Quantifying the range of the dust direct radiative effect due to source mineralogy uncertainty

The large uncertainty in the mineral dust directradiative effect (DRE) hinders projections of future climatechange due to anthropogenic activity. Resolving modeleddust mineral speciation allows for spatially and temporallyvarying refractive indices consistent with dust aerosol com-position. Here, for the first time, we quantify the range indust DRE at the top of the atmosphere (TOA) due to cur-rent uncertainties in the surface soil mineralogical contentusing a dust mineral-resolving climate model. We propagateobserved uncertainties in soil mineral abundances from twosoil mineralogy atlases along with the optical properties ofeach mineral into the DRE and compare the resultant rangewith other sources of uncertainty across six climate models.The shortwave DRE responds region-specifically to the dustburden depending on the mineral speciation and underlyingshortwave surface albedo: positively when the regionally av-eraged annual surface albedo is larger than 0.28 and nega-tively otherwise. Among all minerals examined, the short-wave TOA DRE and single scattering albedo at the 0.44–0.63 μm band are most sensitive to the fractional contribu-tion of iron oxides to the total dust composition. ... mehrThe globalnet (shortwave plus longwave) TOA DRE is estimated to bewithin−0.23 to+0.35 W m−2. Approximately 97 % of thisrange relates to uncertainty in the soil abundance of iron ox-ides. Representing iron oxide with solely hematite opticalproperties leads to an overestimation of shortwave DRE by+0.10 W m−2at the TOA, as goethite is not as absorbing ashematite in the shortwave spectrum range. Our study high-lights the importance of iron oxides to the shortwave DRE:they have a disproportionally large impact on climate con-sidering their small atmospheric mineral mass fractional bur-den (∼2 %). An improved description of iron oxides, suchas those planned in the Earth Surface Mineral Dust SourceInvestigation (EMIT), is thus essential for more accurate es-timates of the dust DRE.