Project 14 (WP1): Impact of dust on Infrared radiation

Project lead: Paola Formenti and Yves Balkanski

Post-doctoral researcher : Claudia Di Biagio

Project Start/End : 5 October 2014 - 5 October 2016

Context: Mineral dust is one of the most abundant aerosol species in the atmosphere and strongly contributes to the total aerosols content and direct radiative effect. At the present time, large uncertainties still exist in the estimation of the dust radiative effect and its climatic impacts. The reduction of these uncertainties is crucial in order to better understand the climate of the present, and in particular the role played by anthropogenic aerosols. Mineral dust can be considered, in fact, as part of the natural aerosol background, so the quantification of its radiative effect is a prerequisite for a more accurate assessment of the impact of the aerosols of anthropic origin on climate. The quantification of the radiative impact of dust aerosols represents also a necessary step to make projections on the climate of the future. Dust emissions are strongly sensitive to climate changes and the concentration of mineral dust is expected to largely change in the next decades as a consequence of changes in surface wind speed, precipitation, vegetation cover, or bare soil fraction. The changes in the radiative effect of dust aerosols associated with the modification in its atmospheric concentration will possibly contribute to the instauration of different climate conditions in the next decades. Due to all these considerations, the reduction in the uncertainties associated to the estimation of the mineral dust radiative effect emerges as a key scientific priority.

Due to its characteristic mineralogical composition and particle size distribution, mineral dust can affect both the solar and the infrared radiation fields by scattering and absorption. At present the main difficulty in estimating the dust direct effect is due to the incapability of quantifying the infrared contribution to its total radiative perturbation. One of the main causes for this is the poor knowledge of the dust optical properties and their variability.

Whereas in the last few years extensive work has been done to characterise the absorption properties of mineral dust in the shortwave, current available data on dust infrared optical properties mainly concern measurements of the refractive index of single minerals composing dust. However, due to the differences in the chemical/crystallographic state between the reference minerals and the minerals in the natural aerosol, and also to the difficulty of finding an appropriate mixing rule, the calculation of the dust refractive index based on information on its single constituents has been proven highly uncertain. On the other side, only very few studies, from a limited number of geographical locations worldwide, has been performed on natural aerosol samples. Hence, to date, the natural variability of the dust infrared refractive index and optical properties remains not represented.

Objectives :

The main aim is to work at improving the knowledge on the infrared optical properties of dust aerosols and associated radiative effects.

The work is performed based on two main tasks:

(i)            the experimental development of parameterizations of the dust infrared optical properties as a function of mineralogical composition and size. This task is based on the mathematical inversion of infrared extinction spectra which are measured in situ IRFT spectrometry in the CESAM simulation chamber at LISA (CESAM, Experimental Atmospheric Multiphasic Simulation Chamber). The particle generation and injection protocols, which control the representativeness of the size distribution and the composition of the generated aerosol with respect to natural conditions, have been optimised with particular attention to the coarse-mode fraction which is the most efficient in interacting with longwave radiation. The particle lifetime of coarse particles is sufficiently long (several hours) in order to investigate the optical properties of the aerosol altered by the different ageing processes. Dust generation is performed by mechanical shaking of the soil grains liberate the grain finer fraction in a similar way that it is done by the sandblasting process during natural aerosol emission. The LISA has an extensive storage bank of natural parent soils from major source regions worldwide. These measurements will provide with the optical properties of mineral dust from different source regions, therefore of different mineralogy, in the fine and coarse size fractions.

(ii)           the integration of those parameterizations in the scheme of LMDzOR-INCA coupled to RRTM radiative module .We will link the work of the first task to the global dust distribution by computing the radiative properties of mineral dust based on an original soil database that has been created at IPSL (Journet et al., 2014). This soil mineralogy database allows estimating the mineralogical composition of the airborne aerosols. The optical properties are calculated knowing, for each mineral fraction, the infrared complex refractive index taken from literature values. For a given source region, the result of these calculations will be constrained to the optical properties estimated experimentally in task 1. This will allow taking into account the regional variability of the mineralogical composition, which at present is not represented in climate models.

By combining (i) and (ii), it will be possible to perform simulations to analyse the climate sensitivity to the dust radiative effect including a realistic representation of the infrared radiative effect. Based upon our knowledge of size distribution and of the mineralogical composition we will also assess the uncertainties of both the SW and the LW radiative effect of aerosols which will allow constraining the whole impact of dust on climate.

Position offer and results (January 2017 update)