Aerosol Light Extinction Measurements by Cavity Attenuated Phase Shift (CAPS) Spectroscopy: Laboratory Validation and Field Deployment of a Compact Aerosol Particle Extinction Monitor

Abstract

We present laboratory and field measurements of aerosol light extinction (σ ep ) using an instrument that employs Cavity Attenuated Phase Shift (CAPS) spectroscopy. The CAPS extinction monitor comprises a light emitting diode (LED), an optical cavity that acts as the sample cell, and a vacuum photodiode for light detection. The particle σ ep is determined from changes in the phase shift of the distorted waveform of the square-wave modulated LED light that is transmitted through the optical cell. The 3-σ detection limit of the CAPS monitor under dry particle-free air is 3 Mm -1 for 1s integration time. Laboratory measurements of absolute particle extinction cross section (σ ext ) using non-absorbing, monodisperse polystyrene latex (PSL) spheres are made with an average precision of ±3% (2-σ ) at both 445 and 632 nm. A comparison with Mie theory scattering calculations indicates that these results are accurate within the 10% uncertainty stated for the particle number density measurements. The CAPS extinction monitor was deployed twice in 2009 to test its robustness and performance outside of the laboratory environment. During these field campaigns, a co-located Multi Angle Absorption Photometer (MAAP) provided particle light absorption coefficient (σ ap ) at 635 nm: the single scattering albedo (ω) of the ambient aerosol particles was estimated by combining the CAPS σ ep measured at 632 nm with the MAAP σ ap data. Our initial results show the high potential of the CAPS as lightweight, compact instrument to perform precise and accurate σ ep measurements of atmospheric aerosol particles in both laboratory and field conditions.

Organic aerosol composition and sources in Pasadena, California, during the 2010 CalNex campaign

“…Volume concentrations of the different PALMS particle classes were calculated by multiplying the volume concentration size distribution determined from the SMPS or WLOPC measurements by the fractions of the different PALMS particle types in each size bin. Particle optical extinction for PM 1 was measured at 532 nm and 630 nm by a Cavity Attenuated Phase Shift instrument [Kebabian et al, 2007;Massoli et al, 2010]. All of the online measurements described in this paragraph were located at the CalNex ground site described above.…”

Vertical distributions of aerosol optical properties during the spring 2016 ARIAs airborne campaign in the North China Plain

“…It has a demonstrated sensitivity of less than 2 Mm −1 for 1 s sampling periods. A method description, including results from laboratory characterization and field deployment, has been reported previously (Massoli et al, 2010). The CAPS extinction and the σ ext measured by the combination of the nephelometer and the PSAP showed a good correlation in both laboratory-generated test particles and ambient aerosols (Petzold et al, 2013).…”

Airborne and laboratory studies of an IAGOS instrumentation package containing a modified CAPS particle extinction monitor

“…The modulation frequency is usually set to 17 kHz which provides an initial phase shift of [35][36][37][38][39][40] if the mirrors are clean. The time response of the monitor is 1-2 seconds and, according to Massoli et al (2010) and Petzold et al (2013b), its limit of detection (LOD D 3s where s is the r.m.s. noise for particle-free air) ranges between 1-3 Mm ¡1 (1 second sample period), depending on the particular instrument, and decreases as the square root of the sample period to at least 100 seconds.…”

“…The CAPS PMex can be readily applied in mobile sampling systems since it is compact and robust (Kebabian et al 2007), with a relatively low-cost set up. The CAPS PM ex operational principle has been successfully assessed in optical closure studies and deployed at ground pressure conditions with artificially generated and ambient aerosols (Massoli et al 2010;Petzold et al 2013b) and high concentration exhaust aerosols (Yu et al 2011).…”

“…An alternative technology, the Cavity Attenuated Phase Shift Particle Extinction Monitor (CAPS PM ex ), was commercially introduced in 2007 to directly measure the extinction of particles (Kebabian et al 2007;Massoli et al 2010). The instrument is available at several operation wavelengths in the visible range.…”