Trends in on-road transportation, energy, and emissions

Introduction

The 2018 Critical Review (CR) (Frey 2018) provided a comprehensive summary of motor vehicle technologies, fuels, emissions, and emission controls. It demonstrated how these have changed over past decades and projected how technology innovations and regulations related to emissions and efficiency are driving the changes. Herein, expert discussants and the CR author provide additional perspectives and information on the topic. The appearance of these discussants as co-authors does not necessarily indicate their agreement with the opinions of other discussants. Transportation has changed several times over past 120 years. Early gasoline engines competed with steam-powered and electric means of locomotion. There were experiments with gas turbines in the 1950s, with diesel-powered passenger cars gaining ground in the 1980s in response to oil embargos and fuel shortages. Natural gas, propane, ethanol, and hydrogen also are being used as fuels. Electric vehicles (EVs) recharged with grid power will continually reduce air pollutant and greenhouse gas (GHG) emissions as power generation becomes more efficient and renewable.

A manufacturer’s perspective on motor vehicle innovations for the future: comments from Susan Collet

As the CR emphasizes, the use of most on-road vehicles results in pollutant emissions with environmental consequences. Toyota’s continental U.S. ozone (O₃) source apportionment study (Collet et al. 2018) applied the Comprehensive Air Quality Model with Extensions (CAMx) with Ozone Source Apportionment Technology (OSAT), and the Community Multi-Scale Air Quality (CMAQ) Integrated Source Apportionment Method (ISAM) to determine relative contributions from different sources. Emissions included non-U.S., natural, global boundary conditions, point sources, other area sources, off-road mobile, and on-road mobile sources. On-road emissions were further divided into light-duty gasoline vehicles (LDGV), heavy-duty diesel vehicles (HDDV), and other on-road mobile categories. Base-year 2011 simulations were evaluated against ambient measurements to demonstrate model performance, and then projections for 2030 O₃ design values were estimated for each U.S. state. Both CAMx and CMAQ estimated similar design values. With projected 2030 emissions, the models predict that most states will attain or be close to attaining the 70 ppb O₃ National Ambient Air Quality Standard (NAAQS). However, California is predicted to reach 135% of the NAAQS. The 2030 LDGV emissions are predicted to contribute <5% of the O₃. Boundary conditions, representing emissions that are unmanageable, contribute up to 70% of O₃ in the eastern United States and up to 90% of O₃ in the western U.S. In the eastern U.S., stationary sources contribute up to 35% and off-road mobile emissions contribute up to 30% of O₃. These projections indicate that further O₃ reductions will require efforts that go beyond reducing LDGV exhaust emissions.

The Toyota Motors (2018) Environmental Challenge for the year 2050 intends to eliminate its environmental footprint. Toyota produces ~10 million vehicles per year globally, and an increasing fraction of this total includes EVs, an essential step to reduce carbon dioxide (CO₂) emissions. Vehicle electrification in the form of hybrid electric vehicles (HEVs), plug-in hybrids (PHEVs), battery electric vehicles (BEVs), and fuel cell vehicles will gain importance, especially beyond the year 2020. Toyota recently announced a goal of 5.5 million EVs by the year 2030. A dedicated or electrified option for all models will be available by 2025, and more than 10 battery electric models will be introduced in the 2020s, first in China, followed by Japan, India, the U.S., and the European Union (EU).

One of the major hurdles is producing larger capacity batteries (Andwari et al. 2017; Zubi et al. 2018), with an increase from the current HEV battery capacity of 0.75 kilowatt-hours (kWh) to the ~40-kWh capacity needed for a sufficient driving range. Toyota has a collaboration with Panasonic Corporation to develop and manufacture globally competitive batteries that have high energy densities, high supply capacities, and reasonable costs. Vehicle manufacturers, electric utilities, and government agencies are readying the social infrastructure that will support EVs, such as provision of charging stations, battery reuse and recycling, and recovery of rare earth metals (Kim et al. 2016; Sarkar, Sarkar, and Bharadwaj 2018). Recycling and reuse actions currently applied to HEVs can be expanded to include BEVs.

Fuel cells offer another zero emissions alternative, depending on how the hydrogen is produced. Toyota’s Mirai (Consumer Reports 2017) fuel cell vehicle, with ~3000 units sold or leased in California, refuels in 3 to 5 minutes, and has a ~500-km range. There are 35 hydrogen stations open in California, with more in various stages of construction. A zero-emission class 8 proof-of-concept truck has completed more than 6,400 successful development kilometers while progressively pulling drayage-rated cargo weight and emitting nothing but water vapor.

A mixture of solutions is needed to meet the 2050 challenge. The concepts mentioned here are steps toward ever-better vehicles that enrich the lives of communities and are contributing to a sustainable society.

Importance of engine exhaust for urban pollution and global warming: Comments from Michael P. Walsh

Because there are so many vehicles on the world’s highways—more than 1.3 billion cars, trucks, and buses—and because more are produced each year, approaching 100 million during 2018, vehicles remain important contributors to urban, regional, and global pollution levels (Walsh 2018). In some cities such as Beijing, China, vehicles are large particulate matter (PM) contributors (China Daily 2018). The urban air pollution problem is most severe in the developing world where used vehicles are frequently dumped to live out their highly polluting second lives.

More than 110 million people in the U.S. live in areas that exceed NAAQS (U.S.EPA 2018). Many European cities have been unable to achieve nitrogen dioxide (NO₂) and PM air quality guidelines. In rapidly industrializing countries such as India and China, cities exceed World Health Organization PM guidelines by factors of 10 or more (WorldAtlas 2018). PM levels in Delhi, India, have exceeded the top limits on the measurement systems. In many African countries, air quality levels are not known because no ambient measurements are recorded; satellite data, however, indicate high concentrations of PM and other pollutants. Smoke belching from buses and trucks is a clear indication of high pollutant emissions.

The most serious air pollution concern is global climate change. Atmospheric CO₂ levels are exceeding 400 ppm, amounts that have not been seen in at least 650,000 years (NASA 2018). The U.S. Fourth National Climate Assessment (GCRP 2018) shows that:

• Average surface air temperatures have increased by 1.8ºF since 1901.

• 2015–2017 was the warmest period on record.

• Human activity is the dominant cause of observed warming.

• Global sea level has risen 18-20 centimeters since 1900.

• Ocean temperatures have increased, glaciers have melted, snow cover is diminished, sea ice is shrinking, and oceans are acidifying.

• Wildfires in the western U.S. and Alaska are increasing.

Driven in part by the more than a 50-year effort by U.S. regulators, and especially those in California, as well as the regulated manufacturers, technology exists and is being incorporated into new vehicles to reduce emissions of carbon monoxide (CO), hydrocarbon (HC), oxides of nitrogen (NO_x), PM, and other toxic substances from cars, trucks, buses, and many non-road vehicles by more than an order of magnitude compared to pre-controlled vehicles. New cars and trucks sold in China and India are becoming almost as clean. The majority of gasoline-fueled cars produced this year will be equipped with a catalytic converter, a technology first introduced in the U.S. in 1975 that dramatically lowers pollutant emissions.

Reductions are also being made for CO₂, vehicle-related halocarbons (Andersen, Halberstadt, and Borgford-Parnell 2013; Eklund et al. 2013), and short-lived climate forcers such as black carbon (BC). Unfortunately, the current pace of progress is not sufficient to attain the targets specified in the Paris Agreement (UNFCCC 2018), which are necessary to contain global warming within 2°C. If the recent proposal (DOT and U.S.EPA 2018) to relax current Corporate Average Fuel Economy (CAFÉ) standards goes into effect, and especially if it prevents California from continuing its global leadership, the global shortfall in climate-forcing emission reductions will be that much greater.

As Volkswagen’s use of defeat devices shows (Muncrief, German, and Schultz 2016), tight vehicle standards certified by laboratory testing don’t necessarily translate into comparable vehicle emissions under real-world driving conditions. Dieselgate, as the discovery of the defeat device is sometimes called, is currently leading to a declining market share of diesels across Europe, which may negatively affect CO₂ emissions.

Converting the entire global fleet to less polluting technologies presents many challenges, especially with regard to cost, supply, and charging infrastructure. Since China is now the largest new vehicle market, its aggressive New Energy Vehicle program (ICCT 2018) has led to major investments in these advanced technologies by all the major vehicle manufacturers. This increases the potential to bring about a paradigm shift away from the internal combustion (IC) engine. If the global sense of urgency regarding climate change persists, the pressure to move away from fossil fuels and IC engines will increase.

Transportation electrification, planning, automation, and emissions enforcement: Comments from Alberto Ayala

Transportation electrification, transportation planning and sustainable land use, the emergence of connected, autonomous, shared, and electric vehicles (CASEVs), and the need for robust compliance with and enforcement of fuel economy and emission standards merit further discussion. Despite being a small share of the total vehicle fleet, the light-duty EV market has grown past early adopters into a broader segment owing to falling costs of batteries, fuel cells, and related technologies. Cost parity between EV and IC vehicles is only 6 to 8 years in the future (Liebreich 2016). EVs have many advantages over ICs, and they are a no-compromise and cheat-proof alternative with respect to emissions. There is no need for sequencing the roll-out of EVs before a full green, clean electric grid is a reality. Even in regions where EV electrons may be coal based, there are still environmental benefits. EVs eliminate the risk of near-source human exposure to IC toxics. EVs also lessen the urban heat island effect (Li et al. 2015) and hold promise for new value streams from emerging vehicle-to-grid and other integrated resource management opportunities. Many of these developments are also occurring in the heavy-duty vehicle and equipment sector. EV markets in China, Japan, Europe, and various U.S. states, including California, serve as evidence of the slow, but steady progress toward a mass EV market. Political leaders from France, Great Britain, and China are speaking openly about a diesel car ban, while others speak more broadly of IC bans. In California, there is a legislative push for banning IC vehicle sales after 2040 (Ting et al. 2018).

The Sustainable Communities and Climate Protection Act of 2008 (California Legislature 2008) mandates the integration of land use, transportation planning at the local level, and environmental review to reduce GHG emissions (Nealer, Reichmuth, and Anair 2015). Targets based on per-capita GHG emission reductions encourage transportation mode shifts and innovation to improve mobility along with cleaner transportation. As more of these smart growth and sustainable development ideas spread across the world, the impacts on the expected trends for global transportation energy can be significant and positive.

To ensure this outcome is closer to the “heaven” described by Sperling (2018), new regulations, city ordinances, and regional incentives can work together to promote CASEVs. These could involve CASEV-only zones, links to transit, beneficial road charges, and congestion mitigation using priority drop off areas. The alternative is a “hell” scenario of more congestion, growth in kilometers traveled, and minimal integration with public transit (Fulton 2018). The decision for policymakers is either to stand on the sidelines and let the transformation go by or to proactively guide it toward the desired, environmentally preferred outcome.

The degree to which real-world vehicle emissions attain the applicable standards will have a definitive impact on the trends noted in the CR. Since IC vehicles will be in use for many decades, strong compliance and enforcement are needed to ensure the integrity of existing standards and certification requirements. Three years after recognition of the Volkswagen diesel defeat device (Ayala 2018), new and very useful approaches are emerging. The European Union’s option for third-party in-use emission testing using Portable Emission Measurement Systems (PEMS) under the Euro 6 Real-world Driving Emissions (RDE) requirements is one of the best ideas for moving forward. Represented by an inverted pyramid, new screening methods and selection of test vehicles will involve approaches like remote sensing or “socializing” the use of PEMS. This should allow for the screening of thousands of on-road vehicles. This screening would identify vehicles for in-depth testing in the emissions laboratory. The objective is to improve the robustness of the existing California program, which yielded the discovery and resolution of environmental violations so well publicized to date.

Defeat devices are not new: Comments from Samuel L. Altshuler

The Volkswagen NO_x defeat device situation is similar to that of HDDV engine manufacturers that took place in the late 1990s (U.S.EPA 1998), as noted in the CR. The manufacturers programmed their engines to recognize the test cycle and then degrade engine performance to attain the NO_x emission standard. After being caught, the seven manufacturers entered into a consent decree (Kennedy 1999) that involved an $83 million fine and a more than $1 billion investment to remove the defeat on more than 1 million engines. It is apparent that NO_x emission control devices can be turned on and off. Diesel NO_x emissions could be controlled by satellite location (using GPS technology), season, air quality/weather, and/or time of day. Urban-scale ambient O₃ and NO_x measurements could be used to switch diesel NO_x controls on or off. Since history seems to repeat itself, there is a need for continuous real-world emission measurements to complement the laboratory-based compliance tests.

In the 1970s the electric utilities and copper smelters advocated curtailment of sulfur dioxide (SO₂) emissions by reducing production based on meteorology and measured ground-level concentrations, termed “intermittent control” (Taylor, Rubin, and Hounshell 2005). This approach was soon abandoned with recognition of the long-range effects of high SO₂ emissions and their reaction products on ecosystems, visibility, and human health (Hidy 1984; Pope and Dockery 2006; Prinz 1985; Vedal 1997; Watson 2002). However, other forms of intermittent control have evolved; the Reid Vapor Pressure (RVP) in gasoline changes twice a year from winter to summer and vice versa. Diesel fuels also have summer and winter blends. The San Francisco Bay Area’s Spare The Air program (BAAQMD 2018) is another form of intermittent control that beseeches residents to curtail driving and wood burning during forecasts of poor air quality.

Several related topics would benefit from elaboration in future reviews. The California experience with EVs, HEVs, renewable natural gas (RNG), and hydrogen merits more attention. Natural gas has been successfully used as a vehicle fuel with an octane rating of ~130 in a gasoline direct injection (GDI) engine. The Westport (2018) dual fuel high pressure direct injection (HPDI) engine combines the attributes of clean natural gas combustion combined with diesel engine efficiency. Integrating vehicle batteries into the grid for energy storage could alleviate the intermittent nature of wind and solar electricity (Coignard et al. 2018).

Emerging technologies will enhance near-road monitoring: Comments from Eric D. Stevenson

Emission contributions on and near roadways exceed those measured at typical compliance stations. Advances in monitoring technologies (Hidy et al. 2017; Kleinman et al. 2017) may permit these measurements to estimate exposures for people who spend time in these microenvironments. EPA’s (2012; 2017) near-roadway monitoring requirements provide a means to “truth-test” exposure models. Tunnel studies yield insight into exhaust emissions and document improvements in emission control technologies (Ban-Weiss et al. 2008; Cui et al. 2018; Gaga et al. 2018; Ho et al. 2009; Kean, Harley, and Kendall 2003; Kirchstetter et al. 1996; Wang et al. 2018; Worton et al. 2014). Cross-plume remote sensing (Bishop et al. 2012; Bishop, Stedman, and Ashbaugh 1996; Bishop, Stedman, and Jessop 1992; Fujita et al. 2012) and satellite detection from space (Hidy et al. 2009; Hoff and Christopher 2009) have also increased knowledge of the near-roadway environment, the effectiveness of emission controls, and emission decreases over time.

While fixed-site compliance monitoring generates excellent temporal information, it lacks the spatial resolution needed to broaden understanding of microenvironmental exposures. Mobile monitoring can provide the spatial coverage, but it does not address temporal variations that occur as traffic and regional contributions change. Commuters and near-road residents are exposed to higher levels of HCs, NO₂, PM₁₀ and PM_2.5 (PM with aerodynamic diameter <10 µm and <2.5 µm), BC, and particle number (PN) concentrations than are measured at neighborhood- and urban-scale compliance stations. As discussed in the following, PN and BC emissions are becoming a larger concern for GDI and diesel engines. More highly time and spatially resolved measurements are needed in near-roadway environments that are accurate, precise, and valid. These measurements need to be acquired for long-enough periods to estimate chronic exposures and their associated health outcomes. California’s Assembly Bill 617 (Garcia 2017) is a step in the right direction, if it is carried out with forethought and consideration of these issues.

In addition to remote sensing techniques mentioned in the preceding, in situ mass spectrometry (Canagaratna et al. 2010; Williams et al. 2014; Zhao et al. 2013) that provides a wide spectrum of gas and particle measurements will increase understanding of the compositional changes in near-roadway environments. Satellite measurements with wide compositional, somewhat time-resolved and finer spatial scales may also provide needed information. Lower cost sensors (SCAQMD 2018) that allow greater spatial resolution may also be of value and can be incorporated on vehicles themselves. All of these techniques need to be further improved and implemented to better assess the health implications of changing vehicle emissions.

The human health perspective: Comments from Rashid Shaikh

The Global Burden of Disease (GBD) project (IHME 2018) provides a systematic review of the effects of exposure to outdoor air pollution, which has been consistently among top-ranked global risk factors. According to the 2016 estimates, outdoor PM_2.5 exposure contributed to more than 4 million premature deaths (7.5% of all deaths) and more than 100 million years-of-healthy-life lost. Although there are many uncertainties, the GBD estimates are the best available assessments of mortality and disability from major diseases, injuries, and risk factors. A case study of source-specific estimates for China highlights some important features of these estimates. Under four different energy efficiency and air pollution control scenarios between now and the year 2030, population-weighted mean exposure to PM_2.5 is projected to decrease. However, the overall health burden is projected to increase under even the most stringent policy scenarios. This is a consequence of an aging Chinese population and disease patterns, and it argues for more aggressive strategies to reduce emissions.

Although U.S. pollution levels have decreased, can human health improvements related to these improvements be quantified? Evaluating the extent to which air quality interventions improve public health is the field of accountability research (HEI 2018). Air pollution policies should reduce pollutant emissions, resulting in air quality improvements. This would, in turn, reduce human exposure and lead to better health of the affected populations. Establishing a reliable association between policies and health effects is challenging. Regulatory changes almost always overlap with other changes and trends—such as economic activity, meteorology, access to health care, smoking prevalence, other regulations, and residential mobility. Some studies have examined these relationships (Gilliland et al. 2017).

Recent investigations are showing adverse health effects in Medicare recipients detected below the current NAAQS, particularly for PM_2.5 and O₃ (Di et al. 2017; Makar et al. 2017). The association between mortality and PM_2.5 exposures was stronger for males than for females and also stronger for minority groups. The association was weaker for exposure to O₃ and the confidence intervals were much wider; there were also some inexplicable “negative” associations for Hispanics and Asians, suggesting a “protective” effect of O₃ exposure. There are important uncertainties with these analyses, particularly since smoking data were not available, nor was information on many other confounders, in the Medicare cohort. Since Medicare data are publicly available, the results of these studies are subject to replication by other investigators.

The U.S. and other developed countries have reduced air pollution from mobile and other sources, even with increases in the number of vehicles and distances driven. The current challenge is to meet increases in mobility demand around the world, particularly in developing countries. Transportation technologies must continue to reduce emissions, particularly of GHGs, and to build the technology and markets for EVs.

Particle number and black carbon measurements for vehicle emissions: Comments from Judith C. Chow and John G. Watson

The measurement and standardization of PN and BC emissions merits greater elaboration. Excessive PN exposures create public health hazards (Biswas and Wu 2005; Chow et al. 2005). BC is a recognized indicator of adverse health effects (Eklund et al. 2014; Grahame, Klemm, and Schlesinger 2014) and contributes to global warming (Fiore, Naik, and Leibensperger 2015; Kleinman et al. 2015).

The Euro 6 light-duty gasoline engine emission standard (DieselNet 2018) limits solid particle number (SPN) emissions to 6 × 10¹² particles/km for engines with model years 2014–2016, which is reduced to 6 × 10¹¹ particles/km for model year 2017 and beyond. As noted in the CR, SPN is limited to solid particles with electrical mobility diameters >23 µm that have been heated to 300–400°C (UN/ECE 2015), thereby removing potentially toxic organic compounds that are adsorbed onto what is often a BC core and neglecting the much smaller particles that can penetrate from the lung and into the blood stream. Emission standards have not yet been established for BC, although opacity tests are administered as part of some states’ diesel inspection and maintenance programs (Chow et al. 2001; Lloyd and Cackette 2001; McCormick et al. 2003).

The CR recognizes that the market penetration of GDI engines is important for both PN and BC emitted to the environment. Although GDI engines have lower CO₂ emissions per distance traveled than conventional port fuel injection (PFI) engines (Alkidas 2007), they tend to emit higher PN, BC, and PM mass concentrations due to incomplete fuel volatilization, partially in fuel-rich zones, and impingement of fuel on piston and cylinder surfaces (An et al. 2016; He, Ratcliff, and Zig 2012; Jiao and Reitz 2015; Karjalainen et al. 2014; Maricq et al. 1999). PM mass from a GDI engine appears to have a large BC component (Maricq, Szente, and Jahr 2012). Depending on engine operating conditions, particles may consist of various amounts of amorphous carbon, semivolatile organic compounds (SVOCs), and ash contents (Karjalainen et al. 2014; Momenimovahed et al. 2015; Momenimovahed and Olfert 2015; Zelenyuk et al. 2016). The majority of PNs are smaller than 100 nm (Amanatidis et al. 2017; Chen et al. 2017; Hassaneen, Samuel, and Whelan 2011; Myung and Park 2012; Myung et al. 2012; Myung, Ko, and Park 2014; Tan et al. 2016; Yamada, Funato, and Sakurai 2015). Wang et al. (2014) illustrate PN and PM mass distributions (derived from the PN assuming spherical particles and densities) from a GDI engine with different cylinder and fuel injection pressures. They show that higher fuel injection pressures reduce BC formation in the larger particle mode, but increase the PN concentrations in the smaller particles. It appears that there is a trade-off between PN and BC concentrations as a function of injection pressure for these more fuel-efficient engines.

Dwyer et al. (2010) further illustrate the effects of volatility of and variability on PN emissions, with emission factors differing by factors of 3 or more depending on the driving cycle and the vehicle tested. Up to 50% of the PN disappeared at temperatures exceeding 200°C. Although these temperatures are not achieved in ambient air, SVOCs in this region can still slowly evaporate as the gas-phase concentrations of these compounds decrease with dilution and aging in ambient air. The volatility for a large fraction of the PN makes it difficult to regulate this indicator in source emissions or ambient air. In their efforts to obtain a more “reproducible” measurement, the European Particle Measurement Program (PMP) method (Giechaskiel et al. 2008; Johnson et al. 2009; Wang et al. 2010) does not obtain a “real-world” PN measurement of what people will inhale. A more realistic estimate of PN emissions to ambient air would facilitate understanding of ultrafine aerosol evolution with distance from the point of emission.

Consistent measurements for BC show greater promise for establishing emission standards. Kamboures et al. (2013) demonstrate high and consistent correlations among different BC measurement methods, even when applied to very low emitting engines. Their conclusions are that “BC measurements in the sub 1 mg/mi range are possible and current PFI and GDI vehicles appear to emit BC at these very low levels.”

The final word: Comments from H. Christopher Frey

The CR focused primarily on empirical evidence for U.S. national and international trends in on-road transportation energy use and emissions, with additional consideration of factors affecting travel demand and vehicle operation, measurement methods, and impacts of emissions on exposure and health. However, given the wide-ranging scope of the subject, it was not possible to treat all topics in the CR in great depth, and some topics were beyond the scope. The discussants have appropriately identified numerous topics that merit further elaboration. On-road transportation and the energy it involves are the subject of much technological change, leading to changing adverse effects of mobile source emissions on the environment. The CR and this discussion set more of a marker in time than a definitive endpoint for this topic, and it is hoped that future contributors to the journal will build on these articles.

Additional information

Notes on contributors

Samuel L. Altshuler

Samuel L. Altshuler is an air quality consultant and current chair of the Critical Review Committee.

Alberto Ayala

Alberto Ayala is air pollution control officer and Executive Director at the Sacramento Metropolitan Air Quality Management District.

Susan Collet

Susan Collet is an executive engineer at Toyota Motors North America, Inc.

Judith C. Chow

Judith C. Chow is a research professor at the Desert Research Institute, past chair of the Critical Review Committee, and author of the 1995 Critical Review.

H. Christopher Frey

H. Christopher Frey is the Glenn E. Futrell Distinguished University Professor of Environmental Engineering at North Carolina State University and author of the 2018 Critical Review.

Rashid Shaikh

Rashid Shaikh is Director of Science at the Health Effects Institute.

Eric D. Stevenson

Eric D. Stevenson is Director of the Meteorology and Measurements Division at the Bay Area Air Quality Management District and vice-chair of the Critical Review Committee.

Michael P. Walsh

Michael P. Walsh is a consultant on clean transportation.

John G. Watson

John G. Watson is a research professor at the Desert Research Institute, past chair of the Critical Review Committee, and author of the 2002 Critical Review.