Instrument artifacts lead to uncertainties in parameterizations of cloud condensation nucleation

Mirrielees, Jessica A.; Brooks, Sarah D.

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 (inline-formulaκapp), 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 inline-formulaκapp values calculated from CCN measurements and the literature value. Discrepancies were found to increase from inline-formula<1 % 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 inline-formula< sheath flow) or larger particle diameters (for excess flow inline-formula> sheath flow) than predicted. For the CPC instrument, undercounting occurred at high concentrations, resulting in calculated inline-formulaκapp 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 inline-formulaκapp are 1.2 % or less for 25 to 200 nm diameter aerosols.

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Mirrielees, Jessica A. / Brooks, Sarah D.: Instrument artifacts lead to uncertainties in parameterizations of cloud condensation nucleation. 2018. Copernicus Publications.

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Rechteinhaber: Jessica A. Mirrielees

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