Development of a Portable Particle Extinctiometer Based on Direct Absorbance in the Visible and Near IR

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Description

A portable particle extinctiometer based on direct absorbance at visible and near IR wavelengths is under development. The instrument is a derivative of the Model 405 nm NO2/NO/NOx Monitor recently introduced by 2B Technologies, which measures NO2 by direct absorbance at 405 nm using a ~2 m path length, LED light source and photodiode detector. The long path length is achieved by use of either a miniature White cell (original Model 405 nm) or by use of a tubular folded path using corner cube mirrors (new version of Model 405 nm). An advantage of the tubular folded path is a much smaller flush volume, allowing measurements every 5 s. For particle extinction measurements, the NO2 scrubber is simply replaced with a particle filter. Both approaches to achieving a long path length are being evaluated for measurements of total aerosol particle extinction (absorbance plus scattering). Preliminary experiments show good correlation with a Cavity Attenuated Phase Shift (CAPS) instrument for laboratory-generated aerosols, and a roadside monitoring experiment shows good correlation of measured particle extinction with PM2.5. The ultimate goal of the project is to provide a lower cost, lower power and more portable alternative to CAPS for measurements of particle extinction, Bext. When used in combination with a miniature integrating nephelometer to measure the scatting coefficient, Bsct, it will be possible to obtain particle absorbance, Babs, at multiple wavelengths by difference, Babs = Bext – Bsct. Our goal is to develop a rugged, robust and inexpensive instrument that is suitable for long-term aerosol optical property measurements for ARM and ASR.