Strengthening Arguments About Air Pollution in Aurelia

The city of Aurelia recently implemented a new public transportation system, which studies indicate has reduced traffic congestion by 30%. City officials claim that the decrease in congestion has led to a significant reduction in air pollution, which they attribute directly to the new transportation system.

However, some environmentalists argue that other factors, such as the increase in green spaces and stringent vehicle emissions regulations enacted in the same period, could also be contributing to the cleaner air.

To strengthen the claim that the new public transportation system is the primary reason for reduced air pollution in Aurelia, which of the following pieces of evidence would be most useful?