- The use of different
air quality markers (metrics) for surface ozone calculated from
the same time series can result in different trend patterns.
This outcome is important to researchers, as well as policymakers
and regulators, who use exposure metrics to assess how changes
in ozone levels affect human health, vegetation, and climate.
one conclusion from a new metrics assessment based on the Tropospheric
Ozone Assessment Report or TOAR, an effort by the International
Global Atmospheric Chemistry Project to create the world's largest
database of surface ozone observations from all available ozone
monitoring stations around the globe. The TOAR paper, Global
surface ozone metrics identified for climate change, human health,
and crop/ecosystem research, was published in the journal
Elementa: Science of the Anthropocene. The list of metrics
used in the TOAR program can be downloaded here. The paper is available at the Elementa
website at: https://www.elementascience.org/article/10.1525/elementa.279/
24 international researchers who worked on the paper anticipate
that their effort will provide scientists, regulators, and policymakers
with better insight about spatial and temporal variation that
relate to climate change, human health, and crop/ecosystem around
paper provides the following:
description of 25 metrics, which are used for assessing spatial
and temporal trends by environmental agencies and researchers
around the world (4 for model-measurement comparison, 5 for characterization
of ozone in the free troposphere, 11 for human health impacts,
and 5 for vegetation impacts).
The scientific rationale for the selection of each of the 25
A detailed description of the statistical methods based on stringent
scientific principles used in the Tropospheric Ozone Assessment
Report (TOAR) program.
A comparison of the trend behavior for each of the ozone impact
metrics when using the same surface ozone concentration time
components of the Tropospheric Ozone Assessment Report (TOAR)
(http://www.igacproject.org/activities/TOAR) are the use
of metrics that are biologically defensible, as well as the use
of statistical methods that adhere to stringent scientific principles.
This paper provides the background for the selection of the metrics
and the statistical methods used in the international TOAR program.
- Background ozone
is an important part of the challenge to attaining the new 0.070
ppm ozone standard. Although the EPA is continuing the ozone
area designation process, the Agency is still concerned about
the effect that background has on attainment of the 2015 ozone
standard. Specifically, the key reasons that the EPA proposed
a delay in 2017 to implementing the new ozone standard were
understanding the role of background ozone levels;
accounting for international transport; and
consideration of exceptional events demonstrations.
is much controversy on what the range of background ozone is
in the United States. Our research is indicating that frequent
occurrences greater than or equal to 50 ppb that occur at both
high- and low-elevation monitoring sites across the US are influenced
by transport from the stratosphere to the lower troposphere.
The enhanced ozone concentrations that appear to be related to
stratospheric transport occur during the springtime and sometimes
during the summertime. In addition, long-range transport of Eurasian
biomass burning, as well as wildfires in the US, contribute to
background ozone concentrations. Estimating the range of background
ozone properly is important because the range of background concentrations
is used in the EPA's risk assessment for human health and vegetation,
as well as assessing the amount of emission reductions required
to attain a specific ozone level (i.e., standard). If the actual
background level of ozone is higher than EPA estimates with models,
then the Agency may overestimate human health, as well as vegetation
risks and present inaccurate information to the public and policymakers.
Our published material on background ozone can be found here. Our current
research continues to address how to integrate background ozone
with the attainment process.
- Since A.S.L.
& Associates' founder, Dr. Allen Lefohn, participated like
others in the first Earth Day on April 22, 1970, we have seen
much progress in controlling environmental pollution and improving
the Nation's health and welfare (i.e., vegetation). For example,
the US EPA began to regulate ozone with the promulgation of a
ground-level National Ambient Air Quality Standard (NAAQS) in
1971, with subsequent revisions in 1979, 1997, 2008, and 2015.
Following promulgation of the 1997 ozone standard, the US EPA
issued a NOx State Implementation Plan (SIP) Call, which reduced
regional summertime NOx emissions from power plants and other
large stationary sources by 57% in 22 Eastern US states. In addition,
the US EPA established national rules that substantially reduced
NOx and VOC emissions from on-road mobile sources by 53% and
77% between 1990 and 2014, respectively. Overall, NOx and VOC
have decreased in the US by 52% and 39% from all sources since
in the magnitude of national and regional emissions, as well
as any long-term changes in international emissions, climate,
and inter-annual meteorological variability, can drive shifts
in the distributions of hourly surface ozone (O3) concentrations.
Changes in the distributions of hourly average O3 concentrations
can result in changes in the magnitude of exposure metrics used
for assessing human health and vegetation effects. Surprisingly,
trend patterns in O3 exposure metrics may be in a similar direction
as emissions changes (e.g., metrics increase as emissions increase)
or may not (Lefohn et al., 2017 - see publications list). This
is a very important observation because if a biologically irrelevant
O3 metric is selected for assessing trends, an incorrect conclusion
may be drawn concerning the relationship between emissions reductions
and the protection of the public's health and welfare. Over the
past 20-30 years, substantial changes in O3 concentrations have
been observed at many sites across the world, likely driven by
a combination of the large emissions changes and potentially
by shifts in various meteorological conditions. A recent paper
by Lefohn et al. (2017) investigated the relationship between
exposure metrics, hourly O3 concentration distributions, and
emission changes. To achieve this, we analyzed the response of
14 human health and vegetation O3 metrics to long-term changes
in the hourly O3 concentration distribution, as measured at 481
monitoring sites in the EU, US, and China. The study provides
insight into the utility of using specific exposure metrics for
assessing emission control strategies. One important aspect of
the study was that trends in mean or median concentrations did
not appear to be well associated with some of the exposure metrics
applicable for assessing human health or vegetation effects.
Additional insights concerning the relationships between emissions
reductions, hourly average concentration distributions, and human
health and vegetation exposure metrics are discussed in Lefohn
et al. (2017) (see publications list).
October 2015, the EPA announced that both the human health and
vegetation ozone standards will now be 70 ppb. Prior to that,
on November 26, 2014, the EPA Administrator proposed an ozone
human health (primary) standard in the range of 65 to 70 ppb
and indicated that she would take comment on a standard as low
as 60 ppb. The EPA Administrator noted that she placed the greatest
weight on controlled human exposure studies, citing significant
uncertainties with epidemiologic studies. Reasons for placing
less weight on epidemiologic-based risk estimates are key uncertainties
about (1) which co-pollutants are responsible for health effects
observed, (2) the heterogeneity in effect estimates between locations,
(3) the potential for exposure measurement errors, and (4) uncertainty
in the interpretation of the shape of concentration-response
functions for ozone concentrations in the lower portions of ambient
distributions. The health standard is mainly based on the controlled
human exposure study of Schelegle et al. (2009) that reported
clinical effects at 72 ppb. Dr. Milan Hazucha of UNC Chapel Hill
and I (Allen S. Lefohn) designed the ozone hourly exposure regimes
used in the Schelegle et al. (2009) study. To the 72 ppb threshold
of effects resulting from the Schelegle et al. (2009) study,
the Administrator applied a Margin of Safety that helped her
establish the ozone health standard below the 72 ppb level. Although
the EPA initially recommended a separate exposure metric for
the secondary standard (the W126 vegetation metric), the EPA
adopted the 8-hour standard of 0.070 ppm to protect vegetation.
The Agency felt that the 3-month, 12-h W126 exposure index
used for assessing vegetation effects could be controlled to
a level of 17 ppm-h or less by using the 8-hour standard. Industry
and environmental organizations
are back in court contesting the decision of the 8-hour ozone
standard set at the 0.070 ppm level.
perspective is important in understanding the background concerning
the events that led to the EPA's Administrator's decision on
revising the 8-hour ozone standard in October 2015. On March 12, 2008, the EPA Administrator
announced a decision on the human health and vegetation ozone
standards. At that time, EPA revised the 8-hour "primary"
ozone standard, designed to protect public health, to a level
of 0.075 parts per million (ppm). The previous standard, set
in 1997, was 0.08 ppm. EPA decided not to adopt the cumulative
exposure index as the vegetation standard (i.e., secondary ozone
standard). Although the EPA Administrator recommended the W126 index as the secondary ozone standard,
based on advice from the White
Post, April 8, 2008; Page D02), the EPA Administrator made the
secondary ozone standard the same as the primary 8-hour average
standard (0.075 ppm). On May 27, 2008, health and environmental
organizations filed a lawsuit arguing that the EPA failed to
protect public health and the environment when it issued in March
2008 new ozone standards. On March 10, 2009, the US EPA requested
that the Court vacate the existing briefing schedule and hold
the consolidated cases in abeyance. EPA requested the extension
to allow time for appropriate EPA officials that were appointed
by the new Administration to review the Ozone NAAQS Rule to determine
whether the standards established in the Ozone NAAQS Rule should
be maintained, modified, or otherwise reconsidered. After an
extensive review process, the Obama Administration decided to
not revise the 0.075 ppm standard that was set during the Bush
Administration. The reason provided was that the EPA would soon
begin a new review cycle of the science associated with surface
ozone and recommend whether the 0.075 ppm standard needed to
- For several
years, A.S.L. & Associates has had an on-going effort to
better understand the range and frequency of occurrence of background
ozone levels that may not be affected by emission reduction strategies.
In a paper published
in May 2001, the research team consisting of Allen Lefohn, Samuel
Oltmans, Tom Dann, and Hanwant Singh discussed that background
ozone levels were higher and that the natural short-term variability
was more frequent and greater than previously believed. The authors
are associated with the following institutions: A.S.L. &
Associates, NOAA, Environment Canada, and NASA, respectively.
In our 2001 paper, we concluded that hourly levels greater than or equal to 50 ppb occur
more frequently as a result from natural sources than previously
2006, the US EPA defined Policy-Relevant
Background (PRB) for ozone as those concentrations that would
occur in the United States in the absence of anthropogenic emissions
in continental North America (i.e., the United States, Canada,
and Mexico). PRB concentrations (renamed by the EPA as North
American Background (NAB)) include contributions from (1) natural
sources everywhere in the world and (2) anthropogenic sources
outside the United States, Canada, and Mexico. In 2008, we published
results, using empirical data, confirming that at some locations
in the US, PRB ozone concentrations are greater than or equal
to 50 ppb. In late September
2009, the National Research Council released the report, Global
Sources of Local Pollution. In the report, the Committee
states that modeling and analysis supports the finding that Policy-Relevant
Background (PRB) is 20-40 ppb for the United States. Unfortunately,
the NRC conclusion does not agree with the peer-review literature
using empirical data that hourly averaged PRB ozone concentrations
are greater than or equal to 50 ppb. Although spatially low-resolution
models have been exercised and indicated that conclusions reached
by Lefohn et al. (2001) were incorrect, our current research
and the results published by other research groups support the
conclusions reached by Lefohn et al. (2001) that PRB ozone
concentrations are greater than or equal to 50 ppb at both high-
and low-elevation monitoring sites. Clearly, low-resolution models
are unable to adequately capture the important processes that
are important for characterizing PRB and therefore, underestimate
policy relevant background concentrations. The latest results
using GEOS-Chem models continue to underestimate PRB ozone concentrations.
An Internet-based slide
is available for purposes of previewing our paper. Also please
be sure to check out the answer to our quiz that identifies
the month in which the highest 8-hour daily maximum concentration
occurred for the 4 remote ozone monitoring sites. Additional
information on background ozone concentrations can be found in
the Air Quality Analyses section of our Table of Contents. In-depth
discussions are provided on this very important topic.
- The range of suggested
values for the W126 ozone vegetation standard is in part
historically based on the recommendations that were made at a
Workshop that took place in Raleigh, North Carolina in 1996.
To better understand what took place at this workshop, please
click here. The EPA recommends an accumulation
over a 12-hour (8 am 8 pm) exposure period over a 3-month
period giving greater weight to exposures at higher levels of
ozone. Our analyses and peer-reviewed published papers indicate
that such a secondary
ozone standard, in the proposed form, would overestimate vegetation
effects. For information about why the use of a 12-hour versus
a 24-hour accumulation period would contribute
to the inconsistency problem of the W126 index, please click here. You can learn more about the subject
of vegetation effects by visiting our Table of Contents web page.
- Lefohn, Shadwick,
and Oltmans (2010) have statistically quantified in a paper published
in the peer-reviewed journal, Atmospheric Environment,
a site-by-site trending analysis for the period 1980-2008 and
1994-2008. Lefohn et al. (2010) point out that many ozone monitoring
sites show no statistical changes over time as well as a small
number of sites show increases in trending. Please see the publications list for the citation.
As indicated above, Lefohn et al. (2010)
published their trending findings for surface ozone monitoring
sites across the United States. Using statistical trending on
a site-by-site basis of the (1) health-based annual 2nd highest
1-hour average concentration and annual 4th highest daily maximum
8-hour average concentration and (2) vegetation-based annual
seasonally corrected 24-hour W126 cumulative exposure index,
they investigated temporal and spatial statistically significant
changes that occurred in surface ozone in the United States for
the periods 1980-2008 and 1994-2008. For more information about
the Lefohn et al. (2010) and Lefohn et al. (2008) (for the period
1980-2005 and 1990-2005) findings, please click
Since 1997, we have
been discussing the "piston effect" in the peer-reviewed
literature (see publications listing).
In 1997, we predicted that there would be a leveling off of improvements
in O3 concentrations as O3 emission precursors were reduced at
some monitoring sites Our prediction apparently has been verified
by the most current trends analysis by the EPA. On EPA's web
the Agency in July 2016 summarized trends for the ozone periods
1980-2015, 1990-2015, and 2000-2015. Note that the national average
for trends for the three time periods were 32%, 22%, and 17%,
respectively. Clearly, the trend is slowing down.
effect", as described in the peer-review literature and
on this web site, affects the ability
of the nation to attain the 8-hour ozone standard as lower and
lower 8-hour standards are established. As we discussed in our
original paper, the peak hourly average concentrations are reduced
much faster than the mid-level concentrations. This pattern is
discussed in our most current publication on trends in the EU,
US, and China (Lefohn et al., 2017-see publications
list). Clearly the "piston effect" heavily influences
the Nation's ability to attain an 8-hour ozone standard as standard
levels are reduced. We discuss more about the "piston effect"
and how it affects the attainability of the ozone standard in
our concerns web area.
- Over the past years, A.S.L. & Associates and its
consultants have commented on the strengths and weaknesses associated
with the mathematical and statistical methodologies used in epidemiological
studies to link exposure with human health effects. Many of the
statistical caveats raised throughout the PM and Ozone Criteria
Documents and the PM and Ozone Staff Papers indicate a pattern
of inconsistent results that is troubling. Examples of the growing
pattern of inconsistent and inconclusive findings include the
- Instability of PM mortality
effect estimates resulting from different model specifications
of weather effects and time trends.
- Instability of PM effect
estimates resulting from different selections of monitoring sites
- Increased heterogeneity
of PM effect estimates across cities.
- Greater diversity of findings
among studies and across study areas.
- Contradictory results
from mortality displacement studies.
- PM effect lags that are
inconsistent across cities and across studies.
- Exposure-response relationships
that are inconsistent across cities and across studies.
- Inconsistencies between
short-term and long-term effect studies, such as respiratory
effects of fine particles.
- Contradictory findings
among long-term studies.
Additional details about
the Team's epidemiological concerns are discussed on our epidemiological
concerns web page. The
Team's comments on the first draft of the PM Staff Paper were
submitted to EPA in October 2003. To read more about our concerns
about the first draft, please visit our web page.
- Sometimes science
and politics mixed together become science fiction. Such is the
case that occurred, when in September 2002, many newspapers across
the United States printed a story summarizing the report, Code
Red: America's Five Most Polluted National Parks, which described
The Great Smoky Mountains as the nation's most polluted national
park, with air quality rivaling that of Los Angeles. For the
period 1997-2001, the report claims that the annual ozone exposure
was higher at Great Smoky Mountains National Park than at Los
Angeles, California. There is a serious technical problem associated
with the report and the report's conclusions are flawed. Please
read "The Rest of the Story."
- In 2000, Haywood County, NC experienced its 4th highest
8-hour ozone concentration at 0.085 ppm.
On May 1, a daily maximum 8-hour average concentration of 0.089
ppm was experienced. A detailed meteorological
that stratospheric ozone played an important role in this ozone
Page | News |
Corporation | Maps
| Publications | Table
of Contents | Multimedia Center
1995-2018 A.S.L. & Associates. All rights reserved.