Abstract
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 (NH4)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.
Highlights
The number concentration of cloud condensation nuclei (CCN) is an important parameter for cloud microphysics
In most CCN studies, the critical dry diameters of laboratory-generated particles are measured using a CCN counter (CCNC) connected to a differential mobility analyzer (DMA) in tandem to calibrate the instruments
RUN3 was performed soon after the CCN measurement at T 1, and we concentrated on the size range (b) that is important for CCN activation under these conditions
Summary
The number concentration of cloud condensation nuclei (CCN) is an important parameter for cloud microphysics. In most CCN studies, the critical dry diameters (the threshold diameters for CCN activation, dc) of laboratory-generated particles are measured using a CCNC connected to a differential mobility analyzer (DMA) in tandem to calibrate the instruments. S values calculated using the two different compounds did not always agree, and the magnitude of the difference was up to 10% (relative) (e.g., Shilling et al, 2007) These discrepancies were possibly caused by the uncertainties described above. Rose et al (2008) have suggested that the shape factor of NaCl particles varies between 1.0 and 1.08 based on measurements of the CCN activity of NaCl particles. S values inside the CCNC were calculated using the measured dc, particle mass, and several thermodynamics models. The calculated S values are compared to investigate the consistency of the experimental results of (NH4)2SO4 and NaCl particles
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