Aerosol Light Extinction Measurements by Cavity Attenuated Phase Shift (CAPS) Spectroscopy: Laboratory Validation and Field Deployment of a Compact Aerosol Particle Extinction Monitor
We present laboratory and field measurements of aerosol light extinction (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 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- 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 (ext) using non-absorbing, monodisperse polystyrene latex (PSL) spheres are made with an average precision of ± 3% (2-) 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 (ap) at 635 nm: the single scattering albedo () of the ambient aerosol particles was estimated by combining the CAPS ep measured at 632 nm with the MAAP ap data. Our initial results show the high potential of the CAPS as lightweight, compact instrument to perform precise and accurate ep measurements of atmospheric aerosol particles in both laboratory and field conditions.
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