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

摘要

Experimental and theoretical uncertainties in the measurement of cloudcondensation nuclei (CCN) with a continuous-flow thermal-gradient CCN counterfrom Droplet Measurement Technologies (DMT-CCNC) have been assessed by modelcalculations and calibration experiments with ammonium sulfate and sodiumchloride aerosol particles in the diameter range of 20–220 nm. Experimentshave been performed in the laboratory and during field measurement campaigns,covering a wide range of instrument operating conditions (650–1020 hPapressure, 293–303 K inlet temperature, 4–34 K m−1 temperaturegradient, 0.5–1.0 L min−1 flow rate). For each set of conditions, theeffective water vapor supersaturation (S_eff, 0.05–1.4%) wasdetermined from the measured CCN activation spectra (dry particle activationdiameters) and K?hler model calculations. High measurement precision wasachieved under stable laboratory conditions, where the relative standarddeviations of S_eff were as low as ±1%. During fieldmeasurements, however, the relative deviations increased to about ±5%,which can be mostly attributed to variations of the CCNC column toptemperature with ambient temperature. The observed dependence of S_eff on temperature, pressure, and flow rate was compared to the CCNC flowmodel of Lance et al. (2006). At high S_eff the relative deviationsbetween flow model and experimental results were mostly less than 10%, butat S_eff≤0.1% they exceeded 40%. Thus, careful experimentalcalibration is required for high-accuracy CCN measurements – especially atlow S_eff. A comprehensive comparison and uncertainty analysis of thevarious K?hler models and thermodynamic parameterizations commonly usedin CCN studies showed that the relative deviations between differentapproaches are as high as 25% for (NH₄)₂SO₄ and 12% for NaCl.The deviations were mostly caused by the different parameterizations for theactivity of water in aqueous solutions of the two salts. To ensurecomparability of results, we suggest that CCN studies should always reportexactly which K?hler model equations and parameters were used. Providedthat the Aerosol Inorganics Model (AIM) can be regarded as an accurate sourceof water activity data for highly dilute solutions of (NH₄)₂SO₄ andNaCl, only K?hler models that are based on the AIM or yield similarresults should be used in CCN studies involving these salts and aiming athigh accuracy. Experiments with (NH₄)₂SO₄ and NaCl aerosols showedthat the conditions of particle generation and the shape and microstructureof NaCl particles are critical for their application in CCN activationexperiments (relative deviations up to 18%).