The direct measurement of aerosol light absorption coefficient is preferable over indirect methods. A photothermal interferometer probes the change of the refractive index caused by light absorption in the sample – the detection is linear and can be traced to first principles. Measurement at two wavelengths allows the determination of its wavelength dependence and the Ångström exponent (AAE). The photothermal aerosol absorption monitor (PTAAM) uses a folded Mach-Zender interferometer. Two pump lasers at 532 and 1064 nm are modulated at different frequencies and focused in the sample chamber using an axicon (patent granted) for simultaneous measurement. The interferometer signal is detected by photodiodes and lock-in amplifiers at the two respective frequencies. The green channel is calibrated traceably to primary standards using ~1 µmol/mol NO2. The calibration is transferred to the IR using aerosolized nigrosin. The uncertainties for absorption coefficients at 532 and 1064 nm and AAE were 4%, 6% and 9%, respectively. We calibrated filter photometers in green and IR. A winter ambient campaign has shown similar multiple scattering parameter values for ambient aerosols and laboratory experiments. We have also determined the absorption enhancement by coatings of BC with non-absorbing secondary organic matter (SOM).
Dr. Griša Močnik is a professor at the University of Nova Gorica in Slovenia. He received his PhD in physics from the University of Ljubljana. He serves as the Head of the Center for Atmospheric Research and the Dean of the School of Environmental Sciences at the University of Nova Gorica. His research interests include measurement techniques of black carbon aerosol and other carbonaceous light absorbing aerosol. He has served as a member of the United Nations ECE expert group on black carbon.
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