Calibration and measurement uncertainties of a continuous-flow cloud condensation nuclei counter (DMT-CCNC): CCN activation of ammonium sulfate and sodium chloride aerosol particles in theory and experiment

Abstract

Abstract. Experimental and theoretical uncertainties in the measurement of cloud condensation nuclei (CCN) with a continuous-flow thermal-gradient CCN counter from Droplet Measurement Technologies (DMT-CCNC) have been assessed by model calculations and calibration experiments with ammonium sulfate and sodium chloride aerosol particles in the diameter range of 20–220 nm. Experiments have been performed in the laboratory and during field measurement campaigns, covering a wide range of instrument operating conditions (650–1020 hPa pressure, 293–303 K inlet temperature, 4–34 K m−1 temperature gradient, 0.5–1.0 L min−1 flow rate). For each set of conditions, the effective water vapor supersaturation (Seff, 0.05–1.4%) was determined from the measured CCN activation spectra (dry particle activation diameters) and Köhler model calculations. High measurement precision was achieved under stable laboratory conditions, where the relative standard deviations of Seff were as low as ±1%. During field measurements, however, the relative deviations increased to about ±5%, which can be mostly attributed to variations of the CCNC column top temperature with ambient temperature. The observed dependence of Seff on temperature, pressure, and flow rate was compared to the CCNC flow model of Lance et al. (2006). At high Seff the relative deviations between flow model and experimental results were mostly less than 10%, but at Seff≤0.1% they exceeded 40%. Thus, careful experimental calibration is required for high-accuracy CCN measurements – especially at low Seff. A comprehensive comparison and uncertainty analysis of the various Köhler models and thermodynamic parameterizations commonly used in CCN studies showed that the relative deviations between different approaches are as high as 25% for (NH4)2SO4 and 12% for NaCl. The deviations were mostly caused by the different parameterizations for the activity of water in aqueous solutions of the two salts. To ensure comparability of results, we suggest that CCN studies should always report exactly which Köhler model equations and parameters were used. Provided that the Aerosol Inorganics Model (AIM) can be regarded as an accurate source of water activity data for highly dilute solutions of (NH4)2SO4 and NaCl, only Köhler models that are based on the AIM or yield similar results should be used in CCN studies involving these salts and aiming at high accuracy. Experiments with (NH4)2SO4 and NaCl aerosols showed that the conditions of particle generation and the shape and microstructure of NaCl particles are critical for their application in CCN activation experiments (relative deviations up to 18%).

Overview: Precipitation characteristics and sensitivities to environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…The total concentration of individual cloud condensation nuclei particles (N CCN (S)) was measured with a continuousflow stream-wise thermal gradient CCN counter (model CCN-200, DMT, Longmont, CO, USA; Rose et al, 2008). The measured aerosol was sampled by the HALO aerosol submicrometer inlet (HASI).…”

“…During the wet season, the atmosphere is mostly clean and convective, and the landscape is referred to as the Green Ocean (Roberts et al, 2001;Williams et al, 2002;Andreae et al, 2004) because the convection there resembles storms over blue oceans, where the warm phase in clouds generally produces rain. Large urban areas, however, introduce perturbations into the pristine air .…”

Measurements of aerosol and CCN properties in the Mackenzie River delta (Canadian Arctic) during spring–summer transition in May 2014

“…A SS calibration of the CCNc had been done at the cloud laboratory of the Leibniz Institute for Tropospheric Research (TROPOS) prior to the campaign to determine the relationship between the temperature gradient along the column and the effective SS. Based on recommendations given in Gysel and Stratmann (2013) and Rose et al (2008), ammonium sulfate particles were size selected using a differential mobility analyzer (DMA-type Hauke medium) and then fed into a CPC (TSI model 3010) and CCNc, which were operated in parallel. This was done for all SS values that were also applied during the measurement campaign.…”

Assessment of cloud supersaturation by size-resolved aerosol particle and cloud condensation nuclei (CCN) measurements

“…CCN size distributions were calculated by multiplying the best-fit cumulative Gaussian distribution function (cf. Rose et al, 2008) of CCN efficiency spectra with the respective total aerosol size distributions, which were interpolated on a grid of 250 size steps . Integral CCN concentrations, N CCN,int , were calculated by integrating the CCN size distributions.…”

“…We used the calibration method described in Rose et al (2008), applying the activity parameterization Köhler model AP3 to derive the corresponding critical supersaturation from the critical dry diameter of CCN activation of the ammonium sulfate particles.…”