Near-UV (365 and 405 nm) CAPS PMssa Monitors for the In Situ Measurement of Particle Optical Properties
ZACHARY PAYNE, Benjamin Moul, Stephen Jones, Andrew Freedman, Timothy Onasch, Aerodyne Research, Inc.
Abstract Number: 470
Working Group: Instrumentation and Methods
Abstract
Heretofore, in situ measurement of the optical properties (e.g., extinction, scattering and absorption) of particulates has been limited to the visible region of the electromagnetic spectrum, typically in the 450 nm to 700 nm region defined by commercially available nephelometers. However, it has become increasingly clear that the effects on climate change by particles which absorb light below ~400 nm need to be quantitatively evaluated. We have been able to extend the range of in situ optical extinction and scattering measurements of Aerodyne’s PMSSA monitor to 405 and 365 nm, taking advantage of the development of powerful LEDs and highly reflective mirrors at these wavelengths.
The monitors utilize highly reflective mirrors (nominal transmission ~ 50 ppm) centered at 365 or 405 nm from FiveNines Optics and 365 nm LED ENGIN or 405 nm Luxeon light emitting diodes. The vacuum photodiode in a glass housing used in older instruments has been replaced by one from Hamamatsu which utilizes a metal can which appears to offer slightly higher sensitivity and is far more robust. As with CAPS PMSSA monitors operating at longer wavelengths, these monitors demonstrates better than 1% linearity out to 1000 Mm-1 in both scattering and extinction channels. The level of detection (3σ, 1 second) in the extinction channel is less than 1 Mm-1 and below 3 Mm-1 in the scattering channel. (The latter value can readily be improved by using a higher count rate in the PMT used to measure the scattering signal.) The noise levels in both channels decreased as the square root of integration out to hundreds of seconds before baseline drift was observed. The measured truncation corrections in the scattering channel markedly increase as a function of wavelength (400 nm < <365 nm), presumably caused by the relatively poor reflectance of the integrating sphere below 400 nm.