Timothy B Onasch — Aerodyne Research
Paul L Kebabian — Aerodyne Research, Inc.
Paola Massoli — Aerodyne Research, Inc.
Andrew Freedman — Aerodyne Research, Inc.
Category
Absorbing Aerosol
Description
Simplified schematic of CAPS PMssa single scattering albedo monitor.
We present data detailing the performance of a particle single scattering albedo (SSA) monitor (the CAPS PMssa monitor) that incorporates both a CAPS (Cavity Attenuated Phase Shift)-based optical extinction measurement and an inverse nephelometer in the same measurement volume. Since an absolute extinction measurement is provided, the scattering channel can be calibrated with any non-absorbing particle (e.g., a PSL or white salt).
The aerosol extinction measurement is identical to that of the CAPS PMex monitor. However, the middle portion of the optical path incorporates a 10 cm i.d. sphere. The sphere walls are coated with Avian-D paint which provides a water-proof surface with >98% diffuse reflectance over the visible light range. Within the sphere region, the aerosol flow is contained within a transparent tube to prevent the particles from contaminating the sphere. This change in geometry from the PMex extinction monitor requires an adjustment in the correction for the presence of a purge flow which protects the mirrors from contamination. Once this adjustment is made, the measured extinction (in combination with a measurement of PSL particle concentration using a CPC) agrees with that calculated using Mie theory to with a few per cent over a range of particle sizes and extinction levels.
The monitor demonstrates noise levels of less than 1 Mm-1 (1σ, 1 s) in both channels while providing 1 s response time (10-90%). The scattering channel has been shown to provide a linear response (<1% deviation) with respect to the measured extinction from 0-1000 Mm-1. It scattering response demonstrates a truncation effect similar in magnitude to that of commercial nephelometers.
The CAPS PMSSA monitor can be operated in any wavelength region compatible with the availability of high reflectivity mirrors, LEDs and photomultiplier tube (PMT) photocathodes. At present, the available wavelength band is restricted to the 400-700 nm region because of the lack of suitable high reflectivity mirrors in the UV and of photocathode materials in the near-IR.
