Methods for determining particle size distribution and growth rates between 1 and 3 nm using the Particle Size Magnifier

The most important parameters describing the atmospheric new particle formation process are the particle formation and growth rates. These together determine the amount of cloud condensation nuclei attributed to secondary particle formation. Due to difficulties in detecting small neutral particles, it has previously not been possible to derive these directly from measurements in the size range below about 3 nm. The Airmodus Particle Size Magnifier has been used at the SMEAR II station in Hyytiala, southern Finland, and during nucleation experiments in the CLOUD chamber at CERN for measuring particles as small as about 1 nm in mobility diameter. We developed several methods to determine the particle size distribution and growth rates in the size range of 1-3 nm from these data sets. Here we introduce the appearance-time method for calculating initial growth rates. The validity of the method was tested by simulations with the Ion-UHMA aerosol dynamic model.