New particle formation and growth to CCN sizes at a coastal site in the Netherlands: insights from the CAINA campaign

New particle formation (NPF) and subsequent growth are key processes controlling cloud condensation nuclei (CCN) number concentrations, as newly formed particles can grow into the CCN size range and thereby influence cloud properties and climate. In this study, we investigate particle number size distributions, CCN activity, and hygroscopicity during the Cloud–Aerosol Interactions in a Nitrogen Dominated Atmosphere (CAINA) campaign conducted in spring 2025 at a coastal site in the northern Netherlands, using a combination of a Scanning Mobility Particle Sizer (SMPS), a Particle Size Magnifier (PSM), and size-resolved CCN measurements. SMPS measurements covering the size range 6.7–969 nm were conducted between 29 March and 13 May 2025, while PSM measurements (1.19–12.0 nm) were available from 4 April to 9 May 2025. Based on visual classification of particle size distribution evolution, 19 NPF events were identified during the 46-day period (41%), 5 days were classified as undefined (11%), and the remaining 22 days as non-event days (48%). In addition, size-resolved CCN measurements were performed between 12 and 23 April 2025 to investigate in more detail the processes governing new particle formation and their growth towards CCN-relevant sizes. The measurements were carried out using a CCN counter operating at supersaturations (SS) of 0.3% and 1% downstream of a Differential Mobility Analyzer (DMA), covering particle diameters between 40 and 140 nm. The data were used to derive CCN activation fractions, characteristic activation diameters (D50), and the apparent hygroscopicity parameter kappa for the two different supersaturations. Our results show a clear size dependence of particle hygroscopicity, with particles activated at 0.3% SS generally exhibiting higher kappa values than particles activated at 1% SS. Average kappa values are around 0.1–0.2 for larger particles and 0.3–0.4 for smaller particles. A detailed case study of a NPF event shows a higher particle hygroscopocity before and during the start of the event, while the hygroscopicity decreases when the particles grow. These findings provide new insights into the link between NPF, particle chemical properties, and their ability to act as CCN.