Sources of Error and Uncertainty in the Use of Cavity Ring Down Spectroscopy to Measure Aerosol Optical Properties

Abstract

The use of cavity ring down spectroscopy to retrieve aerosol complex refractive index from optical property measurements has seen increasing popularity over the past few years. However, few studies have looked at the limit which sources of error and uncertainty inherent in the cavity ring down method place on the accuracy with which the refractive index can be retrieved. In this paper, we consider both experimental sources of error and those which compromise the theoretical models against which measurements are compared, both reviewing previously published work and presenting new data. Our results show that for absolute measurements made using single-cavity instruments, factors such as uncertainty in the length of the ring down cavity occupied by aerosol and the counting efficiency of the CPC can introduce an error of ∼2.5% into the real part of the refractive index retrieved from experiment. This is significantly higher than the typical 1% error quoted in previously published work. We note that due to the dependence of particle extinction efficiency on diameter, the effect of a given error on measurements for different particle sizes is not constant.

Acknowledgments

This study was funded by the NERC through grant NE/C512537/1. Rachael E. H. Miles is grateful to the EPSRC for studentship and postdoctoral funding, and Svemir Rudić acknowledges the NERC for postdoctoral support. Andrew J. Orr-Ewing thanks the Royal Society and Wolfson Foundation for a Research Merit Award, and Jonathan P. Reid acknowledges the EPSRC for Leadership Fellowship funding. Drs Richard Gilham, Marc Coleman, Fabrizio Innocenti, and Tom Gardiner (National Physical Laboratory), and Dr Jin Kim (University of Bristol) are thanked for their assistance in measuring the counting efficiency of the Grimm CPC.