The black arbon in european atmosphere : identification, transfert, deposition and impacts

Black carbon (BC) induces a warming effect (RFBC = +1.1 W m-2 ± 90%) through two main pathways: aerosol-radiation interaction (RFari) and aerosol-cloud interaction (RFaci). Both BC-radiation and BC-cloud interaction are affected by the mixing of black carbon with other non-refractory and non-absorbing matter present in the atmosphere. An estimation of the global radiative forcing of BC rarely accounts for internal mixing of BC while the net global cloud radiative forcing is sensitive to assumptions in the initiation of cloud glaciation, which is mostly unknown for black carbon particles. Within this thesis we investigated the variability of the light absorbing properties of black carbon, the mixing of black carbon, and the impact on light absorption and ice activation.In the first part of this thesis we investigated the spatial and seasonal variability of the mass absorption cross section (MAC) over Europe. MAC values were determined from ambient observations of elemental carbon mass concentrations (mEC) and absorption coefficients (σap). The data had been acquired during several years at different background ACTRIS supersites spread over Europe. Site specific MAC values were found to be spatially homogeneous, suggesting that the overall MAC average 9.5 ± 1.9 m2 g-1 at a wavelength of 637 nm might be representative of BC at European background locations. The MAC values showed a distinct seasonal cycle at every station. This seasonality might be related to chemical composition and aging. We observed that the MAC value has a linear and positive proportionality with the non-absorbing matter mass fraction.The second part of the work focuses on the coating acquisition of BC and the induced absorption enhancement after long-range transport. Within the CLIMSLIP (climate impact of short-lived pollutants and methane in the Arctic) project field experiments were conducted at the Zeppelin research site in Svalbard, Norway, during the Arctic spring. SP2 data were used to characterize the BC size distribution and mixing. BC containing particles having a core diameter between 170 and 280 nm were found to have a median coating thickness of 47 nm. The relationship between coating thickness and BC absorption was simulated. The observed coating thickness enhanced the mass absorption cross section by 46%, which led to a decrease of less than 1% in the single scattering albedo.In the final part of this work, the role of black carbon as ice nuclei in mixed phase clouds was investigated at the high elevation measuring site Jungfraujoch (Switzerland) during the cloud and aerosol characterization experiment (CLACE) held in 2013. The ice-CVI inlet and a single particle soot photometer were used to select and quantify the ice activated BC particles. According to the observations, BC containing particles were depleted in the ice residuals. The activation efficiency showed a size dependency, with larger BC containing particles being activated more efficiently compared to smaller ones. Activated BC cores having a diameter between 170 and 240 nm showed a larger coating thickness (median = 53 nm) compared to the total aerosol (median = 16 nm). The results obtained in this thesis shed new light on the effect of the mixing state on the optical properties and cloud activation of black carbon particles. Absorbing properties of BC showed a distinct seasonal pattern, while aging was found to consistently increase its absorption behavior. However, black carbon was found not to act as ice nuclei in low tropospheric mixed-phase clouds, where the coating thickness might play a role in the activation efficiency. This work provides freshly determined physical properties derived from ambient observations that will improve the accuracy of future aerosol and cloud radiative forcing estimations.