Climate forcing by carbonaceous aerosols: measurements (ARM) enhancing models (CESM)

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Description

Carbonaceous aerosols—black carbon and organic (secondary and primary)—have been identified as key sources of uncertainty in estimates of radiative forcing of climate change. Recent significant advances in measurement techniques and laboratory and field observations (e.g., by ARM) have identified gaps in model treatments of these aerosols. For example, most models underestimate organic carbon, their light absorption at short wavelengths, and the effect of coatings on absorption by black carbon. Most IPCC AR5 models (e.g., CESM) now use size-resolved and speciated treatments of carbonaceous aerosols with first principles calculations of optical properties from refractive indices, size distribution, composition, and mixing state. Some IPCC AR5 simulate an anthropogenic direct aerosol radiative forcing of 0.0 to -0.1 W/m2, which is substantially more positive than most estimates (-0.5 W/m2) by IPCC AR4 models. We will analyze recent laboratory and field observations that have observed (or constrained) size distributions, optical properties, black carbon mass, and even organic composition, and use them to evaluate the treatments of carbonaceous aerosols in CESM, GISS, and ECHAM models. We hypothesize that improved treatments of carbonaceous aerosols in models to better match observations is the reason why the direct forcing aerosol forcing by AR5 models has increased to 0 to -0.1 W/m2 from -0.5 W/m2 for AR4 models. Our paper will test this hypothesis.