Cavity Enhanced Aerosol Extinction Spectroscopy for the Measurement of the Optical Properties of Atmospheric Particles

Abdur Rasheed (1), Kevin B. Sayer (1), DANIEL B. CURTIS (1)

(1) California State University, Northridge

     Abstract Number: 539
     Last modified: May 13, 2010

Abstract
The optical properties of atmospheric aerosol particles are important for climate studies, remote sensing studies of particulate matter, and visibility studies of urban air pollution. Typical values of interest are the extinction coefficient, extinction cross section, extinction efficiency, and real and imaginary parts of the refractive index of the particles. A new technique (to our knowledge), cavity enhanced aerosol extinction spectroscopy (CE-AES), based on cavity enhanced spectroscopy, has been developed to measure the optical properties of atmospheric aerosol particles at a wavelength of 532 nm.

CE-AES measures the extinction of size-selected monodisperse aerosol particles suspended in air within an optical cavity between two highly reflective mirrors (R > 0.9995). Continuous-wave laser light at 532 nm is introduced to the cavity through one of the mirrors and detected through the other mirror with a photomultiplier tube (PMT). The CW laser light passes through the cavity multiple times, establishing an intensity, as monitored with the PMT as a voltage signal. A significantly lower intensity is measured when particles are introduced to the cavity and thus scatter or absorb the light, preventing it from reaching the detector. Based on the difference in intensity between clean air and air with particles, along with their size and number density, the optical properties of the particles can be determined.

Efficacy of the technique has been demonstrated by the measurement of extinction properties of polystyrene latex particles of various diameters. The measured extinction properties match theoretical calculations and previously measured values in the literature. Cavity enhanced aerosol extinction spectroscopy has proven to be simpler to perform than similar techniques such as cavity ringdown spectroscopy, yet offers an excellent sensitivity due to long path lengths on the order of kilometers.