The concentration of carbon monoxide in an urban environment varies throughout the day due to varying rates of input and removal of carbon monoxide.
The main soure of carbon monoxide is emissions from motor vehicles. The rate of input of CO from this source is high during the morning and evening rush hours and lower during the middle of the day and the night.
Carbon monoxide is removed from an urban environment by two main routes
Some carbon monoxide is removed by plants and by bacteria and fungi in the soil, but the contribution from this source is insignificant in an urban environment.
Diffusion occurs throughout the day and night but the rate depends on atmospheric conditions such as wind speed.
Photochemical reactions only occur to any significant extent when the Sun is shining.