Ozone Trends
Over the past several years,
Dr. Lefohn, A.S.L. & Associates, has collaborated with other
researchers from around the world to quantify ozone trends for
world-wide "signature" monitoring sites as well as
anthropogenically influenced monitoring sites in the United States.
The signature" ozone monitoring stations provide good
data for the purpose of assessing possible changes in background
levels of ozone. Some of the "signature" sites offer
the opportunity to study records that are representative of broad
geographic regions where local effects are minimized. We are
continually updating our ozone trending analyses.
Part 1 - Ozone
Trends of Anthropogenically Influenced Monitoring Sites
Introduction
Recently, Lefohn et al. (2008) summarized
their trends analyses 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 1980-2005
and 1990-2005 and explored whether differences in trending occur
depending upon the selection of the exposure metric. Using the
trending results, the analyses quantitatively explore the evidence
for the higher hourly average ozone concentrations decreasing
faster than the mid- and lower-values.
Results
Figure 1 below from Lefohn et al. (2008) summarizes
the findings for the trending of the 4th highest 8-hour ozone
metric for the 1980-2005 and 1990-2005 periods.
Figure 1. Trend of 4th highest 8-hour average
ozone metric for the (a) 1980-2005 and (b) 1990-2005 periods.
This figure was published in Lefohn et. al. (2008). Copyright
Elsevier. Please see reference below. Permission granted by Elsevier
to reproduce the above figure only on this web page.
Most of the surface ozone monitoring sites
analyzed in the study experienced decreasing or no trends. Few
monitoring sites experienced increasing trends. For those monitoring
sites with declining ozone levels, an initial pattern of rapid
decrease in the higher hourly average concentrations, followed
by a much slower decrease in mid-level concentrations was observed.
In some cases, they observed shifts from the lower hourly average
ozone concentrations to the mid-level values. On a site-by-site
basis, the majority of monitoring sites (1) changed from negative
trend to no trend, (2) continued a negative trend, or (3) remained
in the no trend status, when comparing trends for the 1980-2005
to the 1990-2005 time periods. For the three exposure metrics
(i.e., annual 2nd highest 1-hour average concentration, annual
4th highest daily maximum 8-hour average concentration, and vegetation-based
annual seasonally corrected 24-hour W126 cumulative exposure
index, approximately 60% of the monitoring sites shifted from
negative trending to no trending status. All regions of the United
States were equally affected by the shift in status.
Lefohn et al. (2008) in their paper provide
several figures that illustrate the spatial patterns of trends
across the United States. The greatest statistically significant
decreases in the 2nd highest 1-hour average concentrations and
the annual 4th highest daily maximum 8-hour average concentration
for the two temporal periods occurred in southern California.
Monitoring sites in other portions of the United States experienced
lesser decreases than this geographic area. In contrast to the
two exposure indices, the vegetation-based 24-hour W126 ozone
cumulative index for 1980-2005 experienced significant declines
in the midwestern states and the northeastern United States as
well as in southern California. For the 1990-2005 period, monitoring
sites in southern California and the northeastern United States
experienced the greatest decreases in the W126 exposure metric.
When trending was observed, not all months
experienced trending. Lefohn et al. (2008) tested for statistically
significant changes in the number of hourly average concentrations
within specified concentration intervals and identified specific
months that experienced shifts in the distribution of the hourly
average concentrations. As an example, Figures 2 and 3 below
illustrate the changes in the distribution of the hourly average
ozone concentrations for a monitoring site located in Reseda
in Los Angeles County as reductions occurred over the 1980-2005
and 1990-2005 periods.
Figure 2. Distribution of changes by month
for a monitoring site located in Los Angeles County, California
(AQS 060371201) for 1980-2005 for the months with significant
changes.
Figure 3. Distribution of changes by month
for a monitoring site located in Los Angeles County, California
(AQS 060371201) for 1990-2005 for the months with significant
changes.
The two figures show the reductions in the
number of hourly average concentrations in the higher hourly
average concentrations and the increases in the mid-level concentrations
as the peak values were reduced. The Thiel estimate was used
to estimate trending. The Thiel estimate is determined as the
median of slope estimates calculated as the slope of the line
passing through two points for all pairs of points in the data
set of interest. To test for statistical significance, Kendall's
tau test was used to determine significance at the 10% level.
Note that the months of March and April exhibited statistically
significant trending in the 1980-2005 period, but did not exhibit
statistically significant trending over the 1990-2005 period.
Over the 1990-2005 period, the month of September exhibited statistically
significant trending but did not over the 1980-2005 period.
Conclusions and Recommendations
Most of the surface ozone monitoring sites
analyzed in the study experienced decreasing or no trends. Few
monitoring sites experienced increasing trends. Lefohn et al.
(2008) observed that a statistically significant trend at a specific
monitoring site, using one exposure index, did not necessarily
result in a similar trend using the other two indices. The authors
recommended that because different trending patterns were observed
when applying the various exposure indices, a careful selection
of ozone exposure metrics is required when assessing trends for
specific purposes, such as human health, vegetation, and climate
change effects.
Part 2 - Ozone
Trends of Signature Monitoring Stations for Assessing
Background Ozone Trending
Introduction
Over the past several years,
there have been several articles quoting other sources indicating
that surface ozone concentrations are increasing everywhere.
Our most current research results do not support this claim.
Our research efforts monitor the status of worldwide ozone levels
by performing sophisticated analyses using surface ozone and
ozonesonde data (e.g., Oltmans et al., 1998; Oltmans et al.,
2006). Our research focuses on the results from the available
data from four decades (~1967-2007) of observations for the longest
records (ozonesondes). Several signature stations
have 20-30 years of observations for both surface and ozonesondes.
The signature" ozone monitoring stations provide good
data for the purpose of assessing possible changes in background
levels of ozone. Some of the "Signature" sites offer
the opportunity to study records that are representative of broad
geographic regions where local effects are minimized.
In December 2008, we presented
at the AGU Fall Conference in San Francisco a summary of one
of our several research efforts dealing with changes in tropospheric
ozone levels. The talk was presented at the Tropospheric Gaseous
Composition in the Regional and Global Perspective 1 - A11F-03
session. The title of our presentation was: "Tropospheric
Ozone Changes from Surface and Ozonesonde Observations."
The research was presented by the lead author, Samuel Oltmans,
NOAA Earth System Research Laboratory Global Monitoring Division,
Boulder, Colorado. The talk explored the following:
- What are the implications
from this record of observations?
- Are the records consistent
in regions with multiple records?
- At what geographic scale
can conclusions about trends be drawn (global, hemispheric, regional)?
and
- Are changes related to precursor
emissions, transport variability, stratospheric input?
Results and Conclusions
Based on our analyses, our
results and conclusions are
- In the Northern Hemisphere,
there is a different pattern of long-term changes between North
America, Europe, and Japan;
- Over North America there
does not appear to be a significant increase over the 30 years
of measurements, although there have been shorter term fluctuations;
- Western Europe saw the largest
increases prior to 1990 but a significant decrease in growth
rates (and in some cases declines) over the past 15 years over
continental Europe;
- In Japan, increases were
primarily prior to the mid 1980s, but with recent increases in
Okinawa;
- In the Southern Hemisphere
mid latitudes, at Cape Point, Cape Grim, and Lauder, ozone has
increased significantly with the increases coming primarily in
the austral spring;
- In Hawaii (North Pacific
tropics), increases appear to be associated with decadal transport
shifts;
- The tropical south Pacific
(Samoa) has not shown significant changes in tropospheric ozone;
and
- At the South Pole, earlier
declines have reversed so that overall there has been almost
no change.
Some possible implications
of recent (i.e., 30 year) observed changes are
- The relationship between
emission changes and longer term hemispheric and global tropospheric
ozone changes is not adequately understood;
- The climate forcing of tropospheric
ozone changes over the recent past is still very uncertain; and
- It will be difficult to assess
the climate forcing reductions that can be gained by reducing
tropospheric ozone with the current network of observations.
Co-authors of the research
study are A. Lefohn, J. Harris, H.-E. Scheel, E. Brunke, H. Claude,
D. Tarasick, I. Galbally, G. Bodeker, J. Davies, T. Koide, B.
Johnson, C. Meyer, F. Schmidlin, E. Cuevas, A. Redondas, P. Simmonds,
and B. Buchman.
References
Lefohn A. S., Shadwick D., and Oltmans S.
J. (2008). Characterizing long-term changes in surface ozone
levels in the United States (1980-2005). Atmospheric Environment.
42:8252-8262.
Oltmans S. J., Lefohn A. S.,
Scheel H. E., Harris J. M., Levy H. II, Galbally I. E. , Brunke
E. G., Meyer C. P., Lathrop J. A., Johnson B. J., Shadwick D.
S., Cuevas E., Schmidlin F. J., Tarasick D. W., Claude H., Kerr
J. B., Uchino O., and Mohnen V. (1998) Trends of ozone in the
troposphere. Geophysical Research Letters. 25:139-142.
Oltmans S. J., Lefohn A. S.,
Harris J. M., Galbally I., Scheel H. E., Bodeker G., Brunke E.,
Claude H., Tarasick D., Johnson B.J., Simmonds P., Shadwick D.,
Anlauf K., Hayden K., Schmidlin F., Fujimoto T., Akagi K., Meyer
C., Nichol S., Davies J., Redondas A., and Cuevas E. (2006) Long-term
changes in tropospheric ozone. Atmospheric Environment. 40:3156-3173.
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