Limitations in the enhancement of visible light absorption due to mixing state

Absorption by light-absorbing carbon (LAC) particles increases when the carbon is mixed with other material, and this change affects climate forcing. We investigate this increase theoretically over a realistic range of particle sizes. Perfect mixing at the molecular level often overestimates absorption. Assuming that LAC is coated by a concentric shell of weakly absorbing material, we calculate absorption by a range of realistic particle sizes and identify regimes in which absorption behaves similarly. We provide fits to amplification in five regions: (1) small cores and (2) intermediate cores, both with large shells; (3) small to intermediate cores with intermediate shells; (4) cores with growing shells; and (5) intermediate to large cores with large shells. Amplification in region 1 is highest but is physically implausible. Amplification in region 5 is constant at about 1.9 and represents an asymptote for particles with broad size distributions. Because absorption by aggregates is amplified by about 1.3 above spherical particles, and that factor is lost when particles are coated, we suggest that absorption by aged aerosol is about 1.5 times greater than that of fresh aerosol. The rate at which particles acquire sufficient coating to increase their original diameter by 60% is important in determining total absorption during their atmospheric lifetimes. Fitted amplification factors are not very sensitive to assumed refractive index of LAC and can be used even in simple models.