Small area variations in ambient NO 2 concentrations in four European areas

Abstract

Spatial variations in urban air pollution are of considerable significance both because of the growing evidence for associations between exposure and human health and because of the increasing requirement for action to control and reduce levels of air pollution. This study examines sources and patterns of variation in NO 2, as a marker for traffic-related pollution, in four predominantly urban study areas: Amsterdam (Netherlands), Huddersfield (UK), Poznan (Poland) and Prague (Czech Republic). Data on pollution levels were collected using passive samplers, deployed in duplicate for 2 week periods on four occasions over one year. Mixed-effect modelling was used to explore the magnitude of between-survey, between-site and between-sampler and interaction effects, and to provide a measure of mean annual concentration at each sample site. Measured concentrations varied significantly between different surveys, with a tendency to be higher in winter months. Strong correlations were seen, however, between monitored concentrations in successive surveys, suggesting that the geographic pattern of variation in all areas was essentially stable over time. Between-sampler variation was seen to be small (CV generally 5–8%), indicating that the samplers provided consistent measures of NO 2 concentrations. There were also strongly significant between-site and site–survey interaction effects. Between-site variation accounted for between 61% (Poznan) and 84% (Prague) of total observed variation; expressed as a coefficient of variation, between-site variation was least in Amsterdam, the smallest and most uniform study area (22%), and greatest in Prague (42%). The modelled mean annual NO 2 concentration, derived from the mixed-effect model, gave a good prediction of mean annual concentration measured using passive samplers on a continuous basis, at 8–10 reference sites in each study area ( r 2>0.85). Based on the modelled results, it appears that WHO and national air quality standards for NO 2 will be routinely exceeded at a number of sites in each study area. Given the apparent stability of the pollution surface in each area, these `hotspots’ are also likely to be persistent over time, implying the potential for relatively high risks of chronic exposure in local populations. Overall the results indicate the capability to characterise spatial variation in traffic-related pollution in urban areas using a small number of intensive surveys, with low-cost sampling devices.