"For the last 30 years, we've known that the hydroxyl radical is the primary cause of ozone," Saltzman said. "Chlorine plays a similar role. It doesn't take much chlorine to affect ozone chemistry."
This study is the first to extensively measure atmospheric chlorine in Southern California air over several consecutive weeks. Researchers made extremely sensitive chlorine measurements, detecting the chemical at levels as low as three parts per trillion.
Results showed that chlorine is present in the atmosphere both day and night, and that its levels fluctuate with little regularity. Saltzman and Finley couldn't pinpoint the cause of these changes, finding no obvious link between chlorine levels and meteorological conditions such as wind, temperature or relative humidity or existing pollution levels. They also don't know for sure what created the chlorine they detected.
"We found more chlorine in the daytime than expected. It must be rapidly produced, because sunlight destroys it so quickly," Saltzman said. "Most likely the process involves airborne marine particles, but research is needed to understand how it occurs."
Inland ozone levels typically are higher than on the coast, mainly because of meteorological conditions that restrict air circulation and trap pollutants. Ozone is a highly reactive and unstable gas that damages living cells. Even low levels of ozone can harm the upper respiratory tract and the lungs, causing a cough, throat irritation and reduced lung function, and aggravating the effects of asthma, bronchitis and emphysema.
Said Saltzman, "Understanding the chemistry of chlorine is important to understanding its role in ozone chemistry, especially in California where air quality is such an important issue."