02/19/2013

Worldwide Datasets Greatly Improve Constraint on Key Cloud-Aerosol Relation Term

Summary

Cloud formation occurs when aerosol particles take up moisture from the atmosphere. The water uptake rate
is important to constraining the effect of aerosols on cloud brightness, or the “aerosol indirect effect,” resulting
from pollution emissions, but the rate at which this occurs has been poorly constrained and has been formulated in terms of particle size, composition, and humidity. A new study, partially funded by the U.S. Department of Energy, used a large dataset to constrain the kinetics of water uptake as expressed by the condensation coefficient, αc. Estimates of αc for droplet growth from activation of ambient particles vary considerably (over five orders of magnitude, from 10^-5 to 1!) and represent a critical source of uncertainty in estimates of global cloud droplet distributions and the aerosol indirect forcing of climate. The authors analyzed 10 globally relevant datasets of cloud condensation nuclei to constrain the value of αc. They found that rapid uptake kinetics (αc > 0.1) is uniformly prevalent. This finding resolves a long-standing issue in cloud physics, as the uncertainty in water vapor uptake on droplets is considerably less than previously thought.

References

Raatikainen, T., A. Nenes, J. H. Seinfeld, R. Morales, R. H. Moore, T. L. Lathem, S. Lance, L. T. Padró, J. J. Lin, K. M. Cerully, A. Bougiatioti, J. Cozic, C. R. Ruehl, P. Y. Chuang, B. E. Anderson, R. C. Flagan, H. Jonsson, N. Mihalopoulos, and J. N. Smith. 2013. “Worldwide Data Sets Constrain the Water Vapor Uptake Coefficient in Cloud Formation,” Proceedings of the National Academy of Sciences USA 110(10),3760–64.