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. The concentrations of cloud condensation nuclei (CCN) modulate cloudproperties, rainfall location and intensity, and climate forcings. This workassesses uncertainties in CCN measurements and the apparent hygroscopicityparameter ( κapp ), which is widely used to represent CCNpopulations in climate models. CCN measurements require accurate operation ofthree instruments: the CCN instrument, the differential mobility analyzer(DMA), and the condensation particle counter (CPC). Assessment of DMAoperation showed that varying the ratio of aerosol to sheath flow from 0.05to 0.30 resulted in discrepancies between the κapp valuescalculated from CCN measurements and the literature value. Discrepancies werefound to increase from  % to 13 % for both sodium chloride andammonium sulfate. The ratio of excess to sheath flow was also varied, whichshifted the downstream aerosol distribution towards smaller particlediameters (for excess flow   sheath flow) or larger particle diameters(for excess flow  >  sheath flow) than predicted. For the CPC instrument,undercounting occurred at high concentrations, resulting in calculated κapp lower than the literature values. Lastly, undercountingby CCN instruments at high concentration was also assessed, taking the effectof supersaturation on counting efficiency into account. Under recommendedoperating conditions, the combined DMA, CPC, and CCN uncertainties in κapp are 1.2 % or less for 25 to 200 nm diameteraerosols.

Instrument artifacts lead to uncertainties in parameterizations of cloud condensation nucleation