Road-side measurements of aerosol and ion number size distributions : a comparison with remote site measurements

We measured the number concentrations and size distributions of aerosol particles and air ions with a differential mobility particle sizer and air ion spectrometer, respectively, during a roadside campaign conducted in Kuopio (Savilahti), Finland, between 16 June and 2 July 2004. The average cluster ion (0.3-1.8 nm) concentrations were quite low (around 320 cm -3 and 280 cm -3 for negative and positive ions) during the whole period. For comparison, cluster ion concentrations in a rural SMEAR II station in Hyytiala, southern Finland, were at the same time almost three times higher. Negative intermediate ions (1.8-7.5 nm) reached maximum concentrations of 620 cm -3 in Kuopio, while the average concentrations were in the range 60-80 cm -3 depending slightly on the wind direction. Positive intermediate ion concentrations were lower. We observed higher amounts of the intermediate ions usually during rain but also during non-rain periods indicative of short-term secondary particle formation. Large ion (7.5-40 nm) concentrations (average values of 500-800 cm -3 ) were 2-3 times higher in Kuopio than at the SMEAR II station. Straightforward impact of traffic was observed when the wind blew from the road: an increase in the traffic density increased concentrations of large ions.