The behaviour of charged particles (ions) during new particle formation events in urban Leipzig (Germany)

Abstract. Air ions are electrically charged molecules or particles in air. They are found in the natural environment. Charging accelerates the formation and growth of new aerosol particles. A Neutral cluster and Air Ion Spectrometer was deployed in Leipzig, Germany, to measure the number size distribution of ions from 0.8 to 42 nm, between July 27th and August 25th 2022. Charged particles were mobility classified into small (0.8–1.6 nm), intermediate (1.6–7.5 nm), and large (7.5–22 nm) fractions and their mean concentrations (sum of positive and negative polarities) during the campaign were 462, 88, and 420 cm-3, respectively. The study found that small charged particles were primarily associated with radioactive decay during the early hours, while the intermediate and large charged fractions were linked to photochemistry and local air pollution, as indicated via synchronous peaks in sulphuric acid dimer and black carbon concentrations, respectively. NPF events, observed on 30 % of days, coincided with intense solar radiation. Small charged particle concentrations were lower on NPF event days, whereas the intermediate and large charged species exhibited higher concentrations. The apparent contributions of charged species to 3 and 7.5 nm particles formation rates were 5.7 and 12.7 %, respectively, with mean growth rates of 4.0 and 5.2 nm h-1. Although the ratio of apparent formation rates for charged to uncharged nanoparticles of 3 nm suggested a minor role for charged species in NPF, a substantial increase in intermediate and large charged species was associated with NPF events. The findings contribute valuable insights into the complex interplay between charged species and particle formation in urban environments.