Using measurements of air pollution in streets for evaluation of urban air quality - Meteorological analysis and model calculations

Measurements of urban air pollution are usually confined to a few locations within a city area. Monitoring stations are often situated in streets with significant traffic levels or in places where severe pollution problems are expected. Such measurements are naturally influenced by very local conditions and care must be taken in interpreting the results. This is especially important when the measurements are used for estimating urban air pollution levels or comparing air quality in different cities. Another frequent application of street measurements is for public information or warning the population of elevated pollution levels. Estimating the dangers of long-term exposure to air pollution means careful consideration must be given to how representative these measurements actually are. In this paper the influence of local conditions on air pollution concentrations is discussed, regarding especially the dependency of pollution levels on street configuration and meteorolgical parameters. The examples used are based on measurements from locations in Copenhagen and on model calculations using the Danish Operational Street Pollution Model (OSPM). It is shown that large concentration gradients can occur in street canyons with leeward concentrations far higher than windward concentrations. Thus, street measurements are site-dependent and not representative for urban areas. Model calculations with OSPM agree well with measurements.