Regional climate forcing by carbonaceous aerosols: relating optical properties to chemical composition to improve predictions

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Description

The emissions, composition, and optical properties of carbon-containing aerosols are the largest source of uncertainty in estimates of direct aerosol radiative forcing, which vary by a factor of four amongst models. Using a state-of-the-art 3-laser photo-acoustic instrument, we measured the aerosol absorption and scattering and combined it with chemical observations of long-range transport (LRT) pollution in the Arctic (ISDAC) and in Asia (CAPMEX) in 2008. Our analysis develops a process-level understanding of the optical properties as the aerosols mix and age during transport by linking them to changes in observed changes in chemical composition. In Asia we find that transport of mixed sulfate, carbonaceous, and nitrate aerosols from various pollution plumes to Jeju, South Korea accounted for 74% of the deployment days, showing large variations in their measured chemical and optical properties. We show that episodes with high organic carbon/sulfate and nitrate/sulfate ratios exhibit lower single-scatter albedo at shorter wavelengths, significantly lower than predicted by an optical model that assumes constant complex index of refraction with wavelength. Organic carbon absorption accounts for up to 50% of the measured aerosol absorption at 405 nm. In Alaska, in April 2008 we intercepted and interrogated pervasive pollution layers aloft Alaska. The absorption and scattering signals occurred in layers from 1 to 6 km above the surface and approached 30 to 200 (Mm)-1 respectively. Alternating light and dark aerosol layers with single-scatter albedo ranging from 0.7 to 0.95 were evident, and they extended over vast areas. Real-time satellite data assimilated transport models indicate that this pollution was imported from Chinese dust storms and Siberian fires as well as from Eurasian energy sectors. Our wavelength-dependent optical properties are used to diagnose the soot, dust, sulfate, and organic components of this complex soup of pollutants. They are then verified by analysis of size distributions and chemical compositions observed by a single-particle laser ablation spectrometer instrument. We use our optical observations to estimate a direct radiative forcing by pollution of the order of 40 of W m-2 and also significant atmospheric heating rates. We find that aerosols forcings and impacts in Asia and the Arctic are large. Our results underscore the need to predict changes in optical properties from aerosol aging during their long-range transport in models.