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

Category: Air Pollution Modeling

Volume: 15 | Issue: 1 | Pages: 220-233
DOI: 10.4209/aaqr.2014.06.0108
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Patricia Krecl 1, Admir Créso Targino1, Christer Johansson2,3, Johan Ström2

  • 1 Federal Technological University of Paraná, Graduate Program in Environmental Engineering, Apucarana-Londrina, Brazil
  • 2 Department of Applied Environmental Science (ITM), Stockholm University, Stockholm, Sweden
  • 3 Stockholm Environment and Health Administration, Stockholm, Sweden

Highlights

Measurements of air pollutants were conducted in a street canyon in Stockholm. 
Number size distribution mode shifted to larger sizes when diesel fleet dominated.
PMF was applied on particle number size distributions for source apportionment. 
Diesel emissions dominated black carbon load in the early hours of weekends. 


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 (MLAC), PM10, NOx, 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 MLAC of the day, a higher NOx/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 NOx, and particle number concentrations (54–65%), whereas MLAC 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 MLAC, NOx and CO can be used to quantify the contribution from diesel and gasoline vehicles.

Keywords

DMPS Traffic emissions PMF Urban air quality Black carbon


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