Characterisation and Source Apportionment of Submicron Particle Number Size Distributions in a Busy Street Canyon

ABSTRACT

Street canyons are well-known hot spots due to the harmful exposure to high concentrations of atmospheric pollutants emitted mainly by motor vehicles. We report on measurements of air pollutants conducted in a street canyon in Stockholm (Sweden) in spring 2006. Particle number size distributions (PNSD) were measured in the 25–606 nm range, along with total particle number, light-absorbing carbon mass concentration (M_LAC), PM₁₀, NO_x, CO, traffic rate (TR), vehicle speed and meteorological variables. We used PNSD as input to the positive matrix factorisation (PMF) analysis to identify and apportion the pollutant sources. All pollutants showed distinct diurnal patterns, with highest concentrations in weekday mornings (08:00–09:00). TR was always higher on weekdays, except for the early hours (00:00–06:00). The raise in the weekend early-hour TR was accompanied by the largest M_LAC of the day, a higher NO_x/CO ratio compared to weekdays and a modal shift of PNSD towards larger diameters (47–56 nm), indicates a change in the vehicle fleet share to being dominated by diesel-run taxis. The largest contribution to the submicron particles was observed for winds blowing along the canyon, transporting particles emitted by vehicles accelerating from the traffic lights at the intersection, uphill towards the measurement site, and from the nearby streets. Three PMF factors were identified: local emissions from a mixed fleet dominated by gasoline engines, local traffic emissions highly impacted by diesel vehicles, and urban background aerosol. On average, gasoline-fuelled vehicles largely contributed to NO_x, and particle number concentrations (54–65%), whereas M_LAC sources were dominated by diesel emissions, especially at weekends in the early hours (73%). The urban background contribution was rather low (4–13%) and with little dependence on the weekday. This work demonstrated how particle size distribution measurements, together with M_LAC, NO_x and CO can be used to quantify the contribution from diesel and gasoline vehicles.