Extended Range CAPS-Based Single Scattering Albedo Monitor
Authors
Andrew Freedman — Aerodyne Research, Inc.
Timothy B Onasch — Aerodyne Research, Inc.
Paul L Kebabian — Aerodyne Research, Inc.
Paola Massoli — Aerodyne Research, Inc.
Category
Absorbing Aerosol
Description
Bottom panel: Plot of scattering versus extinction measurements (1 s time resolution) as a function of time for 200 nm Ammonium Sulfate particles at a wavelength of 660 nm. Scattering data has been rescaled so that the slope of a least squares fit of the scattering versus extinction over a range of 0-600 Mm-1 is equal to 1.00; Middle panel: Correlation plot of data presented below that has been averaged (40-60 samples) for each particular extinction level. Also shown is a 1:1 line derived from 0-600 Mm-1 data which has been extended over the entire data range; and Top panel: Deviation of the scattering measurement from the 1:1 line. Error bars represent the expected uncertainty based on the precision values measured in the following section. At higher extinction levels, the error bars are approximately the size of the data points.
We describe a robust, compact, field deployable instrument (the CAPS PMssa) that simultaneously measures airborne particle light extinction and scattering coefficients and thus the single scattering albedo (SSA) on the same sample volume. With an appropriate change in mirrors and light source, measurements have been made at wavelengths ranging from 450 to 780 nm. The extinction measurement is based on cavity attenuated phase shift (CAPS) techniques as employed in the CAPS PMex particle extinction monitor; scattering is measured using a integrating nephelometry by incorporating a Lambertian integrating sphere within the sample cell. The scattering measurement is calibrated using the extinction measurement. Measurements using ammonium sulfate particles of various sizes indicate that the response of the scattering channel with respect to measured extinction is linear to within 1% up to 1000 Mm-1 and can be extended further (4000 Mm-1) with additional corrections. The precision in both measurement channels is less than 1 Mm-1 (1s, 1σ).
Results presented here will focus on a monitor that operates at 780 nm, a wavelength at which there are no gas phase interferences and corresponds to a wavelength region near commercial available photo acoustic aerosol absorption monitors will be presented. Results from field studies conducted in Athens, Greece will also be presented if available. Absorption calculated from the difference between extinction and scattering will be compared to that obtained using an aethalometer.
