AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Measuring Light Absorption with Filter-based Instruments: Correction Schemes for Filter Artifacts
APOORVA PANDEY, Yu Yang, Rajan Chakrabarty, Washington University in St Louis
Abstract Number: 562 Working Group: Instrumentation and Methods
Abstract Several filter-based instruments, such as the aethalometer, particle soot absorption photometer and UV-vis spectrophotometer, are commonly used for measuring aerosol light absorption. These techniques involve the estimation of Beer-Lambert light attenuation through a filter sample, and subsequently calculating an absorption coefficient. The conversion of attenuation coefficient to absorption coefficient is subject to several measurement artifacts. Multiple scattering of light by the filter substrate causes an increase in the optical path, leading to an increase in light absorption by aerosol deposits with respect to their absorption as free particles. Further, with increasing loading of absorbing aerosols, the proportion of the scattered light absorbed by the particles increases, effectively decreasing the optical path. In contrast, high loading of highly scattering aerosols results in an increase in the optical path. The net discrepancy between filter-based and non-contact measurements of aerosol absorption coefficients depend upon the type of filter substrate, and the amount and optical nature of aerosol deposits.
In this work, we provide correction factors for filter based absorption coefficient measurements for Black Carbon (BC) and light absorbing Organic Carbon (LAOC). A natural gas fueled Bunsen burner was used to generate BC particles and peat combustion was used to generate LAOC containing organic material. Aerosol samples were collected on Teflon and glass fiber filters. Simultaneously their real-time absorption and scattering coefficients were measured using multi-wavelength Photoacoustic Spectrometers with integrated nephelometers at 375, 405, 532, 671 and 1047 nm wavelengths. A high-resolution UV-vis spectrophotometer equipped with an integrating sphere was used to measure transmittance spectra for blank and sample filters. The calculated attenuation coefficients were compared with the corresponding real-time absorption coefficients, to extract the multiple scattering and loading correction factors. A ray-tracing inversion approach, was used to estimate a scattering factor from transmittance and reflectance measurements on the filters. The correction schemes employed wavelength-dependent attenuation and aerosol single scattering albedo as parameters.