Arctic black carbon from on-road diesel and aircraft emissions

Market strategies have greatly incentivised the use of diesel engines for land transportation. These engines are responsible for a large fraction of black carbon (BC) emissions in the extra-tropical northern hemisphere with significant effects on both air quality and global climate. A sensitivity study is made with the University of L’Aquila climate-chemistry-aerosol model (ULAQCCM) by eliminating on-road diesel emissions of BC (which represent approximately 50% of BC emissions from land transportation). According to the model and using year 2000 emission scenarios, these emissions coupled to large scale atmospheric transport would imply an average tropopause direct radiative forcing (RF) of 53 mWm-2 over the Arctic region, with a peak of 160 mWm-2 during Arctic springtime months. The calculated Arctic BC optical thickness due to on-road diesel emissions is on average 0.54×10-3 at λ=0.55 μm (i.e., 16.5% of the total BC optical thickness over the Arctic), with a peak of 0.96×10-3 during springtime. The same model has been used to compare this direct RF with that produced by BC emissions from global aviation. According to the model and using 2006 aviation emission inventories (from the EC project REACT4C), the calculated annuallyaveraged BC optical thickness change over the Arctic region is 6.5×10-6 at λ=0.55 μm, with a tropopause RF=1.6 mWm-2. The tropopause RF ratio between on-road diesel and aircraft BC (33) is much smaller than the mass loading ratio (73), due to the different vertical distribution of BC particles. This produces a much higher radiative efficiency for aircraft BC, approximately 2.3 times larger with respect to on-road diesel BC (i.e., 2700 .vs. 1200 W/g). The corresponding ratio of surface RFs, on the other hand, is approximately one order of magnitude larger with respect to the ratio of tropopause RFs, reflecting the fact that approximately only half of the aircraft BC mass loading resides below the mean Arctic tropopause.