We find that this level, taking into consideration reasonable uncertainty, serves as an appropriate 

indicator of ambient conditions that may be conducive to the formation of O3 concentrations that 

approach or may exceed the proposed 8-hour average range. Proposed revisions for each state’s required 

monitoring season are based on the number of days equal to or above the 0.060 ppm level in and 

surrounding the state. EPA considered a number of factors including out of season exceedance days 

either before or after the current O3 monitoring season, the pattern of exceedance days in the out of 

season months, and regional consistency. We note that year-to-year variability occurs in seasonal O3

patterns based on highly variable meteorological conditions which can support the formation of early or 

late season elevated O3 concentrations in some years and not others. The EPA believes it is important 

that O3 monitors operate during all periods when there is a reasonable possibility of ambient levels 

approaching the level of the proposed NAAQS.

EPA’s analysis found that a total of 43 states experienced at least 1 day greater than or equal to 0.060 

ppm outside of the current O3 season; 21 states had exceedance days only before the required monitoring 

season; 4 states had exceedance days only after the required monitoring season; and 18 states had 

exceedance days both before and after the required monitoring season. In some cases, the frequency of 

exceedance days before the current O3 season was high, with 4 states (Colorado, South Dakota, Utah, 

and Wyoming) experiencing between 31 and 230 out-of-season exceedance days from 2010 to 2013 at 

monitors operating year-round. Although 43 states had at least one exceedance day outside the current 

monitoring season, changes are proposed for only 33 of those states.

The specific proposed changes to the required state O3 monitoring seasons are provided in Table 1 and 

shown in Figure 2. Table 2 provides the supporting details on the number of sites operating year-round 

by state, the time period of the exceedance days, and number of days with exceedances greater than or 

equal to 0.060 ppm before and after the current season for each state. The number of days greater than or 

equal to 0.060 ppm were used to determine the length of the proposed ozone seasons. EPA also 

considered regional consistency (seasons of surrounding states) in proposing a change to the season. To 

see the regional consistency of the seasons, see Figure 3 for a map of the proposed seasons color-coded 

by season. Please also refer to Attachment A for state-by-state maps of ozone monitors and exceedance 

days, Attachment B for the distribution of exceedance days by month and state, and Attachment C for 

detailed information regarding the AQS site ID numbers, exceedance dates, and daily 8-hour maximum 

values (dmax8) by state (Attachment C).
Figure 1. Map of Year-Round Monitors used in this analysis
Figure 2. Proposed Changes to Ozone Seasons
Figure 3. Map of ozone season regional consistency
Table 1. Ozone Monitoring Season Length – Proposed Changes*

State Name Current Ozone Season Proposed Ozone Season Net Change 

(Months)

Alabama March-October March-October 0

Alaska April-October April-October 0

Arizona January-December January-December 0

Arkansas March-November March-November 0

California January-December January-December 0

Colorado March-September January-December 5

Connecticut April-September March-September 1

Delaware April-October March-October 1

District Of Columbia April-October March-October 1

Florida March-October January-December 4

Georgia March-October March-October 0

Hawaii January-December January-December 0

Idaho May-September April-September 1

Illinois April-October March-October 1

Indiana April-September March-October 2

Iowa April-October March-October 1

Kansas April-October March-October 1

Kentucky March -October March -October 0

LA AQCR** 106 January-December January-December 0

LA AQCR** 019, 022 March -October March -October 0

Maine April-September April-September 0

Maryland April-October March-October 1

Massachusetts April-September March-September 1

Michigan April-September March-October 2

Minnesota April-October March-October 1

Mississippi March-October March-October 0
Missouri April-October March-October 1

Montana June-September April-September 2

Nebraska April-October March-October 1

Nevada January-December January-December 0

New Hampshire April-September March-September 1

New Jersey April-October March-October 1

New Mexico January-December January-December 0

New York April-October March-October 1

North Carolina April-October March-October 1

North Dakota May-September March-September 2

Ohio April-October March-October 1

Oklahoma March-November March-November 0

Oregon May-September May-September 0

Pennsylvania April-October March-October 1

Rhode Island April-September March-September 1

South Carolina April-October March-October 1

South Dakota June-September March-October 4

Tennessee March-October March-October 0

TX AQCR** 106, 153, 213, 214, 

216

January-December January-December 0

TX AQCR** 022, 210, 211, 212, 

215, 217, 218

March -October March -November 1

Utah May-September January-December 7

Vermont April-September April-September 0

Virginia April-October March-October 1

Washington May-September May-September 0

West Virginia April-October March-October 1

Wisconsin April 15-October 15 March-October 15 1.5

Wyoming April-October January-September 2
American Samoa January-December January-December 0

Guam January-December January-December 0

Puerto Rico January-December January-December 0

Virgin Islands January-December January-December 0

* EPA is proposing to require O3 monitors at NCore to operate year-round.

** AQCR – Air Quality Control Region
Table 2. Number of Days ≥ 0.060 ppm Before and After the Current Ozone Season by State

State FIPS State Name Current Ozone Season Proposed Ozone Season

Net Change 

(Months)

No. 

Ozone 

Sites

No. Year 

Round

% Year 

Round

No. 

days Time Period

No. 

days Time Period

1 Alabama March-October March-October 0 26 2 8 1 2/17 1 11/12

2 Alaska April-October April-October 0 5 2 40

4 Arizona* January-December January-December 0 40 35 73

5 Arkansas March-November March-November 0 10 9 90 1 2/13

6 California* January-December January-December 0 201 172 86

8 Colorado March-September January-December 5 53 34 64 60 1/7-2/28 19 10/1-12/31

9 Connecticut April-September March-September 1 13 3 23 3 3/19 -3/21 2 10/12 - 10/13

10 Delaware April-October March-October 1 7 6 86 4 3/10-3/25

11 District Of Columbia April-October March-October 1 4 3 75 3 3/19-3/21

12 Florida March-October January-December 4 62 59 95 13 1/24-2/28 5 11/1-11/23

13 Georgia March-October March-October 0 21 2 10 2 2/14-2/17

15 Hawaii* January-December January-December 0 2 2 100

16 Idaho May-September April-September 1 6 2 33 8 4/4-4/24 1 10/2

17 Illinois April-October March-October 1 44 26 59 6 2/13, 3/15-3/31 1 11/2

18 Indiana April-September March-October 2 46 3 7 2 3/21, 3/31 9 10/5-10/17

19 Iowa** April-October March-October 1 15 1 7

20 Kansas April-October March-October 1 11 11 100 17 2/12-3/31 1 11/1

21 Kentucky March -October March -October 0 32 5 16 3 11/11-11/13

22 N. Louisiana AQCR 019, 022 March-October March-October 0 3 3 100 1 11/1

22 S. Louisiana AQCR 106* January-December January-December 0 24 21 88

23 Maine*** April-September April-September 0 19 5 26 3 3/19, 3/12, 3/31

24 Maryland April-October March-October 1 21 6 29 10 2/17, 3/10-3/30

25 Massachusetts April-September March-September 1 19 5 26 5 3/7-3/31 2 10/9-10/10

26 Michigan April-September March-October 2 30 7 23 5 3/9-3/31 6 10/6-10/11

27 Minnesota April-October March-October 1 18 2 11 5 3/7-3/30

28 Mississippi March-October March-October 0 12 2 17 1 2/13 1 11/2

29 Missouri** April-October March-October 1 25 2 8 2 3/3, 3/11

30 Montana June-September April-September 2 9 7 78 9 4/20-5/17

31 Nebraska April-October March-October 1 7 2 29 4 3/24-3/29

32 Nevada* January-December January-December 0 37 16 43

33 New Hampshire April-September March-September 1 13 7 54 13 2/17, 2/18, 3/11-3/31 6 10/1-10/25

34 New Jersey April-October March-October 1 17 14 82 3 3/19-3/21

35 New Mexico* January-December January-December 0 27 24 89

36 New York April-October March-October 1 38 34 89 16 2/17, 3/7-3/30

37 North Carolina April-October March-October 1 49 7 14 17 2/17, 3/8-3/30

38 North Dakota May-September March-September 2 9 8 89 12 3/8-4/23 1 10/5

39 Ohio April-October March-October 1 53 6 11 13 2/17, 3/7-3/31 2 11/10, 11/13

40 Oklahoma March-November March-November 0 33 16 48 1 1/29

41 Oregon May-September May-September 0 10 1 10

42 Pennsylvania April-October March-October 1 59 50 85 14 3/7-3/31

44 Rhode Island April-September March-September 1 3 1 33 3 3/19, 3/20, 3/31

45 South Carolina April-October March-October 1 24 9 38 23 2/21, 3/3-3/31

46 South Dakota June-September March-October 4 7 7 100 31 3/5-5/30 6 10/3-10/6

47 Tennessee*** March-October March-October 0 27 5 19 6 2/13-2/21 4 11/1-11/13

48 N. Texas AQCR 022, 210, 211, 212, 215, 217, 218 March-October March-November 1 36 31 86 3 11/1-11/8

48 S. Texas AQCR 106, 153, 213, 214, 216* January-December January-December 0 47 44 94

49 Utah May-September January-December 7 30 13 43 230 1/1-4/30 19 10/1-12/31

50 Vermont April-September April-September 0 2 2 100 1 3/31

51 Virginia April-October March-October 1 30 4 13 13 2/17, 3/8-3/30

53 Washington May-September May-September 0 18 5 28 2 10/27, 10/28

54 West Virginia** April-October March-October 1 10 2 20 2 3/20, 3/21

55 Wisconsin April 15-October 15 March-October 15 1.5 37 5 14 15 3/5-4/14

56 Wyoming April-October January-September 2 34 24 71 55 1/29-3/31

72 Puerto Rico* January-December January-December 0 2 2 100

* Current ozone season is year-round. State not included in the ozone season analysis.

**Changed for Regional Consistency

***No Change for Regional Consistency

State Monitoring Info Days ≥ 0.060ppm Before Season, 2010-2013 Days ≥ 0.060 ppm After Season, 2010-2013