What change would have the largest impact on reducing air pollution in a municipality?

Switching city fleets of vehicles to natural gas from gasoline would have a significant impact on reducing air pollution in a municipality for several reasons. Natural gas burns cleaner than gasoline, producing fewer harmful emissions such as nitrogen oxides, particulate matter, and carbon monoxide, which are major contributors to air pollution and associated health issues. By converting municipal vehicles to natural gas, the overall emissions from the city's transportation sector would decrease, improving air quality and potentially lowering the incidence of respiratory diseases among residents.

While implementing strict recycling laws can contribute to reduced waste and lower emissions associated with waste management, it may not directly address the immediate air quality issues stemming from vehicle emissions. Increasing public transportation options is also beneficial, as it can reduce the number of individual cars on the road; however, the transition might take longer to implement on a large scale and may not eliminate emissions from existing vehicle fleets in the short term. Planting more trees in urban areas is valuable for absorbing carbon dioxide and providing various environmental benefits, but the immediate effect on air pollution from vehicular emissions is less direct compared to switching to cleaner fuel sources. Therefore, transitioning to natural gas for city vehicles represents a more direct approach to reducing harmful pollutants and leading to quicker improvements in air quality.