Instrument artifacts lead to uncertainties in parameterizations of cloud condensation nucleation
Abstract. The concentrations of cloud condensation nuclei (CCN) modulate cloud
properties, rainfall location and intensity, and climate forcings. This work
assesses uncertainties in CCN measurements and the apparent hygroscopicity
parameter ( κapp ), which is widely used to represent CCN
populations in climate models. CCN measurements require accurate operation of
three instruments: the CCN instrument, the differential mobility analyzer
(DMA), and the condensation particle counter (CPC). Assessment of DMA
operation showed that varying the ratio of aerosol to sheath flow from 0.05
to 0.30 resulted in discrepancies between the κapp values
calculated from CCN measurements and the literature value. Discrepancies were
found to increase from  % to 13 % for both sodium chloride and
ammonium sulfate. The ratio of excess to sheath flow was also varied, which
shifted the downstream aerosol distribution towards smaller particle
diameters (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, undercounting
by CCN instruments at high concentration was also assessed, taking the effect
of supersaturation on counting efficiency into account. Under recommended
operating conditions, the combined DMA, CPC, and CCN uncertainties in
 κapp are 1.2 % or less for 25 to 200 nm diameter
aerosols.