An Aerosol Tour de Forcing

Bellouin et al. attempt an empirical end-run around this uncertainty by dividing the planet into six regions where aerosol concentration is high, and using a ‘typical’ value of particle absorption based on surface measurements. The measured absorption is a single value that reflects the combined effect of both anthropogenic and natural aerosols, although the six representative sites were chosen where contribution by the former dominates. Regions with a preponderance of sulfates, such as the eastern coast of North America and downwind, were assigned greater reflectance and lesser absorption than particles over the Indian Ocean where dark soot particles are more common. This is based upon contrasting surface measurements at Washington DC and the Maldive Islands in the Indian Ocean. The total aerosol mass was inferred from MODIS estimates of the aerosol optical thickness (AOT), which measures attenuation of a light beam passing through an aerosol layer. To estimate the anthropogenic fraction of aerosols, Bellouin et al. made use of the fact that anthropogenic aerosols such as sulfate and soot are generally smaller than natural aerosols such as soil dust and sea salt. MODIS provides not only the total AOT but also the fractional contribution corresponding to smaller particles whose diameter is less than one micron (a thousandth of a millimeter). Bellouin et al. attributed the total AOT to human influence in regions where the fine fraction AOT exceeds 85% of the total. Conversely, regions where larger particles make the predominant contribution to AOT were excluded from the anthropogenic total. While MODIS is able to make this distinction between small and large particles over ocean, the distinction is more uncertain over land, and here Bellouin et al. resorted to the anthropogenic fraction computed by five aerosol models, a number chosen to reduce the uncertainty associated with any single model.

Despite their different result compared to Bellouin et al., the calculations by Chung et al. and Yu et al. are similar. Chung et al. assign the total AOT using MODIS, and adjust this value using local measurements by the AERONET array of sun photometers. (These instruments point toward the sun and record incident radiation at various wavelengths.) The main difference is that Chung et al. compute the anthropogenic fraction over both land and ocean using a single aerosol model, and they use this model along with AERONET measurements to specify the radiative properties of the combined aerosol population within each column. Consequently, these properties vary within each region as opposed to the regionally averaged values used by Bellouin et al. based upon a single putatively representative site. Yu et al. use an even broader array of measurements and models.

Why do similar methods result in forcing estimates whose uncertainty ranges don’t overlap? This is difficult to know, although here we speculate upon the effect of some of the differing assumptions. Chung et al. specify greater particle absorption compared to all but one of the six regional values used by Bellouin et al. Because the TOA forcing becomes less negative as absorption increases, this accounts for some of the difference. Similarly, Chung et al.’s replacement of their model estimate of anthropogenic particle fraction over the ocean with the MODIS estimate (following Bellouin et al.) narrows the difference.

Treatment of aerosol forcing over cloudy regions also contributes to the difference. Both studies estimate nearly identical forcing at the surface in the absence of clouds. While aerosol absorption and reflection have opposing effects at TOA, they both reduce sunlight beneath the aerosol layer, contributing to negative forcing at the surface. Thus, forcing at the surface is less sensitive to the relative strength of absorption versus reflection. When cloudy regions are included, Chung et al. calculate a much larger reduction of surface radiation than Bellouin et al., who assume that aerosol forcing in these regions is zero. At TOA, Chung et al. calculate positive aerosol forcing within cloudy regions, accounting for some of the global disagreement with Bellouin et al. TOA forcing depends strongly upon the relative position of the cloud and aerosol layer. An absorbing soot layer above a bright cloud absorbs more radiation than if the layer were beneath the cloud. Unlike AOT, the vertical distribution of aerosols is not measured routinely, and is comparatively uncertain.

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