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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Jul 2019

Submitted as: research article | 23 Jul 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Traffic-originated nanocluster emission exceeds H2SO4-driven photochemical new particle formation in an urban area

Miska Olin1, Heino Kuuluvainen1, Minna Aurela2, Joni Kalliokoski1, Niina Kuittinen1, Mia Isotalo1, Hilkka J. Timonen2, Jarkko V. Niemi3, Topi Rönkkö1, and Miikka Dal Maso1 Miska Olin et al.
  • 1Aerosol Physics Laboratory, Physics Unit, Tampere University, FI-33014 Tampere, Finland
  • 2Atmospheric Composition Research, Finnish Meteorological Institute, FI-00101 Helsinki, Finland
  • 3Helsinki Region Environmental Services Authority (HSY), FI-00066 HSY, Finland

Abstract. Elevated ambient concentrations of sub-3 nm particles (nanocluster aerosol, NCA) are generally related to atmospheric new particle formation events, usually linked with gaseous sulfuric acid (H2SO4) produced via photochemical oxidation of sulfur dioxide. According to our measurement results of H2SO4 and NCA concentrations, traffic density, and solar irradiance at an urban traffic site in Helsinki, Finland, the view of aerosol formation in traffic-influenced environments is updated by presenting two separate and independent pathways of traffic affecting the atmospheric NCA concentrations: by acting as a direct nanocluster source, and by influencing the production of H2SO4. As traffic density is frequently correlated with solar radiation, it is likely that the influence of traffic-related nanoclusters has been hidden in the diurnal variation, and is thus underestimated because new particle formation events also follow the diurnal cycle of sunlight. Urban aerosol formation studies should, therefore, be updated to include the proposed formation mechanisms. Additionally, the directly emitted NCA poses a potentially elevated health risk, highlighting the need for quantifying the source and loadings in populated areas. The formation of H2SO4 in urban environments is here separated in two routes: primary H2SO4 is formed in hot vehicle exhaust and is converted rapidly to particle phase; secondary H2SO4 results from the combined effect of emitted gaseous precursors and available solar radiation.

Miska Olin et al.
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Miska Olin et al.
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Publications Copernicus
Short summary
Photochemically formed sulfuric acid is generally considered the main source for new particle formation in the atmosphere. Contrary to current understanding, our measurements of nanoclusters and gaseous sulfuric acid performed in an urban area imply that traffic contributes to sulfuric acid concentration, and that even for the smallest particles, the traffic-emitted fraction mostly exceeds the photochemistry-driven regional new particle formation.
Photochemically formed sulfuric acid is generally considered the main source for new particle...