Abstract
Abstract. In this work, new information has been gained on the laboratory-resuspended PM10 fraction from geological topsoil and outcropped rocks representative of the Rome area (Latium). Mineralogical composition, size distribution, optical properties and the surface radiative forcing efficiency (RFE) of dust types representing the compositional end members of this geological area have been addressed. A multi-disciplinary approach was used, based on chamber resuspension of raw materials and sampling of the PM10 fraction, to simulate field sampling at dust source, scanning electron microscopy/X-ray energy-dispersive microanalysis (SEM XEDS) of individual mineral particles, X-ray diffraction (XRD) analysis of bulk dust samples, building of number and volume size distribution (SD) from microanalysis data of mineral particles and fitting to a log-normal curve, and radiative transfer modelling (RTM) to retrieve optical properties and radiative effects of the compositional end-member dust samples. The mineralogical composition of Rome lithogenic PM10 varies between an end-member dominated by silicate minerals (from volcanics lithotypes), and one mostly composed of calcite (from travertine or limestones). Lithogenic PM10 with intermediate composition derives mainly from siliciclastic rocks or marlstones. Size and mineral species of PM10 particles of silicate-dominated dust types are tuned mainly by rock weathering and, to lesser extent, by debris formation or crystallization; chemical precipitation of CaCO3 plays a major role in calcite-dominated types. These differences are reflected in the diversity of volume distributions, either within dust types or mineral species. Differences are also observed between volume distributions of calcite from travertine (natural source; SD unimodal at 5 μm a.d.) and from road dust (anthropic source; SD bimodal at 3.8 and 1.8 μm a.d.). The volcanics and travertine dusts differently affect the single scattering albedo (SSA) and the asymmetry parameter (g) in the visible (VIS) and near-infrared (NIR) regions. The downward component of the bottom-of-atmosphere (BOA) solar irradiance simulated by RTM for an atmosphere where only volcanics (or only travertine dust) composes the aerosol, shows that the volcanics contribution to the solar irradiance differs significantly from that of travertine in the NIR region, while similar contributions are modelled in the VIS. The RFE (−293 W m−2 for volcanics and −139 W m−2 for travertine, at 50° solar zenith angle) shows that volcanics dust produces a stronger cooling effect at surface than travertine, as expected for more absorbing aerosols.
Highlights
Airborne geological dust sourced from topsoil and surface rocks critically contribute to the total mass, composition, microphysical, and optical properties of the atmospheric aerosol in continental regions, and largely impacts Earth’s different compartments by transport and deposition (Scheuvens and Kandler, 2014)
Results indicate that the microanalysis is less reliable for Na, Mn as well as for Si and Mg in the siliciclastics sample, while in all other cases it shows a good agreement with ED-XRF bulk analysis
A knowledge gap was faced concerning how, and to which extent, the local mineral dust resuspended from rocks outcropped in a site/area may contribute to the PM10 fraction, to the direct interaction of the aerosol with solar radiation, and to the radiative flux at BOA, within the same source area of dust
Summary
Airborne geological dust sourced from topsoil and surface rocks critically contribute to the total mass, composition, microphysical, and optical properties of the atmospheric aerosol in continental regions, and largely impacts Earth’s different compartments by transport and deposition (Scheuvens and Kandler, 2014). While indirect effects depend on the heterogeneous chemistry occurring at particles surface (Levin et al, 1996; Buseck and Pósfai, 1999; Sokolik et al, 2001; Krueger at al., 2004; Kandler et al, 2007), the light scattering and absorption are mostly controlled by the mineralogical composition, shape features and microphysical properties of geological particles (D’Almeida, 1987; Kalashnikova and Sokolik, 2002, 2004; Kokhanovsky, 2008; Hansell Jr. et al, 2011) Most studies facing this issue relate to desert dust from Sahara and Sahel regions (Kandler et al, 2007, 2009; Müller et al, 2009; Papayannis et al, 2012; Wagner et al, 2012; Di Biagio et al, 2014). Results from experimental and modelling analysis are discussed for their consistency with both the lithological nature of major local dust sources and the microphysical properties of the mineral dust samples
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