The relationship between optical attenuation and black carbon concentration for ambient and source particles

The "dual-spot" Aethalometer: an improved measurement of aerosol black carbon with real-time loading compensation

“…The factor 100 is there for convenience only (Gundel et al, 1984). The loading effect was analyzed using a plot of the BC dependence on attenuation (Park et al, 2010;Segura et al, 2014).…”

“…The relationship between attenuation and BC surface loading is linear for low attenuation values. However, this relationship becomes nonlinear when the attenuation values are high due to a filter saturation effect (Gundel et al, 1984). In the presence of saturation, assuming linear proportionality between attenuation and the BC surface loading would lead to underestimation of the BC concentrations.…”

“…Herein, we describe a dual-spot approach, where the data from two filter spots with different loading are used to measure the loading effect, calculate the compensation parameter and apply it to the data to obtain compensated results. Gundel et al (1984) showed that the optical attenuation measured at 633 nm (Rosen et al, 1978) for samples collected on quartz filter started to deviate from the linear relationship as a function of surface loading (B) above 10 µg cm −2 of black carbon. This can be parameterized by an empirical relationship of ATN vs. B:…”

“…where a factor of 100 is required due to the original definition of attenuation in Gundel et al (1984). The above determination is performed simultaneously at seven different wavelengths using mass absorption cross sections listed in the Supplement (Table S2).…”

“…3.3) being C quartz = 2.14 for quartz filter and C TFE = 1.57 for TFE-coated glass fiber filter. BC is historically defined by Aethalometer measurement at 880 nm, so we use parameters at this wavelength to derive the mass equivalent concentration using σ air = 7.77 m 2 g −1 , obtained initially by comparing optical and thermal measurements of filters loaded with refractory carbonaceous material (Gundel et al, 1984). We use the attenuation measured on spot 1 (the spot with the higher flow of the two spots):…”