Elucidating the mechanisms of atmospheric new particle formation in the highly polluted Po Valley, Italy

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Abstract
New particle formation (NPF) is a major source of aerosol particles and cloud condensation nuclei in the troposphere, playing an important role in both air quality and climate. Frequent NPF events have been observed in heavily polluted urban environments, contributing to the aerosol number concentration by a significant amount. The Po Valley region in northern Italy has been characterized as a hotspot for high aerosol loadings and frequent NPF events in southern Europe. However, the mechanisms of NPF and growth in this region are not completely understood. In this study, we conducted a continuous 2-month measurement campaign with state-of-the-art instruments to elucidate the NPF and growth mechanisms in northern Italy. Our results demonstrate that frequent NPF events (66 % of all days during the measurement campaign) are primarily driven by abundant sulfuric acid (8.5×10 6 cm −3 ) and basic molecules in this area. In contrast, oxygenated organic molecules from the atmospheric oxidation of volatile organic compounds (VOCs) appear to play a minor role in the initial cluster formation but contribute significantly to the consecutive growth process. Regarding alkaline molecules, amines are insufficient to stabilize all sulfuric acid clusters in the Po Valley. Ion cluster measurements and kinetic models suggest that ammonia (10 ppb) must therefore also play a role in the nucleation process. Generally, the high formation rates of sub-2 nm particles (87 cm −3 s −1 ) and nucleation-mode growth rates (5.1 nm h −1 ) as well as the relatively low condensational sink (8.9×10-3 s −1 ) will result in a high survival probability for newly formed particles, making NPF crucial for the springtime aerosol number budget. Our results also indicate that reducing key pollutants, such as SO 2 , amine and NH 3 , could help to substantially decrease the particle number concentrations in the Po Valley region.
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Funding Information
  • Research Council of Finland (project no. 307331,)
  • Research Council of Finland (PROFI3 funding no. 311932,)
  • Research Council of Finland (ACCC Flagship no. 337549)
  • Research Council of Finland (346370)
  • H2020 European Research Council (no. 742206)
  • H2020 European Research Council (Consolidator grant INTEGRATE (no. 865799))
  • H2020 European Research Council (CHAPAs (no. 850614))
  • H2020 European Research Council (the European Union’s Horizon 2020 research and innovation programme (no. 821205))
  • H2020 European Research Council (Marie Skłodowska–Curie grant grant agreement no. 895875 (NPF-PANDA))
  • Vienna Science and Technology Fund (VRG22-003)
  • Jenny ja Antti Wihurin Rahasto (none)
  • Knut och Alice Wallenbergs Stiftelse (WAF project CLOUDFORM, (grant no. 2017.0165))

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