Institute of Meteorology and Climate Research

Infrared spectroscopy of aerosol and cloud particles

Mid-infrared extinction spectroscopy is a valuable tool to derive chemical composition, phase (solid or liquid), volume, and size distribution parameters of aerosols and individual aerosol particles and to monitor multi-phase processes such as deliquescence, efflorescence, and ice nucleation. This, however, requires a careful analysis of the size- and shape dependency of the spectral habitus as well as an assessment of the accuracy of the complex refractive indices which are employed in the theoretical models for the computation of the infrared extinction cross sections. For readers interested in the underlying concepts, we recommed our chapter "Infrared Spectroscopy of Aerosol Particles", published in the book "Fundamentals and Applications in Aerosol Spectroscopy" (Editors Ruth Signorell, Jonathan P. Reid) by CRC Press.

Recent AIDA investigations have focused on providing new temperature-dependent data sets of infrared optical constants for atmospherically relevant aerosol particles as well as on assessing the accuracy of data sets that have already been published. In addition, the shape-dependency of the infrared spectra of ice and nitric acid dihydrate particles and its influence on the size distribution retrieval has been analyzed. A summary of recent publications on this subject is given below.

  • A critical test of available infrared optical constants (real and imaginary part of the complex refractive index) for supercooled binary H2SO4/H2O and HNO3/H2O solution droplets.
    Reference:
    Wagner, R., Mangold, A., Möhler, O., Saathoff, H., Schnaiter, M., and Schurath, U.
    Atmospheric Chemistry and Physics, 3, 1147-1164, 2003.
  • Optical constants and N2O5 uptake on supercooled ternary H2SO4/H2O/HNO3 solution droplets at polar stratospheric cloud temperatures.
    Reference:
    Wagner, R., Naumann, K.-H., Mangold, A., Möhler, O., Saathoff, H., and Schurath, U.
    The Journal of Physical Chemistry A, 109, 8140-8148, 2005.
  • Optical characterization of nitric acid hydrate aerosols. Influence of the infrared spectrum of nitric acid dihydrate on the particle shape.
    Reference:
    Wagner, R., Möhler, O., Saathoff, H., Stetzer, O., and Schurath, U.
    The Journal of Physical Chemistry A, 109, 2572-2581, 2005.
  • Mid-infrared extinction spectra and optical constants of supercooled water droplets.
    Reference:
    Wagner, R., Benz, S., Möhler, O., Saathoff, H., Schnaiter, M., and Schurath, U.
    The Journal of Physical Chemistry A, 109, 7099-7112, 2005.
  • Homogeneous and heterogeneous ice nucleation, studied by expansion cooling experiments in the AIDA chamber. Retrieval of ice particle number densities and size distributions from the FTIR extinction spectra; dependence of the retrieval results on the shape of the ice crystals.
    References:
    Wagner, R., Benz, S., Möhler, O., Saathoff, H., and Schurath, U.
    Atmospheric Chemistry and Physics, 6, 4775-4800, 2006 and
    Wagner, R., Benz, S., Möhler, O., Saathoff, H., Schnaiter, M., and Leisner, T.
    The Journal of Physical Chemistry A, 111, 13003-13022, 2007.

The interplay of infrared extinction spectroscopy with other optical measurements like light scattering and depolarization is decribed in the recent review of optical measurements at the aerosol and cloud chamber AIDA by R. Wagner, C. Linke, K.-H. Naumann, M. Schnaiter, M. Vragel, M. Gangl, and H. Horvath in the Journal of Quantitative Spectroscopy and Raditative Transfeer, 110, 930-949, 2009.

For further information please contact Dr. Robert Wagner.