Reevaluation of Mineral aerosol radiative forcings suggests a better agreement with satellite and AERONET data

[摘要] Modelling studies and satellite retrievals do not agree on theamplitude and/or sign of the direct radiative perturbation fromdust. Modelling studies have systematically overpredicted mineral dustabsorption compared to estimates based upon satellite retrievals. Inthis paper we first point out the source of this discrepancy, whichoriginates from the shortwave refractive index of dust used inmodels. The imaginary part of the refractive index retrieved fromAERONET over the range 300 to 700 nm is 3 to 6 times smaller than thatused previously to model dust. We attempt to constrain theserefractive indices using a mineralogical database and varying theabundances of iron oxides (the main absorber in the visible). We firstconsider the optically active mineral constituents of dust and computethe refractive indices from internal and external mixtures of mineralswith relative amounts encountered in parent soils. We then compute theradiative perturbation due to mineral aerosols for internally andexternally mixed minerals for 3 different hematite contents, 0.9%,1.5% and 2.7% by volume. These constant amounts of hematiteallow bracketing the influence of dust aerosol when it is respectivelyan inefficient, standard and a very efficient absorber. These valuesrepresent low, central and high content of iron oxides in dustdetermined from the mineralogical database. Linke et al. (2006)determined independently that iron-oxides represent 1.0 to 2.5% byvolume using x-Ray fluorescence on 4 different samples collected overMorocco and Egypt. Based upon values of the refractive index retrievedfrom AERONET, we show that the best agreement between 440 and 1020 nmoccurs for mineral dust internally mixed with 1.5% volume weightedhematite. This representation of mineral dust allows us to compute,using a general circulation model, a new global estimate of mineraldust perturbation between –0.47 and –0.24 Wm⁻² at the top ofthe atmosphere, and between –0.81 and –1.13 Wm⁻² at thesurface for both shortwave and longwave wavelengths. The anthropogenicdust fraction is thought to account for between 10 and 50% of thetotal dust load present in the atmosphere. We estimate a top of theatmosphere forcing between –0.03 and –0.25 Wm⁻², which ismarkedly different that the IPCC range of –0.6 to+0.4 Wm⁻² (IPCC, 2001). The 24-h average atmospheric heating bymineral dust during summer over the tropical Atlantic region(15° N–25° N; 45° W–15° W) is in therange +22 to +32 Wm⁻² τ⁻¹ which compares well with the30±4 Wm⁻² τ⁻¹ measured by Li et al. (2004) over thatsame region. The refractive indices from Patterson et al. (1977) andfrom Volz (1973) overestimate by a factor of 2 the energy absorbed inthe column during summer over the same region. This discrepancy is dueto too large absorption in the visible but we could not determine ifthis is linked to the sample studied by Patterson et al. (1997) or tothe method used in determining the refractive index.

[学科分类] 大气科学