Reevaluation of mineral aerosol radiative forcings suggests a better agreement with satellite and AERONET data
Modelling studies and satellite retrievals do not agree on the amplitude and/or sign of the direct radiative perturbation from dust. Modelling studies have systematically overpredicted mineral dust absorption compared to estimates based upon satellite retrievals. In this paper we first point out the source of this discrepancy, which originates from the shortwave refractive index of dust used in models. The imaginary part of the refractive index retrieved from AERONET over the range 300 to 700 nm is 3 to 6 times smaller than that used previously to model dust. We attempt to constrain these refractive indices using a mineralogical database and varying the abundances of iron oxides (the main absorber in the visible). We first consider the optically active mineral constituents of dust and compute the refractive indices from internal and external mixtures of minerals with relative amounts encountered in parent soils. We then compute the radiative perturbation due to mineral aerosols for internally and externally mixed minerals for 3 different hematite contents, 0.9%, 1.5% and 2.7% by volume. These amounts represent low, central and high content of iron oxides in dust determined from the mineralogical database. Based upon values of the refractive index retrieved from AERONET, we show that the best agreement between 440 and 1020 nm occurs for mineral dust internally mixed with 1.5% volume weighted hematite. This representation of mineral dust allows us to compute, using a general circulation model, a new global estimate of mineral dust perturbation between -0.40 and -0.21 Wm-2. This range is determined from both optical properties and varying dust size distribution. The broadband shortwave effect varies from -0.78 to -0.53 Wm-2 and the longwave effect between +0.29 and +0.38 Wm-2. The 24-h average atmospheric heating by mineral dust during summer over the tropical Atlantic region (15° N-25° N; 45° W-15° W) is in the range +22 to +32 Wm-2 τ-1 which compares well with the 30±4 Wm-2 τ-1 measured by Li et al. (2004) over that same region. The refractive indices from Patterson et al. (1977) and from Volz (1973) overestimate by a factor of 2 the energy absorbed in the column during summer over the same region.
Atmospheric Chemistry & Physics Discussions
- Pub Date:
- September 2006