INTRODUCTION
Ozone and related pollutants
have long been recognized, in both clinical and epidemiological
research, as affecting public health. In accordance with sections
108 and 109 of the Clean Air Act, the U.S. Environmental Protection
Agency reviewed the air quality criteria and National Ambient
Air Quality Standards (NAAQS) for ozone (O3). Based on its review,
in December 1996, the EPA proposed to change both the human health
(primary) and secondary (welfare/vegetation) standards for O3
(U.S. EPA, 1996a).
Currently the existing primary
and secondary standards for O3 are each set at a level of 0.12
ppm, with a 1-hour averaging time and a 1-expected-exceedance
form. The standards are attained when the expected number of
days per calendar year with maximum hourly average concentrations
above 0.12 ppm is equal to or less than 1, averaged over 3 years
(i.e., no more than 3 exceedances in 3 years) (as determined
by 40 CFR Part 50, Appendix H). Current emissions control strategies
are focused on reducing the highest hourly average concentrations.
One of the proposed revisions announced by the Agency for O3
is a new 8-hour primary standard set at 0.08 parts per million
(ppm) with the third highest daily maximum concentration averaged
over 3 years. Based on rounding conventions, the Agency is considering
two possible versions of the primary 8-hour 0.08 ppm concentration-based
standard. One option that EPA is taking comment on is to use
a 0.081 ppm threshold (hereafter referred to as 081CB3); the
favored option by the Agency is to use a 0.085 ppm threshold
(hereafter referred to as 085CB3).
As of January 1997, under
the current 1-hour O3 standard of 0.12 ppm, there were 66 areas
and 273 counties designated by the U.S. EPA as nonattainment.
For the period 1993-1995, 46 areas violated the current 1-h standard.
Based on its analysis of 1993-1995 monitoring data, the EPA identified
335 counties that would violate the 085CB3 primary standard (Figure
1). No estimate was provided either for the number of areas represented
by these counties or the number of areas and counties that would
violate the 081CB3 primary standard.
For developing an appropriate
emission control strategy for meeting the proposed 8-hour standard,
it is important to identify the unique patterns of hourly average
concentrations that make up the 8-hour violations and investigate
the importance of mid-range concentrations (i.e., 0.06-0.09 ppm)
in defining these violations. For those sites whose 8-hour violation
is defined primarily by hourly average concentrations below 0.09
ppm, control strategies will have to change from focusing on
the higher hourly average concentrations to the mid-level (i.e.,
0.06 to 0.09 ppm) hourly average values (Lefohn, 1997). Given
the potential importance of the mid-level concentrations, it
is important to investigate whether the rate of decline of the
mid-level hourly average concentrations is similar to the rate
experienced by the high hourly average concentrations.
ROM emission control simulations
(U.S. EPA, 1985) have shown that mid-level hourly average concentrations
decline at a slower rate than the high-level values. Roselle
and Schere (1995) have presented similar evidence. The increased
resistance as one attempts to reduce the mid-level hourly average
concentrations is similar to the resistance experienced when
a gas is compressed by a piston. The resistance is initially
low, but the resistance increases as the piston compresses. The
implication of the "piston" effect is that sites whose
8-hour daily maximum violation is influenced by mid-level hourly
average concentrations may have a difficult time attaining the
proposed 8-hour standard.
To investigate the possible
importance of the "piston" effect, this report identifies,
for the period 1993-1995, the number of sites that violated the
proposed 8-hour 0.08 ppm (085CB3) O3 standard. A subset of violating
sites was identified in which the mid-level hourly average concentrations
played an important role in defining the violation. Using data
from the EPA's Aerometric Information Retrieval System (AIRS),
for trending O3 sites, we quantified the rate of decline (at
0.01 ppm increments) of the hourly average O3 concentrations
at the high-level (greater than or equal to 0.09 ppm), mid-level
(0.05-0.09 ppm), and low-level (< 0.05 ppm) ranges.
To investigate the relationship
between the rate of decline of the hourly concentrations and
the 8-hour average concentrations for those sites that experienced
trends, we compared the strength of the hourly trend with the
strength of the trends of the (1) annual 8-hour third highest
concentration, and (2) running 3-year average of the 8-hour third
highest value.
CONCLUSION
The number of potential violating
areas for the proposed 8-hour primary standard was quantified
and those violating sites that experience 4 or more hourly average
concentrations in the mid-range (between 0.06 and 0.09 ppm) that
define the 8-hour violations were identified. For the period
1993-1995, 71% of the areas that violate the 8-hour 081CB3 option
are heavily influenced by mid-level hourly average concentrations
(i.e., < 0.09 ppm); 55% of the areas are heavily influenced
by mid-level hourly average concentrations (i.e., < 0.09 ppm)
for the 085CB3 form of the standard. If one uses the list of
counties that the EPA has identified as violating the 085CB3
standard, 52% (88 of 169 areas) of all potential nonattainment
areas are heavily influenced by mid-level hourly average concentrations
(i.e., < 0.09 ppm). The remaining violating areas have sites
that currently experience hourly average concentrations during
the 8-hour period four or more times greater than or equal to
0.09 ppm. Almost all the design value sites either are presently
influenced by mid-level hourly average concentrations or will
be influenced in the future.
With almost all the design
value sites that violate the proposed primary standard, either
currently influenced by mid-level concentrations or potentially
influenced in the future (after reducing the higher values into
the mid-range), it is important to explore the efficiency of
reducing the mid-level values. For the 82 trending sites we investigated,
77% of the sites indicate a greater resistance to reducing the
hourly average concentrations in the mid-range values (i.e.,
0.06-0.09 ppm) than in the hourly average concentrations above
0.09 ppm.
Of the 82 monitoring sites
that showed trends, almost all experienced a stronger trending
for the hourly concentrations than the 8-hour annual third highest
8-hour daily maximum values or the two forms of the running third
highest 8-hour daily maximum concentration averaged over 3-years.
This indicates that for the 82 monitoring sites, the rate of
decline is slower for the 8-hour average concentrations than
the hourly values. Our results indicate that, for most sites
that violate the proposed 8-hour primary standard, the attainment
of the proposed 8-hour primary standard may be extremely difficult.
REFERENCES
Code of Federal
Regulations (1991). National primary and secondary ambient air
quality standards. C.F.R. 40:§50.
Roselle, S.J. and
K.L. Schere. (1995). Modeled response of photochemical oxidants
to systematic reductions in anthropogenic volatile organic compound
and NOx emissions. Journal of Geophysical Research. 100
(D11):22929-22941.
U.S. Environmental
Protection Agency (1985). Application of the first-generation
regional oxidant model to an assessment of the effects of proposed
1987 emissions reductions on episodic ozone levels in the northeastern
United States. Atmospheric Science Research Laboratory, Office
of Air Quality and Planning, Research Triangle Park, NC.
U.S. Environmental
Protection Agency (1996a) National Ambient Air Quality Standards
for Ozone: Proposed Decision. Federal Register 61, Number 241,
65715-65750. December 13, 1996.
