Scientific rationale: Today, air traffic contributes about 5% to the global anthropogenic radiative forcing, mainly originating from contrail cirrus, aircraft CO 2 emissions and other minor components. This estimate does not include indirect aerosol effects on clouds, as neither the indirect effects of soot and volatile emissions on high altitude cirrus clouds nor effects of fuel sulfur emissions on low liquid clouds are adequately understood – hence preventing their accurate quantification. Indirect aerosol effects on high clouds include soot aerosol emissions modifying existing cirrus or fostering additional cirrus cloud formation. This effect could possibly be enhanced by pre-activation of soot due to processing in contrails. In addition, volatile particles formed and processed in the aircraft exhaust plume may be transported to the (subtropical) lower troposphere, where they might affect low clouds.