In 1994, Drs. Allen S. Lefohn and Paul
J. Lioy, who have published extensively in the peer-review literature
on exposure-response, described in a New
Directions Column, in the distinguished journal Atmospheric
Environment, their concerns about the form of the 8-hour
ozone standard. One of the several concerns mentioned was the
use of averaging to develop a standard to protect vegetation.
The concerns are real and important. In addition, the "piston"
effect, as described elsewhere on the web pages, may make
it difficult to attain the 8-hour ozone standard at specific
levels of protection.
The figures below show the effect of using
8-hour averages to describe ozone exposure.
Both figures summarize the ozone data that
were collected on August 24, 1998. The figure on the right identifies
many more areas of concern than the figure on the left. The hourly
average ozone concentrations are the same in the two figures.
The difference is that the figure on the right averaged the hourly
concentrations over an 8-hour period, while the figure on the
left shows the maximum hourly value for the day. By applying
8-hour averages, the data are smoothed and provide the appearence
of greater areas of concern, which not necessarily be accurate.
Clinical health laboratory studies show that the peak hourly
concentrations are more important than extended-period average
concentrations (see Lefohn et al., 2018-Section 2). Thus, the
figure on the right may not be as relevant as the figure on the
left. Yet, the figure on the right uses the 8-hour average concentrations
similar in the manner that the 8-hour ozone standard is determined.
Some scientists and engineers are aware
that the United States 8-hour ozone standard may present a problem
that is called "unattainability." We discussed this
in our peer-review paper published in
1997. In November 1998, the topic was discussed at an international
meeting in Beijing, China. In the coming years, policymakers
will find that the 8-hour ozone standard will become more and
more difficult to attain as the 8-hour standard level is lowered;
control strategies may not work as planned. As the highest hourly
average concentrations are reduced as a result of emissions controls,
the remaining portions of the highest end of the distribution
of hourly average concentrations decline slower than the previously
highest values and make it more difficult to attain the 8-hour
standards. Independent analyses have confirmed the "piston
effect". EPA reports and papers published in 1985, 1995,
and 1996 confirm the effect. Lefohn et al. (2017) illustrate
the potential for the "piston effect" for some of the
ozone monitoring sites in the EU. A slide
presentation summarizing the "piston effect" is
available. More detailed information about the effect can be
found by clicking here.
On
EPA's web site (https://www.epa.gov/air-trends/ozone-trends),
the Agency in May 2022 summarized trends for the ozone periods
1980-2021, 1990-2021, 2000-2021, and 2010-2021. Note that the
national average for trends for the four time periods were 29%,
21%, 16%, and 5%, respectively. Clearly, the trend is slowing
down.
The "piston effect" makes it difficult to attain
the 8-hour standard for some sites. The
"piston effect" as described in the peer-review literature
and on this website affects the ability
of the nation to atttain the 8-hour ozone standard at many sites.
The peak hourly average concentrations (i.e., hourly average
concentrations greater than or equal to 0.10 ppm) are reduced
much faster than the mid-level concentrations (i.e., 0.06-0.099
ppm).
What is the cause
of the "piston effect"? Research appears to point to
the possibility that natural processes are partly responsibile
for the effect. Possible reasons for it have been discussed in
the literature (Reynolds et al., 2003; Reynolds et
al., 2004). The authors commented on possible chemical explanations
for the observation that more prominent trends in peak 1-h O3
levels than in peak 8-h O3 concentrations or in occurrences of
mid-level (i.e., 0.06 to 0.09 ppm) concentrations have been reported.
The authors noted that when anthropogenic VOC and NOx emissions
are reduced significantly, the primary sources of O3 precursors
are biogenic emissions and CO from anthropogenic sources. Chemical
process analysis results indicated that a slowly reacting pollutant
such as CO could be contributing on the order of 10 to 20% of
the O3 produced. There are other reasons for the "piston
effect" and our research continues on this very important
subject.
Is there a way to
get around the "piston effect". Probably not. We must
realize its existence and deal with it in implementing our national
ozone standards. If we do not, then it is possible that future
8-hour ozone standards, as they are lowered, may become a "goal"
instead of an attainable regulation. Thus, as the 8-hour ozone
standard continues to be lowered, there will be a level at which
the result of further emissions reductions may not yield attainability
because of the "piston effect." To learn more about
the "piston" effect, please click
here.
References
Lefohn, A.S., Shadwick,
D., Oltmans, S.J. (2010). Characterizing Changes of Surface Ozone
Levels in Metropolitan and Rural Areas in the United States for
1980-2008 and 1994-2008. Atmospheric Environment. 44:5199-5210.
Lefohn, A.S., Malley, C.S.,
Simon, H., Wells. B., Xu, X., Zhang, L., Wang, T. (2017). Responses
of human health and vegetation exposure metrics to changes in
ozone concentration distributions in the European Union, United
States, and China. Atmospheric Environment 152: 123-145. doi:10.1016/j.atmosenv.2016.12.025.
Lefohn, A.S., Malley, C.S.,
Smith, L., Wells, B., Hazucha, M., Simon, H., Naik, V., Mills,
G., Schultz, M.G., Paoletti, E., De Marco, A., Xu, X., Zhang,
L., Wang, T., Neufeld, H.S., Musselman, R.C., Tarasick, T., Brauer,
M., Feng, Z., Tang, T., Kobayashi, K., Sicard, P., Solberg, S.,
and Gerosa. G. (2018). Tropospheric ozone assessment report:
global ozone metrics for climate change, human health, and crop/ecosystem
research. Elem Sci Anth. 2018;6(1):28. DOI:
http://doi.org/10.1525/elementa.279.
Reynolds, S. D.; Blanchard, C. L.; Ziman,
S. D. (2003). Understanding the effectiveness of precursor reductions
in lowering 8-hr ozone concentrations. J. Air & Waste Manage.
Assoc. 53: 195-205.
Reynolds, S. D.; Blanchard, C. L.; Ziman,
S. D. (2004). Understanding the effectiveness of precursor reductions
in lowering 8-hr ozone concentrations - Part II. The Eastern
United States. J. Air & Waste Manage. Assoc. 54: 1452-1470.
