Characterization of light-absorbing aerosols from a laboratory combustion source with two different photoacoustic techniques

In this study, we characterized a variety of light-absorbing carbonaceous aerosols generated from a controlled combustion system using two different photoacoustic measurement techniques: the RGB-DPAS instrument operating at 671, 532, and 473 nm with a single optical pass configuration; and the Multi-PAS III instrument operating at 781, 532, and 422 nm with a multiple optical pass configuration. We calibrated the two instruments using Cab-O-Jet aerosols. The two instruments measured similar absorption coefficients at 532 nm for both brown carbon (BrC) and black carbon (BC) aerosols. The linear fit between the determined aerosol absorption coefficients from the two instruments yielded a linear factor of 1.005 ± 0.029, with a correlation coefficient R2 = 0.94. The absorption Ångström exponents (AAE), ranging from 1 to 10, measured by the two instruments also agreed well, with a resulting linear fitting factor of 1.021 ± 0.038 and R2 = 0.92. These results indicate that although the two instruments have significant differences in the acoustic resonator design and optical length within the sample cells, the determined optical properties for BrC and black BC aerosols were nearly identical. We also investigated the effect of difference in calibration scheme on PAS measurements. In addition to Cab-O-Jet, the Multi-PAS III was also calibrated using NO2. We found that NO2 calibration resulted in 24% lower photoacoustic signal response at 532 nm than Cab-O-Jet calibration. The AAE values obtained with NO2 calibration were 16% smaller than with Cab-O-Jet calibration for BC-dominated aerosols (AAE 7.