A Calibration Technique for Improving Refractive Index Retrieval from Aerosol Cavity Ring-Down Spectroscopy

Abstract

Cavity ring-down spectroscopy (CRDS) is a technique that is commonly used to measure the extinction of light by aerosol particles in situ. This extinction, when normalized to particle concentration, yields the extinction cross section, a measure of a single particle's ability to scatter and absorb light. The complex index of refraction can then be retrieved by comparison of the extinction cross sections at several particle diameters with those predicted by Mie theory. This approach requires accurate determination of particle diameter and concentration as well as the length of the extinction region in the cavity, but it is often difficult to quantify the systematic errors in the measurements of these quantities. Here, we introduce a calibration technique using particles of a reference compound to account for these systematic errors. The two calibration parameters are: C f , which scales the measured extinction cross sections, and d, which shifts the particle diameters. It is found that C f correlates strongly with the condensation particle counter (CPC) used to measure particle concentration and that d is associated with the differential mobility analyzer (DMA) used to select particle diameters. Calibration is shown to reduce errors of subsequently-measured extinction cross sections of a test aerosol from 11% to <2% with a concomitant improvement in the accuracy of the retrieved complex index of refraction and corresponding atmospheric radiative forcing estimates.

Sensitivity of Aerosol Refractive Index Retrievals Using Optical Spectroscopy

“…While doublets could be eliminated using an impactor (Toole et al 2013) or by selecting particle sizes far from the peak of the distribution so that doublets have minimal contribution (Freedman et al 2009;Hasenkopf et al 2010;Zarzana et al 2012), such options may not be practical, particularly when one does not have control over the particle distribution. The use of a mass selection instrument behind the DMA could be used to remove doubly charged particles (Nakayama et al 2010a;Radney et al 2013Radney et al , 2014.…”

“…The precision of the CPC is determined by Poisson statistics, and a sufficiently long averaging time can reduce the uncertainty, as characterized by a standard deviation, to low ( §1%) levels (Washenfelder et al 2013). The accuracy of the CPC is given by the manufacturer, and is typically given as §10%, but Toole et al (2013) report a method to reduce this to §2 or 3% (Figure 3 of that paper), and a §3% uncertainty in the concentration will only increase the uncertainty in the cross-section to §5.8%.…”

“…If the DMA is set to pass 200 nm particles but instead passes 205 nm particles, this represents a sizing error of only 2.5%, but for a range of refractive indices this sizing error increases the extinction cross-section by 10%. Errors in the DMA must be corrected (Miles et al 2011;Toole et al 2013), and any sizing issues will increase the uncertainty in the retrieval. Our uncertainties therefore are lower limits.…”

“…Cavity ring-down (CRD) and cavity enhanced extinction spectroscopy (CEES) have been used by numerous groups to determine the refractive index of aerosol particles (Pettersson et al 2004;Dinar et al 2008;Hasenkopf et al 2010;Nakayama et al 2010b;Cappa et al 2011;Miles et al 2011;Redmond and Thompson 2011;Washenfelder et al 2013;Toole et al 2013). CRD and CEES measure extinction, the sum of scattering and absorption, so for measurements made at a single size, a purely scattering substance cannot be distinguished from a substance that also absorbs light.…”

“…Miles et al (2011) and Mason et al (2012) examined several sources of error, including incorrect particle counting and doubly charged particles, while Toole et al (2013) reported a method for removing systematic errors in particle sizing and counting. However, these studies only examined nonabsorbing compounds, and, except for Toole et al (2013), fixed k at 0. Instrument setups have been validated using polystyrene latex spheres (PSLs) or ammonium sulfate (AS), but since neither system is absorbing (and in the case of PSLs is monodisperse), these may not be appropriate test systems.…”