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It is well established that in Europe, high O3 concentrations are most pronounced in southern/Mediterranean countries due to the more favourable climatological conditions for its formation. However, the contribution of the different sources of precursors to O3 formation within each country relative to the imported (regional and hemispheric) O3 is poorly quantified. This lack of quantitative knowledge prevents local authorities from effectively designing plans that reduce the exceedances of the O3 target value set by the European air quality directive. O3 source attribution is a challenge because the concentration at each location and time results not only from local biogenic and anthropogenic precursors, but also from the transport of O3 and precursors from neighbouring regions, O3 regional and hemispheric transport and stratospheric O3 injections. The main goal of this study is to provide a first quantitative estimation of the contribution of the main anthropogenic activity sectors to peak O3 events in Spain relative to the contribution of imported (regional and hemispheric) O3. We also assess the potential of our source apportionment method to improve O3 modelling. Our study applies and thoroughly evaluates a countrywide O3 source apportionment method implemented in the CALIOPE air quality forecast system for Spain at high resolution (4 × 4 km 2) over a 10-day period characterized by typical summer conditions in the Iberian Peninsula (IP). The method tags both O3 and its gas precursor emissions from source sectors within one simulation, and each tagged species is subject to the typical physico-chemical processes (advection, vertical mixing, deposition, emission and chemistry) as the actual conditions remain unperturbed. We quantify the individual contributions of the largest NOx local sources to high O3 concentrations compared with the contribution of imported O3. We show, for the first time, that imported O3 is the largest input to the ground-level O3 concentration in the IP, accounting for 46 %–68 % of the daily mean O3 concentration during exceedances of the European target value. The hourly imported O3 increases during typical northwestern advections (70 %–90 %, 60–80 µ g m -3), and decreases during typical stagnant conditions (30 %–40 %, 30–60 µ g m -3) due to the local NO titration. During stagnant conditions, the local anthropogenic precursors control the O3 peaks in areas downwind of the main urban and industrial regions (up to 40 % in hourly peaks). We also show that ground-level O3 concentrations are strongly affected by vertical mixing of O3 -rich layers present in the free troposphere, which result from local/regional layering and accumulation, and continental/hemispheric transport. Indeed, vertical mixing largely explains the presence of imported O3 at ground level in the IP. Our results demonstrate the need for detailed quantification of the local and remote contributions to high O3 concentrations for local O3 management, and show O3 source apportionment to be an essential analysis prior to the design of O3 mitigation plans in any non-attainment area. Achieving the European O3 objectives in southern Europe requires not only ad hoc local actions but also decided national and European-wide strategies. [ABSTRACT FROM AUTHOR]

The current air quality situation in Europe is reviewed for the most critical pollutants, i.e.: PM10, PM2.5, N02, 03 and BaP. Reasons of incompliance, as well as major actions required for reaching further compliance of the WHO guidelines are discussed. For some pollutants there is a need for further scientific research in order to cost-efficiently abate urban and peri-urban pollution, especially for PMx and03. The policy and scientific challenges are identified for these five pollutants. These include, for policy, the need to solve the air quality problem caused by NOx-diesel-vehicle EU policy failure, the need to reduce PM and BaP emissions from an increasing domestic biomass burning combustion favoured by policy actions, the need to abate the currently high agricultural NH3 emissions, and devising specific and cost-effective 03 abatement strategies. Key scientific challenges include the need for an accurate source apportionment of secondary organic aerosols in urban areas, the abatement of vehicle wear and road dust resuspension, an accurate tropospheric 03 source apportionment identifying the hemispheric/regional/local contributions as well as the accurate modelling of 03 able to reproduce the complex meteorology favouring high 03 episodes around the Mediterranean, and yielding reliable sensitivity analysis for policy actions. Levels, origins and application of other non-regulated air quality parameters such as concentrations of ultrafine particles (UFP) and black carbon (BC) are discussed in view of their potential use in air quality standards. We conclude that monitoring of BC should provide accurate evaluations of the effectiveness of road traffic air quality measures and their associated health gains. In the case of UFP, although road traffic is the major contributing source to urban levels, in high insolation regions a significant fraction of urban UFP can be attributed to photochemical nucleation processes. Thus, caution is necessary when doing metaanalysis of health outcomes linked to urban UFP using data from diverse cities, where the finest aerosol fraction might be considerably different in both composition and origin. Finally a strategy to abate N02 and PM from road traffic in urban areas of Europe, based on 7 major pillars or action types, is presented. [ABSTRACT FROM AUTHOR]

Legislation controlling vehicle emissions has been credited with a general downward trend in NO x (NO 2 +NO) concentrations in Europe since the 1990's. However, recent studies suggest that traffic (roadside) (TR) NO 2 concentrations have not decreased as expected, and in some cases increased, most likely due to the use of oxidation catalysts and particle filters in diesel vehicles (EURO III, IV, V, VI). In this study we describe the time trends in NO x , NO 2 and NO concentrations in 9 European cities comparing TR and urban background (UB) monitoring locations. In each city, we collected hourly city-specific NO x , NO, and NO 2 data from one TR and one UB monitoring site for each year. We describe hourly, weekly, seasonal and inter-annual patterns for periods corresponding to the implementation dates of various EURO vehicle emission standards regulating NO x emissions. The diurnal patterns in all 9 cities strongly reflected morning and evening traffic. In addition, lower weekend concentrations were observed. The NO x concentrations from the TR sites remain unchanged in the majority of the cities over the study period. When stratified by 3 time periods according to the implementation of the EURO standards, an increasing NO 2 /NO x ratio in 7/9 cities with time was noted. However, over the same time period the NO/NO 2 ratio decreased in 8/9 cities. A permanent inversion of the NO/NO 2 ratio was observed to occur in 2003 in 5/9 cities. Our analyses of temporal and diurnal patterns of NO x in European cities show reductions in concentrations consistent with reductions in primary emissions likely arising from the implementation of successive EURO standards. The generally constant or increasing NO 2 concentrations in the majority of the cities assessed over the study period underline the need of further regulative measures to meet the air quality standards and consequently to minimise adverse effects on human health. The ongoing collection and analysis of pollution concentrations across the EU is recommended to monitor trends in pollutants associated with adverse health effects. [ABSTRACT FROM AUTHOR]