Determining the impact of new particle formation events on cloud condensation nuclei (CCN) concentrations

Atmospheric aerosol particles act as cloud condensation nuclei (CCN) and play a crucial role in the climate system and hydrological cycle. New particle formation (NPF) events are a significant source of CCN, though their global contribution to the CCN budget remains uncertain. We propose a novel method to estimate the direct contribution of NPF events to CCN concentrations, distinguishing between NPF-generated particles and background or transported aerosols. Previous methods estimated CCN concentration enhancements by comparing CCN concentration at different times periods (during or after NPF events with CCN concentrations prior to the events or with non-event days). Our method overcomes this limitation by focusing on the ceiling size that newly formed particles can reach, the timing of NPF events, and isolating the NPF mode from the overall aerosol size distribution. The method was developed using size-resolved CCN measurements at the Sierra Nevada Station (SNS) in southeastern Spain (2500 m a.s.l.). We demonstrate that the method is also applicable to polydisperse CCN measurements, showing consistency across both variants. Additionally, it has been applied to NPF events at the Izaña Observatory (IZO) in Tenerife (Canary Islands, 2367 m a.s.l.). Both SNS and IZO frequently experience NPF events, though their environmental and aerosol characteristics differ, providing insights into the advantages and limitations of the method. Comparison between the two sites reveals that the mean NPF contribution to CCN is 6.2 ± 4.8 % (IQR = 9 %) at SNS and 24 ± 25 % (IQR = 28 %) at IZO. The higher NPF contribution at IZO is attributed to the ability of newly formed particles to grow to larger sizes and the presence of fewer transported boundary layer particles acting as CCN. Both sites show significant variability in the NPF contribution percentage due to the distinct characteristics of each NPF event.