A Conceptual Model for Eight-Hour Ozone Exceedances in Houston, Texas Part I: Background Ozone Levels in Eastern Texas
Abstract
Key findings and recommendations of general interest are in boldface. Page numbers
are shown in [brackets].
From Chapter 1:
1. As has been done by TCEQ, it is useful to consider the daily ozone levels in a
particular area as being the sum of two quantities: the background ozone and the
local contribution. (page 10)
2. The background ozone is defined as the ozone level that would be attained if there
were no local anthropogenic (or unusual biogenic) emissions of ozone precursors. (page
10)
3. The local contribution is the difference between the 8-hour maximum background
ozone and the 8-hour maximum actual ozone. (page 10)
From Chapter 2:
4. The lowest 8-hour maximum cannot be taken to be the background ozone level in
Dallas-Fort Worth (DFW) or Houston-Galveston-Brazoria (HGA) because the lowest
maximum is often found in the urban core or other sites affected by local emissions.
(page 13)
5. A particular limited set of stations, assumed to be measuring pristine ozone under
appropriate wind conditions, is used in DFW and HGA for estimating the background
ozone in those two areas. (page 13-14)
6. Individual stations within the limited set typically, but not always, record the
lowest 8-hour ozone when the wind blows from the station toward the metropolitan area.
(page 15-19)
7. Other regions had few monitors, so none were excluded. (page 15)
8-Hour, Part I Page 4 of 52 1/29/05
From Chapter 3:
8. Taking DFW as an example, there is year-to-year variability in the timing and
magnitude of peaks of background ozone, but its interannual variability is sufficiently
small that a multi-year average is an appropriate measure of typical conditions. (page 19)
9. Day-to-day variability of background ozone can be a factor of two or more,
especially during late summer and early fall. (page 20)
10. There is relatively little variability of background ozone during the winter and
during occasional periods in the early summer. (page 20)
11. The regular annual variation is a fundamental component of background
ozone. (page 20)
12. All regions of eastern Texas have the same basic pattern of annual variation
of background ozone. Background ozone is low in December but starts rising
steadily in mid-January. A secondary maximum of background ozone is reached in
mid-May, followed by a period with lower ozone values. The overall maximum in
each region occurs in August or September, followed by a decline through the fall.
(page 21-22)
13. Northeast Texas (NETX) appears to have the highest background ozone
concentrations through the year. (page 22)
14. Differences in background ozone between NETX and DFW suggest that, because
of sampling problems, absolute background ozone levels cannot be compared between
regions with many sensors and regions with few sensors. (page 22)
15. DFW reaches a relative minimum in background ozone around the end of June
and an absolute maximum in late August, while HGA reaches a relative minimum in
early July and an absolute maximum in mid-September. (page 23)
16. The decline in ozone during the summer is largest in the southern regions and
smallest in the northern regions. (page 23)
17. Background ozone levels as computed by the present method are suspiciously low
at Beaumont-Port Arthur, suggesting that the sampling network has local source issues.
(page 23)
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18. The local contribution at DFW makes up less than a third of the total 8-hour
ozome maximum on average days. (page 24)
19. The local contributions at DFW and HGA peak in July and August. (page 24-25)
20. The local contribution at HGA is generally a greater percentage of the total ozone
than at DFW and reaches an average value of 0.035 ppmv in August. (page 25)
21. The local contribution at HGA averages 0.010 ppmv greater than at DFW,
while the background at HGA averages 0.010 ppmv less than at DFW, leading to
similar mean 8-h ozone levels. (page 25)
22. Background ozone levels at HGA and DFW correlate most strongly with the wind
direction (and component from the north) on the previous day, with slightly lower but
still highly significant correlations on the same day and two days previous. (page 26)
23. Also contributing to background ozone are weak winds at HGA and a lack of
precipitation at DFW. (page 27)
24. The local contribution at DFW and HGA is positively correlated with high
temperatures, low wind speeds, and a lack of precipitation. (page 27)
25. On days without precipitation, the average 8-h maximum ozone in HGA in
early September exceeds the 8-h standard. Ozone in DFW comes close. (page 28)
From Chapter 4:
26. Principal Component Analysis (PCA) is useful for representing the large-scale
winds, which may be expected to strongly control background ozone, as a small number
of continuously-varying patterns. (page 30)
27. Mean ozone-season winds are from the southeast in the HGA area and from the
south in the DFW area, rotating clockwise around a center of high pressure in the
southeastern United States. (page 31)
28. The leading principal component (PC1), which explains the greatest amount of
variability in the wind pattern, represents winds from the southwest (if positive) or
northeast (if negative). It therefore is an indicator of the presence (or absence, if positive)
of transport from the central and eastern United States. (page 31)
8-Hour, Part I Page 6 of 52 1/29/05
29. The second principal component (PC2) is positive with winds from the northwest.
(page 31-32)
30. The total wind on any given day is the sum of the mean wind and the daily
amplitudes of the various principal components. (page 33)
From Chapter 5:
31. The principal component most strongly correlated with background ozone is
PC1, followed by PC5 and PC2. (page 37)
32. The correlations are strongest with the wind pattern one day before the ozone
event. (page 37)
33. By itself, PC1 explains background ozone variations of about 0.017 ppmv at
DFW and over 0.020 at HGA. (page 38)
34. The mean value of PC1 declines steadily from spring to fall, consistent with
the expected increasing prevalence of continental transport in late summer and
early fall. (page 41)
35. The intraannual variation of PC1 explains the early fall background ozone peak,
but does not explain why spring experiences higher background ozone than early
summer. (page 42)
36. PC1 is much less variable in the summer, so days with PC1 less than –8
(favorable for high background ozone) are somewhat more common in spring than
summer. The summer sees extended periods of onshore transport. (page 42)
37. Even with a given wind pattern (PC1), background ozone levels are higher by
close to 0.010 ppmv in spring than in early fall. (page 43)
38. The PC1 values on two days together are a better predictor of background
ozone than a single-day PC1 value. This finding is consistent with the expectation
that extended transport from the continent is more favorable for high background
ozone than single days with favorable winds. (page 45)
39. Winds from the southwest carry high ozone if they were recently from the
northeast. (page 46)
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40. At given values of PC1 at zero and two-day leads, background ozone in HGA is
0.010 ppmv to 0.018 ppmv higher in spring than in summer. This suggests that the
relatively high background ozone in springtime high regardless of the day-to-day weather
patterns. (page 47)
41. Previous research suggests that high springtime ozone values are a consequence
of the high lifetime of ozone in spring combined with a wintertime buildup of NOx. (page
48-49)
42. DFW background ozone is less sensitive to details of transport than HGA
background ozone. (page 49)
Department
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