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FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe

Periodic Reporting for period 2 - FRAGMENT (FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe)

Reporting period: 2020-04-01 to 2021-09-30

Soil dust aerosols are a key component of the Earth system. Climate perturbations by dust aerosols depend fundamentally upon their physical and chemical properties. Dust aerosols are indeed a mixture of different minerals, whose relative abundances, particle size distribution (PSD), shape, surface topography and mixing state influence their effect upon climate. However, Earth System models typically assume that dust aerosols have a globally uniform composition, neglecting the known local and regional variations in the mineralogical composition of the sources.

The overarching goal of FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe (FRAGMENT) is to understand, constrain and calculate the global mineralogical composition of dust along with its effects upon climate. FRAGMENT aims at fundamentally advancing the treatment of dust mineralogy by fulfilling several objectives: 1) It will contribute new fundamental understanding to reduce the large uncertainties in the emitted dust PSD and mineralogy by evaluating and extending current theoretical paradigms, based on an unprecedented ensemble of coordinated measurement campaigns and laboratory analyses. 2) Precise knowledge of dust mineral content requires more detailed and widespread measurements of soil mineralogy. We are evaluating airborne and spaceborne hyperspectral imaging to improve global atlases of soil mineralogy for dust modelling. The new methods and understanding will anticipate the coming innovation of retrieving soil mineralogy through high-quality spaceborne hyperspectral measurements. 3) While the impact of dust mineralogy upon climate is potentially large, the interconnection of mechanisms has been neglected in the few models that represent mineral variations. FRAGMENT will generate integrated and quantitative knowledge regarding the effects of dust mineral composition based on modelling experiments constrained with the theoretical innovations, field measurements and the new soil mineralogical map.
As planned, we are performing an unprecedented set of coordinated and interdisciplinary field campaigns over remote dust sources and detailed laboratory analyses across three laboratories (CSIC,TUDA and Caltech). After two instrument tests in Aragón, Spain, the first complete field campaign took place in the northern tip of the Sahara Desert (M’hamid, Morocco) in September-October 2019, where we acquired a wealthy dataset including close to 20 dust events of different magnitudes and types. Our results show how soil mineralogy and particle size are strongly dependent on the basin scenario and location. Finer particles are observed in crusts and paved sediments and coarser in sediments and dunes. Crusts show an enrichment in clays, iron oxides and salts mainly and are therefore more susceptible to be ejected into the atmosphere. The average emitted size distribution is consistent with previous studies, although as opposed to most previous studies our estimated range covers a wider range of super-coarse dust particles. We observe moderate emitted dust size distribution variability depending on meteorology and we are proposing methods to infer it inferred from optical particle counters while showing the high sensitivity to the index of refraction and shape of the particles. We also find strong dependencies of key mineral groups with size for the emitted dust that are consistent with key FRAGMENT hypotheses. Dust particles are aspherical and show considerable heterogeneity. The optical properties of the emitted dust are dependent upon the type of event, dominated by changes in size and potentially size-resolved mineralogy.

FRAGMENT is intimately linked to EMIT, a NASA instrument mission that will sample the Earth’s surface mineral composition using hyperspectral imaging spectroscopy on the International Space Station (ISS) by 2022. Analyses of all spectral data thus far include statistical principal component analyses and analyses of the depths of key absorption features indicative of different minerals, focusing on understanding the compositional diversity by grain size and geographic location. We are investigating correlations between spectral proxies for mineral abundance and results from XRD measurements. FRAGMENT is also extending Tetracorder, which is the key mineralogical mapping tool that is used in EMIT. We are developing three additional models beyond the default EMIT model that assumes that linear band depth is proportional mineral abundance. The four models have been coded and results are being refined by obtaining better optical constants. In a next step the models will be tested against laboratory spectra of known samples and applied to airborne spectroscopy data.

In terms of climate modelling, two models (MONARCH and EC-Earth3) are being further developed to include dependencies upon the dust mineralogical composition. We have enhanced and evaluated our MONARCH model with additional emission schemes and optical properties. We have also extended and implemented soil mineralogical maps using available data and quantified the associated uncertainties. We are using inversion modelling and data assimilation techniques to constrain the dust emission spatiotemporal variability. For future assimilation and evaluation work, we have co-developed new global estimates of dust optical depth. We have contributed to quantify for the first time the range in dust direct radiative effect at the top of the atmosphere due to current uncertainties in the surface soil mineralogy. We have also shown that part of the range of dust absorption retrieved from sun photometers can only be explained by regional variations in aerosol mineral composition.
FRAGMENT tries to make a leap forward in understanding and constraining the effects of dust and its composition upon climate. The NASA EMIT mission has the potential of creating a paradigm shift by allowing the production of an accurate near-global database of surface mineralogy that exceeds, by six orders of magnitude, the mineralogical analyses that underpin the current mineralogical atlases. And in this context FRAGMENT attempts to cover the poorly known relationship between emitted dust and the parent soil in terms of size and mineralogical composition. A key methodological aspect that makes FRAGMENT unconventional is the unprecedented combination of theory, field campaigns, laboratory analyses, soil-surface spectroscopy and climate modelling towards the vision of constraining the global dust mineralogical composition and its effects. The set of field campaigns over natural dust sources along with the comprehensive combination and detail of the soil and airborne measurements and laboratory analyses are unprecedented. Particularly important is the development of new approaches based on spectroscopy to improve the mineralogical mapping for dust modeling and to properly adapt dust emission schemes to this new type of information. In this context, the limits of spectroscopy to retrieve information on soil particle size and mineralogy are being massively pushed within FRAGMENT. Our framework to represent the emitted size distributed mineral is an extension of brittle fragmentation theory. While brittle fragmentation theory is not novel, our extensions are and are being tested. By the end of FRAGMENT, we expect to have greatly enhanced our fundamental and quantitative knowledge on dust mineralogical composition and its effects.
Looking for a measurement location in M'Hamid (Morocco)
The main measurement site in L'Bour (M'Hamid, Morocco)
Dust in the Draa Valley (M'Hamid, Morocco)
Tetracorder mapping results for clays, carbonates, sulfates and other minerals from AVIRIS data
Tetracorder mapping results for iron oxide grain sizes from AVIRIS data for the Salton Sea region, C
Saltation in L'Bour (M'Hamid, Morocco)
Local and regional soil sampling locations in Morocco
Dust devils in Lake Iriki (Morocco)
Part of the research team in M'hamid (Mocorcco)