Environmental Engineering Reference
In-Depth Information
Table 1.
Reduction of peak ozone concentrations (ppb)
Urban areas
Area
Point
On-road
Non-road
99
02
99
02
99
02
99
02
Austin
3.4
3.7
8.3
7.2
9.8
23.4
3.5
3.8
San Antonio
<1
<1
8
7.9
15.5
21.5
6.5
3.2
Corpus Christi
1.5
<1
1.4
3.3
1.6
2.5
17
3.9
Victoria
1.4
<1
3.0
6.0
9.8
3.4
1.2
<1
4.2. Impact of zero out emissions on ozone exceedances (>75 ppb)
The number of exceedances is greater in 2002 especially in Victoria and Corpus
Christi indicating higher average ozone concentrations (
Table 2)
. These two regions
are neither NO
x
nor VOC sensitive and the model is less responsive to emission
reductions in these areas in 2002 with further decrease in total NO
x
and VOC
emissions. The uncertainty in response of modeled ozone to controls is enhanced
by meteorological variability. San Antonio and Austin are more responsive to
mobile emissions reductions in 2002 due to increased NO
x
sensitivity as explained
earlier. Pattern of spatial extent of exceedances is similar. The relatively low impact
area emissions have not been considered in this study.
Table 2.
Number of ozone exceedances (>75 ppb)
Urban areas
Base Case
Point
On-road
Non-road
99
02
99
02
99
02
99
02
Austin
1112
2019
870
1647
397
117
870
1486
San Antonio
1013
1708
620
1276
395
503
787
1514
Corpus Christi
325
1252
273
1252
285
1252
0
1252
Victoria
136
4585
101
4432
133
4517
133
4559
5. Summary
The preliminary results clearly depict the importance of considering multiple
ozone episodes in devising effective emission control strategies. There are sub-
stantial spatial differences in the effectiveness of control strategies with San
Antonio and Austin being most responsive to mobile emissions, Corpus Christi to
non-road and Victoria to point sources. Biogenic VOC emissions play a crucial
role in model sensitivity to controls. The study also reveals the challenge to reduce
exceedances under EPA's revised standards. The non-linear response of ozone to
NO
x
-VOC chemistry coupled with meteorological variations makes it imperative
that emission strategies be based on averaged response of model to multiple
episodes.
