High concentrations of sub-3nm clusters and frequent new particle formation observed in the Po Valley, Italy, during the PEGASOS 2012 campaign

Abstract. The concentrations of neutral and charged sub-3nm clusters and their connection to new particle formation (NPF) were investigated during the PEGASOS campaign (7 June–9 July 2012) at the San Pietro Capofiume measurement station in the Po Valley, Italy. Continuous high concentrations of sub-3nm clusters were detected during the measurement period, although the condensation sink was relatively high (median value 1.1 × 10⁻² s⁻¹). The median cluster concentrations were 2140 and 7980 cm⁻³ in the size bins of 1.5–1.8 and 1.8–3 nm, and the majority of them were electrically neutral. NPF events were observed during the measurement period frequently, on 86 % of the days. The median growth rates of clusters during the events were 4.3, 6.0 and 7.2 nm h⁻¹ in the size ranges of 1.5–3, 3–7 and 7–20 nm. The median formation rate of 1.6 nm clusters was high, 45 cm⁻³ s⁻¹, and it exceeded the median formation rate of 2 nm clusters by 1 order of magnitude. The ion-induced nucleation fraction was low; the median values were 0.7 % at 1.6 nm and 3.0 % at 2 nm. On NPF event days the neutral cluster concentration had a maximum around 09:00 (local winter time), which was absent on a non-event day. The increase in the cluster concentrations in the morning coincided with the increase in the boundary layer height. At the same time radiation, temperature and SO₂ concentration increased, and RH and condensation sink decreased. The concentrations of neutral and charged clusters were observed to have a positive correlation with sulfuric acid proxy, indicating the significance of sulfuric acid for the cluster formation in San Pietro Capofiume. The condensation sink had a negative correlation with the concentration of charged clusters but no clear relation to the neutral cluster concentration. This finding, together with back-trajectory analysis, suggests that the precursor vapors of the clusters and background aerosol particles, acting as their sink, have possibly originated from the same sources, including e.g., power plants and industrial areas in the Po Valley.