Abstract
Abstract. This paper presents an approach to study droplet activation kinetics from measurements of CCN activity by the Continuous Flow Streamwise Thermal Gradient CCN Chamber (CFSTGC) and a comprehensive model of the instrument and droplet growth. The model, which can be downloaded from http://nenes.eas.gatech.edu/Experiments/CFSTGC.html , is evaluated against a series of experiments with ammonium sulfate calibration aerosol. Observed and modeled droplet sizes are in excellent agreement for a water vapor uptake coefficient ~0.2, which is consistent with theoretical expectations. The model calculations can be considerably accelerated without significant loss of accuracy by assuming simplified instrument geometry and constant parabolic flow velocity profiles. With these assumptions, the model can be applied to large experimental data sets to infer kinetic growth parameters while fully accounting for water vapor depletion effects and changes in instrument operation parameters such as the column temperature, flow rates, sheath and sample flow relative humidities, and pressure. When the effects of instrument operation parameters, water vapor depletion and equilibrium dry particle properties on droplet size are accounted for, the remaining variations in droplet size are most likely due to non-equilibrium processes such as those caused by organic surface films, slow solute dissociation and glassy or highly viscous particle states. As an example of model application, data collected during a research flight in the ARCTAS 2008 campaign are analyzed. The model shows that water vapor depletion effects can explain changes in the observed average droplet size.
Highlights
Aerosols are the precursors of cloud droplets and can profoundly affect cloud albedo, lifetime, and droplet size distribution
The analysis here focuses on the Droplet Measurement Technologies (DMT) Continuous Flow Streamwise Thermal Gradient cloud condensation nuclei (CCN) chamber (Roberts and Nenes, 2005; Lance et al, 2006), the approach presented here can be applied to any CCN instrument
In the two sections, we present the comparison of model predictions with experimental data obtained for ammonium sulfate aerosol at two different CCN instrument flow rates (0.5 and 1.0 L min−1) and pressures (500 and 960 mbar) and 7–11 different temperature gradients, which are typical of those used in past field measurements
Summary
Aerosols are the precursors of cloud droplets and can profoundly affect cloud albedo, lifetime, and droplet size distribution. Droplet size does not depend only on the water vapor uptake coefficient, and on instrument operation parameters (e.g. supersaturation, temperature, pressure and flow rates), aerosol properties (e.g. hygroscopicity and size distributions), and CCN concentration. The second limitation of TDGA is that it cannot provide numerical values for vapor uptake coefficients; a droplet growth model such as those used by Shantz et al (2010), Ruehl et al (2008, 2009), Asa-Awuku et al (2009) and Kumar et al (2011a) is instead required. We present an approach to study droplet activation kinetics and quantify kinetic parameters from measurements of CCN activity combined with a comprehensive model of the process For the latter, an augmented version of the coupled DMT CCN instrument and droplet growth model used previously in Roberts and Nenes (2005), Lance et al (2006), and Lathem and Nenes (2011) is developed and applied. We demonstrate this using an airborne CCN sample data set collected in the vicinity of intense biomass burning plumes during the 2008 ARCTAS experiment
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