Source apportionment of aerosol particle number in background Stockholm: impact of aviation and shipping

Ultrafine particles (UFP) have gained increased attention during recent years due to their adverse health effects (WHO, 2021). Both the WHO and the EU have therefore recommended systematic measurements of particle number concentration (PNC) and particle number size distributions (PNSD) in cities.

The nPETS project (nanoparticles Emissions from the Transport Sector) aimed to study the lifecycle of sub-100 nm particles from different sources. The nPETS project in Stockholm included measurements of PNSD and PNC in urban background air on a rooftop. PNSDs were measured during two years with 16 channels between 10 and 410 nm and Positive Matrix Factorization (PMF) source apportionment was used to analyse these data.

 

Four different emission source profiles were attributed by the PMF analysis:

 

• The most important factor with on average 38 % of the PNC had a peak between 25 – 45 nm. It was most dominant during periods with high PNC, high NOx and low wind speed. It also followed the local rush hours and was identified as traffic with a typical diesel contribution.

 

• The second most important factor with 36 % of the PNC had at peak 60- 150 nm. It was dominating with northeasterly and higher wind speeds with strongest signal late afternoons and evening. The size distribution showed similarity with ship emissions from other nPETS measurements. Trajectories also showed influence from the Baltic Sea.

 

• The third factor was associated with a peak below 20 nm in size. It showed similarity to aircraft particles from other nPETS measurements and had also strongest impact during moderate northwesterly winds which corresponds to the direction of the local Bromma airport. This factor was contributing to 17 % of the PNC.

 

• The fourth factor had a peak in particles larger than 200 nm. It showed strong correlation with PM1 mass and air mass trajectories from easterly Europe. This factor was thought to be long-range transport and was contributing to 8 % of the PNC.

 

The PMF analysis was compared with dispersion modelling of PNC in the Stockholm region. The emission database (EDB) used was from Eastern Sweden Air Quality Association with inputs from the nPETS project. Based on the dispersion modelling the traffic is the largest source, while shipping is a minor source and aviation negligible. Long range transport was not included in the modelling. The magnitude of UFP emissions from aviation in Stockholm is still largely unknown with low emissions in the EDB. Shipping in central Stockholm is limited to passenger ships in port only during some hours per day which explains the relatively small emissions in the EDB.

The discrepancy between the PMF analysis and the dispersion modelling for shipping and aviation demands further investigations.

 

Reference.

WHO (2021). WHO global air quality guidelines