Air quality trends in Europe over the past decade: a first multi-model assessment

Abstract

Abstract.We discuss the capability of current state-of-theart chemistry and transport models to reproduce air quality trends and interannual variability. Documenting these strengths and weaknesses on the basis of historical simulations is essential before the models are used to investigate future air quality projections. To achieve this, a coordinated modelling exercise was performed in the framework of the CityZEN European Project. It involved six regional and global chemistry-transport models (BOLCHEM, CHIMERE, EMEP, EURAD, OSLOCTM2 and MOZART) simulating air quality over the past decade in the Western European anthropogenic emissions hotspots.Comparisons between models and observations allow assessing the skills of the models to capture the trends in basic atmospheric constituents (NO 2 , O 3 , and PM 10 ). We find that the trends of primary constituents are well reproduced (except in some countries -owing to their sensitivity to the emission inventory) although capturing the more moderate trends of secondary species such as O 3 is more challenging. Apart from the long term trend, the modelled monthly variCorrespondence to: A. Colette (augustin.colette@ineris.fr) ability is consistent with the observations but the year-to-year variability is generally underestimated.A comparison of simulations where anthropogenic emissions are kept constant is also investigated. We find that the magnitude of the emission-driven trend exceeds the natural variability for primary compounds. We can thus conclude that emission management strategies have had a significant impact over the past 10 yr, hence supporting further emission reductions.

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…They both report that the fraction of sites with an increasing trend is smaller when looking at peaks compared to daily means, but their findings suggest that the earlier reports of opposite signs of trends do not hold for this more recent period using this alternate monitoring data set. By including urban stations in their analysis, Colette et al (2011) also find an enhanced contribution of upwards trends, hence reinforcing the results of Derwent and Hjellbrekke (2013) regarding the observation that urban ozone concentrations are gradually becoming closer to rural levels.…”

“…The rate of ozone removal at the surface strongly influences the exposure of vegetation and the human population to ozone and is responsible for much of the nocturnal decline in surface ozone in rural areas as deposition to the surface consumes ozone beneath a nocturnal temperature inversion. During the day, vertical transport of ozone to the surface layers is generally sufficient to maintain mixing ratios within 10 % of the boundary layer mean values, except in urban areas or near major roads, where local nitric oxide sources remove ozone by titration (Colette et al, 2011). Thus the interplay between dry deposition of ozone in the surface layers and mixing from higher levels in the atmosphere plays a major role in regulating ozone exposure of ecosystem and the human population.…”

“…This behaviour has been interpreted as resulting from the combined effect of regional pollution controls since 1990 and increasing hemispheric ozone levels (Derwent and Hjellbrekke, 2013). In contrast, episodic ozone levels in the high-NO x environments of major European towns and cities are now rising towards the levels found in the rural areas surrounding them, as exhaust gas catalysts fitted to petrol and diesel motor vehicles reduce the Colette et al, 2011). Whereas the vast majority of the scientific literature on ozone trends relies on rural supersites dedicated to the assessment of long-term atmospheric change, the records of regulatory monitoring networks are becoming progressively long enough to complete such assessments.…”

“…The size and diversity of the data gathered in the Airbase repository (maintained by EEA) offers an unprecedented view into air quality trends in Europe, provided that (1) 10-year records can be considered long enough to assess trends, and (2) the station representativeness of thousands of urban and rural background sites can be compared to the handful of supersites used elsewhere. Colette et al (2011) and Wilson et al (2012) propose such assessments and conclude that the dominating picture is towards an increase in ozone in urban areas or non-significance of the rural background trends. They both report that the fraction of sites with an increasing trend is smaller when looking at peaks compared to daily means, but their findings suggest that the earlier reports of opposite signs of trends do not hold for this more recent period using this alternate monitoring data set.…”

“…A variety of interactions, feedbacks and ecosystem effects confound the picture for NO x , resulting in significant uncertainty and often times competing effects on climate. A variety of modelling studies have evaluated the various direct and indirect effects Shindell et al, 2009;Fry et al, 2012;Colette et al, 2011) and a summary can be found in von Schneidemesser and Monks (2013). More research is needed on this topic.…”