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

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 Δd, 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 Δd 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 with a concomitant improvement in the accuracy of the retrieved complex index of refraction and corresponding atmospheric radiative forcing estimates.

Copyright 2013 American Association for Aerosol Research     

An Asymptotic Analysis of Differential Electrical Mobility Classifiers

An asymptotic analysis of balanced flow operations of differential mobility analyzers (DMAs) and a new class of instruments that includes opposed migration aerosol classifiers (OMACs) and inclined grid mobility analyzers (IGMAs) provides new insights into the similarities and differences between the devices. The characteristic scalings of different instruments found from minimal models are shown to relate the resolving powers, dynamic ranges, and efficiencies of most such devices. The resolving powers of all of the instruments in the nondiffusive regime of high voltage classifications, , is determined by the ratio of the flow rate of the separation gas (sheath or crossflow) to that of the aerosol. At low voltage, when diffusion degrades the classification, the OMAC and the IGMA share an factor advantage in dynamic range of mobilities over the DMA, although the OMAC also suffers greater losses because diffusion immediately deposits particles onto its porous electrodes. On the basis of this analysis, a single master operating diagram is proposed for DMAs, OMACs, and IGMAs. Analysis of this operating diagram and its consequences for the design of differential electrical mobility classifiers suggests that OMACs and IGMAs also have advantages over DMAs in design flexibility and miniaturization. Most importantly, OMACs and IGMAs may outperform DMAs for the currently difficult classification of particles with diameters less than 10 nm. On the other hand, DMAs are more amenable to voltage scanning-mode operation to enable accelerated size distribution measurements, whereas it is most convenient to operate OMACs and IGMAs in voltage stepping-mode operation.     

Extending the Capabilities of Single Particle Mass Spectrometry: II. Measurements of Aerosol Particle Density without DMA

Particle density is an important and useful property that is difficult to measure because it usually requires two separate instruments to measure two particle attributes. As density measurements are often performed on size-classified particles, they are hampered by low particle numbers, and hence poor temporal resolution. We present here a new method for measuring particle densities using our single particle mass spectrometer, SPLAT. This method takes advantage of the fact that the detection efficiency in our single particle mass spectrometer drops off very rapidly as the particle size decreases below 100 nm creating a distinct sharp feature on the small particle side of the vacuum aerodynamic size distribution. Thus, the two quantities needed to determine particle density, the particle diameter and vacuum aerodynamic diameter, are known. We first test this method on particles of known compositions and densities to find that the densities it yields are accurate. We then apply the method to obtain the densities of particles that were characterized during instrument field deployments. We illustrate how the method can also be used to measure the density of chemically resolved particles. In addition, we present a new method to characterize the instrument detection efficiency as a function of particle size that relies on measuring the mobility and vacuum aerodynamic size distributions of polydisperse spherical particles of known density. We show that a new aerodynamic lens used in SPLAT II improves instrument performance, making it possible to detect 83 nm particles with 50% efficiency.