Measurements of particle masses of inorganic salt particles for calibration of cloud condensation nuclei counters

Abstract

We measured the mobility equivalent critical dry diameter for cloud condensation nuclei (CCN) activation (dc-me) and the particle mass of size-selected (NH4)2SO4 and NaCl particles to calibrate a CCN counter (CCNC) precisely. The CCNC was operated downstream of a differential mobility analyzer (DMA) for the measurement of dc-me. The particle mass was measured using an aerosol particle mass analyzer (APM) operated downstream of the DMA. The measurement of particle mass was conducted for 50-150-nm particles. Effective densities (ρeff) of (NH 4)2SO4 particles were 1.67-1.75 g cm -3, which correspond to dynamic shape factors (χ) of 1.01-1.04. This shows that (NH4)2SO4 particles are not completely spherical. In the case of NaCl particles, ρeff was 1.75-1.99 g cm-3 and χ was 1.05-1.14, demonstrating that the particle shape was non-spherical. Using these experimental data, the volume equivalent critical dry diameter (dc-ve) was calculated, and it was used as an input parameter for calculations of critical supersaturation (S). Several thermodynamics models were used for the calculation of water activity. When the Pitzer model was employed for the calculations, the critical S calculated for (NH4)2SO4 and NaCl agreed to well within the uncertainty of 2% (relative). This result demonstrates that the use of the Pitzer model for the calibration of CCNCs gives the most accurate value of S.