Contribution of road traffic emissions to the atmospheric black carbon burden in the mid-1990s

We developed a global inventory for black carbon (BC) emissions from road traffic for 1993. Global emissions of 2.4 TgC were found in this year. The inventory was implemented in the atmosphere general circulation model ECHAM4.L39(DLR), together with inventories for BC emissions from fossil fuel combustion, biomass burning, and air traffic. The transport of BC particles and the concentration changes due to these emissions were investigated. Particularly, we focused on the contributions of road traffic emissions from North America, Europe, and Asia to the atmospheric BC burden. The sink of BC was parameterized as an exponential decay process with a half-lifetime depending on altitude. Because of the simplified BC cycle, the current investigation should be regarded as a pilot study. However, it is the first study dealing specifically with global road traffic BC emissions. The model results indicate that less than 20% of the BC mass concentrations predicted to be in the planetary boundary layer of the northern midlatitudes arise from road traffic. The road traffic fraction of BC concentrations over North America, Europe, and Asia originated mostly from emissions in the respective region. However, road traffic BC particles from Asia also significantly influence the tropopause region. In July their relative contribution is about 10%. A very rough estimate of the globally averaged direct radiative forcing of BC particles from global and Asian road traffic is given. As road traffic and its emissions in Asia are expected to rise in the near future, their relative importance will also grow and may, eventually, become a significant factor in anthropogenic climate change, unless the specific emissions are drastically reduced.