On Measuring the Critical Diameter of Cloud Condensation Nuclei Using Mobility Selected Aerosol

Abstract

Cloud condensation nuclei (CCN) instruments determine the so-called “critical diameter” for activation of particles into cloud droplets at a fixed water supersaturation. A differential mobility analyzer is often used to size-select particles for purposes of scanning for the critical diameter. Usually the diameter where 50% of the particles have activated to cloud droplets is assumed to be equal to the critical diameter. We introduce a model that describes the transfer of polydisperse charge-equilibrated particles through an ideal differential mobility analyzer followed by transit through an ideal CCN instrument. We show that if the mode diameter of the polydisperse size distribution exceeds the critical diameter of the particles, multiply-charged particles may lead to nonmonotonic CCN counter response curves (plots of CCN-active fraction vs. mobility diameter) that exhibit multiple peaks, rather than a simple sigmoidally-shaped curve. Hence, determination of the 50% activation diameter is ambiguous. Multiply-charged particles significantly skew the CCNc response curves when sampling particles with critical diameters exceeding 0.1 μ m from particle size distributions with mode diameters also larger than the critical diameter. We present a method for inversion of CCN counter data that takes multiple-charging effects into account, and demonstrate its application to laboratory data. Our calculated CCN counter response curves are in good agreement with observations, and can be used to infer the critical activation diameter for a specified supersaturation.

Acknowledgments

We thank Paul J. Ziemann, Yong B. Lim, and Aiko Matsunaga for the aerosol generation and SMPS size distribution measurement. We also thank Jefferson R. Snider for reviewing an early draft of this manuscript. This research was supported by the U.S. National Science Foundation under grant ATM-0436196. Partial support was supplied by the Office of Science (BER), U.S. Department of Energy, under DE-FG02-05ER63984 and the National Aeronautics and Space Administration, under NNG04GR44G. We thank two anonymous reviewers for helpful comments. In particular we thank the reviewer who alerted us to the possibility that standard inversion techniques could be applied with no further modifications to also retrieve the activation diameter. We also thank Ulrike Dusek for calculating the activation diameters for our data based on the Frank et al. (2006) model. Finally we thank Rick Flagan for useful discussions about this manuscript. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.