What causes summer smog?
On hot and sunny summer days, stagnant air can trap pollutants and, when combined with sunlight, result in unacceptable levels of ground-level ozone. Since ground-level ozone concentrations are generally highest when the suns intensity is at its maximum, smog levels peak in mid-afternoon, and fall rapidly in the evening when the sun goes down. The pollutants build up over time, so anything people can do to reduce individual contributions to smog is beneficial to air quality, not only on high pollution days, but every day.
