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

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 the growth mechanisms in Northern Italy. Our results show that abundant sulfuric acid, ammonia and amines from agricultural activities may be the dominant components driving the frequent NPF events (66 % of all days during the measurement campaign) in this area. In contrast, organics seem to have a smaller role in cluster formation but contribute to the consecutive growth process. According to ion cluster measurements and kinetic model results, dimethylamine is not sufficient to stabilize all of the sulfuric acid during springtime in the Po Valley, suggesting that other amines and ammonia can also be involved. 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) together with the relatively low condensational sink (8.9 × 10⁻³ s⁻¹) will result in a high survival probability of newly formed particles, making NPF crucial for the springtime aerosol number budget in the Po Valley region.