Scaling laws for Light Absorption by Atmospheric Black Carbon Aerosol

TL;DR

This paper develops a scaling law for black carbon aerosol light absorption, revealing how internal mixing affects absorption cross-section and providing corrections for climate models to improve radiative forcing estimates.

Contribution

It introduces a power-law scaling theory for MACBC and EMACBC evolution with mixing, improving parameterizations in climate models.

Findings

MACBC scales with mixing ratio as a 1/3 power law.

MACBC remains inversely proportional to wavelength.

Current core-shell models underestimate MACBC due to photon shielding.

Abstract

Black carbon (BC) aerosol, the strongest absorber of visible solar radiation in the atmosphere, contributes to a large uncertainty in direct radiative forcing estimates. A primary reason for this uncertainty is inaccurate parameterizations of BC mass absorption cross-section (MACBC) and its enhancement factor (EMACBC), resulting from internal mixing with non-refractory and non-light absorbing materials, in climate models. Here, applying scaling theory to numerically-exact electromagnetic calculations of simulated BC particles and observational data on BC light absorption, we show that MACBC and EMACBC evolve with increasing internal mixing ratios in simple power-law exponents of 1/3. Remarkably, MACBC remains inversely proportional to wavelength at any mixing ratio. When mixing states are represented using mass-equivalent core-shell spheres, as is done in current climate models, it results in significant under prediction of MACBC. We elucidate the responsible mechanism based on shielding of photons by the skin depth of a sphere and establish a correction factor that scales with a 3/4 power-law exponent.