Analysis of the Influence of Film-Forming Compounds on Droplet Growth:
Implications for Cloud Microphysical Processes and Climate

Dr. Graham Feingold

NOAA Environmental Technology Laboratory

Decades of cloud microphysical research still have not provided conclusive understanding of the physical processes responsible for droplet spectral
broadening. Numerous mechanisms have been proposed, including entrainment mixing, vortex shedding, giant cloud condensation nuclei (CCN), chemical processing of CCN, and radiative cooling, all of which are likely candidates under select conditions.

In this talk it is suggested that variability in the composition of CCN, and in particular, the existence of condensation inhibiting compounds, is another
possible candidate. The inferred potential abundance of these amphiphilic film-forming compounds (FFCs) suggests that their effect may be important. Using a cloud parcel model with a simplified treatment of the effect of FFCs, it is shown that modest concentrations of FFCs (on the order of 5% of the total aerosol mass) can have a marked effect on drop growth and can cause significant increases in spectral dispersions. Moreover, it is shown that FFCs may in some cases reduce
the number concentration of cloud droplets, with implications for cloud-climate feedbacks. This trend is at least in qualitative agreement with results from the ACE-2 field campaign.