Article pubs.acs.org/JPCA
Comparison of the Accuracy of Aerosol Refractive Index Measurements from Single Particle and Ensemble Techniques Bernard J. Mason, Simon-John King, Rachael E. H. Miles,* Katherine M. Manfred,† Andrew M. J. Rickards, Jin Kim,‡ Jonathan P. Reid, and Andrew J. Orr-Ewing* School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K. ABSTRACT: The ability of two techniques, aerosol cavity ring down spectroscopy (ACRDS) and optical tweezers, to retrieve the refractive index of atmospherically relevant aerosol was compared through analysis of supersaturated sodium nitrate at a range of relative humidities. Accumulation mode particles in the diameter range 300−600 nm were probed using A-CRDS, with optical tweezer measurements performed on coarse mode particles several micrometers in diameter. A correction for doubly charged particles was applied in the A-CRDS measurements. Both techniques were found to retrieve refractive indices in good agreement with previously published results from Tang and Munkelwitz, with a precision of ±0.0012 for the optical tweezers and ±0.02 for the A-CRDS technique. The coarse mode optical tweezer measurements agreed most closely with refractive index predictions made using a mass-weighted linear mixing rule. The uncertainty in the refractive index retrieved by the A-CRDS technique prevented discrimination between predictions using both mass-weighted and volume-weighted linear mixing rules. No efflorescence or kinetic limitations on water transport between the particle and the gas phase were observed at relative humidities down to 14%. The magnitude of the uncertainty in refractive index retrieved using the A-CRDS technique reflects the challenges in determining particle optical properties in the accumulation mode, where the extinction efficiency varies steeply with particle size.
I. INTRODUCTION Atmospheric aerosols affect the Earth’s energy balance through their interaction with solar and terrestrial radiation (the direct effect) and by modifying cloud properties through their role as cloud condensation nuclei (the indirect effect).1 The direct effect is governed by the aerosol optical properties, for which the key determining parameter is the complex refractive index (RI) of the constituent particles. The RI depends on the chemical composition of the aerosol and is composed of real (nr) and imaginary (ni) parts that determine the wavelengthdependent scattering and absorption of incident light by the aerosol, respectively. If the refractive index of an aerosol particle is known, scattering and absorption coefficients, single scattering albedos, and scattering asymmetry parameters can be predicted for climate modeling.2−5 The single scattering albedo and asymmetry parameter are especially important in governing the level of atmospheric cooling or warming associated with the aerosol. The relative humidity (RH) of the vapor phase determines the mass fraction of water partitioned in the condensed aerosol phase and, thereby, the refractive index, further influencing the aerosol radiative forcing. Atmospheric RH varies significantly with geographical location and altitude, and thus, it is important to quantify the response of aerosol RI to variations in RH over a broad range of environmental conditions if the optical properties and radiative forcing of the aerosol are to be quantified and assessed. Although estimates of RI with variation in RH can be obtained from pure component values for the solutes and water and the assumption that the mixed © 2012 American Chemical Society
component properties follow a linear mixing rule, pure component and mixed composition RI measurements are still required to validate these model treatments. Notably for atmospheric aerosol, the ambient RH is often such that solute concentrations in an aerosol droplet are above the bulk solubility limit, precluding the possibility of performing bulk measurements of the RI. Such supersaturated conditions are accessible in the aerosol state as phase transitions of particles do not always follow those observed in bulk solutions, the prime example being the hysteresis observed in the deliquescence/ efflorescence behavior of common salts. Further, when in such supersaturated nonideal states, the linear mixing rules that are applied to calculate mixed component properties must be called into question and the influence of phase separation and inhomogeneity on particle optical properties considered. Cavity ring down spectroscopy is increasingly used in atmospheric science for the determination of mixing ratios of trace gases in air.6−8 Over the past decade, it has also found applications in the study of aerosol optical properties.9−32 Aerosol cavity ring down spectroscopy (A-CRDS) has been applied to measurements of the RI for a variety of aerosol samples, including absorbing and nonabsorbing species,33−38 single and multicomponent aerosols,39−41 and particle ensembles and single particles.42−44 The extinction efficiency (Qext) is a relative measure of the ability of a particle to remove Received: May 22, 2012 Revised: July 31, 2012 Published: August 3, 2012 8547
dx.doi.org/10.1021/jp3049668 | J. Phys. Chem. A 2012, 116, 8547−8556
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Figure 1. A-CRDS experimental setup. (a) Aerosol nebulizer: MFC, mass flow controller. (b) Relative humidity control: NV, needle valve; RH, Honeywell relative humidity probe. (c) Aerosol size selection: DMA, differential mobility analyzer; VRH, Vaisala relative humidity probe. (d) Cavity ring down spectrometer: MML, mode matching lens; CPC, condensation particle counter; PMT, photomultiplier tube. The shading illustrates schematically the transverse mode profile of the laser beam within the optical cavity.
studies, simultaneous measurements of the variation in mass of an aerosol particle and the scattered light intensity at a fixed angle (often near 90°) with change in RH have been used to determine the change in RI with composition for a wide range of inorganic component aerosol, including sodium chloride, ammonium sulfate and sodium nitrate. Tang surveyed EDB RI measurements made on binary aqueous aerosol droplets of 27 inorganic compounds with solute mass fractions spanning the range 0.15−0.37, all corresponding to solute concentrations below saturation for which accurate bulk solution measurements could be directly made.50 The average error in RI retrieved from the EDB measurements was found to be 0.048%, equivalent to a typical uncertainty less than ±0.001 for aqueous aerosol below saturation. The uncertainties associated with RIs were not reported for more complex mixtures, or for RHs below which the solute concentrations are higher than the bulk solubility limit. In a recent paper, we described the retrieval of both the real and imaginary parts of the refractive index from optical tweezers studies of solution phase aerosol.51 By using the unique information obtained from the whispering gallery modes (WGMs) of Raman scattering spectra, both the size and complex refractive index of a particle can be obtained simultaneously. As with the light scattering methods described above, accurate retrieval of RI information is only possible if it is assumed that the particle composition is homogeneous. We demonstrated that the RI could be retrieved with an accuracy of better than ±0.11%, or ±0.0012, for typical aqueous based aerosol over a broad RH range extending to highly supersaturated solute states. This level of accuracy is consistent with the earlier assessment made of single particle EDB measure-
incident light and is defined as the ratio of the extinction (σext) and geometrical (σgeom) cross sections (for a spherical particle, σgeom = πr2 where r is the particle radius). At a given wavelength (λ), values of Qext can be determined from A-CRDS measurements of the extinction coefficient (α) as a function of particle number concentration (N), for monodisperse aerosol samples of known particle radius and, thus, known size parameter x = 2πr/λ. The complex RI can then be retrieved by comparison of the experimental data with Mie scattering theory calculations. 45 Recently, we have assessed the uncertainties associated with the absolute retrieval of RIs from the A-CRDS approach for measurements made on ensembles of accumulation mode aerosol and concluded that a typical error of ±0.02 in the retrieved value of the real part of the RI can be expected.32 All subsequent discussion of the RI will be limited to the real part, unless otherwise stated. If relative measurements of aerosol properties are performed, such as determination of the change in optical extinction with RH due to hygroscopic growth, a more precise assessment of the optical properties can be made. Alternative methods for retrieving the RI of aerosol particles are based on angularly resolved measurements of light scattering from ensembles of particles, usually using nephelometers, or from single particles, typically captured in an electrodynamic balance (EDB).46,47 In the former case, light scattering from a flowing aerosol sample is recorded over a wide angular range.48 The variation in scattered intensity with angle can be used to infer the RI of the aerosol and the particle size distribution. Typical quoted uncertainties in the retrieved RI are ±0.02,48 although differences as low as ±0.01 from expected values have been achieved for measurements of the RH dependent RI of ammonium sulfate aerosol.49 In EDB 8548
dx.doi.org/10.1021/jp3049668 | J. Phys. Chem. A 2012, 116, 8547−8556
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have been described in detail in a previous publication,31 and only a brief summary is given here. Aerosol particles were generated by nebulization of aqueous solutions of SN (purity >99.5%, Fisher Scientific) using a constant output atomizer (TSI 3076). The RH of the aerosol flow was reduced to the required measurement RH using a Nafion dryer (Perma Pure, PD-100T-12SS) with a humidified sheath flow, the RH of which was controlled by mixing wet and dry nitrogen gas flows. The particle number density was varied inside a mixing chamber by combining the aerosol sample flow with an additional flow of nitrogen humidified to the desired RH for the measurements. The RHs of the Nafion sheath flow, aerosol flow, and dilution mixing gas were all monitored by capacitance probes (HIH-3602A, Honeywell,
