Optical Constants of Aerosols from Synchrotron Infrared Spectroscopy
Date:
Talk at 11TH AUSTRALIAN CONFERENCE ON VIBRATIONAL SPECTROSCOPY & THE 5TH ASIAN SPECTROSCOPY CONFERENCE, Sydney, Australia
See the abstract of the talk here
Abstract: Aerosols are important constituents of the atmosphere. They affect the radiation budget and hydrological cycle of earth. Depending on the composition and size, aerosols have either net warming or cooling effect. Aerosols also have been observed in the interstellar medium, and in the atmosphere of other planets such as Titan, a moon of Saturn [1]. Aerosol size and complex refractive indices are two important parameters from the perspective of understanding radiation budget and remote detection. These parameters dictate the amount of light that aerosols absorb/scatter and the pattern of such interactions. The absorption/scattering of light by aerosols is responsible for some everyday phenomena, such as low visibility smog in cities and the blue haze in Blue Mountains [2]. Complex refractive indices of aerosols are needed for understanding radiation budget and interpreting spectral data whether it is of earth’s atmosphere or of extraterrestrial environments. Refractive indices for the material of interest can be retrieved from conventional spectroscopic techniques, such as FTIR. Traditionally, such measurements are conducted on thin films deposited on substrates where interference effects at interfaces can cause distortion and baseline shifts in spectra. Although it is called ‘thin film’, in practice the films are not ‘thin’ enough compared to the IR wavelength. The refractive indices extracted from such thin film spectra can be quite different from those obtained from aerosols which are usually suspended in a buffer gas, similar to atmospheric aerosols. In this study, we present various methods to retrieve the complex refractive indices of aerosols from FTIR spectra and discuss the ease of application and accuracy for each method. Small aerosols (<100 nm) attenuate the IR beam mainly by absorption which can be easily simulated by Rayleigh scattering, whereas scattering is dominant for larger particles (> a few micron) where more rigorous Mie scattering theory is needed [3]. The analysis and discussions are corroborated using aerosol FTIR spectra taken at the Australian Synchrotron over the years. These aerosols are of atmospheric or astrophysical importance, which include water ice (H2O), carbon dioxide (CO2), formic acid (HCOOH), acetonitrile (CH3CN), propionitrile (CH3CH2CN), and ethane (C2H6).
Acknowledgement We acknowledge the assistance of staff of the THz/Far-IR Beamline at the Australian Synchrotron, and the facility for providing access. M. Ruzi acknowledges the Latrobe University LTUPRS scholarship.
References [1] K. Rages, J. B. Pollack, P. H. Smith, J. Geophys. Res. 88 (1983), 8721-8728. [2] F. W. Went, Nature 87 (1960), 641-643 [3] C. F. Bohren and D. R. Huffman, Absorption and scattering of light by small particles, John Wiley & Sons, New York, 1983