The Ozone Weekend Effect in California

This page last reviewed January 7, 2014

This website provides information regarding research on the “ozone weekend effect”, the phenomenon of ozone concentrations tending to be higher on weekends than on weekdays in many urban areas around the world. This tendency is counter to expectations because the emissions of ozone precursors (NOx and ROG) are lower (NOx more so than ROG) on weekends than weekdays. This counter-intuitive phenomenon is due to the fact that NOx emissions are primarily in the form of NO, which reacts to destroy ozone (O3) present in the air (i.e., suppresses ambient concentrations) to form nitrogen dioxide (NO2). However, this NO2, as well as additional NO2 created from the NO emissions by photochemical catalytic reactions involving ROG (i.e., a single ROG molecule can recycle several times and oxidize multiple NO molecules to NO2 molecules) can react in sunlight to increase ozone concentrations. The photolysis of NO2 (i.e., sunlight splitting the NO2 molecule into NO and O) is the only known significant pathway for creating the proper oxygen atom for reacting with the abundant oxygen molecules (i.e., O2) in the atmosphere (18%) to form ozone. The ROG emissions effectively enhance the conversion of NO to NO2 and ultimately the formation of O3 concentrations significantly above natural continental background levels (30-40 ppb). Thus, although fresh NOx emissions initially reduce ambient O3 concentrations, it is the magnitude of NOx emissions that control global background and regional maximums in O3 concentrations.

A weekend effect workgroup was formed in 1999 to investigate the phenomenon and coordinate research efforts. Participants in the workgroup included the Coordinating Research Council, the U.S. Department of Energy (NREL), the South Coast Air Quality Management District, private companies (e.g., Chevron, Pacific Gas and Electric), and other interested parties (e.g., academia). The ARB published a peer-reviewed report and several of the participants published results in peer-reviewed literature. Links to the activities and products of this workgroup, as well as other weekend effect resources, are provided below:

This website posts products and links related to research on the ozone weekend effect. Questions related to this website can be directed to:


Mr. Leon Dolislager

(916) 323-1533
Atmospheric Processes Research Section
Research Division
California Air Resources Board
P.O. Box 2815
Sacramento, CA 95812

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