A coupled observation – modeling approach for studying activation kinetics from measurements of CCN activity

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.

Gas–particle partitioning of atmospheric aerosols: interplay of physical state, non-ideal mixing and morphology

“…However, a value of t hg of this order can impact results inferred from aerosol instruments with residence times of seconds, such as the hygroscopicity tandem differential mobility analyzer (HTDMA) 15,32 and cloud condensation nuclei counter (CCNC). 42 D H 2 O is estimated to decrease to values as low as 10 À20 cm 2 s À1 at temperatures characteristic of the middle to upper troposphere, resulting in kinetic limitations of water mass transport with t hg E hours to days. 40,43 As a consequence, glassy organicinorganic aerosol particles (not fully equilibrated with the ambient RH) may act preferentially as heterogeneous ice nuclei, rather than as cloud condensation nuclei forming liquid water droplets.…”

Ammonium nitrate evaporation and nitric acid condensation in DMT CCN counters

“…In the DMT Continuous Flow Streamwise Thermal Gradient Chamber (CFSTGC, or CCN column, Roberts and Nenes, 2005), we also have to take account the wetted walls, which act not only as a source for water vapour but also as a sink for semivolatile compounds. To take account of this, we have updated the most recent version of the CFSTGC model (Raatikainen et al, 2012) to also include semivolatile gases. With the augmented CCN counter model, we are able to calculate trajectories for aerosol particles inside the CCN column, and these trajectories are used to drive the air parcel model to simulate the condensation/evaporation of different compounds on/off the particles.…”

“…Aerosol trajectories are calculated from the velocity fields, and these, with related gas phase concentrations and temperature profiles, are saved to be used in the cloud parcel model. The current model version is based on the original instrument model of Lathem and Nenes (2011), later updated by Raatikainen et al (2012). An additional section similar to that of water vapour was added to the model for the semivolatile gas, which is here either ammonia or nitric acid.…”

CCN Properties of Organic Aerosol Collected Below and within Marine Stratocumulus Clouds near Monterey, California

“…According to TDGA, the water uptake coefficient is similar to that of the water uptake coefficient of (NH4)2SO4 ( c ~ (NH4)2SO4). Given that the water uptake coefficient, (NH4)2SO4 ~ 0.2 [50,51], this suggest that the WSOC would not slow down activation kinetics of CCN compared to (NH4)2SO4. This is consistent with the analysis of CCN data collected from a wide range of environments [51].…”

“…We adopt the method of Threshold Droplet Growth Analysis (TDGA) (e.g., but not limited to [39,48,49]) to detect the possible presence of droplets growing more slowly than calibration (NH4)2SO4 aerosol [50]. If present then, a coupled measurement modeling approach can be used to infer the effective water vapor uptake coefficient [14,51].…”