A219-0002
A dual wavelength photo-thermal interferometer for measurements of Brown Carbon

Wednesday, 16 December 2020
Poster
Luka Drinovec1,2, Bradley Visser3, Luca Ferrero4, Luka Pirker1, Ernest Weingartner3 and Grisa Mocnik1,5, (1)Jozef Stefan Institute, Ljubljana, Slovenia, (2)Haze Instruments d.o.o., Ljubljana, Slovenia, (3)University of Applied Sciences and Arts Northwestern Switzerland, School of Engineering, Windisch, Switzerland, (4)University of Milan - Bicocca, Earth and Environmental Sciences, Milan, Italy, (5)University of Nova Gorica, Ajdovscina, Slovenia
Abstract:
Measurement of aerosol light absorption is still challenging due to systematic artifacts of the current measurement methods. A photo-thermal interferometer (PTI) probes the change of the refractive index caused by light absorption in (and the subsequent heating of) the sample – the detection is linear and can be traced to first principles. On the other hand, filter photometers require calibration by fixing the filter multiple-scattering parameter (Weingarthner et al., 2003). Measurement of optical absorption at two wavelengths allows for the determination of its wavelength dependence, parametrized by the Ångström exponent (AAE).

Methods

The PTI instrument is based on a folded Mach-Zender interferometer design (similar to Moosmüller & Arnott, 1996; Sedlacek, 2006). The He-Ne laser probe beam is split into the sample chamber and reference beams. Pump lasers at 532 and 1064 nm are modulated at different frequencies and focused in the sample chamber using an axicon (patent EP3492905). The quadrature point is maintained using a pressure cell. The interferometer signal is detected by two photodiodes and resolved by a dual-channel lock-in amplifier measuring at the two respective frequencies.

The green channel is calibrated using NO2. The calibration is transferred to the IR channel using aerosolized nigrosin and its relative green-to-infrared absorption ratio, determined using a Mie calculation based on size distribution measurements (range: 12-570 nm).

Results

Instrument performance was validated using several lab aerosol sources: propane soot, nigrosin dye and ammonium sulphate. Winter ambient measurements were performed at an urban background site. Results (Fig. 1) show that the absorption coefficient can be measured with 1 min resolution and AAE can be measured reliably for concentrations above 5 Mm-1.

Conclusions

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.

This work is supported by the Swiss Natl. Sci. Foundation, EMPIR BC project, MIUR Project “Dipt. di Eccellenza 2018-22”.

Moosmüller, H. & Arnott, W. (1996). Opt. Lett., 21, 438-440.

Sedlacek, A.J. (2006). Rev. Sci. Instrum., 77, 064903, 1-8.

Weingarthner et al. (2003). J. Aerosol Sci., 34, 1445–1463.

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