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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:
- ARB Report on the Ozone Weekend
Effect in California
- Final
Weekend Effect Reports
- Work
Group Meetings
-
EM Articles (July 2003)
- Chow
- Introduction to Special Topic: Weekend and Weekday
Differences in Ozone Levels, p 16
- Croes
- Forum—The O3 “Weekend Effect” and NOx
Control Strategies—Scientific and Public Health Findings and Their
Regulatory Implications, p 27
- Lawson
- Forum—The Weekend Ozone Effect—The Weekly Ambient Emissions
Control Experiment, p 17
-
Journal
of Air & Waste Management special issue on the weekend effect
(July 2003; V53N7)
- Austin
- Day-of-Week Patterns in Toxic Air Contaminants in Southern
California, p 889
- Chinkin - Weekday versus Weekend Activity Patterns for Ozone
Precursor Emissions in California’s South Coast Air Basin, p
829
- Chow - Introduction to Special Topic: Weekend and Weekday
Differences in Ozone Levels, p 771
- Fujita 1 - Evolution of the Magnitude and Spatial Extent of the
Weekend Ozone Effect in California’s South Coast Air Basin, 1981-2000,
p 802
- Fujita 2 - Diurnal and Weekday Variations in the Source Contributions
of Ozone Precursors in California’s South Coast Air Basin, p
844
- Heuss - Weekday/Weekend Ozone Differences: What Can We Learn from
Them? p 772
- Motallebi
- Day-of-Week Patterns of Particulate Matter and Its
Chemical Components at Selected Sites in California, p 876
- Pun - Day-of-Week Behavior of Atmospheric Ozone in Three U.S.
Cities, p 789
- Yarwood -Modeling Weekday/Weekend Ozone Differences in the Los
Angeles Region for 1997, p 864
- Additional
Weekend Effect References
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