Beijing on Monday completed the renovation of its 8,800-strong fleet of diesel buses, which will now discharge significantly less emissions.

This latest effort, led by Beijing Municipal Environmental Protection Bureau, means that the buses will discharge 60 percent less nitrogen oxide, or 2,800 tons, annually.

The project follows standards on automobiles and their emissions, released by the city in 2013, that aimed to address loopholes in diesel vessel standards.

Observers and officials say vehicles are a major contributor to Beijing's air pollution. The city has 5.57 million vehicles, which churn out 700,000 tons of pollutants annually.

The city has promised to phase out 200,000 vehicles that fail to meet emission standards, as well as close more than 300 polluting factories this year.