‘Mayors have already stood up to say climate change is one of the greatest challenges we face,’ said Anne Hidalgo, mayor of Paris, France, in a recent speech. ‘Today, we also stand up to say we no longer tolerate air pollution and the health problems and deaths it causes – particularly for our most vulnerable citizens.’
Hidalgo’s comments came as she and leaders of three other major cities – Madrid, Athens and Mexico City – announced in December 2016 they would be prohibiting diesel vehicles in their streets from 2025. Once promoted as more energy efficient than gasoline, diesel fuels are now known to cause dangerous levels of air pollution, especially harmful particulate matter (PM).
In the EU, transport accounts for 25% of greenhouse gas emissions. Despite regulatory and other measures, it remains the largest contributor to nitrogen oxides (NOx) emissions and a major source of discharges of particulate matter (PM). PM and other air pollutants are linked to cardiovascular diseases, cancers and respiratory conditions, such as asthma.
However, creating ‘dual-purpose’ clean fuels that reduce both CO2 and reduce air pollution is a considerable challenge. Especially in the confined spaces of modern cities, it is more difficult to develop fuels that reduce air pollution. Recent research delving into the chemical interactions between fuel components and derivatives, both inside and outside vehicles, has highlighted the complexities.
But fuel chemicals are not the only factor contributing to air pollution. Engine design, post-combustion treatments by catalysts and filters, and external influences, such as geography, topography and atmospheric conditions, can all have a big impact on air quality.
Scientists are still struggling to pinpoint exactly which chemical reactions are the root causes of air pollution. With PM, for example, it has been difficult to identify the key precursors to the formation of particulates, especially the fine and small PM 2.5 particles, which are the most hazardous to humans. This detection work is complicated by the number of stages in the formation of PM, both within the vehicle itself and then outside, when chemicals in the exhaust fumes react with pollutants already in air. All these uncertainties pose difficulties for the developers of catalysts, filters and additives to keep fuels clean.
These difficulties are exacerbated by the stringent tests being applied by regulators, especially after the ‘dieselgate’ scandal revealed in 2015 that Volkswagen and possibly other leading car manufacturers had been using hidden software programmes to give false laboratory test results on emissions. Independent research groups discovered that road test emissions were up to 40 times higher than those in the lab tests.
To close the gap between the results of lab approval tests and real-world tests – averaging around 40% with CO2 emissions, according to studies by the International Council on Clean Transportation (ICCT) in Berlin – new vehicle models are having to undertake road trials. From September 2017, all new vehicle types in the EU will be subject to real driving emissions (RDE) tests.
Meanwhile, the political pressures for improvements to air quality are intensifying across the world. The latest figures from the World Health Organisation (WHO) database covering air-pollution monitoring programmes in 3000 cities in 103 countries showed that 80% of city dwellers live in areas with air pollution levels above WHO safety limits. Even in cities in high income countries, the proportion was 56%. Maria Neira, WHO’s public health director, says that urban air pollution is rising at an ‘alarming rate’ and ‘wreaking havoc on human health’. Worldwide, the WHO estimates air pollution causes 3m premature deaths annually.
Since the turn of the century, reductions in air pollution in the developed world have been levelling out. Particulate matter, especially PM2.5, has been increasing in some areas.
In its 2015 review of air quality,1 the EU’s European Environment Agency ( EEA) said that declines in total emissions of PM and its precursors had not been matched by equivalent falls in PM concentrations, which it blamed on cross-border flows of pollutants combining with emissions from localised sources.
Meanwhile, analysis by epidemiologists and statisticians indicate that even limited exposure to air pollution can be dangerous to health. ‘We’re interested in finding out what levels of exposure to certain pollutants have health impacts and whether even low-dosages of these pollutants could be hazardous,’ explained Maria Costantini, principal scientist at Health Effects Institute (HEI) in Boston, US, which specialises in air pollution research. ‘This is a major area of uncertainty governing decisions on new anti-pollution standards.’
The HEI is among several research bodies shifting attention to multi rather than single pollutants. The conventional approach to air pollution has been to focus on individual chemicals or compounds – such as PM, sulphur dioxide, NOx, ammonia, ozone, carbon monoxide and dioxide, benzene and certain toxic metals like arsenic, cadmium, lead and nickel. Particulate matter is split between PM10 with a diameter of 10 microns or less, and PM2.5 with a diameter of 2.5 microns or less. SO2, NOx and NH3 are considered key precursors of PM2.5, mainly during the secondary stage of particulate formation outside the vehicle.
Regulators and government environmental agencies have also been highlighting the hazards of aromatics, like polycylic aromatic hydrocarbons (PAHs) with multiple aromatic rings. In some vehicle engines, the presence of certain aromatics, such as PAHs, can lead to the formation of PM. They have been found to be major precursors of soot or black carbon (BC).
All these different chemicals, classified in regulations as single substances, come together to form lethal mixtures of pollutants. ‘Since the air we breathe is a mixture, the scientific community has considered the possibility that the observed adverse health effects associated with individual pollutants may be partly attributable to the combined effects of multiple pollutants,’ says the HEI.2
A change in spotlight
The trend towards closer investigation of air pollution and the contribution of fuels through all stages of pollutant formation has put the spotlight on certain fuels previously regarded as relatively clean. This is especially the case with biofuels which have been helping to lower CO2 and other GWG emissions.
Research indicates that in low proportion blends bioethanol, for example, has a cooling effect in some engines which hinders combustion of other fuel ingredients – leading to formation of PAHs and PM.
Biofuels can differ in their effect on emissions, sometimes due to the environmental conditions outside the vehicle. High percentage blends of ethanol can decrease PM emissions, but fuels blended with a high proportion of isobutanol have the opposite effect of increasing PM quantities. Iso-butanol also leads to the creation of butyraldehyde, which is mutagenic. Bioethanol emissions can produce ozone, which can damage the lungs, in certain urban environments, according to research in Brazil, which is a big consumer of ethanol-blended fuels.
These research findings reinforce scepticism about the benefits of biofuels, especially biodiesel even when it is advanced or second generation. ‘For blends below 10%, there is close to no effect on local pollutants from first or second generation biodiesel,’ says Julia Poliscanova, clean vehicles and air quality manager at Transport and Environment in Brussels. ‘With higher blends—above 20% – biodiesel makes a small difference to NOx and SOx emissions with vehicles with the latest Euro V and Euro VI fuel standards while the difference grows in older vehicles. In new vehicles on Euro V or VI fuels the effect of blending any type of biodiesel is insignificant.’
Life cycle analyses (LCAs) casting doubts on the sustainability of biofuels, mainly because of the logistics needed to transport raw materials, have deterred investment, while regulators have been reluctant to support high percentage biofuel blends. Governments have also been cautious about funding the infrastructure needed for advanced biofuels production.
‘[Government] policies in the US and EU have not been major drivers of biofuel blends beyond 5-10,’ John German, senior fellow at ICCT, told a recent HEI workshop in Chicago, US, on fuels and particulate matter. ‘High blend biofuels dramatically reduce PM, but aren’t likely to penetrate the market in any significant amount.’
Meanwhile, large market shares are no guarantee of long-term success. Diesel has been accounting for around half the vehicle fuel market in Europe because of its capacity to boost vehicle performance with lower energy consumption. But now EU drivers are switching back to gasoline because of diesel’s emissions difficulties.
Even better performance advantages have been demonstrated by gasoline direct injection (GDI), which delivers fuels at high pressure directly into each cylinder of the combustion chamber. GDI has already taken a 40% share of the US gasoline engine market and is forecast by some automobile market pundits to account for the majority of car sales.
However, although GDI offers low energy consumption, CO2 emissions and engine power, it has a poor performance with emissions of air pollutants, especially PM2.5.
‘OEMs [car manufacturers] are looking for trouble,’ warned Georgios Karavalakis, of California University’s Center for Environmental Research and Technology, at the HEI meeting. ‘GDI can lead to increased PM [due to] fuel impingement onto piston surfaces and cylinder walls, fuel leakage from injectors and over-fueling during accelerations.’
Nonetheless, car makers are confident they can redesign the system so that there are tighter controls on the injectors. But, as for diesel engines, the most effective solution could be the use of filters to trap particles.
This could be good news for chemical companies making combined catalysts and filters. BASF, for example, markets a four-way conversion catalyst (FWCTM) that can remove PM from gasoline-engine exhaust through filtration, while also eliminating CO, NOx and hydrocarbon emissions.
Doubts among some consumers about the cleanliness of fuel exhausts from car combustion engines will probably accelerate demand for electric vehicles (EVs) or their hybrid variety.
This trend to EVs will undermine supplies of chemical feedstocks made as byproducts of fuels production. Without the economies of scale of fuel output, there is concern such chemical feedstocks may no longer be commercially viable. Could that be the sacrifice the industry will have to make as the fight against global warming and air pollution becomes inextricably bound together?
Sean Milmo is a freelance science writer based in Braintree, Essex.
1. European Environment Agency Air Quality in Europe - 2015 Report. Copenhagen, 2015.
2. Health Effects Institute. HEI’s Research Program to Develop Methods for Analyzing Multiple Air Pollutants and Health Outcomes, Boston, MA, 2015.