It is convenient to measure the optical attenuation <i>A</i> 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 <i>A</i> to the absorption and scattering coefficients <i>k</i> and <i>s</i> 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, <i>A</i> is a nonlinear function strongly dependent on both <i>s</i> and <i>k</i>, for a limited range of <i>s</i> and sample thickness <i>d</i>, <i>A</i> can be a practically linear function of <i>k</i>. Fortunately, this range includes that common to atmospheric particulate samples. Furthermore, it is shown that if the filter’s reflectance is sufficiently high, <i>A</i> can be nearly independent of <i>s</i>. 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 <i>A</i> as a means of determining the black carbon mass loading <i>C</i> on a filter is also investigated. It is shown that when the black carbon mass fraction <i>f<sub>c</sub></i> is high, as it is for samples collected in large urban areas, <i>A</i> is a predictable and practically linear function of <i>C</i>. However, when <i>f<sub>c</sub></i> is low, as it is for many rural locations, then the slope of the function <i>A</i>(<i>C</i>) is strongly dependent on <i>f<sub>c</sub></i>, leading to possible overestimates of <i>C</i>. This problem can be alleviated by making the measurement of <i>A</i> at near-infrared wavelengths rather than in the visible spectrum.
[Optical Society of America ]
(010.0010) Atmospheric and oceanic optics : Atmospheric and oceanic optics
(010.1120) Atmospheric and oceanic optics : Air pollution monitoring
(010.1280) Atmospheric and oceanic optics : Atmospheric composition
(350.4990) Other areas of optics : Particles
James D. Lindberg, Rex E. Douglass, and Dennis M. Garvey, "Atmospheric Particulate Absorption and Black Carbon Measurement," Appl. Opt. 38, 2369-2376 (1999)