cc-by-4.0Markku Kulmala2025-04-292023-06-202023-06-20https://datakatalogi.helsinki.fi/handle/123456789/5618Data for Measurement report: Ion clusters as indicator for local new particle formation. There are two files, negative_ion_concentrations.csv and positive_ion_concentrations.csv. The former (latter) includes absolute number concentrations for 1.87, 2.16, 2.49, and 2.88 nm negative (positive) ions. The unit for these concentrations is #/cm-3. Contact Santeri Tuovinen (santeri.tuovinen@helsinki.fi) for more details.New particle formationAtmospheric ionsNumber concentrationSMEAR IIdataset