Wavelength dependent mass-specific optical absorption coefficients of laser generated coal aerosols determined from multi-wavelength photoacoustic measurements

Mass-specific optical absorption coefficients (MSOAC) of aerosols generated from samples of coal used for residential purposes by UV laser ablation are measured as a function of wavelength with the help of a novel multi-wavelength photoacoustic system operating at 266, 355, 532, and 1064 nm wavelengths. The spectral dependencies of the measured MSOAC are quantified by assuming a power-law wavelength dependence (i.e. with the help of ngstrom exponents). The MSOAC and the ngstrom exponents of the investigated residential coal samples vary between 2.9 and 26.6 m(2)/g, and 1.05 and 2.05, respectively. Furthermore it is shown that in the UV region MSOAC deviate characteristically from the wavelength dependence found in the visible and in the near-infrared ranges. These findings underline the uniqueness of the used multi-wavelength photoacoustic system, as no other instrument can measure in such a wide wavelength range. Therefore, this photoacoustic system has a potential to unambiguously differentiate between these (and other) types of aerosols. Supplementary scanning electron microscopy measurements indicate that the measured aerosols mimic those that are generated during residential coal burning and have a large impact on the global radiation budget of the Earth, yet they have not been sufficiently characterized.