Optical-Feedback Cavity Ring-Down Spectroscopy Measurements of Extinction by Aerosol Particles

Abstract

Optical feedback cavity ring-down spectroscopy (OF-CRDS) using a continuous wave distributed feedback diode laser at 1650 nm has been used to measure extinction of light by samples of monodisperse spherical aerosol particles <1 mum in diameter. The OF-CRDS method allows measurements of low levels of extinction of incident light to be made at repetition rates of 1 kHz or greater. A statistical model is proposed to describe the linear relationship between the extinction coefficient (alpha) and its variance (Var(alpha)). Application of this model to experimental measurements of Var(alpha) for a range of alpha values typically below approximately 1 x 10(-6) cm(-1) allows extinction cross-sections for the aerosol particles to be obtained without need for knowledge of the particle number density. Samples of polystyrene spheres with diameters of 400, 500, 600, and 700 nm were used to test the model by comparing extinction cross-sections determined from the experiment with the predictions of Mie theory calculations. Fitting of ring-down decay traces exhibiting amplitude noise to extract cavity ring-down times introduces additional quadratic and higher order polynomial dependencies of the variance that become significant for larger particle number densities and thus extinction coefficients (typically for alpha > 1 x 10(-6) cm(-1) under our experimental conditions). Aggregation of particles at larger number densities is suggested as a further source of variance in the measurements. Extinction cross-sections are severely underestimated if the measurements are made too rapidly to sample uncorrelated distributions of particle numbers and positions.

Intercomparison of a Cavity Attenuated Phase Shift-based extinction monitor (CAPS PMex) with an integrating nephelometer and a filter-based absorption monitor

“…A detailed introduction to the CRD technique for aerosol extinction measurement is given by Strawa et al (2003), whereas Moosmüller et al (2005) provide an overview over the various CRD and cavity-enhanced detection approaches. Driven by a rapid technology development, CRD instruments are now available as multi-wavelength systems for atmospheric measurements (Schmid et al, 2006;Baynard et al, 2007;Atkinson et al, 2010) and laboratory studies Butler et al, 2009;Cross et al, 2010).…”

Aircraft Instrument for Comprehensive Characterization of Aerosol Optical Properties, Part I: Wavelength-Dependent Optical Extinction and Its Relative Humidity Dependence Measured Using Cavity Ringdown Spectroscopy

“…The error in τ is somewhat more complex to evaluate, as the time between successive laser shots at 1 kHz is insufficient for particles to redistribute themselves throughout the laser beam. Under these conditions, successive τ measurements are correlated and therefore Poisson statistics are not directly applicable (Butler et al 2009). Indeed, evaluating the error in τ from the 1-s standard error of the mean of 1000 laser shots (as for τ 0 ) would significantly underestimate the true degree of uncertainty.…”

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

“…Several different methods of drying have been reported in the cavity ring down literature to date, including mixing of the aerosol flow with large volumes of dry air (Radney et al 2009), use of Nafion dryers (Butler et al 2009;Miles et al 2010aMiles et al , 2010b or those containing molecular sieves, activated alumina or silica gel crystals (Baynard et al 2006;Bulatov et al 2006;Garland et al 2007;Riziq et al 2007;Spindler et al 2007;Riziq et al 2008;Thompson et al 2008;Adler et al 2010), or a combination of several of the above (Freedman et al 2009;Khalizov et al 2009;Xue et al 2009;Beaver et al 2010). The absorption crosssection of water at λ = 560 nm is very low (∼5 × 10 −27 cm 2 molec −1 (HITRAN)); thus, the effect of any residual solvent in our study is likely to be minimal; for τ 0 = 14 µs, a relative humidity of 50% at 20…”

“…By measuring the extinction coefficient b ext as a function of particle number density N for size-selected particles of known diameter, the size-dependent values of the particle extinction efficiency can be extracted from the experimental data. Measuring the value of Q ext at a range of particle radii, r, or size parameter (x = 2π r/λ) allows the complex refractive index of the aerosol to be determined through comparison with theoretical Q ext values calculated from Mie theory Dinar et al 2008;Riziq et al 2008;Butler et al 2009;Freedman et al 2009;Lang-Yona et al 2009Adler et al 2010;Hasenkopf et al 2010;Miles et al 2010aMiles et al , 2010b. This approach relies critically on knowing the size distribution of particles within the cavity, their morphology, and their number density.…”

“…A-CRDS is now routinely used in laboratory (Sappey et al 1998;Vander Wal and Ticich 1999;Smith and Atkinson 2001;Bulatov et al 2002Bulatov et al , 2003Bulatov et al , 2006Strawa et al 2003;Pettersson et al 2004;Baynard et al 2006;Garland et al 2007;Riziq et al 2007;Rudić et al 2007;Dinar et al 2008;Riziq et al 2008;Butler et al 2009;Freedman et al 2009;Khalizov et al 2009;Lang-Yona et al 2009Radney et al 2009;Xue et al 2009;Adler et al 2010;Hasenkopf et al 2010;Miles et al 2010aMiles et al , 2010b and field studies (Thompson et al 2002(Thompson et al , 2003Strawa et al 2006;Garland et al 2008;Nakayama et al 2010) to measure extinction by both aerosol ensembles (Vander Wal and Ticich 1999;Thompson et al 2002Thompson et al , 2003Thompson et al , 2008Strawa et al 2003;Pettersson et al 2004;Baynard et al 2006Baynard et al , 2007Garland et al 2007;Riziq et al 2007;Beaver et al 2008;…”