MEMORANDUN (2/11/04)

SUBJECT: API comments regarding relation between ozone 1-hour and 8-hour trends

FROM: Fred Dimmick, Group Leader

Air Quality Trends Analysis Group

API offered the following comment on the 8-hour NAAQS implementation proposal. 

First, the 8-hour standard is more stringent than the 1-hour standard and that past air 

quality trends information for 11 metropolitan areas show that, on average, 8-hour 

average ozone concentrations declined over the 1980-2002 period at about half the rate 

that 1-hour averages declined. API concludes that attainment for areas with 8-hour 

values a certain percentage above the standard to take relatively more time to attain the 8-

hour standard than areas with 1-hour values the same percentage above the 1-hour 

standard would take to attain the 1-hour standard. 

In API comments on the 6/2/03 notice, they provided the figure below as a comparison 

for selected metropolitan areas of median decadal trends in annual maximum 1-hour and 

4th-highest maximum daily 8-hour ozone concentration, 1980 through 2002.

API notes that for all areas, 8-hour design values decrease at a slower rate than the 1-hour 

values. API goes on to find, on average 8-hour values have decreased at about half the 

rate as 1-hour values, attesting to the fact that the 8-hour standard will be much more 

difficult to attain
Response

 

First, from a program implementation perspective, it is not necessarily informative to 

project future program effectiveness based on an analysis of progress from the 1980's to 

2002. Programs designed to address the 1-hour ozone NAAQS were not necessarily 

designed to reduce 8-hour ozone levels at some prescribed rate. See Figures 1 and 2 for 

long-term national trends. Note, the relationship between 1-hour and 8-hour trends for 

recent years differs from the relationship early in this 20-year comparison. The 1-hour 

ozone programs were most effective in the early years. In the most recent 10-years, 

neither the 1-hour nor the 8-hour ozone levels have decreased as they did in the first 10 

years. Also, different rates of improvement occurred at different locations; program 

effectiveness varied across the country -- more or less NOx and VOC bringing about 

differing program needs/results. The EPA is currently focusing on NOx reductions as 

needed to address the lingering 8-hour ozone problems1. In the 1980s and 1990s, the 

focus was on VOC reductions (which can be seen in national emission trends where NOx 

was reduced considerably less than VOC). With the emerging national ozone programs 

addressing NOx much more than previously, the rate of progress for 8-hour (contrasted to 

1-hour) is not expected to be well related to previous progress. 

Second, the technical analyses provided do not support a broadly applicable percentage 

such as the 50% suggested. The API developed a ratio of 50% for comparisons using the 

1-hour ozone design values compared to 8-hour ozone design value for identified 

metropolitan trends; in the same figure, they showed widely varying ratios for the 

identified metropolitan trends. Georgia DNR's performed a similar analysis for their 

comments found a different relationship. With this variability it is hard to say that the 

50% ratio represents a broadly applicable relationship. The ratio of 8-hour to 1-hour 

ozone levels changes over time, approaching a ratio of 8/10 for 90th percentile 8-hour 

ozone levels to 1-hour ozone levels. By comparing the annual 2nd (1-hour) and 4th (8-

hour) maxima at the higher ozone levels across the country, Figures 3 and 4 show how 

they have compared over time. Figure 4 focuses on the most recent 10 years and shows

the kind of relation that occurred as the 1-hour ozone programs were implemented. If 

this pattern continues, and the 1-hour ozone program does not adapt to an 8-hour ozone 

program, then there may well be a constant relationship and both 8-hour and 1-hour 

would make similar progress over time. On the other hand, with the emerging ozone 

program addressing NOx much more than previously, the rate of progress for 8-hour 

ozone levels (contrasted to 1-hour ozone levels) would not be well related to previous 

progress. 

 

1

In developing the NOx SIP call, EPA followed work done by the Ozone Transport Assessment Group 

(OTAG), which was active from 1995-1997 and consisted of representatives from the 37 States in that 

region; the District of Columbia; EPA; and interested members of the public, including industry and 

environmental groups. The OTAG completed the most comprehensive analysis of ozone transport that had 

ever been conducted, developing technical data, including up-to-date inventories and state-of-the-art air 

quality modeling, to quantify and identify the sources of interstate ozone transport. The OTAG concluded 

that regional NOx emissions reductions are effective in producing ozone benefits, while VOC controls are 

effective in reducing ozone locally and are most advantageous to urban nonattainment areas.
In summary, the suggested “percent” adjustment does not address current program 

direction; would be difficult to develop analytically (considering the wide variations 

found), and therefore does not appear to represent a broadly applicable relationship.

1hr

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Mean

10th%

90th%

Linear (Mean)

Figure 1. Plot of 20-year trend for 1-hour ozone (annual 2nd max) national air quality 

levels

8hr

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Mean

10th%

90th%

Linear (Mean)

Figure 2. Plot of 20-year trend for 8-hour ozone (annual 4th max) national air quality 

levels
Ratio of 8-hr to 1-hr Ozone Levels

0.6

0.65

0.7

0.75

0.8

0.85

1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

90th%

10th%

Mean

Figure 3. Plot of Ratio of 10th%, Mean, and 90th% ozone levels for 20 years.

Mean Ozone Levels 10-yr Trend

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

ppm

1hr

8hr

Ratio/10

Ratio of 8-hr ozone to 1-hr ozone 

concentrations divided by 10

Figure 4. Plot of trend of mean ozone 1-hour and 8-hour levels with ratio of the two 

levels.