10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Estimating Aerosol Light Absorption from Filter-based Spectrophotometer Measurements
APOORVA PANDEY, Nishit Shetty, Rajan K. Chakrabarty, Washington University in St Louis
Abstract Number: 1341 Working Group: Carbonaceous Aerosol
Abstract Filter-based instruments, such as the aethalometer, particle soot absorption photometer and UV-vis spectrophotometer, estimate aerosol light absorption by measured attenuation through an aerosol loaded filter. The conversion of attenuation coefficient to absorption coefficient is subject to several measurement artifacts, including multiple scattering of light by the filter and “shadowing” effects from increasing loading of aerosols. In general, the magnitude of these artifacts depends on the properties of the filter substrate, the aerosol optical loading, and the aerosol single scattering albedo (SSA). The relationship between aerosol light absorption and filter based transmittance or reflectance measurements needs to be established for each instrument and filter substrate combination. In this work, we evaluate the use of Teflon membrane filters in a UV-visible spectrophotometer to estimate absorption coefficients. Particles with SSA values ranging 0.4-0.99 were generated using flaming/smoldering phase combustion of a variety of biomass types. Aerosol samples were collected on Teflon and quartz fiber filters in parallel with in situ measurements of absorption and scattering coefficients using multi-wavelength Photoacoustic Spectrometers with integrated nephelometers at 375, 405 and 532 nm. A high-resolution UV-vis spectrophotometer equipped with an integrating sphere was used to measure transmittance and reflectance spectra from the Teflon filters. An attenuation value was calculated using the above measurements. The ratio of the aerosol absorbance (calculated from photoacoustic measurements) and the filter attenuation is commonly designated filter correction factor; this factor was positively correlated with the attenuation (loading effect), but showed an inverse relationship with SSA. We found that for a wide range of aerosol properties (i.e. SSA), the absorbance can be conveniently expressed as a simple function of the attenuation measurements, with no need of additional parameters. The behavior of the aerosol-filter system was also modeled by applying the solution of two-stream model of radiative transfer in one dimension to a two-layer approximation of the system. The predicted relationship between absorbance and attenuation was consistent with the experimental data.