Abstract
Abstract. Airborne mineral dust is a key player in the Earth system and shows manifold impacts on atmospheric properties such as the radiation budget and cloud microphysics. Investigations of smoke plumes originating from wildfires found significant fractions of mineral dust within these plumes – most likely raised by strong, turbulent fire-related winds. This study presents and revisits a conceptual model describing the emission of mineral dust particles during wildfires. This is achieved by means of high-resolution large-eddy simulation (LES), conducted with the All Scale Atmospheric Model (ASAM). The impact of (a) different fire properties representing idealized grassland and shrubland fires, (b) different ambient wind conditions modulated by the fire's energy flux, and (c) the wind's capability to mobilize mineral dust particles was investigated. Results from this study illustrate that the energy release of the fire leads to a significant increase in near-surface wind speed, which consequently enhances the dust uplift potential. This is in particular the case within the fire area where vegetation can be assumed to be widely removed and uncovered soil is prone to wind erosion. The dust uplift potential is very sensitive to fire properties, such as fire size, shape, and intensity, but also depends on the ambient wind velocity. Although measurements already showed the importance of wildfires for dust emissions, pyro-convection is so far neglected as a dust emission process in atmosphere–aerosol models. The results presented in this study can be seen as the first step towards a systematic parameterization representing the connection between typical fire properties and related dust emissions.
Highlights
1.1 Occurrence and characteristics of wildfiresBiomass burning and other types of natural and prescribed wildland fires are very common in semiarid regions almost all over the world
Fire impacts on the soil and surface conditions and the lower tropospheric circulation can lead to conditions that are favorable for dust emissions
Some studies indicate the importance of such wildfires to the atmospheric dust load, the process of the dust uplift during such fire events is not well understood so far and currently not considered as a source of airborne mineral dust in climate or aerosol models. This is all the more astonishing since the mixture of raised dust particles with fresh combustion aerosol such as soot or black carbon can lead to changes in the chemical, optical, and microphysical properties of the dust particles. This would have impacts on the particle aging processes, the dust-radiative forcing, and the suitability of dust particles to act as ice nuclei particles (INPs) or as cloud condensation nuclei (CCN), which influence atmospheric residence time and microphysical properties of clouds and are related to health hazards (e.g., Chalbot et al, 2013; Hand et al, 2010; McCluskey et al, 2014; Levin et al, 1996; Winton et al, 2016)
Summary
Biomass burning and other types of natural and prescribed wildland fires (in the following referred to altogether as wildfires) are very common in semiarid regions almost all over the world. Grassland and shrubland fires are usually smaller in size and intensity compared to forest fires, for which the typical forest vegetation provides a higher fuel load, resulting in a higher energy consumption by the fire (Reid et al, 2005). R. Wagner et al.: Wildfires as a source of airborne mineral dust pact the surface properties in the fire-affected area, which can change the vulnerability of that area to wind erosion during and after the fire. During such high-intensity fires a breakdown of soil structures and crusts was observed, which would increase the number of fine particles available for mobilization (McNabb and Swanson, 1990; Levin et al, 2012). The higher vulnerability of burned landscapes to dust emission was found by other studies (e.g., Whickler et al, 2002, 2006; Ravi et al, 2012; Merino-Martín et al, 2014), suggesting this as a possible important source of airborne mineral dust
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