A previously unknown chemical reaction that scientists at the University of California Irvine (UCI) recently discovered in a new study could account for up to 40 parts per billion of ozone.
This reaction is not included in computer models used to predict air pollution levels and the effectiveness of ozone control strategies that can cost billions of dollars.
Ozone can cause coughing, throat irritation, chest pain and shortness of breath. Exposure to it has been linked to asthma, bronchitis, cardiopulmonary problems and premature death.
"We still don't really understand important elements of the atmosphere's chemistry," she added.
When fossil fuels burn, compounds called nitrogen oxides are generated.
Previously, scientists believed these would be eliminated from the atmosphere upon contact with surfaces.
But, UCI scientists discovered that when nitrogen oxides combine with hydrochloric acid from airborne sea salt on buildings, roads and other particles in the air, highly reactive chlorine atoms are created that speed up smog formation.
Hydrochloric acid also is found indoors in cleaning products.
When it interacts with nitrogen oxides from appliances such as gas stoves, chlorine compounds form that cause unusual chemistry and contribute to corrosion indoors.