple, cheap, and the information are given in quasi-real time, but the nature of the event (sea-salt or mineral dust) is not detectable. The second indicator relies on the chemical analysis of the collected PM by X-ray fluorescence (XRF) and allows a robust identification of sea-salt and crustal components. The third one is based on diagnostic ratios of elemental fractions: Mgextractable/Tiresidue for sea-salt and Tiresidue/Sbresidue for mineral dust. It requires skilled staff but it is most accurate and sensible. The last indicator, constructed on the basis of natural radioactivity data, is not diagnostic for the nature of the event but it is able to estimate the increase in PM concentration with respect to the expected concentration in the absence of natural events.The relevance of natural events and the variations in PM concentration and composition during the study are discussed. The joined use of the four indicators allowed the identification of about 20 natural PM episodes. In general, sea-salt aerosol events did not cause exceedance of the daily EU limit value for PM10. Saharan dust events, instead, were in most cases responsible for the exceedance of the limit value at all stations.
Keywords: Particulate matter composition; Natural aerosol; Sea-salt; Saharan dust; Lazio region (Central Italy);
Trend analysis of urban NO2 concentrations and the importance of direct NO2 emissions versus ozone/NO x equilibrium by Menno Keuken; Michiel Roemer; Sef van den Elshout (4780-4783).
The annual air quality standard of NO2 is often exceeded in urban areas near heavy traffic locations. Despite significant decrease of NO x emissions in 1986–2005 in the industrial and harbour area near Rotterdam, NO2 concentrations at the urban background remain at the same level since the end of the nineties. Trend analysis of monitoring data revealed that the ozone/NO x equilibrium is a more important factor than increasing direct NO2 emissions by traffic. The latter has recently been identified as an additional NO2 source due to the introduction of oxy-catalytic converters in diesel vehicles and the growing number of diesel vehicles. However, in Rotterdam over the period 1986–2005 direct NO2 emissions by road traffic only increased 3–4%. Due to the importance of the ozone/NO x equilibrium, it is concluded that local NO x emissions in Rotterdam need substantial reduction to achieve lower NO2 urban background levels. This is a relatively costly abatement strategy and, therefore, a “hotspot” approach aiming at reducing NO x emissions by local traffic measures is more effective to meet European air quality standards.
Keywords: Urban air quality; Nitrogen dioxide; Traffic emissions;
Ozone production from Grenoble city during the August 2003 heat wave by Eric Chaxel; Jean-Pierre Chollet (4784-4792).
The heat wave from 1 to 16 August 2003 is considered in the city of Grenoble (French Alps). The modelling system (PREVALP) is based on several models operating on nested domains: MM5 for dynamics, CHIMERE for chemistry (18 km and 6 km grid size) and METPHOMOD for both dynamics and chemistry (2 km grid size). The analysis of the results shows that during the heat wave, the mixing layer is thicker, up to 3500 m agl, hence inducing transport of ozone to high altitude. Two regimes were diagnosed: (1) a freely developing convective layer, (2) a layer trapped under south wind which makes ozone precursors accumulate in the city. Local ozone production is estimated to be 40% of the plume maximum in case (2) and only 30% in case (1). Sensitivity analysis by step increase for temperature at the boundary of the inner domain shows the non-linearity of the response; in this case most of the effect comes from chemistry. By changing biogenic emission significant changes are observed in restricted areas.
Keywords: Ozone; Heat wave; Atmosphere; Modelling; Urban;
Coupling GEOS-CHEM with a regional air pollution model for Greece by M. Tombrou; E. Bossioli; A.P. Protonotariou; H. Flocas; C. Giannakopoulos; A. Dandou (4793-4804).
The sensitivity of regional air quality modeling simulations to boundary conditions over Greece is investigated, for various synoptic conditions. For this purpose, a global to mesoscale model-chain is developed and applied, coupling the individual models' simulations. The global chemical transport model GEOS-CHEM, applied in a one-way nested procedure, is used to drive the regional UAM-V chemical dispersion model with time-varying lateral and top boundary conditions. The results of the coupling procedure are compared with the MINOS campaign measurements at Finokalia (Southern Greece) during the period from 1 to 16 August 2001 which is mainly characterized by an interchange of two synoptic types, High-Low and Long Wave trough.The comparison between the simulation results and the measurements reveals that the coupling procedure captures satisfactorily the range of observed CO concentrations at the southern part of Greece. The most severe deviations are observed under strongly variable atmospheric circulation, when no distinct synoptic circulation is allowed to be established in the area. Regarding O3, the highest, though underestimated, surface concentrations are simulated under Long Wave trough conditions due to the influence of the ozone inflow predicted by GEOS-CHEM at the western boundary of the innermost domain and/or under enhanced NOy emissions arriving at Finokalia from urban and ships plumes.
Keywords: Global to mesoscale model-chain; Boundary conditions; Synoptic categories; Global GEOS-CHEM model; CO; O3; NOy;
On the importance of the meteorological coupling interval in dispersion modeling during ETEX-1 by U.S. Korsholm; A. Baklanov; A. Gross; J.H. Sørensen (4805-4810).
Traditionally, transport and dispersion models are offline coupled to meteorological drivers, receiving pre-processed output at regular coupling intervals. However, today meteorological models have reached urban and cloud resolving scales and online models integrating meteorological and dispersion models have been developed. In this study the online coupled model, Enviro-HIRLAM, which can also run in offline mode, was used to compare online and offline representations of meso-scale disturbances. The online model was evaluated using data from the first European Tracer Experiment (ETEX-1) and produced satisfactory results. Meso-scale influences during the simulation pertube the plume during long-range transport, leading to a double peak structure at a specific measurement station. The meso-scale influence was investigated by varying the offline coupling interval which was shown to be important in constraining the influence of meso-scale disturbances on plume structure in coarse resolution.
Keywords: Online; Offline; ETEX; Meso-scale; Urban;
Neuro-fuzzy and neural network systems for air quality control by Claudio Carnevale; Giovanna Finzi; Enrico Pisoni; Marialuisa Volta (4811-4821).
In order to define efficient air quality plans, Regional Authorities need suitable tools to evaluate both the impact of emission reduction strategies on pollution indexes and the costs of such emission reductions. The air quality control can be formalized as a two-objective nonlinear mathematical problem, integrating source–receptor models and the estimate of emission reduction costs. Both aspects present several complex elements. In particular the source–receptor models cannot be implemented through deterministic modelling systems, that would bring to a computationally unfeasible mathematical problem. In this paper we suggest to identify source–receptor statistical models (neural network and neuro-fuzzy) processing the simulations of a deterministic multi-phase modelling system (GAMES). The methodology has been applied to ozone and PM10 concentrations in Northern Italy. The results show that, despite a large advantage in terms of computational costs, the selected source–receptor models are able to accurately reproduce the simulation of the 3D modelling system.
Keywords: Multi-objective optimization; Particulate matter; Ozone; Source–receptor models; Neural networks; Neuro-fuzzy models;
Skill and uncertainty of a regional air quality model ensemble by R. Vautard; M. Schaap; R. Bergström; B. Bessagnet; J. Brandt; P.J.H. Builtjes; J.H. Christensen; C. Cuvelier; V. Foltescu; A. Graff; A. Kerschbaumer; M. Krol; P. Roberts; L. Rouïl; R. Stern; L. Tarrason; P. Thunis; E. Vignati; P. Wind (4822-4832).
Recently several regional air quality projects were carried out to support the negotiation under the Clean Air For Europe (CAFE) programme by predicting the impact of emission control policies with an ensemble of models. Within these projects, CITYDELTA and EURODELTA, the fate of air quality at the scale of European cities or that of the European continent was studied using several models. In this article we focus on the results of EURODELTA. The predictive skill of the ensemble of models is described for ozone, nitrogen dioxide and secondary inorganic compounds, and the uncertainty in air quality modelling is examined through the model ensemble spread of concentrations.For ozone daily maxima the ensemble spread origin differs from one region to another. In the neighbourhood of cities or in mountainous areas the spread of predicted values does not span the range of observed data, due to poorly resolved emissions or complex-terrain meteorology. By contrast in Atlantic and North Sea coastal areas the spread of predicted values is found to be larger than the observations. This is attributed to large differences in the boundary conditions used in the different models. For NO2 daily averages the ensemble spread is generally too small compared with observations. This is because models miss highest values occurring in stagnant meteorology in stable boundary layers near cities. For secondary particulate matter compounds the simulated concentration spread is more balanced, observations falling nearly equiprobably within the ensemble, and the spread originates both from meteorology and aerosol chemistry and thermodynamics.
Keywords: Ensemble modelling; Air quality model; Model evaluation; Uncertainty;
Multi-criteria analysis for PM10 planning by Enrico Pisoni; Claudio Carnevale; Marialuisa Volta (4833-4842).
To implement sound air quality policies, Regulatory Agencies require tools to evaluate outcomes and costs associated to different emission reduction strategies. These tools are even more useful when considering atmospheric PM10 concentrations due to the complex nonlinear processes that affect production and accumulation of the secondary fraction of this pollutant. The approaches presented in the literature (Integrated Assessment Modeling) are mainly cost-benefit and cost-effective analysis. In this work, the formulation of a multi-objective problem to control particulate matter is proposed. The methodology defines: (a) the control objectives (the air quality indicator and the emission reduction cost functions); (b) the decision variables (precursor emission reductions); (c) the problem constraints (maximum feasible technology reductions). The cause-effect relations between air quality indicators and decision variables are identified tuning nonlinear source–receptor models. The multi-objective problem solution provides to the decision maker a set of not-dominated scenarios representing the efficient trade-off between the air quality benefit and the internal costs (emission reduction technology costs). The methodology has been implemented for Northern Italy, often affected by high long-term exposure to PM10. The source–receptor models used in the multi-objective analysis are identified processing long-term simulations of GAMES multiphase modeling system, performed in the framework of CAFE-Citydelta project.
Keywords: Particulate matter; Emission control; Multi-objective optimization; Air quality;
The effects of congestions tax on air quality and health by Christer Johansson; Lars Burman; Bertil Forsberg (4843-4854).
The “Stockholm Trial” involved a road pricing system to improve the air quality and reduce traffic congestion. The test period of the trial was January 3–July 31, 2006. Vehicles travelling into and out of the charge cordon were charged for every passage during weekdays. The amount due varied during the day and was highest during rush hours (20 SEK = 2.2 EUR, maximum 60 SEK per day). Based on measured and modelled changes in road traffic it was estimated that this system resulted in a 15% reduction in total road use within the charged cordon. Total traffic emissions in this area of NO x and PM10 fell by 8.5% and 13%, respectively. Air quality dispersion modelling was applied to assess the effect of the emission reductions on ambient concentrations and population exposure. For the situations with and without the trial, meteorological conditions and other emissions than from road traffic were kept the same. The calculations show that, with a permanent congestion tax system like the Stockholm Trial, the annual average NO x concentrations would be lower by up to 12% along the most densely trafficked streets. PM10 concentrations would be up to 7% lower. The limit values for both PM10 and NO2 would still be exceeded along the most densely trafficked streets. The total population exposure of NO x in Greater Stockholm (35 × 35 km with 1.44 million people) is estimated to decrease with a rather modest 0.23 μg m−3. However, based on a long-term epidemiological study, that found an increased mortality risk of 8% per 10 μg m−3 NO x , it is estimated that 27 premature deaths would be avoided every year. According to life-table analysis this would correspond to 206 years of life gained over 10 years per 100 000 people following the trial if the effects on exposures would persist. The effect on mortality is attributed to road traffic emissions (likely vehicle exhaust particles); NO x is merely regarded as an indicator of traffic exposure. This is only the tip of the ice-berg since reductions are expected in both respiratory and cardiovascular morbidity. This study demonstrates the importance of not only assessing the effects on air quality limit values, but also to make quantitative estimates of health impacts, in order to justify actions to reduce air pollution.
Keywords: NO x ; PM10; Road traffic; Exposure; Premature mortality; Stockholm;