Laboratory Instrument for Aerosols Polarimetry and Spectroscopy (LIAPS)
Theme headed by Alain Miffre
Atmospheric aerosols are particularly complex objects. They indeed present a wide range of sizes, shapes and composition, which impact the Earth's climate and public human health as underscored by the latest IPCC report. Figure 1 shows microscopic images taken at iLM of these particles.
Figure 1: Examples of atmospheric aerosols (electron microscopy photographs taken by the OET team at the ILM):
a: Ammonium sulphate (-, climate cooling), b: Sea-salt, c: Desert dust, d: Volcanic ash (courtesy of Muñoz et al), e: Soot aggregate (+, climate warming), f: biogenic.
Whereas the traditional approach to observing these solid or liquid particles is to take samples and/or use optical microscopy on a substrate, the approach adopted involves observing the aerosol in the atmosphere (in-situ conditions, without taking samples) and in real time, so as not to alter the size or shape of these particles, which governs their ability to cool the climate via the albedo effect. The major scientific challenge, overcome during the theses of G. David, T. Mehri and D. Cholleton, was to develop a laboratory instrument, the only one of its kind in the world (Miffre et al., 2016), revealing the optical backscatter (and therefore albedo) properties of inhomogeneous particles of any shape contained in the atmosphere (i.e. in a dilute medium). There is in fact no analytical solution to Maxwell's equations for optical backscatter for inhomogeneous, non-spherical particles; in particular, Mie's theory is inapplicable.
The LIAPS-instrument (Miffre et al., 2016)
Overcoming the obstacles posed by the coaxial source-detector geometry (180.0 ± 0.2°) and the very low intensity of the backscattered wave in a dilute medium, the LIAPS instrument (Laboratory Instrument for Aerosol Polarimetry and Spectroscopy), shown in Figure 1, provides real-time aerosol metrology, by measuring the elements of the scattering matrix of a set of inhomogeneous particles of any shape in the air, thus revealing their spectral (UV, VIS, IR) and polarimetric signatures, accurately (1%) and unequivocally (bias < 10-7), laying the foundations for an optical database for these particles. LIAPS has thus become a reference instrument for the scientific community with major publications (see references below) either from iLM-ATMOS group or in cooperation with several groups which have come to the iLM to study optical backscattering: ONERA [39] and IRCELYON [38]. These developments have made the headlines on the iLM website (PNAS, ACP, OptLett, PCCP, AMT).
Figure 2 : LIAPS (Laboratory Instrument for Aerosol Polarimetry and Spectroscopy) developed at Université Claude Bernard Lyon 1 and at iLM as part of the ATMOS team.
As a world first, LIAPS performs laboratory aerosol metrology by evaluating the optical backscatter of a set of inhomogeneous particles of any shape suspended in the air.
Its accuracy and spectral versatility have made this instrument a benchmark for the community (see publications).
References :
Mineral particles (silica, hematite, inhomogeneous mixtures) light backscattering (AMT 2023, selected as iLM spotlight 2023
The mineral particles in our atmosphere contribute to the Earth's current climate by interacting with electromagnetic radiation. However, because of the diversity of their size, their irregular shape and their inhomogeneity, it is difficult to interpret the imprint left by these particles on the propagation of electromagnetic waves. Within this inhomogeneous mixture, which notably includes silica and iron oxides, such as hematite, the authors revealed the crucial role played by the imaginary part of the complex refractive index in the imprint left by these particles on the backscattering of electromagnetic waves. This backscatter governs the albedo effect, which cools the Earth’s climate.
Sulfates, core-shell organic sulfates light backscattering (PCCP, 2021), selected as iLM spotlight 2021
Because of its ability to backscatter light, sulphate aerosol is responsible for a net cooling of the Earth's atmosphere, potentially offsetting somewhat the global warming effect of greenhouse gases. By taking advantage of the extreme sensitivity of the unique iLM-built laboratory aerosol LIAPS instrument with exact backscattering, we were able to reveal a clear decrease in the backscattering of light by sulphate aerosol in the presence of organic compounds, giving rise to core-shell structures. These core-shell structures of organosulphates, synthesised by our colleagues at IRCELYON, are important because they are among the most important precursors of organic aerosols in the atmosphere. This laboratory discovery, which is supported by numerical core-shell light scattering simulations, is therefore decisive for quantifying the impact of these organic sulphate aerosol particles on the climate.
Soot aggregates light backscattering (JQSRT, 2021)
Light backscattering by fractal soot aggregates has also been measured in the laboratory using the Pi-Scatt polarimeter. The figures below show respectively the soot aggregates generated by our colleagues at ONERA (L. Paulien, F. Foissard and R. Céolato), a collaboration initiated as part of the Soot GDR, and the size distribution associated with these aggregates. Optical backscatter is measured in the UV and VISible wavelengths, as shown in the figure below, which shows the variation in backscatter intensity for different modulations of the polarisation of the incident electromagnetic wave. By fitting these variations to the scattering matrix formalism, we are able to provide, for the first time to our knowledge, quantitative data on the ability of these aggregates to depolarise incident light. This information is decisive for the interpretation of lidar data.
Evaluating polarized light scattering by aerosols from 176° to 180.0° in laboratory (JQSRT, 2019)
UV-VIS depolarization from Arizona Test Dust particles at exact backscattering angle, JQSRT, (2016).