Effective Mass Extinction Efficiency b

The mass extinction efficiency b for each aerosol component represents the conversion factor, which multiplied with the aerosol component mass, determines the component aerosol optical depth. Inversely, data for aerosol mass and for the associated aerosol optical depth permit a derivation of the 'effective' mass extinction efficiency. These 'effective' b of models are compared in Table 12.

Table 12. Comparison of mass extinctions efficiencies b (m/g2) in models ([..-..] indicate typical range)_










no H20




.5 *

.5 *







Org. Carbon








8 *

Black Carbon

7.7 *

7.7 *





9 *

9 *

Sea Salt





0.4 *



0.3 *









* assumed constant

O.CARBON- b: GISS is the only model that prescribes the organic carbon conversion at 8m2/g. This is a large value in comparison to that of size-resolved conversions, which usually range between 3 and 6m2/g, The GISS value is only reached and slightly exceeded in GOCART or ECHAM4, when ambient relative humidity in models is higher.

B.CARBON- b: GISS and ECHAM4 prescribe the black carbon conversion with values of 9m2/g and 7.7m2/g. The size-resolved conversion of GOCART is slightly larger between 7.5 and 9m2/g. The surprising low values of MIRAGE (between 2 and 4m2/g) are an artifact in the derivation, as MIRAGE considers aerosol internally mixed.

DUST- b: ECHAM4 prescribes a dust conversion value of 0.5m2/g. Values near 0.8m2/g are typical for the GOCART and MIRAGE. The lack of seasonal variability suggest that changes to dust aerosol size are small. This is surprising and quite in contrast to GISS. Strong seasonal variations but also the largest values, ranging from 1.0 to 1.8m2/g, in GISS seem to indicate a frequent dominance of sub-micrometer dust sizes.

SULFATE- b: All tested models use size-resolving schemes for the sulfate conversion. The largest conversions at about 10m2/g are suggested by GOCART and on occasion by ECHAM4. Both models are based on the same assumption for size and humidification, thus, differences largely reflect the ambient relative humidity of the model. Values for GISS at 4m2/g and for CCSR at 3m2/g reflect assumptions of a larger sulfate aerosol size

SEASALT- b: Model values for sea-salt mass conversion are near 0.5m2/g. MIRAGE and ECHAM4 values are slightly larger and GOCART values are slightly lower. GISS assumes a relatively large constant conversion of 2m2/g.

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