1. Weekday/Weekend O 3 and PM Differences in Three Cities Outside California CRC Project A-36B Betty K. Pun and Christian Seigneur AER, San Ramon, CA Warren White Washington University, St. Louis, MO “Weekend Effect” Research Workgroup Meeting 6 September 2000

2. Introduction CARB, NREL, CRC Data analysis Modeling Field study CRC Project A-36B is a data analysis project to study weekday/weekend differences in O 3 and PM in areas outside CA CRC Project A-36B is a data analysis project to study weekday/weekend differences in O 3 and PM in areas outside CA Chicago Philadelphia Atlanta

3. Objectives At 3 urban locations outside CA, study the day-of- the-week dependence of –Diurnal profile of hourly O 3 concentrations –Daily maximum 1-hour and 8-hour O 3 –PM 10 and PM 2.5 Test hypotheses for the “weekend effect” Identify changes in the weekday/weekend difference over a longer period

4. Technical Approach

5. Summary from Last Progress Report Ozone Day-of-the-week analysis –Atlanta, GA High Fridays, low Mondays –Chicago, IL High Sundays, low Tuesdays, Wednesdays –Philadelphia, PA High Sundays, low Fridays PM 10 (24-hour average) –Chicago, IL High during week, low Sundays

6. Atlanta PM 10 Day-of-the-week Behavior 1 station; July 98 to June 99 Jagged diurnal profiles 24-hour average : –Highest on Tuesdays –Lowest on Sundays –Difference significant at 10% level

7. Philadelphia PM 10 Diurnal Profiles 2 stations, 1995 to 1998

8. Philadelphia PM 10 Day-of-the-week Behavior 24-hour average: –Highest on Wednesdays or Thursdays ; lowest on Sundays –Difference between all weekday-weekend pairs significant at 5% level

9. Atlanta PM 2.5 Day-of-the-week Behavior 24-hour average PM 2.5 data 8 sites in 1999 Highest late work week and Saturdays –Thursdays (3), Saturdays (2), Fridays (2)*, Wednesdays (1) Lowest early work week and Sundays –Mondays (4), Sundays (3), and Tuesdays (1)* Statistically significant differences at 5 sites * statistically insignificant at 10% level

10. Chicago PM 2.5 Day-of-the-week Behavior 24-hour average PM 2.5 data 13 sites in 1999 High concentrations on Saturdays (6), Wednesdays (5), and Fridays (2) Low concentrations on Tuesdays (11), Thursdays (2) Statistically significant differences at all sites

11. Philadelphia PM 2.5 Profiles 1 station, 1999

12. Philadelphia PM 2.5 Day-of-the-week Behavior 24-hour average: –Highest on Saturdays, Tuesdays; lowest on Thursdays –Differences significant at 10% level

13. Hypothesis Testing

14. Hypothesis: VOC/NO x Ratio VOC/NO x ratio increases during weekend AtlantaChicagoPhiladelphia Weekday5.6 - 6.62.3 - 2.59.4 - 10.5 Weekend7.9 - 8.43.1 - 3.511.0 - 14.1 Increase in the VOC/NO x ratio during the weekend leads to increase in O 3 in a VOC-sensitive airshed

15. Hypothesis: VOC/NO x Ratio Correlation between O 3 and VOC/NO x ratio (grouped by day of the week) Atlanta ChicagoPhiladelphia Correlation coef. 0.04 0.98 0.67 R 2 0.00 0.96 0.45

16. Hypothesis: Friday/Saturday Night Traffic Predawn NO x (ppb) lower on weekends AtlantaChicagoPhiladelphia Weekday28 - 38 38 - 51 36 - 43 Weekend26 - 33 37 - 46 29 - 39 Predawn CO (ppm) higher on weekends AtlantaChicagoPhiladelphia Weekday0.76 - 0.830.69 - 0.73 0.72 - 0.75 Weekend0.90 - 0.990.75 - 0.78 0.87 - 0.91 Increased traffic on Friday and Saturday evenings leads to increased carry-over of precursors and increased O 3 formation potential on the weekends

17. Hypothesis: Friday/Saturday Night Traffic Correlation between O 3 and predawn NO x Atlanta ChicagoPhiladelphia Correlation coef. 0.70 - 0.58 - 0.57 R 2 0.50 0.34 0.32 Correlation between O 3 and predawn CO Atlanta ChicagoPhiladelphia Correlation coef. 0.11 0.77 0.92 R 2 0.01 0.59 0.85

18. Hypothesis: Precursor Carry-over Atlanta (sensitive to predawn NO x ) Correlation coef. R 2 –O 3 vs. previous day NO x 0.750.56 –O 3 vs. previous day NO y 0.870.76 Chicago, Philadelphia (sensitive to predawn CO) Correlation coef. R 2 –O 3 vs. previous day CO - 0.09, - 0.07 0.01, 0.00 –O 3 vs. previous day VOC 0.56, 0.31 0.31, 0.09 Carry-over of precursors from one day to the next is the driving force for the weekly cycle of O 3

19. Hypothesis: PM Affects Photochemistry In Atlanta, Chicago, and Philadelphia, PM 2.5 is high on Saturdays; evidence is weak for the decrease of fine PM emissions during weekends Weak correlation between O 3 and solar radiation Atlanta ChicagoPhiladelphia Correlation coef. 0.23 0.63 0.56 R 2 0.06 0.39 0.31 PM 2.5 correlates positively with solar radiation Reduction in fine PM emissions during weekends results in increased photochemical activity and O 3

20. Future Work Finish testing hypotheses –Changes in activity patterns Temporal distribution Spatial distribution –Changes in emission sources (VOC/PM composition) Analyze historical trends in O 3 weekend effect Complete final report