Traffic-originated nanocluster emission exceeds H<sub>2</sub>SO<sub>4</sub>-driven photochemical new particle formation in an urban area

Abstract

<p><strong>Abstract.</strong> 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 (H<sub>2</sub>SO<sub>4</sub>) produced via photochemical oxidation of sulfur dioxide. According to our measurement results of H<sub>2</sub>SO<sub>4</sub> 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 H<sub>2</sub>SO<sub>4</sub>. 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 H<sub>2</sub>SO<sub>4</sub> in urban environments is here separated in two routes: primary H<sub>2</sub>SO<sub>4</sub> is formed in hot vehicle exhaust and is converted rapidly to particle phase; secondary H<sub>2</sub>SO<sub>4</sub> results from the combined effect of emitted gaseous precursors and available solar radiation.</p>

Emerging Investigator Series: COVID-19 lockdown effects on aerosol particle size distributions in northern Italy

“…In the absence of regional NPF, particles below 50 nm, and even below 10 nm mainly come from traffic emissions, as shown in previous studies [19][20][21][22]42 and to a certain extent corroborated by this study.…”

“…17 Traffic is a major source of atmospheric nanocluster aerosols (particles smaller than 3 nm), and it directly emits aerosol particles including the core mode (particles smaller than 10 nm) and soot mode (particles between 30 and 100 nm in diameter), [18][19][20] what we call primary aerosol particles. In addition, traffic emissions have been shown to signicantly produce sub-3 nm particles, and also to contribute to secondary aerosol particle formation by emitting various aerosol precursors, 21,22 including SO 2 (sulfur dioxide), VOCs (volatile organic compounds) and NO x . The traffic sector is considered as one of the major sources of NO x emission, 23,24 which can affect O 3 concentration by O 3 titration with NO and O 3 nonlinear formation chemistry.…”