Cavity ring-down spectroscopy measurements of single aerosol particle extinction. I. The effect of position of a particle within the laser beam on extinction

Abstract

A continuous wave distributed feedback diode laser operating in the near infrared at wavelengths close to 1650 nm has been used to measure the extinction of light by single aerosol particles. The technique of optical feedback cavity ring-down spectroscopy (CRDS) was used for measurement of CRDS events at a repetition rate of 1.25 kHz. This very high repetition rate enabled multiple measurements of the extinction of light by single aerosol particles for the first time and demonstrated the dependence of light scattering on the position of a particle within the laser beam. A model is proposed to explain quantitatively this phenomenon. The minimum detectable dimensionless extinction coefficient epsilonmin was determined to be 3x10(-6). Extinction values obtained for single spherical polymer beads from a monodisperse sample of particles of diameter of 4 microm are in near-quantitative agreement with the values calculated by the Mie scattering theory. The deviations from the Mie theory expected for measurement of extinction by CRDS using a continuous wave laser are discussed in the companion paper.

Error Analysis and Uncertainty in the Determination of Aerosol Optical Properties Using Cavity Ring-Down Spectroscopy, Integrating Nephelometry, and the Extinction-Minus-Scattering Method

“…This has been investigated for a continuous mode-locked beam, and the force on a 4 μm particle was shown to have a negligible perturbation on particle motion (Butler et al 2007). Determining the error of N CRD from the flow error (1%) under flows used in this work, results in a propagated uncertainty of <0.1% due to mass flow controller (MFC) accuracy.…”

“…CRDS was initially focused on the study of gaseous samples (Brown 2003;Berden and Engeln 2009) due to its high sensitivity and ability to measure absolute extinction. It is increasingly applied to the study of aerosols (Smith and Atkinson 2001;Thompson et al 2002Thompson et al , 2008Strawa et al 2003;Pettersson et al 2004;Moosmüller et al 2005;Strawa et al 2006;Butler et al 2007;Miller and Orr-Ewing 2007;Riziq et al 2007Riziq et al , 2008Rudic et al 2007;Dinar et al 2008). CRDS is able to directly measure the total extinction by the sample of particles contained within the optical cavity; this extinction is the sum of scattering and absorption losses.…”

“…The extinction-minus-scattering technique has been used for airborne ambient measurements , characterization of optical properties of biomass burning aerosols (Riziq et al 2007Rudich et al 2007Rudich et al , 2009, retrieving complex refractive indices of humic-like aerosols (Dinar et al 2008), and determining the SSA of isolated aerosol particles (Butler et al 2007;Miller and Orr-Ewing 2007). Instruments have been developed for SSA and aerosol light absorption measurements that combine aerosol cavity ring-down spectrometers (CRDS) and integrating sphere nephelometers using the same laser beam and sample volume (Smith and Atkinson 2001;Thompson et al 2002Thompson et al , 2008Moosmüller et al 2005;Strawa et al 2006).…”

Measurement of aerosol optical extinction using diode laser cavity ringdown spectroscopy

“…Most of these pervious works were based on using either a pulsed YAG laser, which is relatively bulky and limited to only 355, 532 or 1064 nm laser radiation [12][13][14], or a pulsed tunable dye laser, which is cumbersome and not suitable for field study [15]. The cw-CRDS using narrow linewidth diode laser in visible (690 nm) [16] and near-IR (1550 and 1650 nm) [16,17] was also utilized for the aerosol extinction measurements. However, these setups required complex electronics to control the optical cavity length and to modulate the diode laser output in order to couple the cw diode laser radiation into the cavity.…”

“…In particular, in situ measurements of the extinction and other optical parameters of atmospheric aerosols are crucial, and these require real-time, accurate, and sensitive techniques that can detect the ambient aerosols and their rapid temporal and spatial changes. Recent years have seen a rapid rise in the use of cavity ringdown spectroscopy (CRDS), a sensitive and direct optical extinction technique, to determine the optical properties of both laboratory generated and atmospheric ambient aerosols [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”