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A Dual-wavelength Photo-thermal Interferometer for the Determination of Aerosol Optical Absorption Coefficient and the Absorption Ångström Exponent

G. Mocnik1,2, U. Jagodic2,3, L. Pirker3, M. Kurtjak3, K. Vidovic4, L. Ferrero5, D.M. Kalbermatter6, K. Vasilatou6, B. Visser7, J. Rohrbein7, E. Weingartner7 and L. Drinovec2,3

1University of Nova Gorica, Nova Gorica, Slovenia
2Haze Instruments d.o.o., Ljubljana, Slovenia
3Jozef Stefan Institute, Ljubljana, Slovenia
4National Institute of Chemistry, Ljubljana, Slovenia
5University of Milano Bicocca, Milan, Italy
6Swiss Federal Institute of Metrology (METAS), Berne-Wabern, Switzerland
7University of Applied Sciences Northwestern Switzerland, Windisch, Switzerland

Measurement of aerosol light absorption is challenging due to systematic artifacts influencing current measurement methods. A photo-thermal interferometer (PTI) probes the change of the refractive index (RI) due to heating as light is absorbed. The detection is linear, can be calibrated with gases and traced to first principles, while filter photometers require calibration by assuming the filter artifacts and a reference measurement. Measurement at two wavelengths determines the wavelength dependence of optical absorption and its Ångström exponent (AAE).

The PTI features a folded HeNe laser Mach-Zender interferometer. Two pump lasers at 532 and 1064 nm are modulated at different frequencies and focused using an axicon (patented) for concurrent sample measurement. The quadrature point is maintained by a pressure cell. The signal is detected by two photodiodes and a lock-in at the two frequencies.

We use NO2 to calibrate the PTI at 532 nm. The calibration was transferred to 1064 nm with aerosolized nigrosin using a Mie model based on size distribution measurements and measured solid thin film nigrosin real and imaginary RI. The difference between the calculated and PTI measured nigrosin absorption coefficient at 532 nm was found to be in very good agreement, within 6%.

Instrument performance was validated in the laboratory using propane and diesel soot, and ammonium sulphate. Winter ambient measurements were performed at an urban background location in Central Europe. The results (Figure 1) show that the absorption coefficient and AAE can be reliably measured.

We demonstrate an artifact-free determination of the aerosol absorption coefficient and AAE in laboratory and ambient studies using our new dual-wavelength PTI instrument.

Research support: Eurostars, European Space Agency, Swiss Natl. Sci. Foundation, EMPIR Black Carbon and AeroTox.

Figure 1

Figure 1. The time series of the optical absorption of ambient aerosol.