Calibration and measurement uncertainties of a CCN counter

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 5 particles in the diameter range of 20–220 nm. Experiments have been performed in the laboratory and during field measurement campaigns, extending over a period of more than one year and covering a wide range of operating conditions (650-1020 hPa ambient pressure, 0.5–1.0 L min −1 aerosol flow rate, 20–30 • C inlet temperature, 4–34 K m −1 temperature gradient). For each set of conditions, the effective water vapor 10 supersaturation (S eff) in the CCNC was determined from the measured CCN activation spectra and K ¨ ohler model calculations. High measurement precision was achieved under stable laboratory conditions, where relative variations of S eff in the CCNC were generally less than ±2%. During field measurements, however, the relative variability increased up to ±5–7%, which can 15 be mostly attributed to variations of the CCNC column top temperature with ambient temperature. To assess the accuracy of the K ¨ ohler models used to calculate S eff , we have performed a comprehensive comparison and uncertainty analysis of the various K ¨ ohler models and thermodynamic parameterizations commonly used in CCN studies. For 20 the relevant supersaturation range (0.05–2%), the relative deviations between different modeling approaches were as high as 25% for (NH 4) 2 SO 4 and 16% for NaCl. The deviations were mostly caused by the different parameterizations for the activity of water in aqueous solutions of (NH 4) 2 SO 4 and NaCl (activity parameterization, osmotic coefficient, and van't Hoff factor models). The uncertainties related to the 25 model parameterizations of water activity clearly exceeded the CCNC measurement precision. Relative deviations caused by different ways of calculating or approximating solution density and surface tension did not exceed 3% for (NH 4) 2 SO 4 and 1.5% Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion EGU for NaCl. Nevertheless, they did exceed the CCNC measurement precision under well-defined operating conditions and should not be neglected in studies aimed at high accuracy. To ensure comparability of results, we suggest that CCN studies should always report exactly which K ¨ ohler model equations and parameterizations of solution properties were used. 5 Substantial differences between …