Ozone. No, not the ‘good’ ozone up in the stratosphere that protects us from cosmic rays. I mean the ‘bad’ ozone down here where we breathe. And it is truly bad: it’s a bleach, which of course makes it good in cleaning swimming pools, and not so good when we inhale it. It particularly preys on the young, the old, the weak and the disadvantaged. Every year worldwide, says a recent study by the Stockholm Environment Institute, ground-level ozone causes some 1 million premature deaths. Ozone also is a significant global warmer, but it’s the inhalation issue that most concerns regulators.
So I set out to compare the common automotive fuels on their propensity to create ground-level ozone [Johnson, E., 2017. Cars and ground-level ozone: how do fuels compare? Eur. Transp. Res. Rev. 9, 47. doi:10.1007/s12544-017-0263-7]. That obviously means gasoline and diesel, also liquefied petroleum gas (LPG), which in countries such as Poland, South Korea, and Turkey is significant. And I threw in battery-electric ‘fuel’, which seems to be undergoing a groundswell, especially in China.
Comparing Different Fuel Types
As a basis for comparison, I chose the humble Ford Focus, one of the world’s most common cars, which means emissions data for it are plentiful – and for all the fuel types studied. The cars are very similar across fuel types, so this is an ‘apples to apples’ comparison. To assess the environmental impact of their emissions, I used two methods: the US Environmental Protection Agency’s ‘Tool for the Reduction and Assessment of Chemical and other environmental Impacts’ (TRACI), and the ‘ReCiPe’ method developed for The Netherlands’ Ministry of the Environment.
Battery-electric cars, of course, have no tailpipe emissions. But the power plants generating their electricity do have emissions, so we measured all ozone-precursors from ‘well to wheel’, i.e. from the extraction of resources from the environment to the emission of wastes and pollutants to the environment. We did not include the production of the car itself or its non-fuel-related emissions, but we did include a factor for battery production, which is very energy-intensive. All emissions were normalized according to the distance driven, i.e. reported as pollutants per kilometer.
Ground-level ozone is unusual among pollutants. Very little of it is produced by human activity directly. A baseline of it comes down to us from the stratosphere, circulated by the weather. The rest comes from a complex chain of atmospheric reactions triggered by sunlight and mainly involving NOx and hydrocarbons. Car exhausts used to be the main source of both, but the introduction of catalytic converters in the 1970s-80s tamped that down mightily. Tailpipes and the refined-products supply-chain are still significant generators of NOx and hydrocarbons, but today, more NOx spews out of heavy-duty engines and boilers (such as those used in power plants) and more hydrocarbons are emitted from aerosols and solvents.
So the question comes down to this: do the pollutants emitted by power plants to fuel battery-electric cars outweigh those emitted from the supply chain and tailpipe of gasoline, diesel, and LPG? As already suggested, for ozone precursors, yes – at least in the United Kingdom, the region picked for this study. This would also be the case in countries with similar or ‘dirtier’ generating stations, say, China, Germany, India, and the United States. In the base-case UK, LPG comes out best, less than half the ozone impact of the battery-electric, gasoline comes out slightly higher, and diesel significantly higher than those first two, but still well below battery-electric.
Since this peer-reviewed study was published, several readers have commented that tailpipes tend to be concentrated in urban areas and power plants in rural ones. That’s generally true, but the consequences of that are counterintuitive. You see, the countryside, especially a forested one, often has plentiful hydrocarbons emitted mostly by trees. Add in NOx from power plants and voila, it makes an ozone soup in the air. Indeed, average UK concentrations of ozone over time are highest in rural, not urban areas. So to protect the people, animals, and plants who live there, the results of this study should be instructive.
This study, Cars and ground-level ozone: how do fuels compare? was recently published in the journal European Transport Research Review.*NOTE: New DOI's are registered weekly Friday and may not function until then.