Which environmental condition can contribute to higher levels of air pollution in urban areas?

High temperatures can significantly contribute to higher levels of air pollution in urban areas due to several interrelated processes. When temperatures rise, it enhances the formation of ground-level ozone, a major component of smog. This occurs because heat accelerates the chemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx), which are emitted from vehicles, industrial facilities, and other sources.

Additionally, high temperatures can increase energy consumption, particularly from air conditioning systems. This increased energy demand often leads to a rise in emissions from power plants, further exacerbating air quality issues. Furthermore, higher temperatures can lead to atmospheric conditions that trap pollutants closer to the surface, creating stagnant air situations that allow for the accumulation of harmful substances.

In contrast, increased greenery generally helps improve air quality by absorbing CO2 and filtering pollutants. Low humidity can affect the dispersion of air pollutants, but it does not have as direct an impact on their formation as high temperatures do. Night-time cooling can help to disperse pollutants as cooler air descends, aiding in reducing pollution levels. Thus, the direct link between high temperatures and increased air pollution makes the selection of that environmental condition particularly relevant.