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BLACARAT Report Summary

Project ID: 615922
Funded under: FP7-IDEAS-ERC
Country: Switzerland

Mid-Term Report Summary - BLACARAT (Black Carbon in the Atmosphere: Emissions, Aging and Cloud Interactions)

Atmospheric aerosol particles have been shown to impact the earth's climate because they scatter and absorb solar radiation (aerosol radiation interactions) and because they can modify the microphysical properties of clouds by acting as cloud condensation nuclei or ice nuclei (aerosol cloud interactions). Radiative forcing by anthropogenic aerosols through these effects remains poorly quantified, thus leading to considerable uncertainty in our understanding of the earth’s climate response to the radiative forcing by greenhouse gases. This project addresses the climate effects of atmospheric aerosols with a particular focus on black carbon (BC), which is an important component of atmospheric aerosols and mostly emitted by anthropogenic combustion processes and biomass burning. Estimates show that BC may be the second strongest contributor (after CO2) to global warming. Adverse health effects due to particulate air pollution have also been associated with traffic-related BC particles.

The mass absorption cross section (MAC) of black carbon is a key parameter for its climate effects through aerosol radiation interactions. The spatio-temporal variability of the MAC value was determined by analysis of a comprehensive data set from aerosol monitoring sites across Europe.
In this manner, we could set substantially tighter constraints on the possible range of MAC values for atmospheric aerosols, compared to the large variability found in previous literature. Furthermore, part of the residual variability of the MAC values could be attributed to variations of the mixing state of black carbon with other aerosol components, providing evidence that the so-called lensing effect increases the light absorption of internally mixed BC.
The number of droplets formed in a cloud depends in nonlinear manner on the available cloud condensation nuclei (CCN) and updraft velocity. Statistical analyses of results from a series of aerosol-cloud interaction experiments the Jungfraujoch were combined with box model simulations. It could be shown that a microphysical parametrization commonly used in global model simulations reproduces the observations well and that cloud formation at this alpine site occurs in a CCN sensitive regime. This means that the cloud properties are susceptible to anthropogenic emissions such as black carbon particles. The key players triggering nucleation at the free tropospheric site Jungfraujoch have been identified and the contribution of new particle formation to the CCN budget has been assessed. How the droplet activation of black carbon particles depends on its mixing state with organic and inorganic aerosol components and how it compares with the droplet activation of BC-free aerosol particles will be investigated. Further topics that remain to be addressed in this project include climate-relevant properties of fresh and aged combustion emissions.

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