Monitoring of black carbon and size-segregated particle number concentrations at 9-m and 65-m distances from a major road in Helsinki

In February and August 2003, black carbon (BC) and size-segregated particle number concentrations were monitored simultaneously at 9-m and 65-m distances from a major road in Helsinki, Finland, using aethalometers and electrical low-pressure impactors, respectively. During weekdays in winter, the average total particle number concentrations in the diameter range 0.007–1 ?m increased during morning rush hours from the nighttime values of 17000 and 12000 cm–3 to 190000 and 130000 cm–3 at the 9-m and 65-m stations, respectively. The corresponding BC concentrations increased from 730 and 430 ng m–3 to 2800 and 1550 ng m–3. Compared with those in winter, the average rush-hour particle number concentrations were much lower in summer, the likely reason being enhanced nucleation in cold winter conditions. BC concentrations were slightly higher during summer than during winter. Number size distributions measured at the 9-m and 65-m distances and at a background site had similar modal characteristics with the highest peak occurring below 0.03 ?m. Despite the different wind conditions in winter and summer, concentrations of total particle number and BC decreased similarly between the 9-m and 65-m stations, the likely principal mechanism being mixing with background air. The strong diurnal variation in concentrations during the weekdays, together with the large concentration difference between the 9-m and 65-m distances, suggests that local traffic was the main source of the measured pollutants, especially during rush hours at the 9-m site. In winter, the decrease in the particle number concentrations from the 9-m site to the 65-m site was most pronounced for the smallest exhaust particles. During an episodic pollution event in winter there were indications of condensational growth of 0.007–0.03 ?m particles, which increased the number concentration of 0.03–0.06 ?m particles at the 65-m site.