Size resolved traffic emission factors of submicrometer particles

Size resolved emission factors for submicrometer particles related to trace gases have been obtained from measurement data at a suburban road side, with a traffic intensity of 18,000 vehicles per day. Number of particles with diameter 10–368 nm, trace gases (NO, NOx,O3 and SO2), traffic and meteorology parameters were measured outside of Göteborg, Sweden. Size distributions of small particles at the site are presented and their relation to meteorological and traffic related variables was evaluated. Wind speed correlated negatively with 10–368 nm particles and temperature correlated negatively with the smaller particles (10–60 nm). Nitric oxide was shown to be a better tracer for traffic related ultrafine particles, than traffic intensity itself. The calculated emission factor, with errors at 95% confidence level, for particles in the range 10–368 nm is presented in relation to nitrogen oxides. The emission factors were 268±60 and 176±37 particles cm−3 per ppb NO and NOx, respectively. The particle emission factors for 10–100, 10–50, 50–170 and 170–368 nm were 260±70, 228±52, 41±11 and <1 particle cm−3 per ppb NO, respectively. The size distribution of the emissions is given by number of particles normalised by the width of the size bin, i.e. in units of dNd log Dp−1 ppb−1. The maximum normalised emission factor was 450 cm−3 per ppb NO for 20 nm particles. The shape of the size distribution of emissions revealed one sharp peak at 20 nm, with a small shoulder at 70 nm.