Dispersion of particles and trace gases nearby a city highway: Mobile laboratory measurements in Finland

Traffic particles, total number concentration and size distribution in the range 3 nm–10 μm, as well as concentrations of carbon monoxide and nitrogen oxides were measured by a mobile laboratory at a major highway and in its vicinity, 0–140 m from the road side, in Helsinki, during the summer and winter campaigns in 2003 and 2004. The data have been analysed according to the sampling point and temperature, and classified to three wind sectors; wind blowing perpendicular to the road from northwest (S1) or to the opposite direction (S3), and wind blowing along the road (S2). The average concentrations in winter were 2–3 times higher than the concentrations in summer. In winter, 90–95%, and in summer, 86–90% of particles were smaller than 50 nm. The equations for the concentration curves in sectors S1 and S2 were derived, starting point being at the roadside. When moving 65 m from the roadside, the average concentration reduced to 39% in winter and to 35% in summer in wind sector S1 whilst in sector S2, it already decreased to 19% in summer. The size distributions showed 2–3 modes peaking at 14–23 nm (nucleation mode), 40–50 nm (Aitken mode) and 120–125 nm (accumulation mode). In wind sector S1, in winter, the nucleation mode shifted to larger sizes when moving away from the road.