Variation of NO2 and NOx concentrations between and within 36 European study areas: Results from the ESCAPE study
Highlights
► We measured NO2 and NOx in 36 European study areas using standardized method. ► Significant contrast in NO2 and NOx levels between and within areas were found. ► Concentrations were generally lower in Northern than in Southern Europe. ► Street/urban background contrast was higher than for the particle metrics. ► Epidemiological studies should characterize intra-urban contrasts.
Introduction
There is now increasing evidence from epidemiological studies that exposure to ambient air pollution is associated with adverse health effects (Brunekreef and Holgate, 2002; Heinrich and Wichmann, 2004; Pope and Dockery, 2006; WHO, 2006; Rückerl et al., 2011). Adverse effects include pre-mature mortality and morbidity from cardiovascular and respiratory causes. Based upon experimental studies, plausible mechanisms for these associations have been proposed, including oxidative stress in particular (Brunekreef and Holgate, 2002; Brook et al., 2010; Rückerl et al., 2011). Most studies from the USA have focussed on PM10 and PM2.5 (Brook et al., 2010; Pope and Dockery, 2006). In several European studies, significant associations between adverse health effects and NO2 or NOx concentrations (Brunekreef, 2007) have been reported. In these studies, air pollution exposure was assessed at the residential address using dispersion models, land use regression models and traffic indicator variables. Other epidemiological studies on long-term exposure to NO2 and other air pollutants compared the health status of populations using the contrast in city-average air pollution levels between different areas (e.g. Pope et al., 2002; Laden et al., 2006; Sunyer et al., 2006; Götschi et al., 2008). These studies generally assigned one overall average concentration to all subjects living in each city. For NO2 this likely results in significant misclassification as high spatial variability within urban areas has been documented previously for nitrogen dioxide (NO2) in specifically designed studies (Lebret et al., 2000; Monn, 2001; Lewne et al., 2004). There are a substantial number of studies that have used traffic indicators as exposure variables, including distance to a major road, and traffic intensity on the nearest road (HEI, 2010). A major limitation of these traffic indicators is that their value in characterizing actual air pollution exposure contrasts may differ between study areas (Jerrett et al., 2005). Some studies have made use of the spatial variation of air pollution within metropolitan areas (Gauderman et al., 2005; Gehring et al., 2006; Morgenstern et al., 2008; Jacquemin et al., 2009; Modig et al., 2009). These within-city studies often characterized air pollution with the concentration of NO2 and NOx obtained from either spatially dense monitoring networks, land use regression models based upon such networks, or dispersion models (Jerrett et al., 2005; Hoek et al., 2008; Modig et al., 2009; Levy et al., 2010).
In 1999, the European Commission established limit values for NO2, NOx, PM10 and some other air pollutants in the Air Quality Daughter Directive 1999/30/EC (EC, 1996), which was replaced in 2008 by the new Directive 2008/50/EC on ambient air quality and cleaner air for Europe (EC, 2008). The existing air quality guidelines for NO2 and PM10 are currently being exceeded at many locations throughout Europe (Giannouli et al., 2011; European Environment Agency, 2006; Airbase, 2007; Velders and Diederen, 2009). There is therefore substantial interest at the EU policy level in the health effects of current air pollution levels including NO2, focussing especially on European studies.
A comparison of NO2 concentrations measured either in study specific monitoring programmes (Hazenkamp-von Arxa et al., 2004) or in routine monitoring networks across Europe (e.g. Airbase data used in Beelen et al., 2009) showed significant contrast across Europe. The concentrations were generally lowest in Northern Europe and highest in the major cities and Southern Europe.
NO2 is often used as an indicator of the complex mixture of traffic-related air pollution containing also fine and ultrafine particles. The ratio of NO2 to other components e.g. soot in emissions of motorized road traffic has changed in the last decade (Williams and Carslaw, 2011). Specifically, the fraction of primary NO2 emissions has increased.
In 2008 we embarked upon a European-wide study of long-term air pollution exposure health effects. The ESCAPE study – European Study of Cohorts for Air Pollution Effects – assesses exposure–response relationships between long-term exposures to ambient air pollution using prospective cohort studies in 15 different European countries (http://www.escapeproject.eu). As the estimation of the within-urban variation of air pollution is a key interest of epidemiological long-term studies and the most routine monitoring networks are not sufficiently dense to characterize intra-urban concentration gradients, we decided to carry out study specific monitoring, which was independent of routine monitoring networks.
In the framework of this study we conducted measurements of NO2 and NOx concentrations in 36 study areas during a one year long measurement period. The aim of this paper is to assess the variation of measured NO2 and NOx concentrations between and within 36 European study areas. We further assessed the difference of NO2 and NOx concentrations at traffic stations versus urban background stations as a source of within study area spatial variability. The third aim was to study the variability of the NO2/NOx ratio. The companion paper focuses on the PM measurement (Eeftens et al., submitted for publication).
Section snippets
ESCAPE exposure assessment
In all study areas, NO2 and NOx were measured with passive samplers (see Section 2.4). The measured average concentrations were combined with geographic predictors to develop land use regression (LUR) models (Jerrett et al., 2005; Hoek et al., 2008). In all study centres a common protocol was used to ensure high standardization of all procedures across the 36 European study areas. The standardization of the measurements and the selection of the locations using a common protocol across a wide
Quality control
Detection limits were low for NO2 for all study areas and very few samples were below the limit of detection (Table 2). Detection limits were slightly higher for NOx but in nearly all study areas very few samples were below the limit of detection. Only in Turin, Catalunya and Albacete was a sizable fraction below the detection limit. This was partly due to unexplained outliers which were all included in the detection limit calculations presented in Table 2. Samples below the detection limit
Discussion
Overall, we found significant contrasts in annual average NO2 and NOx concentrations between and especially within 36 study areas across Europe. NO2 concentrations at street locations were on average between 1.22 and 3.6 times higher than at urban background stations. The NO2/NOx ratio varied between 0.47 and 0.72 across study areas. Concentrations were generally lower in Northern than in Southern Europe.
Conclusion
We found significant contrasts in annual average NO2 and NOx concentrations between and especially within 36 study areas across Europe. The within-area spatial variability contributed significantly to the overall variance of NO2 and NOx concentrations (60% and 70%, respectively). It was mostly determined by differences between street and urban background concentrations. The street/urban background concentration ratio varied between 1.09 and 3.16 for NO2 and between 1.14 and 4.24 for NOx across
Competing interest
The authors declare they have no competing financial interest.
Acknowledgements
We would like to thank Kees Meliefste, Geert de Vrieze, Marjan Tewis (IRAS, Utrecht University, The Netherlands) for the sampler preparation, analysis and data management.
Furthermore, we thank all those who were responsible for air pollution measurements, data management and project supervision in all study areas and especially:
Helsinki/Turku: Arto Pennanen, Tarja Yli-Tuomi.
Manchester: Haytham Alhamwi, Nuthchywach Sanguanchaiyakrit.
Munich/Augsburg: Thomas Kusch, Stephanie von Klot, Guido
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