A Conceptual Model for Eight-Hour Ozone Exceedances in Houston, Texas Part II: Eight-Hour Ozone Exceedances in the Houston-Galveston Metropolitan Area
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Key findings and recommendations of general interest are in boldface. From Chapter 2: 1. Background ozone may be defined as the lowest 8-h maximum ozone level on a given day within a region. (page 19) 2. When estimating background ozone for Houston, certain monitors with anomalously low ozone levels must be excluded. (page 19-21) 3. Annual variations in background ozone levels in Houston have a double peak, with high levels in the spring and late summer/early fall and low levels in early winter and early summer. (page 21) 4. The local contribution may be defined as the difference between the background ozone on a given day and the highest 8-h average on that day. (page 21) 5. In Houston, the highest local contributions occur in summer and the smallest occur in winter. (page 21) 6. The 8-h maximum ozone in Houston is a combination of these two annual cycles, resulting in a primary peak in August/September and a secondary, broader peak in May. (page 21) 7. The daily average 8-h maximum ozone in Houston at the August/September peak is 0.089 ppmv, which exceeds the 8-h ozone standard. (page 21) 8-h Ozone in Houston iv 1/29/05 8. Unlike other parts of central and eastern Texas, average local contributions to 8-h ozone in Houston during the summer are as large as background ozone concentrations. (page 22) From Chapter 3: 9. 8-h ozone exceedances occur about 10 times per year each in August and September and about 20 times per year during the period April-July. (page 24) 10. The average annual number of 8-h ozone exceedances at the various monitors ranges from 4 per year to 18 per year. (page 26) 11. Because some ozone monitors in Houston are strongly influenced by instrumental or local effects, it is not possible to determine the true spatial distribution of high levels of ozone within Houston. (page 28) 12. Efforts should be made to determine the true cause of the systematic station-tostation variations in ozone concentration. (page 28) From Chapter 4: 13. The highest 8-h design value for Houston peaked at 0.118 ppmv in 1998 and has fallen every year since then. (page 30) 14. The current (2002-2004) ozone design value for Houston is 0.102 ppmv, at station C53 (Bayland Park). (page 30) 15. A downward trend in design values is found at almost all monitors. (page 30) 8-h Ozone in Houston v 1/29/05 16. The characteristics of high ozone days may be determined by compiling statistics on the third through sixth highest 8-h days each year at each monitor. This is more robust than simply tracking the characteristics of the fourth highest 8-h day each year. (page 32) 17. At the individual monitors examined in detail, the background ozone levels on high 8-h ozone days has been steady over the past 11 years at around 0.065 ppmv. Over a shorter (7-year) period of record, the background ozone high 8-h days has been declining. (page 38) 18. The contribution of transient high ozone events to ozone levels on high 8-h ozone days has been relatively minor since 2002. (page 35) 19. The decline in importance of transient high ozone events may simply be a consequence of transient high ozone events preferentially occurring on low ozone days. (page 43) 20. At the examined stations, background ozone comprises 60% to 75% (on average) of the total ozone measured on high 8-h ozone days. This percentage was lower in the past. (page 32-38) 21. The average of the third to sixth highest annual background ozone levels (a measure of the design value in the absence of Houston emissions) has been falling steadily since the late 1990s and now stands at 0.062 ppmv. (page 41) 22. Days that violate the 8-h standard are more frequent than days that violate the 1-h standard. (page 44) 23. It is not clear from the statistical data whether emissions controls that bring Houston into compliance with the 1-h ozone standard would also bring it into compliance with the 8-h ozone standard. (page 44) 8-h Ozone in Houston vi 1/29/05 From Chapter 5: 24. Background ozone in Houston is most strongly correlated with meteorological parameters relating to a component of wind from the north on the day of the ozone and on each of the previous two days. (page 47) 25. Weaker winds also favor higher levels of background ozone. (page 47) 26. The local contribution to ozone is most strongly correlated with temperature (a positive correlation), with wind speed and the occurrence of precipitation both strongly negatively correlated. (page 47) 27. Temperature is not significantly correlated with background ozone levels. (page 47) 28. The difference in meteorological parameters related to background ozone and local contributions supports the approach of attempting to understand background ozone and local contributions separately. (page 47) 29. When the effects of wind speed and direction are excluded, precipitation appears to be an important suppressor of background ozone levels. (page 48) 30. When meteorological variables are controlled for by stepwise regression, there is significantly less local contribution on Sunday than on other days. (page 50) 8-h Ozone in Houston vii 1/29/05 From Chapter 6: 31. Regional-scale wind patterns are dominated by the sea breeze rotation, in which winds trace a circle or ellipse over the course of a 24-hour period. (page 52) 32. This rotation is not as apparent in surface observations over land, because nighttime winds tend to become calm in the lowest few tens of meters. (page 55) 33. The wind rotation leads to recirculation when large-scale mean resultant winds are smaller in magnitude than the amplitude of the sea breeze rotation, which is about 3 m/s (6 mph). (page 57) 34. The timing of recirculation is determined by the direction of the large-scale wind. (page 57) 35. The wind rotation leads to stagnation when large-scale mean resultant winds are only slightly weaker than the sea breeze rotation. (page 58) 36. Average 1-h maximum ozone levels are 0.090 to 0.110 when the 24-hour mean resultant wind at a nearby offshore buo
