## 4 Mitigation Measures for Road Traffic PM Emissions and Its Impact on Carbonaceous Emissions
### Co-Benefits of Air Quality and Climate Change Mitigation Strategies  
Recent contributions of the Task Force on Integrated Assessment Modelling (TFIAM) and the Task Force on Techno-Economic Issues (TFTEI) to the review of the Gothenburg Protocol in the frame of prioritising actions to reduce PM and BC emissions [22; 198; 199] underline that BC emission factors are still uncertain, and future research might come to change their results slightly. According to current legislation, there will be PM\({}_{2.5}\) and BC emission reductions, through absolute co-benefit technologies, of 73% and 79% between 2020 and 2030 in regions of the EU27 + Norway + Switzerland + United Kingdom. In the ECCCA region, these figures are 36% and 64%, respectively; and in the non-EU-Balkan + Turkey region, 39% and 73%, respectively. In the EU, co-benefit technologies contribute to 2% of the emission reduction for both PM\({}_{2.5}\) and BC and are engine exhaust technologies in diesel engines and high-grade coal in stoves. To improve communication and start developing a common understanding on BC, there is a clear need to develop simple metrics for BC, and establishing a \"BC Footprint\" concept, as proposed by a recent study [200], could be a first step. A BC Footprint would allow the comparison of different BC emission sources and levels of atmospheric BC concentrations, and would enable more efficient communication regarding the climate, health, and air quality impacts of BC. A recent analysis [201] confirms that for mitigation to be rapidly effective at reducing the rate of global warming, targeting BC emissions (where they are not co-emitted, e.g., with SO\({}_{2}\)) would be efficient but with a low final payoff. However, in a recent study [202], it was shown that the decline in surface air temperatures with reduced BC emissions is weaker than would be expected from the magnitude of its instantaneous radiative forcing at the top of the atmosphere. To explore the conceivable range of future air quality and, in particular, of population exposure to PM\({}_{2.5}\), which has been associated with the most harmful health impacts, a recent study [203] develops a series of alternative emission scenarios up to 2040 at the global level. As described in this study, in addition to improved human health (SDG 3--Improve human health and well-being), particularly in urban areas (SDG 11--Sustainable cities and communities), a clean-air scenario would deliver a host of co-benefits in other policy areas through several pathways. For mitigating climate change (SDG 13--climate action), some of the measures will not only reduce emissions of PM\({}_{2.5}\) precursors but will simultaneously reduce emissions that contribute to temperature increase. In particular, CO\({}_{2}\) emissions of the clean-air scenario will be approximately 40% lower than in the reference case in 2040, CH\({}_{4}\) 33%, and particularly BC by 90%. Another recent study [204] contributed with an integrated assessment model-based scenario analysis of BC-focused mitigation strategies aimed at maximising air quality and climate benefits. The impacts of these policy strategies have been examined under different socioeconomic conditions, climate ambitions, and BC mitigation strategies. The study finds that measures targeting BC emissions (including reduction of co-emitted OC, sulphur dioxide, and nitrogen dioxides) result in a significant decline in premature mortality due to ambient air pollution, in the order of 4 to 12 million avoided deaths between 2015 and 2030. Under certain circumstances, BC mitigation can also reduce climate change, i.e., mainly by lowering BC emissions in the residential sector and in high-BC-emission scenarios. Still, the effect of BC mitigation on global mean temperature is found to be modest at best (with a maximum short-term global mean temperature decrease of 0.02 \({}^{\circ}\)C in 2030) and could even lead to warming (with a maximum increase of 0.05 \({}^{\circ}\)C in case of a health-focused strategy, where all aerosols are strongly reduced).