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

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 in many areas is generally 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. The formation of H₂SO₄ in urban environments is here separated into two routes: primary H₂SO₄ is formed in hot vehicle exhaust and is converted rapidly to the particle phase; secondary H₂SO₄ results from the combined effect of emitted gaseous precursors and available solar radiation. A rough estimation demonstrates that ∼85 % of the total NCA and ∼68 % of the total H₂SO₄ in urban air at noontime at the measurement site are contributed by traffic, indicating the importance of traffic emissions.