NREL Publishes Weekend Ozone Study
For years air quality researchers have been puzzled by two pieces of seemingly contradictory information. Although scientists knew that vehicle emissions were responsible for elevated ozone levels in Los Angeles, studies showed that ozone levels were actually higher on weekends—when fewer people were driving.
Beginning in 1999, NREL, in cooperation with the Coordinating Research Council and the California Air Resources Board, tested several theories that might explain the so-called "weekend ozone effect." They examined data from ozone-monitoring sites in Southern California, studied traffic counts from sensors embedded in the area's freeways, and surveyed citizens about their weekend use of barbeques, fireplaces, and lawn and garden equipment.
The results were startling. Although the study indicated that emissions of ozone-forming chemicals were lower on weekends, ozone levels were as much as 25% higher on Saturdays and up to 50% higher on Sundays. The studies also showed that oxides of nitrogen (NOx) levels dropped by as much as 44% on Sundays. This was unexpected, as NOx is commonly believed to be a significant ozone precursor.
If NOx levels were as strongly linked to ozone levels as expected, then a reduction in NOx should lead to a reduction in ozone. Because the opposite was happening, researchers now need to reexamine their assumptions about the formation of ozone in Los Angeles. This is significant because many air quality authorities are considering stricter NOx regulations as a way to combat ozone levels. Further studies will be needed to develop air quality models that more accurately reflect the relationship between NOx and ozone.
For additional information on the weekend ozone effect and NREL's research in this area, see our Publications page.
