According to the California Air Resources Board (CARB) and the South Coast Air Quality Management District (SCAQMD), ozone concentrations in the Los Angeles area actually increased in May to levels not seen in the past five years despite dramatic reductions in vehicle travel due to the COVID-19 shutdown.

CARB examined State data from embedded roadway monitors and traffic detectors, and found that driving, which accounts for about half of the State’s emissions, started falling statewide about 10 days before Governor Newsom issued the stay-at-home order on March 19.  According to CARB, the data showed that car traffic fell precipitously (State-wide) as much as 80 percent, while commercial trucks reduced their activity by 15 to 20 percent.  Separately, the SCAQMD, which covers Los Angeles, Orange County and parts of Riverside and San Bernardino counties to the East, found that car traffic there fell by 25 to 43 percent, while truck traffic fell by 12 to 26 percent.

Other emissions sources dropped as well.  Airports in the Los Angeles area saw traffic fall by 60 percent, while the ports of Los Angeles and Long Beach reduced cargo by 11 percent.

In May, as traffic started picking up again, ozone levels in the SCAQMD region spiked to a 5 year high, despite the previous declines in the types of pollution that combine to form ozone: nitrogen oxides and volatile organic chemicals.

Researchers are still trying to figure out how the counterintuitive result occurred when both car and truck traffic fell in Southern California, but preliminarily said a combination of unfavorable weather conditions and chemistry may be to blame.

CARB and SCAQMD researchers hypothesized the spike was partly due to a regular seasonal uptick caused by warm weather and partly to the return of “stagnant atmospheric conditions” that encourage ozone formation.  Researchers also said that having less nitrogen oxide relative to volatile organic chemicals can actually increase ozone production — an effect commonly seen on weekends when truck traffic falls.  Despite the dramatic rise in ozone, traffic declines did reduce various other forms of harmful pollution, including particulate matter and carbon dioxide, according to CARB.

We will be watching for a follow up and greater in depth analysis from both CARB and SCAQMD and offer more thoughts as more data is published.