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Bus Transit

Based on simulation case studies for both Finland and California, the life cycle CO, emissions of transit buses, based on the fuel cycle and vehicle operation, per vehicle mile are lowest for batteiy electric buses, approximately similar for diesel hybrid and hydrogen fuel cell buses, and highest for conventional diesel and CNG buses. Electric buses may be cost-competitive with diesel buses within 10 to 20 years, whereas fuel cell buses are likely to continue to be more expensive than diesel buses. Buses that can take advantage of opportunity charging at bus stops or bus terminals could har e smaller batteries, and lower costs, than buses that rely on overnight charging for a frill day of driving range (Lajimeu and Lipman,


A comparison of transit bus GHG emissions along a corridor found that operational emissions contribute the largest share of total life cycle emissions, and that life cycle emissions increased in order for hybrid, compressed natural gas, biodiesel, and conventional diesel transit buses (Chan et al., 2013). Comparisons of transit bus options should take into account local driving cycles, terrain, electricity generation mix. and meteorology (Xu et al., 2015).


So-called “eco-driving” is a technique, typically based on reducing the rate of acceleration among other strategies, for reducing vehicle energy consumption for a given trip. However, in practice, eco- driving appears to be of limited effectiveness. There have been numerous studies that har e evaluated the effect of есо-drivmg training on bus drivers. A Calgary-based municipal fleet trained drivers to reduce idling, which was reduced by 4% to 10% per vehicle per day (Rutty et al., 2013). In Sweden, a control group, a group who received in-vehicle feedback, and a group who received feedback plus training were compared. The latter two groups reduced frequency of high acceleration and high speed and adher ed 6.8% fuel savings (Stromberg and Karlsson. 2013). A study focusing on private car owners found that eco-driving training led to city and highway fuel consumption decreases of 4.6% and 2.9%, respectively, but that these reductions dropped to 2.5% and 0%, respectively, 10 months later (Barla et al., 2017).

Air Conditioning

Even though ah conditioning (AC) is the largest individual auxiliary load in a typical passenger vehicle, few studies have examined the effect of AC operation on vehicle fuel use and emission rates. Data used to model the effect of ah conditioning on exhaust emissions in the МОЛ'ЕЗ model was collected in

1997 and 1998 (EPA, 2015) and, thus, is out of date. Vehicles have typically been using HFC-134a as the refrigerant since the early 1990s. Leakage rates for refrigerants are typically estimated to be approximately 5% of the refrigerant charge. Uncertainty in the CFC-12 emissions from vehicles from Flong Kong was estimated to be ±29%, and uncertainties for HFC-134a emissions were reported to be similar (Yan et al., 2014). Under current U.S. fuel economy and GHG emission standards, manufacturers are given AC leakage credits, counted toward their corporate average GHG emissions, for improvements to AC systems based on О-rings, seals, valves, and fittings, and substitution of lower global warming potential (GWP) refrigerants. For example, HFO-1234yf, first introduced in 42,384 model year 2013 vehicles, has a GWP of 4 compared to 1,430 for HCF-134a. In 2016, 2.2 million new vehicles were produced using HFO-1234yf (EPA, 2018b).


Rail tends to be among the most energy efficient of transportation modes for both passenger and freight sendees. Improved operational practices, such as chokepoint relief and idle reduction, could reduce energy consumption and CO, emissions (USDOT, 2010). Infrastructure and technologies that make rail more attractive than more energy intensive transport modes, such as high-speed rail versus aircraft, could lead to inter-modal substitutions that lead to net GHG emission reductions. Methods for reducing the carbon intensity of train operations include improved aerodynamics, weight reduction, more efficient power electronics, regenerative braking, and electrification. For diesel rail, the use of drop-in biofuels can reduce carbon intensity of train operations. Hydrogen fuel cell power trains have been proposed for locomotives (Sims et al.. 2014).

Electrification of existing long-distance rail sen-ices is prohibitively expensive. Possible alternatives to reducing CO, emissions from rail transport in such cases is the substitution of biofuels for diesel fuels, hybridization, or a transition to hydrogen-fueled locomotives (Hall et al.. 2018: Graver and Frey, 2016). However, the current hydrogen fuel cycle, which is mostly based on steam reforming of natural gas, is not favorable with regard to GHG emission reductions.


International shipping by marine vessels is estimated to have contributed 812 million metric tons of CO, emissions globally in 2015 (Olmer et al., 2017). Marine emissions are expected to increase from approximately 10% of transport related emissions in 2018 to 20% by 2060 (Hall et al., 2018). GHG emissions from maritime bunker fuels sold in Europe have increased since the 2008 global economic recession, reaching 147 Mt CO, in 2016. Although 10 percent below 2005 levels, these emissions would har e to be reduced by 34% to meet the European 2050 target (EEA, 2018).

Over a short period from 2013 to 2015, the world fleet of ships used internationally grew by 1.5%, the amount of shipping in deadweight tons per nautical mile increased by 7%, and main engine power increased by 6% to 10%, especially for tankers, general cargo ships, and container ships. Fuel efficiency has increased slightly for some ship classes. For example, general cargo ship CO, intensity decreased by 5%. However, total emissions of general cargo ships increased by 9% because of increased transport supply. Although on average ship speeds did not change, the cruising speed of tankers of greater than

200,000 deadweight tons and of containers ships with more than 14,500 Twenty-foot Equivalent Units (TEU) increased by 4% and 11%, respectively. Faster cruising speed is associated with higher emission rates. In addition to emitting CO,, approximately 21% of the total CO,-equivalent emissions from ships includes BC (Olmer et al., 2017).

According to an estimate, CO, emissions from marine vessel activity centered on China is expected to increase by 8.3% per year from a baseline of 2007 to 2035. Bulk carrier, container, and tanker vessels are expected to comprise over 90% of these emissions. These increases could be partly mitigated by reduction in transport demand, reduction in energy intensity of transport, and substitution of lower carbon energy sources. A key step toward implementing policies would be regulations for monitoring, reporting, and verifying vessel activity in territorial waters. Possible policy options for reductions in marine emissions could include a maritime carbon tax or a cap-and-trade scheme (Yang et al., 2017).

Battery-electric vessels, including femes in several European countries, have recently been introduced on a limited basis. For sliort-distance routes, battery-electric and hydrogen-fueled vessels may be feasible and could offer substantial GHG emissions reductions depending on the upstream energy cycle (Hall et al., 2018).


The estimated GHG emissions from aviation har e approximately doubled from 1980 to 2012. There were nearly 20,000 commercial passenger aircraft in 2010. By 2050, the number of aircraft is expected to reach nearly 70,000. with GHG emissions approximately tripling compared to 2010 (Kliarina et al.. 2016). In Europe, GHG emissions from international aviation increased at an average rate of over 2% per year from 2013 to 2017 (EEA, 2018).

In 2016, the International Civil Aviation Organization (ICAO) completed a CO,, or fuel efficiency, standard for aircraft that individual countries are expected to adopt by 2020. The standard is based on adhering carbon-neutral growth. While there are many potential technologies that could improve aircraft fuel efficiency, some are mutually exclusive. General categories of fuel-saving technologies include: (a) materials, such as advanced composites, that reduce weight; (b) fuselage, such as low-friction coatings and riblets; (c) modifications to control surfaces, such as wingtip devices and variable camber; (d) laminar flow; (e) changes to the external surfaces of the engine such as the nacelle and nozzles; and (f) internal changes to the engine such as higher pressure ratio, higher firing temperature, and others. Although some of these types of features exist in some form on current production aircraft, such as wingtip devices, improvements in all of these could further improve fuel efficiency compared to current production aircraft. The deployment of cost-effective packages of compatible technologies could reduce fuel consumption by as much as 25% for new aircraft in 2024 and 40% for new aircraft in 2034. These potential reductions are much greater than the more modest reductions of current new aircraft designs. The aircraft manufacturing industry typically prefers evolutionary designs rather than “clean sheet” designs that would lead to, for example, entirely new airframes. Other potential options to reduce CO, emissions from aircraft include biofuels, operational practices, and improved air traffic control (Kharina etal., 2016).

Perhaps the most feasible approach to decarbonizing aircraft operation is to deploy low-carbon jet fuels. The key barriers to wide-scale deployment of such fuels is the lack of feedstock supply chain and adequate capacity of an advanced fuel industry. In the short-term, low-carbon fuel policy might be better aimed at on road transportation which would facilitate longer-term adoption in the aviation sector. For example, the development of policy, infrastructure, and capacity to produce biofuels for other transportation modes, such as on road vehicles, would enable evolutionary changes to provide biofuels for aircraft. Policy options that would address other transport modes would enable decarbonization of aircraft operation including investments in sustainable biomass followed by a longer-term increasing share of advanced fuels used in aviation (Searle et al., 2019).

Aircraft are unique among transportation modes in that then emissions at 8 km to 13 km altitude can create aircraft produced condensation trail (“contrail”) line-shaped ice clouds that enhance climate change. Such clouds form in part because of condensable vapors produced by aircraft engine, including water vapor, sulfuric acid, nitric acid, and low volatility hydrocarbons. Ultrafine soot particles produced in aircraft engine exhaust can also serve as condensation nuclei. However, the radiative forcing implications of these aircraft induced clouds is not well quantified, because their effect has to be disentangled from that of naturally occurring cunts clouds. Simulation results indicate that contrails that do not retain their linear shape, known as contrail emus clouds, have substantially more potential climate impact than the linear contrails. Possible techniques to mitigate aircraft induced cloud formation in the short term include lean combustion technology and alternative fuels, such as biofuels with lower sulfur and lower aromatic species content. Longer term mitigation could include, for example, adoption of engine technology that would use fuels such as liquid hydrogen or liquefied natural gas, which would eliminate or reduce soot and sulfur emissions, coupled with more aerodynamic airframes, such as blended wing body technology, which would reduce the amount of energy consumed (Karclier, 2018).

Aviation operations include the use of ground support equipment at airports. Such equipment is amenable to electrification (Hall et al., 2018).

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