Constraining thresholds of Dust Optical Properties on Radiative Forcing over Source Regions

Description automatically generatedMineral dust is a principal global export from Earth's deserts (Washington and Wiggs, 2011; Wiggs et al., 2022). Dust is emitted from these arid regions when winds blow over dry surface sediments, deflating particles into the atmosphere. Once airborne, mineral dust can be suspended for thousands of miles; traversing oceans and continents (Mahowald et al., 2014) and, when conditions permit, can circumnavigate the planet. During its residency in the atmosphere, and when it is later deposited, dust can impact numerous Earth Systems, both positively and negatively. On the one hand, fine dust grains can cool the Earth by scattering shortwave solar radiation during whilst resident in the atmosphere whilst, on the other, coarser particulates can accelerate global warming through absorption of longwave radiation (Milton et al., 2008; Chen et al., 2021); and, thus, affect mean global temperature (Hansen et al., 1997). However, upon its deposition, coarse iron-rich dust grains can generate CO2 drawdown by encouraging increased marine productivity through oceanic fertilisation (Jickells et al., 2005; Dansie et al., 2018, 2022), to some degree compensating for some of its potential heating effect (Chen et al., 2021). Therefore, from its initial entrainment, suspension in the atmosphere and ultimate deposition, mineral dust has numerous, complex impacts on the Earth System through its ability to influence atmospheric, biological, terrestrial, and oceanic processes (Washington and Wiggs, 2011); which makes dust and its dynamics essential for us to understand.

Whilst there is consensus that mineral dust emissions have significant influences on the Earth System, close to and far from their sources, there remain uncertainties when quantifying their effect on regional and global climate. This is because their influence on atmospheric dynamics can depend on the microphysical and optical properties of individual dust grains, including size distribution (Tegen and Lacis, 1996; Liao and Seinfeld, 1998; Mahowald et al., 2014; Chen et al., 2021), chemical composition (Mahowald et al., 2011; Klingmuller et al., 2019), and albedo (Shi et al., 2021; Tian et al., 2023). For instance, darker particulates suspended in the atmosphere absorb solar insolation, and thus can cause a warming effect, while light-coloured minerals can have a cooling effect (Smith, 2023), as illustrated in Figure 1. This makes the geochemistry of emissions vital to understand - since the albedo of dust grains depends on their mineralogy and composition - to ascertain whether mineral dust suspended in the atmosphere has a net positive or net negative effect on climate. This is known as the direct radiative forcing (DRF) efficiency of dust and is an important parameter to measure the effect of dust aerosols on regional and global climate (Tian et al., 2021).

To date, research into the influence of dust aerosols on Earth's climate has generally been restricted to studies which examine the extinction of the solar beam by dust and haze, known as dust aerosol optical depth (DAOD), through remote sensing. Hence, few datasets exist concerning the quantities, fluxes, and spatiotemporal variability of desert dust emissions. This creates two closely related research problems for existing terrestrial-based investigations and models of global dust emissions, transport, and deposition. Firstly, there remain uncertainties in identifying the processes driving atmospheric dust emissions from specific desert source areas. This is primarily due to uncertainties in mineral dust composition and the characterisation of its dynamics within global climate models (GCMs), including dust particle sizes, optical properties, and surface roughness elements (Connelly et al., 2021; Leung et al., 2023). This problem requires a solution since it limits efforts to explore dust transport pathways given that highly localised dust emissionscan have significant impacts on clima