Many cities around the world are facing huge challenges with mobility. The main problems encountered are traffic jams and pollution and both are linked. Today I want to focus on the second point: pollution. And we have a designated culprit: thermal engines.

Fossil fuels (diesel and gasoline mainly) are cheap, easily available and allow long distance trips. But these are hydrocarbons. Their combustion in the engine creates on the one hand energy to make it work and on the other hand, smoke with a lot of things inside: CO2, PM, NO2, NOx, HC, CO… The combustion creates pollution.

But what are we talking about when we talk about pollution?

We are actually dealing about two major issues: air quality and CO2 emissions, both topics being equally important, but totally different from each other.

When we talk about air quality, we are referring to the famous “pollution peaks” that we hear about in the news. Few people know exactly what it is about. “Pollution” as it is understood in this case, is a set of physical, biological and chemical elements resulting from the combustion of thermal engines, but also from the abrasion of tires, the aging of the road wear, brakes or everyday objects/products… which remain suspended in the air and which can be dangerous for health when they are concentrated durably. Only some of these elements, the most harmful to health, are generally remembered: fine particles (PM) and nitrogen oxides (NOx). If we focus on vehicles, fuel combustion (for combustion engines) accounts for 10 to 15% of fine particle emissions[1]. Tires and brakes are responsible for 30 to 46% of fine particle emissions from road transport, regardless of the engine type.

WHO[2] estimates that this pollution is responsible for 4.2 million of premature deaths worldwide and that 91% of the population is exposed daily to a high level of pollutants. An increasingly important problem, especially in urban centers.

The second subject is CO2, which is, in general, better understood: it is one of the greenhouse gases (GHGs), partly responsible for global warming. There are others, including methane, nitrous oxide, ozone, halogenated hydrocarbons … To facilitate the monitoring of GHG emissions and their impacts on the climate, it was decided to use a common unit of measure: the gram of CO2. Each GHG has been classified with a coefficient to calculate its impact on global warming in gram of CO2. For instance, one gram of nitrous oxide (N2O) is equivalent to 298 g of CO2.

Cities are looking at both subjects: air pollution for the health issues and CO2 emissions for climate change. And in the mobility sector, many options have appeared: electric vehicle, biofuels, hydrogen, natural gas…. And a good branding selling them as zero-emission, ecological, green cars.

First of all, speaking of a zero emission or an ecological vehicle is thus not the same thing: the former does not emit particles (which isn’t true since tires produce particles), while the latter does not produce no CO2.

Okay. And so, what about fuels? Are there “clean” fuels?

Before going any further, I would like to introduce another concept: the life-cycle analysis (LCA). LCA is a calculation method that analyses vehicle pollution from the extraction of the necessary materials used for its production to its recycling and adds up the pollution generated by its use. Until now, we used two fuels on the road: gas and diesel. Roughly speaking, the manufacturing methods and the vehicle components are identical. It therefore made sense to calculate tailpipe emissions if we wanted to compare both technologies. With electric vehicles and hydrogen, the equation is changed: the engine is different, the amount of battery explodes … In short if we want to compare what is comparable and keep a neutral and scientific look, the traditional method has no more meaning. We should use the LCA approach. At European level, this concept will be studied for a potential adoption (in 2023 at best …). In this case, we will talk about CO2 emissions.

Let us now have a look to the vehicles.

Electric vehicles

It is the vehicle that is currently marketed “ecological” par excellence. As there is no combustion in an electric engine, it emits only very few local pollutants compared to diesel, according to the tailpipe emission method. Nevertheless, a 2016 study [3] shows that fine particle emission levels are equivalent to diesel by taking tires and road wear aging into account. This is due to the higher weight of an electric vehicle compared to a diesel one: for example, a Tesla S weighs 2.3 T while a Citroen C5 weighs 1.3 T.

According to the same approach, an electrical vehicle emits no CO2 (produced from the combustion of carbon chains gas, gasoline, diesel). On the other hand, if we use the LCA, the calculation differs since the origin of the battery manufacturing materials is considered, together with their recycling and the pollution generated by the energy used to produce the vehicle itself (even though in France we mainly use a “decarbonated” nuclear power). The CO2 balance is then quite different. It is considered that a new electric vehicle will have produced as much CO2 as a diesel vehicle traveling 80,000 km[4] (100,000 km in Germany because of their more carbon-intensive electricity mix).

Moreover, the APPR[5] (Professional Advertising Regulatory Authority) has condemned for misleading advertising Citroën, Opel, Nissan and Bolloré on commercials presenting the electric vehicle as a “clean”, “zero emission” or “ecological” vehicle”.


Hydrogen (H2) has been discussed about for many years but struggles to find its place in the French and global energy landscape. It is the same use as the electrical vehicle: hydrogen is associated with the oxygen present in the air via a fuel cell to generate electricity supplying an electric engine. Same observation as for the electric vehicle, the reaction Oxygen + H2 gives water. No combustion. No CO2 and no PM emission at the exhaust pipe.

Let’s have a look at H2 production. 96% of its production is obtained by steam reforming, that is to say from natural gas (CH4), hydrocarbon or coal (more complex composition of C and H for coal and hydrocarbons). The goal of steam reforming is to break the carbon chains to get H2 on one side … and CO2 on the other. According to the French Environment & Energy Management Agency[6], H2 production represents 7.5% of the total CO2 emissions of the French industry.

The principle is thus to use an energy that could be used directly as a fuel to transform it (with the energy required for its transformation and the related losses) in another fuel for a similar type of vehicle. From an energetic point of view, there is a loss of 30 to 50% of the final energy. To avoid relying on carbon energies, the argument raised is to promote electrolysis: using water and electricity surplus from renewable energy to produce H2. The ratio between the electrical energy required for H2 production and the electrical energy at fuel cell level is 25% (excluding compression loss, transport, etc.). Looking at the LCA, we must also consider transportation of the hydrogen molecule from its production site to the service station: this transport is currently – and will remain for at least the next 40 years – done by truck and the pollution it generates. Transporting hydrogen through existing gas networks is still at research stage: H2 molecules are thinner and more corrosive than gas, which can cause problems with sealing and premature infrastructure aging and present obvious safety risks (hydrogen is 5 times more explosive than gas).


Biofuels are another decarbonated option that is quite complex to address. In fact, any driver of gasoline or diesel vehicles already consumes biofuels. The E5 (former SP95 or SP98) available in all service stations contains 5% bioethanol and 95% traditional gasoline. Then comes the E10 with 10% and E85 with 85% of bioethanol. On the diesel side, we have B7, B10, B30 and B100 (with respectively 7%, 10%, 30% and 100% of biodiesel). Another one is ED95, with 95% of biofuel and 5% additives. In France, we recently have heard about another biofuel with the Mede plant: HVO100, a premium biodiesel mainly produced from palm oil imported from Indonesia and/or Brazil. With the 2019 French Finance Law, the latter has been downgraded from the biofuel category[7], but things could change and not necessarily in the right direction…

Of all the fuels above, the most common remain the E5, E10, B7.

Biofuels are very controversial. The first reason is that 90% of biofuel production comes from dedicated agricultural production of rapeseed, soybean, beet or corn. Land is therefore only used to produce biofuels with associated impacts on deforestation, loss of biodiversity or on food prices. Using the LCA approach, bioethanols are on average 40% less CO2 emitters, while biodiesels are 75% more[8]! Biodiesel produced from palm oil emits 3 times more than a traditional diesel.

In terms of local pollutants, emissions of fine particles are reduced, while NOx emissions are equivalent. We reach levels around 10% for B10 for example and 24% for B30. Let’s not forget that biofuels are mainly transported by truck from its production plant to consumption sites.

Last but not least, the NGV – for Natural Gas Vehicleand its biosourced counterpart, the bio-NGV.

NGV is nothing more than gas that you can consume for cooking or heating. France imports its natural gas mostly from the North Sea/the Netherlands (~ 50%), Russia (~ 20%) and Algeria (~ 10%). It is extracted, processed and transported before being injected into the French network.

Biomethane, for its part, currently accounts for only 0.3% of the French methane consumption. It is mainly produced from agricultural waste and sludge from treatment plants, through a degradation process of the natural resource. 10% of French biomethane is nevertheless produced today from dedicated culture. Consumed NGV is therefore mainly still of fossil origin.

NGV-fuelled vehicles use a thermal engine, close to the gasoline ones, and its combustion emits CO2, PM and NOx, in a smaller quantity than biofuels, diesel or gasoline. In the leading industries’ figures, CO2 emissions are reduced by 20% (80% with bio-NGV in a LCA approach), PM by 95% and NOx by 50%[9].

Cities have to deal with this equation: CO2 emissions and air quality by maintaining a good level of mobility within the city. Technology could help but it is not the solution. Every alternative will generate at one time or another pollution or CO2 emissions. This is not going to change, whatever new technologies that may appear. Alternative fuels help to improve air quality issues, some more than others. The CO2 challenge is controversial as each fuel will pollute during its life-cycle, whether directly (thermal engines) or indirect (fuel production or vehicle components). If CO2 emissions or local pollutants cannot be eliminated, less-pollutant solutions already exist: developing of public transportation, constraining private car use, improving existing engines, and of course favouring alternative fuels that remains less harmful than diesel and gasoline. What about stop polluting?

In fact, there is no miracle solution: any energy consumption emits CO2 and degrades the quality of the air. The only option is to reduce the use of any kind of engine and promoting “soft mobility” as bikes and foot. Or limit any kind of mobility. Because in the end, the least polluting energy is still the one we do not consume. 


[1] Observatoire de la Qualité de l’air de Paris :

[2] WHO website:

[3] Non exhaust PM emissions from electric vehicles study

[4] ADEME:






Share This