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Significant number fraction of urban aerosol particles is nanoclusters from traffic

In a publication in the Proceedings of the National Academy of Sciences of the United States of America, Topi Rönkkö, Aerosol Physics Laboratory of TUT, Finland, and his colleagues summarize findings of their studies about traffic emitted aerosol particles and introduce the concept of Delayed Primary Particles.

Not that long ago, we were all amazed by the researchers who showed that sub-3 nm particles are constantly present in urban air. Now Rönkkö, together with his colleagues from Tampere University of Technology, Finnish Meteorological Institute, Helsinki Region Environmental Services Authority, Metropolia University of Applied Sciences and Turku University of Applied Sciences, report that traffic is the major source of nanocluster aerosol in urban air.  They also state that the number of sub-3 nm particles can be more than half of the total number of submicron aerosol particles. This indicates e.g. that particle emissions from traffic are likely underestimated in all emission scenarios.

In this research program, 1.3–3 nm particles were monitored by the roadside in a semiurban location and an urban street canyon, during a mobile measurement campaign through western Europe, and in laboratory measurements of a modern diesel engine. In the stationary and mobile measurements, the researchers were able to prove, using diurnal variations, air trajectories and e.g. data from traffic tunnels, that the majority of the observed nanocluster aerosol particles were emitted by traffic.

The diesel engine laboratory experiments in turn showed that most of the nanocluster aerosol particles detected after the engine were not yet present in the hot exhaust, and that atmospheric oxidation was not needed for the nanocluster formation. Based on the results the researchers introduce three categories of vehicle and engine emissions:

  • Primary particles – in particle phase already in the hot exhaust
  • Delayed primary particles – from gaseous species that condense or nucleate to particle phase immediately after the exhaust is released to the atmosphere, not depending significantly on atmospheric processing or photochemistry
  • Secondary aerosol particles – are formed through atmospheric processing and photochemistry

According to Rönkkö et al., in urban air the number of traffic-emitted nanocluster aerosol particles likely exceeds the number of particles formed by photochemical nucleation. On global scale, they estimate the annual road traffic related nanocluster emissions to be >4.2*1027 a-1. This is a significant amount when compared to the estimate for global anthropogenic particle sources (17*1027 a-1).

The reported findings help increase understanding of the aerosol processes that have an impact on climate, air quality and public health.


To read the article by Rönkkö et al., please visit the PNAS website: Rönkkö et al. 2017 Traffic is a major source of atmospheric nano cluster aerosol. Proc. Natl. Acad. Sci. U.S.A., vol. 114, issue 29, pp. 7549-7554. DOI: 10.1073/pnas.1700830114

Rönkkö and his colleagues at TUT have been measuring the small particles for a few years now. Nowadays the group considers the sub-3 nm particle measurements a regular part of their vehicle emission investigations. For more information on aerosol research at TUT, please visit their website: Aerosol Physics Laboratory, Tampere University of Technology



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