Why Ozone is Higher on Weekends in the South Coast Air Basin: a 20-Year Perspective Eric Fujita, William Stockwell, Robert Keislar, and Dave Campbell Division of Atmospheric Sciences Desert Research Institute University and Community College System of Nevada Reno, Nevada SCOS97-NARSTO Data Analysis Conference Diamond Bar, CA February 13-15,

Trends in Average Maximum 1-Hour Ozone from 1980 to 1997 in Western, Central and Eastern South Coast Air Basin Western: Los Angeles - N. Main, Lynwood, N. Long Beach, Anaheim, and La Habra Central:Azusa, Glendora, Pomona, and Upland Eastern: Riverside and Lake Gregory. 7

Trend in Number of Annual Exceedances of the Federal 1-hour Ozone Standard in the South Coast Air Basin from 1976 to

Average Daily Maximum Ozone (ppb) in the SoCAB June to October

Average Diurnal Variations in Ozone and HC/NOx Ratios at Azusa in Summer 1995

Twelve-site Average NO and NMHC (from CO) at 4-5 a.m. and 7-8 a.m. (PDT) by Day of the Week in the SoCAB during 1981 to 1998

Source: Zielinska, 1999 Health Assessment Document for Diesel Emissions: Chapter 2 - Diesel Emissions, atmospheric concentrations, transport and transformation. U.S. EPA Office of Health and Environmental Assessment. Heavy-Duty Diesel Hydrocarbon Emission Rates Measured on Chassis Dynamometer from Mid-1970s to Mid-1990s for 4-Stroke Engines

Source: Zielinska, 1999 Health Assessment Document for Diesel Emissions: Chapter 2 - Diesel Emissions, atmospheric concentrations, transport and transformation. U.S. EPA Office of Health and Environmental Assessment. Heavy-Duty Diesel NOx Emission Rates Measured on Chassis Dynamometer from Mid-1970s to Mid-1990s for 4-Stroke Engines

Twelve-Site Average NO 2 /NOx Ratios at 7-8 a.m. (PDT) by Day of the Week

3-Year Running Averages of Ozone Accumulation Rates (ppb/hr) in the South Coast Air Basin for Sunday and Wednesday

Duration and Rate of Ozone Accumulation on Wednesday During for

Duration and Rate of Ozone Accumulation on Sunday During for

3-Year Running Averages of Ozone Accumulation Rates (ppb/hr) in the South Coast Air Basin for Sunday Minus Wednesday

NO 2 + h = O + NO O + O 2 + M = O 3 + M O 3 + NO = NO 2 + O 2 High VOC/NOx Low VOC/NOx

 O 3 inhibition ends about 0.5 to 0.7 hours earlier on Saturdays and about 1.1 to 1.3 hours earlier on Sundays.  O 3 inhibition ends earlier in downwind areas and later in areas of highest density of NO emissions.  The magnitude and spatial patterns of the ozone inhibition periods has not changed significantly between 1981 to  Duration of O 3 accumulation is shortest in the western basin and longest in the eastern basin.  Timing of maximum [O 3 ] has not changed over the last two decades in downwind areas of the basin, but has shifted about 1 to 1.5 hours later in the western and central basin.  Timing of maximum [O 3 ] does not vary significantly by day of the week. FINDINGS

 During , O 3 accumulation rates were highest in the central basin and higher on WDs than WEs at all sites. By , rates were highest in the eastern basin and higher on WEs than WDs in most of the basin.  On average, O 3 accumulation rates were cut in half during the 18-year period with largest reductions in the central basin.  Lower weekend O 3 accumulation rates in the 1980s offset shorter O 3 inhibition periods on WEs at central and eastern locations resulting in either no change or slightly lower [O 3 ] on WEs (i.e., no weekend effect).  In the 1990s, the O 3 accumulation rates were generally higher on WEs than WEs. Coupled with shorter inhibition periods, [O 3 ] was consistently higher on weekends during the 1990s.  A switch from lower to higher O 3 accumulation rates on WEs relative to WDs coincides with an increase in the magnitude and spatial extent of the weekend effect. FINDINGS (Continued)

 VOC/NOx 6-9 a.m.(Sat) /VOC/NOx 6-9 a.m.(Wed) ~ 1.05, 1.06, 1.17, and 1.18 VOC/NOx 6-9 a.m.(Sun) /VOC/NOx 6-9 a.m.(Wed) ~ 1.10, 1.17, 1.27 and 1.42 for , , , and , respectively. Similar differences in VOC/NOx ratios during time of maximum ozone.  VOC/NOx 6-9 a.m. during the 1990s are about 7 on weekdays, 8-9 on Saturdays and 9-10 on Sundays. Corresponding ratios at maximum [O 3 ] are on weekdays, on Saturdays, and on Sundays.  While greater carryover of VOC on weekends contributes to larger weekend morning VOC/NOx ratios, most of the increase is due to lower NO emissions during weekend mornings. FINDINGS (Continued)

Status of Project Weekend/Weekday Ozone Observations in the South Coast Air Basin PHASE I: Retrospective Analysis of Ambient and Emissions Data and Refinement of Hypotheses – Draft Report 8/23/00 – Final report 12/29/00 PHASE II: Field Measurements – Conducted field measurements in late-September to early-October, 2000 – Data Report on 4/15/01 PHASE III: Data Analysis and Final Report –Draft Reports on 8/15/01 –Synthesis of Results and final report due 11/15/01

Acknowledgments Douglas Lawson, National Renewable Energy Laboratory Brent Bailey, Coordinating Research Council Desert Research Institute –Barbara Zielinska, John Sagebiel, Mark McDaniel, Wendy Goliff, Larry Sheetz, and Mike Keith – Organic Analytical Laboratory –John Bowen California Air Resources Board –CCOS and CRPAQS –Monitoring and Laboratory Division –Research Division and Technical Bay Area Air Quality Management District South Coast Air Quality Management District