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Road Transport emissions (RTE) are a significant anthropogenic global NOx source responsible for enhancing the chemical production of tropospheric ozone (O3) in the lower troposphere. Here we analyse a multi-model ensemble which adopts the realistic SRES A1B emission scenario and a "policy-failure" scenario for RTE (A1B_HIGH) for the years 2000, 2025 and 2050. Analysing the regional trends in RTE NOx estimates shows by 2025 that differences of 0.2-0.3TgNyr-1 occur for most of the world regions between the A1B and A1B_HIGH estimates, except for Asia where there is a larger difference of ~1.4TgNyr-1. For 2050 these differences fall to ~0.1TgNyr-1, with shipping emissions becoming as important as RTE. Analysing the seasonality in near-surface O3 from the multi-model ensemble monthly mean values shows a large variability in the projected changes between different regions. For Western Europe and the Eastern US although the peak O3 mixing ratios decrease by ~10% in 2050, there is an associated degradation during wintertime due to less direct titration from nitric oxide. For regions such as Eastern China, although total anthropogenic NOx emissions are reduced from 2025 to 2050, there is no real improvement in peak O3 levels. By normalizing the seasonal ensemble means of near-surface O3 (0-500m) with the recommended European Commission (EC) exposure limit to derive an exceedence ratio (ER), we show that ER values greater than 1.0 occur across a wide area in the Northern Hemisphere for boreal summer using the year 2000 emissions. When adopting the future A1B_HIGH estimates, the Middle East exhibits the worst regional air quality, closely followed by Asia. For these regions the area of exceedence (ER>1.0) for 2025 is ~40% and ~25% of the total area of each region, respectively. Comparing simulations employing the various scenarios shows that unmitigated RTE increases the area of exceedence in the Middle East by ~6% and, for Asia, by ~2% of the total regional areas. For the USA the area of exceedence approximately doubles in 2025 as a result of unmitigated RTE, with the most exceedences occurring in the southern USA. The effects across the various regions implies that unmitigated RTE would have a detrimental effect on regional health for 2025, and potentially offset the benefits introduced by mitigating e.g. international shipping emissions. By 2050 the further mitigation of non-transport emissions results in much cleaner air meaning that mitigation of RTE is not critical for achieving the defined limits in many world regions. © 2014 Elsevier Ltd.

Road Transport emissions (RTE) are a significant anthropogenic global NOx source responsible for enhancing the chemical production of tropospheric ozone (O3) in the lower troposphere. Here we analyse a multi-model ensemble which adopts the realistic SRES A1B emission scenario and a "policy-failure" scenario for RTE (A1B_HIGH) for the years 2000, 2025 and 2050. Analysing the regional trends in RTE NOx estimates shows by 2025 that differences of 0.2-0.3TgNyr-1 occur for most of the world regions between the A1B and A1B_HIGH estimates, except for Asia where there is a larger difference of ~1.4TgNyr-1. For 2050 these differences fall to ~0.1TgNyr-1, with shipping emissions becoming as important as RTE. Analysing the seasonality in near-surface O3 from the multi-model ensemble monthly mean values shows a large variability in the projected changes between different regions. For Western Europe and the Eastern US although the peak O3 mixing ratios decrease by ~10% in 2050, there is an associated degradation during wintertime due to less direct titration from nitric oxide. For regions such as Eastern China, although total anthropogenic NOx emissions are reduced from 2025 to 2050, there is no real improvement in peak O3 levels. By normalizing the seasonal ensemble means of near-surface O3 (0-500m) with the recommended European Commission (EC) exposure limit to derive an exceedence ratio (ER), we show that ER values greater than 1.0 occur across a wide area in the Northern Hemisphere for boreal summer using the year 2000 emissions. When adopting the future A1B_HIGH estimates, the Middle East exhibits the worst regional air quality, closely followed by Asia. For these regions the area of exceedence (ER>1.0) for 2025 is ~40% and ~25% of the total area of each region, respectively. Comparing simulations employing the various scenarios shows that unmitigated RTE increases the area of exceedence in the Middle East by ~6% and, for Asia, by ~2% of the total regional areas. For the USA the area of exceedence approximately doubles in 2025 as a result of unmitigated RTE, with the most exceedences occurring in the southern USA. The effects across the various regions implies that unmitigated RTE would have a detrimental effect on regional health for 2025, and potentially offset the benefits introduced by mitigating e.g. international shipping emissions. By 2050 the further mitigation of non-transport emissions results in much cleaner air meaning that mitigation of RTE is not critical for achieving the defined limits in many world regions. © 2014 Elsevier Ltd.

The future impact of traffic emissions on atmospheric ozone and OH has been investigated separately for the three sectors AIRcraft, maritime SHIPping and ROAD traffic. To reduce uncertainties we present results from an ensemble of six different atmospheric chemistry models, each simulating the atmospheric chemical composition in a possible high emission scenario (A1B), and with emissions from each transport sector reduced by 5% to estimate sensitivities. Our results are compared with optimistic future emission scenarios (B1 and B1 ACARE), presented in a companion paper, and with the recent past (year 2000). Present-day activity indicates that anthropogenic emissions so far evolve closer to A1B than the B1 scenario. As a response to expected changes in emissions, AIR and SHIP will have increased impacts on atmospheric O3 and OH in the future while the impact of ROAD traffic will decrease substantially as a result of technological improvements. In 2050, maximum aircraft-induced O3 occurs near 80° N in the UTLS region and could reach 9 ppbv in the zonal mean during summer. Emissions from ship traffic have their largest O3 impact in the maritime boundary layer with a maximum of 6 ppbv over the North Atlantic Ocean during summer in 2050. The O3 impact of road traffic emissions in the lower troposphere peaks at 3 ppbv over the Arabian Peninsula, much lower than the impact in 2000. Radiative forcing (RF) calculations show that the net effect of AIR, SHIP and ROAD combined will change from a marginal cooling of −0.44 ± 13 mW m−2 in 2000 to a relatively strong cooling of −32 ± 9.3 (B1) or −32 ± 18 mW m−2 (A1B) in 2050, when taking into account RF due to changes in O3, CH4 and CH4-induced O3. This is caused both by the enhanced negative net RF from SHIP, which will change from −19 ± 5.3 mW m−2 in 2000 to −31 ± 4.8 (B1) or −40 ± 9 mW m−2 (A1B) in 2050, and from reduced O3 warming from ROAD, which is likely to turn from a positive net RF of 12 ± 8.5 mW m−2 in 2000 to a slightly negative net RF of −3.1 ± 2.2 (B1) or −3.1 ± 3.4 (A1B) mW m−2 in the middle of this century. The negative net RF from ROAD is temporary and induced by the strong decline in ROAD emissions prior to 2050, which only affects the methane cooling term due to the longer lifetime of CH4 compared to O3. The O3 RF from AIR in 2050 is strongly dependent on scenario and ranges from 19 ± 6.8 (B1 ACARE) to 61 ± 14 mW m−2 (A1B). There is also a considerable span in the net RF from AIR in 2050, ranging from −0.54 ± 4.6 (B1 ACARE) to 12 ± 11 (A1B) mW m−2 compared to 6.6 ± 2.2 mW m−2 in 2000.

The impact of future emissions from aviation and shipping on the atmospheric chemical composition has been estimated using an ensemble of six different atmospheric chemistry models. This study considers an optimistic emission scenario (B1) taking into account e.g. rapid introduction of clean and resource-efficient technologies, and a mitigation option for the aircraft sector (B1 ACARE), assuming further technological improvements. Results from sensitivity simulations, where emissions from each of the transport sectors were reduced by 5%, show that emissions from both aircraft and shipping will have a larger impact on atmospheric ozone and OH in near future (2025; B1) and for longer time horizons (2050; B1) compared to recent time (2000). However, the ozone and OH impact from aircraft can be reduced substantially in 2050 if the technological improvements considered in the B1 ACARE will be achieved. Shipping emissions have the largest impact in the marine boundary layer and their ozone contribution may exceed 4 ppbv (when scaling the response of the 5% emission perturbation to 100% by applying a factor 20) over the North Atlantic Ocean in the future (2050; B1) during northern summer (July). In the zonal mean, ship-induced ozone relative to the background levels may exceed 12% near the surface. Corresponding numbers for OH are 6.0 × 105 molecules cm−3 and 30%, respectively. This large impact on OH from shipping leads to a relative methane lifetime reduction of 3.92 (±0.48) on the global average in 2050 B1 (ensemble mean CH4 lifetime is 8.0 (±1.0) yr), compared to 3.68 (±0.47)% in 2000. Aircraft emissions have about 4 times higher ozone enhancement efficiency (ozone molecules enhanced relative to NOx molecules emitted) than shipping emissions, and the maximum impact is found in the UTLS region. Zonal mean aircraft-induced ozone could reach up to 5 ppbv at northern mid- and high latitudes during future summer (July 2050; B1), while the relative impact peaks during northern winter (January) with a contribution of 4.2%. Although the aviation-induced impact on OH is lower than for shipping, it still causes a reduction in the relative methane lifetime of 1.68 (±0.38)% in 2050 B1. However, for B1 ACARE the perturbation is reduced to 1.17 (±0.28)%, which is lower than the year 2000 estimate of 1.30 (±0.30)%. Based on the fully scaled perturbations we calculate net radiative forcings from the six models taking into account ozone, methane (including stratospheric water vapour), and methane-induced ozone changes. For the B1 scenario, shipping leads to a net cooling with radiative forcings of −28.0 (±5.1) and −30.8 (±4.8) mW m−2 in 2025 and 2050, respectively, due to the large impact on OH and, thereby, methane lifetime reductions. Corresponding values for the aviation sector shows a net warming effect with 3.8 (±6.1) and 1.9 (±6.3) mW m−2, respectively, but with a small net cooling of -0.6 (±4.6) mW m−2 for B1 ACARE in 2050.

The year 2000 radiative forcing (RF) due to changes in O3 and CH4 (and the CH4-induced stratospheric water vapour) as a result of emissions of short-lived gases (oxides of nitrogen (NOx), carbon monoxide and non-methane hydrocarbons) from three transport sectors (ROAD, maritime SHIPping and AIRcraft) are calculated using results from five global atmospheric chemistry models. Using results from these models plus other published data, we quantify the uncertainties. The RF due to short-term O3 changes (i.e. as an immediate response to the emissions without allowing for the long-term CH4 changes) is positive and highest for ROAD transport (31 mW m−2) compared to SHIP (24 mW m−2) and AIR (17 mW m−2) sectors in four of the models. All five models calculate negative RF from the CH4 perturbations, with a larger impact from the SHIP sector than for ROAD and AIR. The net RF of O3 and CH4 combined (i.e. including the impact of CH4 on ozone and stratospheric water vapour) is positive for ROAD (+16(±13) (one standard deviation) mW m−2) and AIR (+6(±5) mW m−2) traffic sectors and is negative for SHIP (−18(±10) mW m−2) sector in all five models. Global Warming Potentials (GWP) and Global Temperature change Potentials (GTP) are presented for AIR NOxemissions; there is a wide spread in the results from the 5 chemistry models, and it is shown that differences in the methane response relative to the O3 response drive much of the spread.

To estimate the impact of emissions by road, aircraft and ship traffic on ozone and OH in the present-day atmosphere six different atmospheric chemistry models have been used. Based on newly developed global emission inventories for road, ship and aircraft emission data sets each model performed sensitivity simulations reducing the emissions of each transport sector by 5%. The model results indicate that on global annual average lower tropospheric ozone responds most sensitive to ship emissions (50.6%±10.9% of the total traffic induced perturbation), followed by road (36.7%±9.3%) and aircraft exhausts (12.7%±2.9%), respectively. In the northern upper troposphere between 200–300 hPa at 30–60° N the maximum impact from road and ship are 93% and 73% of the maximum effect of aircraft, respectively. The latter is 0.185 ppbv for ozone (for the 5% case) or 3.69 ppbv when scaling to 100%. On the global average the impact of road even dominates in the UTLS-region. The sensitivity of ozone formation per NOx molecule emitted is highest for aircraft exhausts. The local maximum effect of the summed traffic emissions on the ozone column predicted by the models is 0.2 DU and occurs over the northern subtropical Atlantic extending to central Europe. Below 800 hPa both ozone and OH respond most sensitively to ship emissions in the marine lower troposphere over the Atlantic. Based on the 5% perturbation the effect on ozone can exceed 0.6% close to the marine surface (global zonal mean) which is 80% of the total traffic induced ozone perturbation. In the southern hemisphere ship emissions contribute relatively strongly to the total ozone perturbation by 60%–80% throughout the year. Methane lifetime changes against OH are affected strongest by ship emissions up to 0.21 (± 0.05)%, followed by road (0.08 (±0.01)%) and air traffic (0.05 (± 0.02)%).Based on the full scale ozone and methane perturbations positive radiative forcings were calculated for road emissions (7.3±6.2 mWm−2) and for aviation (2.9±2.3 mWm−2). Ship induced methane lifetime changes dominate over the ozone forcing and therefore lead to a net negative forcing (−25.5±13.2 mWm−2).