Articles | Volume 14, issue 21
Research article
11 Nov 2014
Research article |  | 11 Nov 2014

A meta-analysis of particle water uptake reconciliation studies

J. D. Whitehead, M. Irwin, J. D. Allan, N. Good, and G. McFiggans

Abstract. Water uptake by aerosol particles controls their ability to form cloud droplets, and reconciliation between different techniques for examining cloud condensation nuclei (CCN) properties is important to our understanding of these processes and our ability to measure and predict them. Reconciliation between measurements of sub-saturated and supersaturated aerosol particle water uptake was attempted at a wide range of locations between 2007 and 2013. The agreement in derived number of CCN (NCCN or particle hygroscopicity was mixed across the projects, with some data sets showing poor agreement across all supersaturations and others agreeing within errors for at least some of the supersaturation range. The degree of reconciliation did not seem to depend on the environment in which the measurements were taken. The discrepancies can only be attributable to differences in the chemical behaviour of aerosols and gases in each instrument, leading to under- or overestimated growth factors and/or CCN counts, though poorer reconciliation at lower supersaturations can be attributed to uncertainties in the size distribution at the threshold diameter found at these supersaturations. From a single instrument, the variability in NCCN calculated using particle hygroscopicity or size distribution averaged across a project demonstrates a greater sensitivity to variation in the size distribution than chemical composition in most of the experiments. However, the discrepancies between instruments indicate a strong requirement for reliable quantification of CCN in line with an improved understanding of the physical processes involved in their measurement. Without understanding the reason for discrepancies in the measurements, it is questionable whether quantification of CCN behaviour is meaningful.

Short summary
Water uptake of ambient particles was measured by 2 independent techniques at a wide range of locations between 2007 and 2013. The agreement between the techniques was mixed and hence the number of potential cloud seeds calculated from the measurements frequently showed discrepancies. Whilst there is sensitivity to how well we measure the size of the particles, much of the difference depends on how the particles behave when exposed to moisture in the different techniques (and in the atmosphere).
Final-revised paper