Photoacoustic Analysis of Particulate Carbon in the Atmosphere

Photoacoustics offers a distinctive combination of features that recommends its use in analyzing the visible light absorption properties of particles collected on filters. The technique is nondestructive and responds only to absorbed light. Unloaded Teflon filters of the type used routinely by the USEPA in dichotomous samplers do not absorb visible light so that no pre-loading measurement is necessary. Previous studies of the photoacoustic (PA) response to soot loaded on Teflon filters have shown that: (1) at 100 Hz and 1000 Hz chopping frequencies the PA signal due to soot loading does not saturate even for loadings at which the sample is totally opaque, i.e., βℓ>31; (2) thermal wave interference effects are evident in the photoacoustic analysis of soot samples2; (3) the perturbation of the signal from soot, when typical ambient loadings of (NH4)2SO4 are admixed is considerably less (±10%) than the perturbation that occurs in light transmission measurements.3 Item (1) precludes the use of simplifying assumptions in relating PA response to the light absorption parameter βℓ. Item (2) explains item (1). Item (3) suggests that a calibration curve relating photoacoustic response and soot loading could be used to translate PA readings on ambient samples to elemental carbon content.