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"Black Carbon in the Atmosphere: Emissions, Aging and Cloud Interactions"

Final 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, 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.
A first focus was on the characterization of particulate emissions from combustion sources. Unexpectedly, tar ball particles were for the first time identified in the exhaust of marine ship engines operated at low engine loads. These tar balls were shown to dominate aerosol light absorption and they may potentially become climate relevant in the Arctic as shipping emissions in this region are expected to increase in the near future.
A second focus was on the mass absorption cross section (MAC) of black carbon, a key parameter for black carbon climate effects through aerosol radiation interactions. Substantially tighter constraints could be set on the possible range of MAC values for atmospheric aerosols at European background sites, compared to the large variability found in previous literature. Furthermore, the major 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 clear experimental evidence that the so-called lensing effect indeed increases the light absorption as theoretically expected.
A third focus was on the activation of black carbon particles to form cloud droplets. In-situ measurements in fog and clouds revealed that cloud peak supersaturation is the main parameter determining the black carbon mass scavenged fraction in clouds, which plays a key role for the life-cycle of black carbon. It was also shown how the observed black carbon scavenging can be predicted using simplified Köhler theory, if the black carbon size distribution and mixing state with water-soluble particulate matter are known.
A forth focus was on the glaciation of mixed-phase clouds. In-situ measurements of ice crystal residual particles in mixed phase clouds revealed that atmospheric black carbon particles are poor heterogeneous ice nuclei at moderate below-zero temperatures. This resolved previous contradictory literature findings and shows that anthropogenic black carbon particles are unimportant for the glaciation of supercooled liquid and mixed phase clouds under these conditions.