Spatial extent of ozone exceedances

Ozone exceedance values are important from the standpoint of health and hence from a regulatory perspective. Figures 4 and 5 characterize the spatial pattern of 8-h ozone exceedances greater than 84 ppb and 74 ppb for a

Table 3. Model-predicted maximum ozone concentrations (ppb) at South Texas urban sites.

Perturbed temperatures

Table 3. Model-predicted maximum ozone concentrations (ppb) at South Texas urban sites.

NNA sub domain

CAMS

Base case

Future case

+2°C

+3° C

+4°C

+5°C

+6°C

Austin

03

81.36

76.41

81.41

82.86

84.1

85.19

86.17

38

82.07

77.12

82.62

84.36

85.88

87.24

88.51

San Antonio

23

88.86

81.6

87.15

88.77

90.2

91.51

92.71

58

94.43

85.52

91.04

92.57

93.91

95.11

96.22

59

70.06

64.58

68.86

69.66

70.3

70.85

71.33

Corpus Christi

04

71.82

67.21

70.7

71.83

72.85

73.75

74.55

21

76.79

70.84

74.77

76.11

77.3

78.37

79.34

Victoria

87

76.29

71.98

76.34

77.35

78.21

78.96

79.63

Fig. 4. Spatial pattern of ozone exceedances >84 ppb.

Fig. 4. Spatial pattern of ozone exceedances >84 ppb.

6°C temperature perturbation, which was chosen as a representative case for the future scenario. The maximum impact is noted downwind of the San Antonio region (^94ppb), and is shown in Fig. 4. This is followed by northwest Austin and the counties north-east of Victoria, which are affected by transboundary pollution from Houston. Corpus Christi is less affected compared to the inland urban regions due to sea-breeze influence on air pollutants. In the case of the extreme 6°C perturbed scenario, the

Fig. 5. Spatial pattern of ozone exceedances >74ppb.

ozone exceedances become considerably more pronounced and widespread for levels greater than 74ppb throughout the NNA domain, as shown in Fig. 5. The magnitude of surface ozone concentrations is once again highest in the San Antonio region, although ozone exceedances tend to be more prevalent in Austin, as shown in Table 5. The dependence of ozone formation on its precursors is nonlinear as described by Lin et al.,17 and hence the ozone formation in each urban region is dependent on the VOC/ NOx concentration ratios in the atmosphere. The ozone formation and destruction can occur in regions far away from their sources of emissions.

Furthermore, an analysis of the number of grid cells or area impacted by the 8-h ozone exceedances with rising temperature perturbations was undertaken. As shown in Table 5, for the first 2°C temperature augmentation there is a substantial increase of area with exceedances relative to the base case for the major portion of the 4 km domain. Thereafter, with each degree rise in temperature, the percent increase in area with 8-h ozone exceedances relative to the immediately preceding perturbation case declines, reinforcing the fact that certain portions of the modeling domain are more affected than others by temperature perturbations. As expected, the spatial extent of ozone exceedances increases significantly in comparison to the base case with rising temperatures as shown in Tables 4 and 5.

Table 4 portrays the fact that amongst the NNA regions, the San Antonio counties are the only area impacted by the 8-h ozone exceedances

Table 4. Number of grid cells exceeding 84ppb.

Perturbed temperature

Perturbed temperature

Table 4. Number of grid cells exceeding 84ppb.

Future case

+2 ° C

+3°C

+4° C

+5°C

+6°C

Austin

0

0

10

39

63

97

San Antonio

10

44

60

74

88

103

Corpus Christi

0

0

1

12

19

35

Victoria

0

0

0

0

0

0

Entire domain

10

63

98

161

247

409

Table 5. Number of grid cells exceeding 74ppb.

Perturbed temperature

Table 5. Number of grid cells exceeding 74ppb.

Perturbed temperature

Future case

+2 ° C

+3°C

+4° C

+5°C

+6°C

Austin

76

331

480

564

630

687

San Antonio

113

195

210

225

236

257

Corpus Christi

48

109

132

149

165

182

Victoria

0

37

93

126

134

146

Entire domain

430

1570

2220

2751

3150

3466

in the future case. Austin, Corpus Christi, and Victoria NNA regions have no instances of ozone exceedances in this case. However, Austin and Corpus Christi regions reveal a significant increase in the number of grid cells with ozone exceedances for the 4°C-6°C temperature perturbation cases relative to the unperturbed case. Overall, the inland areas show a much greater impact with increasing temperature perturbations than the coastal regions which is also consistent with the base case results.

Table 5 shows that in the event the standards are lowered in the future to 74 ppb, and that the pattern of potential spatial representation of exceedances in the future also changes. Austin will now have a greater spatial extent of exceedance values compared to San Antonio with increasing temperature perturbations. In the unperturbed case, although San Antonio still possesses the largest number of grid cells exceeding the standards with rising temperature perturbations, Austin has the largest percent of area affected by ozone exceedances. Lefohn et al.18 analyzed that implementation of controls triggers fastest deterioration of ozone concentrations in those sites with highest daily maximum 8-h averaged concentrations relative to other sites. San Antonio seems to have benefited the most in the future scenario relatively. With the increasing temperature perturbations the ozone exceedances in the event of a lowered standard is not as extensive in

San Antonio as in Austin. Corpus Christi and Victoria also show substantial impact in this case in comparison to the case where ozone is greater than 84ppb. This illustrates the increasing need of considering climate change scenarios while evaluating the impact of a tighter ozone standard.

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