Light Source Effects on Aerosol Photoacoustic Spectroscopy Measurements

Simultaneous Transmission/Absorption Photometry of Particle-Laden Filters from Wildland Fires during the Biomass Burning Observation Project (BBOP) Field Campaign

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 apply this measurement approach, referred to as simultaneous transmission/absorption photometry (STAP), to estimate the aerosol extinction coefficient from actual field-campaign filter aerosol, and compare results with the PSAP. The STAP approach offers several advantages over the PSAP, including estimation of the extinction coefficient from temperature measurements (avoiding the complexities associated with filter reflectivity/scattering measurements), as well as determination of the filter optical properties and filter effects on particle absorption (resulting in particle absorption enhancement). The experimental arrangement included a laser probe beam impinging normal to the particle-coated surface of a vertically mounted filter, and a thermocouple placed flush in the middle of (and in thermal contact with) the filter uncoated back surface. With this simple arrangement, the transmissivity and absorptivity were determined simultaneously at a given laser beam wavelength. The measurement repeatability was better than 0.3 K (95 % confidence level) for temperature and 0.4 mW for laser power. The limit of detection for the extinction coefficient was estimated to be (8 to 12) Mm^-1 (95 % confidence level) at about 1.9 mW laser power. The extinction 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, landscapes, and wildland fires. The STAP extinction coefficient matched the darkness of the filter coating, however the PSAP-filter results did not follow the same order. Although there were differences in transmissivity between the two techniques, the estimated values for absorption coefficient were in good agreement.

Simultaneous transmission and absorption photometry of carbon-black absorption from drop-cast particle-laden filters

Simultaneous transmissivity and absorptivity measurements were carried out in the visible at a laser wavelength of 532 nm on drop-cast, carbon-black-laden filters under ambient (laboratory) conditions. The focus of this investigation was to establish the feasibility of this approach to estimate the mass absorption coefficient of the isolated particles and compare results to earlier work with the same carbon-black source. Transmissivity measurements were carried out with a laser probe beam positioned normal to the particle-laden filter surface. Absorptivity measurements were carried out using a laser-heating approach to record in time the sample temperature rise to steady-state and decay back to the ambient temperature. The sample temperature was recorded using a fine-wire thermocouple that was integrated into the transmission arrangement by placing the thermocouple flush with the filter back surface. The advantage of this approach is that the sample absorptivity can be determined directly (using laser heating) instead of resolving the difference between reflectivity (filter surface scattering) and transmissivity. The current approach also provides the filter optical characteristics, as well as an estimate of filter effects on the absorption coefficient due to particle absorption enhancement or shadowing. The approach may also be incorporated into other filter-based techniques, like the particle/soot absorption photometer, with the simple addition of a thermocouple to the commercial instrument. For this investigation, measurements were carried out with several blank uncoated quartz filters. A range of solution concentrations was prepared with a well-characterized carbon black in deionized water (i.e., a water-soluble carbonaceous material referred to as a surrogate black carbon or 'carbon black'). The solution was then drop cast using a calibrated syringe onto blank filters to vary particle loading. After evaporation of the water, the measurements were repeated with the coated filters. The measurement repeatability (95% confidence level) was better than 0.3 K for temperature and 3 × 10^-5 mW for laser power. From the measurements with both the blank and coated filters, the absorption coefficient was determined for the isolated particles. The results were then compared with an earlier investigation by You et al. and Zangmeister and Radney, who used the same carbon-black material. The measurements were also compared with Lorenz-Mie computations for a polydispersion of spherical particles dispersed throughout a volume representative of the actual particles. The mass absorption coefficient for the polydispersion of carbon-black particles was estimated to be about 7.7 ± 1.4m² g^-1, which was consistent with the results expected for these carbon black particles.

Comparing Aerosol Refractive Indices Retrieved from Full Distribution and Size- and Mass-Selected Measurements

Refractive index retrievals (also termed inverse Mie methods or optical closure) have seen considerable use as a method to extract the refractive index of aerosol particles from measured optical properties. Retrievals of an aerosol refractive index use one of two primary methods: 1) measurements of the extinction, absorption and/or scattering cross-sections or efficiencies of size- (and mass-) selected particles for mass-mobility refractive index retrievals (MM-RIR) or 2) measurements of aerosol size distributions and a combination of the extinction, absorption and/or scattering coefficients for full distribution refractive index retrievals (FD-RIR). These two methods were compared in this study using pure and mixtures of ammonium sulfate (AS) and nigrosin aerosol, which constitute a non-absorbing and absorbing material, respectively. The results indicate that the retrieved complex refractive index values are correlated to the amount of nigrosin in the aerosol but can be highly variable with differences in the real and imaginary components that range between -0.002 and 0.216 and -0.013 and 0.086; the average and standard deviation of the differences are 0.046 ± 0.046 and 0.023 ± 0.033, respectively. Forward calculation of the optical properties yielded average absolute values of the relative deviation of ≈ 15 % and ≈ 26 % for FD-RIR data using the MM-RIR values and contrariwise. The range of retrieved refractive indices were used to calculate the normalized global average aerosol radiative forcing of a model accumulation mode remote continental aerosol. Deviations using the refractive indices of the pure materials range from 9 % to 32 % for AS and 27 % to 45 % for nigrosin. For mixtures of nigrosin and AS, deviations were all > 100 % and not always able to capture the correct direction of the forcing; i.e., positive versus negative.