Black carbon aerosol in the Arctic. Ageing, transport and radiative effects

The anthropogenic impact on climate has led to rising global temperatures. This warming is enhanced in the changing Arctic compared to the global mean. Black carbon (BC) is an aerosol type of particular interest, because it efficiently absorbs solar radiation and thus contributes to the atmospheric warming. BC is released into the atmosphere through incomplete combustion of fossil fuels and biomass including wildfires. The objective of this work is to investigate the sources and transport of BC to the Arctic using global modelling and to provide an up-to-date estimate of its effect on the radiation budget of the Arctic. For this purpose the global aerosol-climate model ECHAM-HAM is used. A comprehensive evaluation of the model using ground-based and airborne observations of BC concentrations in the Arctic shows that it is mostly able to realistically reproduce the observations, but produces an overestimation in the upper Arctic troposphere. The typical uncertainties of current aerosol-climate models are addressed with sensitivity studies: The range of uncertainty in the distribution and radiative effects of BC aerosol due to the assumptions on BC sources is quantified by comparing different emission setups. In addition, the uncertainties related to the wet deposition parametrisation are estimated. It is found that daily, satellite-based biomass combustion emissions are crucial for the reproduction of the vertical distribution of Arctic BC mass concentrations. Moreover, these emission data allow better temporal correlation between observations at Arctic stations and model. A new model configuration, developed in this study, with slower ageing and more efficient scavenging of aerosol in clouds leads to a more realistic BC distribution in the upper Arctic troposphere. The DRE of atmospheric BC in the Arctic (>60°N) amounts to a net energy gain (solar and thermal) at the TOA of +0.31 watt per square meter on average over the years 2007 to 2018, that of the BC-in-snow albedo effect to a gain of +0.12 watt per square meter. The effective radiative impact (direct effects plus rapid adjustments and aerosol-cloud interactions) of BC on the Arctic at top of the atmosphere (TOA) is estimated at -0.2 watt per square meter on the multi-year average. However, the aerosol-cloud radiation interactions are highly uncertain. Improved emission assumptions increase the modelled Arctic BC burden by 25%, while the optimised aerosol microphysics and wet deposition decrease it by 10%. However, both uncertainty factors affect the direct radiative effect (DRE) with 22% to 24% approximately equally, which shows the importance of an accurate description of the vertical distribution of BC in the model. This work thus allows a more complete assessment of the DRE of BC in the Arctic. The newly developed model extensions and methods applied provide a basis for further aerosol-climate research in the Arctic and elsewhere.