PNNL

Abstract

Transmissivity and absorptivity measurements were carried out simultaneously in the visible (wavelength of 532 nm) at laboratory conditions using particle-laden filters obtained from a three-wavelength particle/soot absorption photometer (PSAP). The particles were collected on filters from wildland fires over the Pacific Northwest during the Department of Energy Biomass Burning Observation Project (BBOP) field campaign in 2013. The objective of this investigation was to use a novel measurement approach, referred to as simultaneous transmission/absorption photometry (STAP), to estimate the particle extinction coefficient from actual field-campaign filters, and compare results with the PSAP. The STAP approach offers several advantages over the PSAP, including estimation of the particle extinction coefficient from temperature measurements (avoiding the complexities associated with filter reflectivity/scattering measurements), as well as estimation of the filter optical properties, and filter effects on particle extinction (resulting in particle absorption enhancement). The experimental arrangement included a laser probe beam impinging normal to the particle-coated surface of a mounted filter, and a thermocouple placed flush in the middle of the filter uncoated back surface. With this simple arrangement, the transmissivity and absorptivity were determined simultaneously at a given laser bean wavelength. The measurement repeatability was better than 0.2 K (95 % confidence level) for temperature and 2 x 10-3 mW for laser power. The particle absorption coefficient was determined through measurement of both PSAP blank and exposed filters. Filters were obtained from nine different aircraft flights conducted during the BBOP campaign, representing different flight patterns, days, stages of burning, and wildland fires. The STAP absorption coefficient compared well with the observed particle loading, however the PSAP-filter results did not follow the same order, requiring further investigation as to the reason. The STAP results also indicated that absorption enhancement effects were significant and diminished with increased filter particle loading.