Which environmental factor contributes to higher CO levels during colder months?

The presence of inversion layers is a significant environmental factor contributing to higher carbon monoxide (CO) levels during colder months. Inversion layers occur when a layer of warm air traps cooler air at the surface. This phenomenon often takes place in winter when the ground cools quickly after sunset, and warmer air settles above it.

During such conditions, air pollutants, including CO, which is typically emitted by vehicles and heating systems, become trapped within the cooler air layer. Because the usual vertical air mixing is suppressed, these pollutants accumulate close to the ground instead of dispersing into the atmosphere, leading to higher concentrations of CO in urban areas where emissions are high.

In comparison, increased air circulation generally promotes the dispersion of pollutants, making it less likely for CO levels to rise. Heavy rainfall can wash away pollutants and also contribute to lower levels, while humidity does not have a direct causal relationship with elevated CO levels in winter; it is more associated with comfort and perception rather than pollution concentration. Thus, inversion layers play a crucial role in the accumulation of CO during colder months, resulting in the elevated levels observed in such conditions.