Soot-free urban bus fleets

Bus fleets provide widely affordable low carbon transportation around the world. But urban buses are powered predominantly by diesel engines, accounting for approximately 25% of the black carbon emitted by the transportation sector. Future investments in low-carbon urban bus fleets should be coupled with clean fuels and soot-free engine technologies. In this way, local officials can preserve the clean air and climate benefits of their investments in urban bus fleets.

The Soot-Free Urban Bus Fleets project aims to accelerate the global transition to soot-free engine technology in the urban bus fleet. The core activity of this project is to directly inform, motivate, secure, and support the implementation of commitments made by cities to shift toward soot-free engines. This includes not only developing the soot-free commitments together with cities, but also taking the further step of making these commitments a reality – whether through advanced diesel, natural gas, hybrid-electric, electric buses or other soot-free technologies.


Due to rapidly growing urban populations and increasing demand for efficient and affordable mobility, urban bus activity is predicted to increase by nearly 50% by 2030. This will translate into an estimated additional 26,000 tonnes of black carbon emitted in 2030.

The black carbon in diesel exhaust poses a significant health risk and has been listed as a known human carcinogen by the World Health Organisation (WHO). Concentrations of black carbon are typically highest in urban areas where emission sources are most densely located. For urban residents, exposure to diesel exhaust can constitute up to 70% of their risk of exposure to air toxins.

In many countries where soot-free engines are not required by national or local laws, buses can emit more than 250 times as much black carbon as a gasoline passenger vehicle traveling the same distance. Urban buses will travel up to 10 times as much than the average passenger vehicle, increasing emissions by the same factor. Since buses can stay in service for 20 years or longer, poor emissions performance can persist for decades.


City officials have significant power to reduce urban bus emissions. A survey of 57 cities in the C40 network finds that over 80% own or operate the bus fleet, 86% set or enforce bus fleet policies, and 77% control the local budget for bus transport.

The Soot-Free Urban Bus Fleet project supports city officials to carry out the shift toward soot-free engines with activities that:

  • Inform, motivate, and secure a public commitment to shift to soot-free urban bus fleets
  • Provide implementation support
  • Establish an industry partnership with engine manufacturers and suppliers of commercially available soot-free engines
  • Develop an urban bus fleet database in order to assess current and future market demand for clean buses
  • Support a cross-linkage with the Financing of SLCP Mitigation initiative to estimate total financial assistance needed for their deployment
  • Expand the deployment of soot-free engines


The Coalition's Heavy-Duty Diesel Initiative defines “soot-free” buses  as any fuel and vehicle combination that meets emission levels for particulate matter set by Euro VI or US 2010. This can include compressed natural gas or electric-powered buses, alongside other fuel/engine types including conventional diesel engines.

The soot-free bus fleet project is technology neutral and performance-based in order to seek the maximum feasible cost-effective solutions to black carbon control in all regions.

Diesel black carbon emissions can fall dramatically through the combined use of “soot-free” engines and cleaner fuels that are readily available today.

Not all cities can immediately purchase Euro VI engines or utilize diesel particulate filters. These technologies can be damaged if currently available fuels do not meet minimum specifications. For diesel engines the most important limiting factor is fuel sulfur content.

Cities with access to diesel fuel containing no more than 50 parts per million (ppm) fuel sulfur content can allow the operation of a Euro IV or V engine that will reduce black carbon emissions by 75% compared against a baseline Euro III vehicle. But a city with access to diesel fuel containing no more than 10 ppm sulfur content can leapfrog to soot-free engine technology at the Euro VI emission level and achieve greater than a 98% reduction in black carbon emissions compared with any previously existing engine technology.

Cities can also rapidly eliminate diesel black carbon emissions by:

  • Switching to engines using compressed natural gas, electricity, biodiesel or others that achieve minimum Euro VI emissions
  • Selecting engines that are the cleanest allowed by currently available fuels
  • Scrapping, retrofitting, and replacing existing high-emitting diesel buses with cleaner engines that meet more stringent emission standards

New opportunities to deploy soot-free engines are emerging. Today nearly one-quarter of all cities with a population greater than 1 million have access to ultra-low-sulfur fuels (less than 15ppm diesel sulfur content).

Download: Soot-free bus Euro VI fuel availability by city

Impacts & results

The Soot-Free Urban Bus Fleet project engages cities with a population of 3 million or larger to encourage shifts to soot-free engines. Seeking out cities through regional public meetings, online and other coordinated outreach activities, the project is actively targeting 20 cities in 20 countries. Eleven of these cities are in CCAC member states.

Access to fuel determines the actions expected of cities, the following three categories define the specific commitments cities are asked to make:

  • Tier 1 cities: Have access over the next three years to fuels that will allow an immediate shift to soot-free engines.
  • Tier 2 cities: Have access over the next three years to Euro IV fuel, such as low sulfur fuel containing 50ppm sulfur content.
  • Tier 3 cities: Are not expected to have Tier 2 status within the next three years, such as cities with diesel fuel sulfur content greater than 50 ppm.

Preliminary estimate of benefits from a shift to soot-free urban bus fleets in 20 target cities


Category Black carbon reduction (Kt) Climate benefit

Avoided early deaths
    GWP-20 GWP-100  
Tier 1 cities 0.27 0.87 0.23 1,100
Tier 2 cities 0.96 3.02 0.80 1,300
Tier 3 cities 0.85 2.66 0.70 1,390
Total 2.09 6.56 1.73 3,700

Project updates