UV-VIS-NIR Spectroscopy of Aerosols

Atmospheric aerosols directly affect the radiative budget of the earth by absorption and scattering of shortwave solar radiation.

UV and visible absorption spectroscopy is of particular importance for this climate impact since absorbing particles modify the radiation fluxes in two different ways: directly, by absorption of shortwave solar radiation and semi-directly, by modifying the temperature distribution of the atmosphere which initiates a number of feedback mechanisms. The contribution of absorption by refractory carbonaceous particles (black carbon) to global warming is still poorly understood and induces a significant uncertainty . Moreover, the determination of the spectrally absorption properties of aerosols becomes increasingly important especially for weakly absorbing aerosols, such as organic aerosols (like brown carbon or humic like substances) and mineral dust aerosol.

Carbonaceous aerosols

The UV-VIS spectral optical properties of carbonaceous aerosols from different combustion sources (like diesel engines and flames) and their alteration by different atmospheric processes are investigated at the AIDA facility. In these investigations we combine broadband extinction measurements in the 230 - 1000 nm spectral regime with photoacoustic spectroscopy at discrete wavelength positions in the UV-VIS-NIR range to assess accurate spectral extinction and absorption coefficients which are needed to assess the influence of these aerosols on the atmospheric radiation budget. Instruments that measure the integral and angular dependent scattering coefficients, so-called nephelometers, complete the optical instrumentation in these sudies.

First investigations focused on the specific optical properties of fresh, non-processed particle emissions from a graphite spark-discharge generator, a diesel engine and from a propane diffusion flame. These investigations reveal that the mass specific absorption cross section of the emitted aerosols can differ by several factors depending on the particle formation conditions which has also an influence on the spectral dependence of the absorption and extinction coefficients. For instance, the increase of the fuel-to-air ratio of the diffusion flame leads to the formation of organic particulate matter, presumably consisting of condensed polyaromatic molecules, that results in a decrease of the visible absorption cross section and to an increase of its spectral dependence.

Reference:
Strong spectral dependence of light absorption by organic carbon particles formed by propane combustion
M. Schnaiter, M. Gimmler, I. Llamas, C. Linke, C. Jäger, H. Mutschke
Atmos. Chem. Phys. 6, 2981 (2006)

In a second type of experiments, diesel emission aerosol was coated with refractive secondary organic mass generated by the in situ ozonolysis of α-pinene. These experiments nicely mimics the ageing processes that fresh combustion aerosol experiences during its lifetime in the atmosphere. In these experiments it could be clearly shown that the mass specific absorption cross section of the processed aerosol is amplified by a factor of two which has a direct consequence for the assessment of the aerosol impact on the radiative budget of the atmosphere.

Reference:
Absorption Amplification of Black Carbon Internally Mixed with Secondary Organic Aerosol
M. Schnaiter, C. Linke, O. Möhler, K.-H. Naumann, H. Saathoff, R. Wagner, U. Schurath, B. Wehner
J. Geophys. Res. 110 (D19), 19204 (2005)

Mineral dust

Due to the fact that the optical properties of mineral dust depend on size, shape, and on mineralogical composition, the resulting radiative forcing in the atmosphere could either be positive or negative. Mineral dust aerosols mostly consist of a mixture of different mineral phases, like quartz, carbonates, sulfates, and clay minerals. Depending on their mineralogical composition, the particles can be strong or weak light absorbers. In particular the contribution of iron oxide phases like hematite and goethite significantly increases the absorption cross sections of the mineral dust aerosols at visible and near UV wavelengths.

In a series of measurement campaigns at the AIDA facility, the wavelength-dependence of the specific extinction and absorption cross sections of a variety of Saharan mineral dust samples has been measured. The absorption cross sections were directly measured with a novel multi-wavelength photo-acoustic spectrometer at wavelengths of 1064, 532, 355, and 266 nm. The extinction spectra were recorded from 230 to 1000 nm. In a recent publication (Linke et al. 2006, see below), we have discussed the correlation between the deduced spectral quantities and the iron oxide content of the samples. Currently, a retrieval algorithm is developed to deduce the wavelength-dependent real and imaginary parts of the complex refractive index for the various dust probes from the measured extinction and absorption spectra.

Reference:
Optical properties and mineralogical composition of different Saharan mineral dust samples: a laboratory study
C. Linke, O. Möhler, A. Veres, Á. Mohácsi, Z. Bozóki, G. Szabó, M. Schnaiter
Atmos. Chem. Phys. 6, 3315 (2006)

In view of the importance of iron oxide contributions, ongoing AIDA investigations also focus on the optical characterization of these pure iron oxide minerals like hematite. Natural dust probes feature a high diversity of the dust grains in terms of size, shape, and surface structure. In contrast, hematite samples can also be synthesized as a monodispersed ensemble of particles with uniform and well-defined particle shapes like cubes or ellipsiods, thereby allowing for an accurate modeling of their extinction and absorption cross sections with suited numerical models like the T-matrix method and Discrete Dipole Approximation. Some preliminary results from these studies are presented 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. Claudia Linke, Dr. Martin Schnaiter, or Dr. Robert Wagner.