Mass specific optical absorption coefficient of HULIS aerosol measured by a four-wavelength photoacoustic spectrometer at NIR, VIS and UV wavelengths

The mass specific optical absorption coefficient (MAC) of Humic-Like Substances (HULIS), isolated from a fine aerosol fraction (PM1) collected at a typical Central European rural background site (Kpuszta) was measured in the re-dispersed aerosol phase at 1064, 532, 355 and 266 nm wavelengths by our recently developed four wavelength photoacoustic spectrometer. It is found to be practically negligible in the visible (0.03 m2 g−1 @532 nm), while in the ultraviolet (4.9 m2 g−1 @266 nm) it becomes comparable with that of black carbon (BC), a major absorbing fraction of the ambient aerosol. This type of wavelength dependency was already hypothesized for HULIS aerosol, but it was proved previously only by indirect measurements on HULIS samples dissolved in the aqueous phase. On the other hand, the other generally accepted hypothesis, that this wavelength dependency can be described by a single, wavelength independent absorption Angström-exponent (AAE) is not justified by the presented measurements.