Atmospheric particulate absorption and black carbon measurement

Abstract

It is convenient to measure the optical attenuation A of the combination of a layer of atmospheric particulate matter and the quartz fiber filter on which it has been collected. The problem of relating A to the absorption and scattering coefficients k and s of the particulate matter itself is treated as a problem in diffuse reflectance spectroscopy using the KubelkaMunk theory. The results show that although, in general, A is a nonlinear function strongly dependent on both s and k, for a limited range of s and sample thickness d, A can be a practically linear function of k. Fortunately, this range includes that common to atmospheric particulate samples. Furthermore, it is shown that if the filter's reflectance is sufficiently high, A can be nearly independent of s. This is in agreement with experimental and, for the limiting case when the substrate filter reflectance is unity, theoretical results obtained by other researchers. Use of such measurements of A as a means of determining the black carbon mass loading C on a filter is also investigated. It is shown that when the black carbon mass fraction f(c) is high, as it is for samples collected in large urban areas, A is a predictable and practically linear function of C. However, when f(c) is low, as it is for many rural locations, then the slope of the function A(C) is strongly dependent on f(c), leading to possible overestimates of C. This problem can be alleviated by making the measurement of A at near-infrared wavelengths rather than in the visible spectrum.

Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

“…We conclude that, because of the problems related to aerosol/filter interactions, uncertainties in the mass absorption coefficient, and the different spectral properties of C soot and C brown , a unique relationship between light absorption and "black carbon" cannot be expected to exist (Lindberg et al, 1999). This is the reason why we have introduced the term "equivalent black carbon", BC e , which is that amount of C soot that would cause the same amount of light absorption as that observed in the sample, using the value of α ATN and correction factors applied with a particular instrument.…”

“…2.1. Theoretical considerations as well as laboratory experiments show that, even with the same instrumental setup, α ATN varies considerably as a function of the LAC content of the aerosol, and becomes most variable and uncertain for aerosol with low LAC fractions, in the range commonly encountered in non-urban and remote regions (Horvath, 1997;Lindberg et al, 1999). These problems increase toward lower wavelengths, and therefore most strongly influence measurements made with instruments using green light (e.g., the PSAP at 565 nm) or the shorter-wavelength channels of the 7-wavelength aethalometer AE-31.…”

Optical Calibration and Equivalence of a Multiwavelength Thermal/Optical Carbon Analyzer

“…The R(λ) final and T(λ) final may differ from those of a true blank filter due to refractory residues after the thermal analysis, and these rare cases are flagged when observed after analysis is complete. Nonetheless, in the first-order approximation (Hansen et al, 1984;ISO, 1993;Lindberg et al, 1999;Quincey, 2007), b abs (λ) of the thermally liberated fraction including OC and EC can be expressed as:…”

Modification, Calibration and a Field Test of an Instrument for Measuring Light Absorption by Particles

“…There are several methods for handling these effects see references by Bond et al (1999) and Weingartner et al (2003). Lindberg et al (1999) investigated this problem theoretically using the Kubelka-Munk theory and showed that light attenuation through a filter sample is a function of scattering and absorption properties of the particle layer and the reflectance of the filter. In the MAAP this problem is solved by measuring the decrease of transmission and also light scattering from the aerosol-filter system.…”