City buses going nowhere: scientists warn that investing in gas today is a mistake

At the start of 2022 Madrid City Council boasted about its “historic investment” of 1,000 million euros for a “technological revolution” of the city buses that are part of the capital’s Municipal Transport Company (EMT), claiming that the new strategic plan was “the most sustainable” yet of the public company. However, of the 2,099 buses that will run in the capital by 2025, 80% will still run on natural gas, a fossil fuel whose price has soared to such an extent –even more so since Russia's invasion of Ukraine– that three days after Madrid City Council's announcement, the main employers' association for the bus transport sector, Confebus, called on the government to take urgent measures to alleviate a situation that is “unsustainable” due to the soaring prices of gas and oil.

In what is a very complicated European context due to our dependence on gas, we have analysed the current fleets of urban buses and the medium-term investment plans of the municipal transport companies in the country's main cities: Madrid, Barcelona, Valencia and Seville. Buses are a key means of transport for the environmental transformation of cities, as they carry the greatest number of people per surface journey and have the greatest potential for reducing greenhouse gas emissions. However, at the same time, in the most populated cities millions of people are exposed every day to the toxic pollutants that come out of the exhaust pipes of combustion vehicles, including buses.

According to data we have requested from the urban transport companies themselves, Madrid currently has 2,089 urban buses, of which 80% run on Compressed Natural Gas (CNG). Barcelona has a fleet of 1,135 buses, of which 363 run on gas. In Valencia, where 489 buses circulate, just 6.1% or 30 are gas, and 254 are hybrids. In Seville, with a population equivalent to Valencia, of the 407 urban buses, 300 are gas - 73.7%. In all their public channels, corporate websites, press releases, videos, social networks or in signs embedded on the buses themselves, the message of the municipal companies that are most committed to gas is the same: “it is sustainable”, “eco”, “it does not generate pollutants harmful to health” or “it is better than diesel”. These assumptions translate into millions of euros of public money invested in fleets and new infrastructure, as gas vehicles need their own refuelling stations.

Felipe Rodríguez is a mechanical engineer and mathematician. He works as head of the heavy transport unit at the International Council on Clean Transportation (ICCT), an independent organisation dedicated to mobility research on health and climate change. Speaking to us by videoconference from Berlin he said: “Until Euro 6, diesel performed terribly in real emissions compared to the same category of gas vehicles, especially in particulate matter, so there was this general perception that gas is cleaner. It doesn't emit these puffs of black smoke, but as we move to new diesel technology that difference no longer exists. I don't know how much of it is not wanting to see the evidence or looking the other way, but it's known that gas is not cleaner anymore.”

Since when has it been known? “In technical circles, it has been known since 2015 and 2016, when Euro 6 vehicles began to be tested”, says Rodríguez, referring to the European emissions reduction regulation, the sixth version of which came into force in September 2015, requiring manufacturers to install particulate and NOx filters in vehicles.

It is difficult to find studies on the performance of urban buses that have measured emissions and pollutants in real-time driving. One of the most recent and comprehensive has been carried out by the French organisation Airparif, an air quality observatory in the Ile-de-France region that includes Paris. Researcher Laure Deville Cavellin explains on a presentation that one of the aims of the study is “to make sure that the technological choices made respond to the expectations of improvement in air quality and greenhouse gases”. The study is also aimed at the scientific community, “because the emissions of buses in real driving are not very well documented”.

In this region, home to 12 million people, researchers analysed more than 1,600 journeys by urban buses with diesel, hybrid and Compressed Natural Gas (CNG) technologies of Euro 4 and Euro 6 categories. The latter is the current standard. The results are striking. On average, the most recent gas buses emitted 50% more particulate matter than diesel buses of the same category (see graph below).

For the researchers consulted, this result, which does not surprise them at all, is clearly due to ultrafine particles, which are much more numerous in gas vehicles than in diesel vehicles. These particles, which are still under-regulated in Europe, are the most serious problem of gas vehicles. “Ultra-fine particles are the smallest particles that can be measured and observed. Their size is less than 0.1 microns, which corresponds to the size of a virus or a DNA molecule,” according to Airparif. They add: “These particles are even more harmful than larger particles, as they penetrate deeper into the body. Data collected since 2013 confirms or reinforces their link to respiratory and cardiovascular problems and premature deaths”.

Xavier Querol, one of the leading experts on air pollution in Spain, confirms this result and the impact of gas vehicles on health: “More ultrafines come out of CNG cars than from other fuels, and the problem is that they not only reach the pulmonary alveoli, but also enter the bloodstream”. In the small circle of top air pollutant researchers is also Greece's Constantinos Sioutas, who currently co-directs the Southern California Particulate Centre (SCPC). We spoke to him by video conference and he was emphatic: “Particulate matter is the number one killer of all pollutants. And as soon as you put filters on, particulate matter drops dramatically. The cleanest [combustion vehicle] is diesel with filters, then compressed natural gas, then petrol.  

 Given this evidence, the move to gas buses by the municipal transport companies of major cities in Spain is surprising. Seville, which currently has no electric or hybrid buses, plans to renew its fleet with buses using this fossil fuel – gas - to replace the older diesel. Tussam, the company that manages the city buses, explains that, effectively, none of its CNG buses have filters, not even in the new purchases of Euro 6 e gas buses, the most recent one. The Barcelona municipal company TMB confirms this reality: “High-performance filters, which retain both NOx and particles, were applied in 2012 and 2013 to 425 diesel vehicles in the TMB fleet, not to those powered by CNG because of their reduced emissions of these pollutants harmful to health”. Madrid's EMT directly lists CNG as part of its “green fleet”. Valencia has fewer gas buses in proportion, most of its fleet are diesel hybrids and it plans to replace 20 diesel buses with electric ones in 2023.

 In London, researcher Nick Molden runs one of the few independent centres for real driving vehicle emissions analysis in Europe, Emissions Analytics. Here they take vehicles of different technologies out onto the tarmac, put them on the road and test their emissions. Three years ago, Molden led one of these tests with trucks of various technologies, including Compressed Natural Gas. For him it is obvious why it is not worth investing in gas vehicles: “The argument is to stop using diesel and switch to gas because gas is much better from a CO2 point of view, but this is not true”. The reduction that came out of the test, in which the trucks travelled long distances, offers barely a 10% reduction in CO2 from gas compared to diesel.

 In the ICCT's analysis of trucks, the results are similar, only a 6.5% emissions reduction gain for gas over diesel, the same as the study carried out by the Air Quality Observatory of the French organisation Airparif. For Georg Bieker, a chemist and researcher at the International Clean Transport Centre, “it doesn't matter that they are 10% better, it's not the 1990s anymore, we need to radically reduce emissions. This is not the solution; it just prolongs the bad business model of combustion vehicles”.

 Bieker has been studying greenhouse gas emissions in the entire production cycle of vehicles of different technologies beyond what comes out of the tailpipe. And he provides another reason why researchers refuse to accept gas as a transitional energy towards a zero-emission transport model: methane. “When you extract the gas from the ground, in Russia, in the United States, wherever, there are losses in the process, and then also in the transport, sometimes even in the vehicle itself, although this does not happen much in Europe. But it does in China. Methane has 30 times more impact than CO2 on climate change when you look at it over a 100-year period, so in the end, these losses are very harmful,” Georg explains.

 The only way to reduce this impact of gas would be through biomethane. But the reality is that this alternative, which for example Barcelona and Seville are considering in their strategic plans for investment, is only at an experimental stage. It is impossible to know right now whether biogas extracted from waste can be developed on an industrial scale to move part of the urban fleets. “You can't supply the entire European bus fleet with this alone. It might be enough for two or three per cent of the fuel mix, but the idea that you can move buses with this option does not make sense on a large scale,” Georg explains.

Despite the scientific knowledge available, the medium-term strategy of cities such as Madrid, where EMT transported around 300 million passengers in 2021, according to its own figures, is to invest massively in gas. The 534 million euros foreseen in its 2025 strategic plan for the purchase of new buses will create a fleet with 1,560 CNG buses, 529 electric and 10 hydrogen buses. Given that the lifespan of an urban bus is around 10 years, it is very likely that in 2030 and beyond, the fleet will still be mostly gas buses.

“Gas is an expensive technology and competes with electrification. Its promotion is contradictory to the goal of climate neutrality by mid-century. New vehicles entering fleets must no longer be diesel or gas, they have to be electric,” says Felipe Rodríguez of the ICCT.

The organisation highlights the excessive costs of the gas buses business model that is being built with everyone's money. It makes no sense, argues Rodríguez, to invest in building a whole new infrastructure and generation of buses that will just last ten years. Municipalities are investing public money in what analysts call “stranded assets”, assets that have no future. According to analysts the current situation will lead to two possible scenarios, neither of which is very positive: “Either this public money will be wasted because the compressed gas refuelling stations are ten years old and will have to be eliminated, or it generates the so-called lock-in effects: as we already have the infrastructure and also the buses, we continue to buy this technology instead of making a transition to zero-emission vehicles,” argues engineer Rodríguez.

 In international comparisons Spain is at the bottom of the list in Europe for the deployment of electric buses. Yet these are only ones that offer zero direct emissions, and the only ones suitable for the necessary task of decarbonising transport. Given our current climate crisis we need to eliminate the greenhouse gas emissions generated by both gas and oil (petrol and diesel) vehicles. 

Municipalities that opt for gas for their fleets argue that they comply with or are even more ambitious than European regulations. But looking beyond this framework we find inspiring examples. Santiago de Chile leads the world in the transition to electric buses after China, with a fleet of 800 vehicles, and Bogotá has attained a fleet of 655 electric buses. There, there is no European Commission that considers gas a clean technology, and the clean transition has been achieved by national measures and the municipalities themselves, with initiatives such as banning the purchase of combustion buses in the tendering process and giving tendering incentives for electric buses. 

Russia's invasion of Ukraine has already made it clear what it means to depend on gas from third countries like Vladimir Putin's. Spain does not have gas, but it does have plenty of renewables and sufficiently proven electric mobility technology to raise huge questions about any bet on a fossil fuel that only leads to the past, not the present or the future. 

Madrid, the gas capital

From 2023, EMT will no longer provide services with diesel buses. Two years later, by 2025, the “technological revolution” announced by Madrid City Council will mean that 80% of the fleet will continue to be made up of Compressed Natural Gas buses, with 1,560 vehicles using this technology. At present, gas buses, Euro 5 diesel buses and hybrids all fall into the “green fleet” category.

Barcelona: gas hybrids, electric and a bit of hydrogen

 Barcelona is also committed to gas as a transitional technology for the decarbonisation of its fleet. Of the 508 buses to be purchased by 2024, there will be almost the same proportion of electric buses as compressed natural gas hybrids. Why make such an effort to invest in gas? TMB responds that “the speed of the electrification process is limited by the resources available for big investments, the time needed to adapt the depots and the technological development of the manufacturers”. In Europe, other cities such as Amsterdam, London or Paris have much more ambitious plans to electrify 100% of their fleet.

Valencia, based on information available for 2023

 The conversion plans for Valencia's fleet until 2023 will replace 20 old diesels with electric vehicles. However, the Empresa Municipal de Transportes will continue to rely on Compressed Natural Gas hybrids. The city aspires to be climate neutral by 2030, and says it is considering the possibility of incorporating the use of green hydrogen, without specifying much more about what steps it will take towards this goal.

Seville, the uncertainty of biogas

Seville has only two types of buses: compressed natural gas (73.7%) and diesel (26.3%). Like Madrid, Barcelona and Valencia, this city also has plans to renew its fleet towards less polluting solutions. Its commitment for 2025-2030 is to replace CNG vehicles with biogas vehicles, which it plans to obtain from the distillation of waste and refuse gases. Tussam, the company that manages the city's bus fleet, has signed an agreement with Biocam for the construction of a waste treatment plant to produce the biogas. 

However, as Carlos Bravo of Transport and Environment points out, although this alternative can be considered CO2-neutral, the same is not true of its impact on health, as methane, when burned in the engine, still generates ultrafine particles. Moreover, many doubts remain about the large-scale use of waste gas.