The importance of street traffic and meteorological conditions on the concentrations of particulate matter (PM) with an aerodynamic diameter smaller than 10 m (PM 10 ) was studied in the city of Münster in northwest Germany. The database consisted of meteorological data, data of PM 10 mass concentrations and fine particle number (6 -225 nm diameter) concentrations, and traffic intensity data as counted with tally hand counters at a four-to six-lane road. On working days, a significant correlation could be found between the diurnal mean PM 10 mass concentration and vehicle number. The lower number of heavy-duty vehicles compared with passenger cars contributed more to the particle number concentration on working days than on weekend days. On weekends, when the vehicle number was very low, the correlation between PM 10 mass concentration and vehicle number changed completely. Other sources of PM and the meteorology dominated the PM concentration. Independent of the weekday, by decreasing the traffic by approximately 99% during late-night hours, the PM 10 concentration was reduced by 12% of the daily mean value. A correlation between PM 10 and the particle number concentration was found for each weekday. In this study, meteorological parameters, including the atmospheric stability of the boundary layer, were also accounted for. The authors deployed artificial neural networks to achieve more information on the influence of various meteorological parameters, traffic, and the day of the week. A multilayer perceptron network showed the best results for predicting the PM 10 concentration, with the correlation coefficient being 0.72. The influence of relative humidity, temperature, and wind was strong, whereas the influence of atmospheric stability and the traffic parameters was weak.Although traffic contributes a constant amount of particles in a daily and weekly cycle, it is the meteorology that drives most of the variability.
INTRODUCTIONParticulate matter (PM) with an aerodynamic diameter smaller than 10 m (PM 10 ) can be harmful. PM 10 , and particularly PM with diameters less than 2.5 m (PM 2.5 ), are associated with a risk of cardiovascular and respiratory diseases. 1-3 For preservation of health, the European Union (EU) Directive established the long-and short-term standards for PM 10 . According to those standards, no more than 35 exceedances of 50 g ⅐ m Ϫ3 (daily average PM 10 concentration in air at a given station) per year are allowed. 4 Many European cities cannot comply with these regulations. After 2011, sanctions will be imposed on cities not in accordance with the air quality standards.In Münster, Germany, the PM 10 concentration was exceeded 35 times in 2005, 29 times in 2006, and 31 times in 2007. 5 The corresponding measurements were made at sites comparable to the one used for this study. All sites were traffic sites; one of them was located only 50 m to the southeast from this study site. Yet, by just reaching but not exceeding the legal limit, the city of Münster presently has no urgent ne...