Particulate matter in the underground of Stockholm

The concentrations of PM10 and PM2.5 were measured during 2 weeks at an underground station in central Stockholm. The instrument, an automatic TEOM monitor (Tapered Element Oscillating Microbalance), was placed on the platform in the centre of the station. During weekdays between 7 a.m. and 7 p.m. the average PM10 and PM12.5 concentrations were 470 and 260 mug/m(3), respectively. These levels are a factor 5 and 10 times higher than the corresponding values measured in one of the busiest streets in central Stockholm. The concentrations in the underground followed closely the train traffic intensity. The levels were very similar from one day to the next. During Saturdays and Sundays the levels decreased slightly due to less frequent train passages. Additional measurements were performed right after the tunnel had been washed. Tunnel walls and railway tracks between the platforms of the underground system were washed using water. Only a slight reduction of the PM10 levels (approximately 13%) could be observed during a few days after the water treatment. For PM12.5 the reduction was even less, about 10% lower levels could be seen. This might indicate that particles from tunnel walls and tracks make only a minor contribution to the observed levels. These results confirm earlier unpublished measurements showing high levels of PM in the underground of Stockholm. Substantially, elevated particle exposure levels have also been reported in several earlier studies in the underground