Tropospheric Ozone
A Growing Threat
Acid Deposition and Oxidant Research Center

Foreword
Decreasing concentrations of ozone in the stratosphere are causing a serious problem of
depletion of the ozone layer. Meanwhile, ozone concentrations are increasing in the
troposphere—the ozone within about ten kilometers from the Earth’s surface. A strongoxidant, tropospheric ozone is harmful to humans and ecosystems. It is also a powerfulgreenhouse gas. Average concentrations of ozone are increasing in Japan and othercountries. Even in remote places like the Oki islands in the Sea of Japan, far from any
urban center, ozone concentrations in air transported from far away sometimes exceed
environmental standards in Japan.
Ozone is produced from the photochemical reaction of nitrogen oxides (NOx) and volatileorganic compounds (VOCs) under sunlight. Those pollutants are emitted from automobiles
and industries. Since the emission of nitrogen oxides in East Asia is rapidly increasing due
to a rapid rise in the use of automobiles, the concentrations of tropospheric ozone are
predicted to increase in the future.
Experiments in Europe, the United States and Japan indicate that yield losses ofagricultural crops and suppression of tree growth may occur even in under current ozone
concentrations. It is projected that if concentrations continue to rise in East Asia,
vegetation will be exposed to higher levels of ozone stress, and economic losses will affectnot only Japan but also China, the Republic of Korea and other countries in this region.
During the summer seasons in the 1970s, Japan experienced serious problems from
pollution by photochemical oxidants, which are composed mainly of ozone. After
concentrations gradually decreased, the problems were generally considered to beresolved. However, since around 1980, average ozone concentrations have again been on
an increasing trend year-round. This problem is beginning to appear as a global
environmental issue, not simply a local one.
Meanwhile, noticing that humans and plants may be affected by long-term exposure toozone, even in comparatively low concentrations, European and North American countries
have developed new environmental standards and indexes. Cooperation among
contracting parties in the Convention on Long-Range Transboundary Air Pollution isanother example of efforts to reduce precursor pollutants. Some voices are starting to callfor efforts to address ozone exposure as a hemispheric issue. Today’s issue of
tropospheric ozone deserves to be recognized as a transboundary environmental issue. Inthat context, an appropriate framework is needed so that East Asian countries can also
cooperate to tackle this problem.

References
Akimoto, H., Impacts of ozone pollution from East Asia on Japan (in Japanese), Measures of Resources andthe Environment, pp. 90-96, Vol.39, No.11, 2003.

 Tropospheric Ozone: A Growing Threat

Ozone and photochemical oxidants
Ozone (O3, a molecule formed by three atoms of oxygen) is a gas that is harmful to human health.
Oxidant is a general term for oxidizing substances in the atmosphere. With the exception of nitrogendioxide, these oxidants are known as photochemical oxidants. Ozone is the main component ofphotochemical oxidants, which also contain peroxyacetylnitrate (PAN) and other substances. Damage toplants is expressed differently by ozone and PAN. With lettuce, for example, ozone causes spots to appear
on the tips of leaves, while PAN discolors the leaf joints.
Japan has established environmental quality standards for photochemical oxidants, treating it as amajor air pollutant. The standards defined photochemical oxidants as all oxidizing substances that freeiodine from neutral potassium iodide solution, and originally required measurement by spectrophotometry
and that solution. However, it was later clarified that concentrations of PAN are generally much lower thanthose of ozone and that PAN is less sensitive to spectrophotometry using neutral potassium iodide thanozone. Meanwhile, it was found that other measurement methods of ozone concentrations, such as byphotometric method using ultraviolet, are advantageous in terms of procedures and maintenance.
Considering these facts and developments overseas, on 25 October 1996 the then-Environment Agency(later promoted to Ministry of the Environment) notified the governors of Japan s prefectures and mayorsof the country s major cities that photometric methods using ultraviolet light can be used to measure
photochemical oxidant levels.

ppm

and

ppb

Both ppm and ppb are units used to indicate the presence or concentration of a substance.
ppm means parts per million and ppb parts per billion. Therefore, 1 ppm is equivalent to 1,000 ppb.
The relation between ppm･h and ppb･h ( h means hour) is similar: 1ppm･h is equivalent to 1,000
ppb･h. This booklet uses whichever unit is appropriate in the discussion, depending on the number ofsignificant figures.

Photochemical oxidant warnings and serious warnings in Japan
Photochemical Oxidant Warnings are issued to prevent damage to human health and theenvironment when hourly concentrations of photochemical oxidants exceed 0.12 ppm and such conditions
are expected to continue, based on meteorological conditions. Governors of prefectures inform the generalpublic of the warnings through the media, including television and radio, and seek cooperation to mitigate theproblem, for example by asking factories to take measures to reduce the emissions from stationary sources,and drivers to refrain from using automobiles (Paragraph 1, Article 23 in the Air Pollution Control Act).
On the other hand, regulations concerning Photochemical Oxidant Serious Warnings are stipulated
in guidelines issued by prefectures and other local authorities. These warnings are generally issued when
the hourly concentrations of photochemical oxidants exceed 0.24 ppm and such conditions are expected tocontinue, based on meteorological conditions.
Many municipalities also use the terminology of Photochemical Smog Warnings and Photochemical
Smog Serious Warnings.

Tropospheric Ozone: A Growing Threat1. Increasing Tropospheric Ozone■Stratospheric ozone and tropospheric ozone
Many people know about ozone in the atmosphere, but while they are most likely thinking about
stratospheric ozone, tropospheric ozone is not so well known. However, considering theimportance of its direct effects on humans and vegetation, the environmental issue of troposphericozone is likely to become a greater concern in the future. About 90 percent of ozone in the atmosphereon the Earth is found in the stratosphere, with approximately the remaining 10 percent in the
troposphere. Although both are ozone, its effects on the global environment are quite different
depending on where it is found.
With regard to physical properties, ozone has bands of absorption in both the ultraviolet andinfrared spectra. Stratospheric ozone absorbs ultraviolet rays in sunlight and prevents them fromreaching the surface of the Earth. The ultraviolet light blocked by stratospheric ozone is harmful for living
things, and it is commonly known as
good ozone.
The loss of this ozone, caused bychlorofluorocarbons (CFCs) and other substances, constitutes a serious problem for the global
environment.
In contrast, tropospheric ozone absorbs infrared rays emanating from the Earth and works as a
powerful greenhouse gas. The greenhouse effect of this ozone is more significant in the upper
troposphere than in other layers. According to the Intergovernmental Panel on Climate Change (IPCC
2001), tropospheric ozone is regarded as the third most powerful greenhouse gas in the atmosphere,
after carbon dioxide and methane. Meanwhile, in terms of chemical properties, ozone is a strongoxidant. At the ground level, it is a major air pollutant, and one of the main oxidants. It is also known tocause photochemical smog, which disturbs human respiratory functions and plant photosynthesis. Dueto its greenhouse effect and toxicity, troposphericozone is known as bad ozone.
The sources of tropospheric ozone are (1)influx from the stratosphere and (2) generation byphotochemical reactions in the troposphere. The
influx of ozone from the stratosphere takes placemainly in middle and high latitudes and is mostactive in early spring. The generation of ozone in
the troposphere is most active in summer, since it
is caused by photochemical reactions involving
nitrogen oxides (NOx), carbon monoxide (CO)and volatile organic compounds (VOCs).
Meanwhile, ozone disappears as a result ofphotochemical reactions with HOx (OH + HO2)
Figure 1-1. Global distribution of ground-level ozone
radicals, etc., over the ocean, where NOx
concentrations based on computer model (May-August1993). (Lelieveld and Dentener 2000)
concentrations are generally low, and throughdecomposition by contact with the ground. The
amount of ozone generated by photochemical reactions in the troposphere is much greater than theinflux from the stratosphere, although the global average net amount of ozone supplied by both sources
is about the same, after counting the amount removed by decomposition in the troposphere. Figure 1-1
shows the results of computer modeling of the distribution of ozone concentrations at ground level in a
recent year. It reveals that ground-level ozone concentrations are high in middle and high latitudes of
the northern hemisphere, especially in regions where air pollutant emissions from human activities arelarge in volume.
References
IPCC, Climate Change 2001, P. 7, 2001.
Lelieveld, J. and F. Dentener, Journal of Geophysical Research, 105, 3531-3551, 2000.
Note: Radicals -In atoms and molecules, electrons generally exist in the orbit in pairs. If one of the two is lost,the atom or molecule becomes extremely unstable and chemically active. A radical is an atom or molecule inthis condition.

 Tropospheric Ozone: A Growing Threat

■Rising tropospheric ozone
concentrations worldwide
As shown on the preceding page,
because the amount of ozone generated byphotochemical reaction of air pollutants ismuch larger than the inflow from thestratosphere, the concentrations of ozone
are strongly affected by human activities.
How
didtroposphericozone
concentrations increase globally as societies
became more industrialized? Figure 1-2
plots springtime ozone concentrations at
ground level in middle and high latitudes,
Figure 1-2. Trend of springtime ground-level ozone
using data from Europe and Japan from theconcentrations in middle and high latitudes of the northernend of nineteenth century to the present. The
hemisphere from the nineteenth century to the present.
(Akimoto 2002)
figure shows that the concentrations ofground-level ozone were around 10 ppb in
Europe at the end of nineteenth century. By
contrast, ozone concentrations in rural areasof Europe and Japan in the recent years
have reached averages of 40 to 50 ppb fromspring to summer. These facts mean thatground-level ozone concentrations haveincreased to four to five times the levels in
the preindustrial era. On the other hand,some argue that data suggesting
concentrations of 10 ppb at the end ofnineteenth century may have some faults inmeasurement, because the modelingsuggests
thatground-levelozone
concentrations exceeded 20 ppb even if the
Figure 1-3. Modeling prediction on geographical distributionof ground-level ozone concentrations in 2025 (average fromanthropogenic
emissionsofozone
May to August). (Lelieveld and Dentener 2000)
precursors like NOx and VOCs are set atzero, as they would have been in thepreindustrial era. Nevertheless, even if ozone concentrations were as high as 20 ppb at the time, recent
ozone concentrations at remote and rural sites in the northern hemisphere have at least doubled since
the preindustrial era.
If human activities continue to cause emissions of pollutants that generate ozone, what will happen
to ground-level ozone concentrations in the future? Figure 1-3 is one example of modeling predictions
for tropospheric ozone. It shows that the average concentration of ozone in North America, Europe andEast Asia, including Japan, from May to August will likely exceed 60 ppb, which would exceed accepted
levels under Japan s current environmental quality standards. To avoid such a situation, it is urgent toconsider countermeasures.
References
Akimoto, H., Chemistry of the troposphere and global environment (in Japanese) (edited by Akimoto H.,
Kawamura K., Nakazawa T. and N. Washida, p. 97, Gakkai Shuppan Center, 2002.
Lelieveld, J. and F. Dentener, Journal of Geophysical Research, 105, 3531.

Tropospheric Ozone: A Growing Threat■Rising concentrations of ground-level ozone in Japan
As mentioned above, tropospheric ozone concentrations have been increasing globally and there
are concerns that they may continue to increase in the future. What is the situation in Japan? Figure 1-4
shows the trend of photochemical oxidant
concentrations measured at air pollution
monitoring stations in Japan, mostly in urban
areas. Ozone is considered the primary
photochemical oxidant. A comparison of
average concentrations between 1985 to 1990
and 1994 to 2002 shows an increase of about
3.5 ppb, which has been observed all overJapan. Ozone concentrations monitored atremote sites in Japan also show an upward
trend since 1990. The increases in averageozone concentrations for the ten years fromyear
1992 to 2002 in Happo (a mountainous area)
and Ryouri (a coastal area) were 9.0 ppb andFigure 1-4. Yearly average daytime concentrations of
7.3 ppb, representing annual increases of 2.0
photochemical oxidants in Japan (326 continuous ambient
air pollution monitoring stations nationwide). Vertical bars
and 2.1 percent, respectively.
show standard deviation. (Based on Ohara and Sakata
In the Tokyo metropolitan area as well,
2003)
concentrations of photochemical oxidants are
increasing. According to a survey by the Bureau
of Environment of the Tokyo Metropolitan
Government (2005), daytime concentrations ofphotochemical oxidants increased by 5.6 ppb(about a 0.5 ppb increase per year) from 1990
to 2002. Since 2000, the number of days ofphotochemical smog warnings per year has
been increasing, as has the number of days
when photochemical oxidant concentrations
were 120 ppb or higher. Today, photochemical
oxidants reach high levels more often than in
the late 1970s (Figure 1-5). In fact, in July 2002,year
Chiba Prefecture, also located in the Tokyo
metropolitan area, issued a photochemical
Figure 1-5. Photochemical smog warnings and days withsmog serious warning for the first time since
photochemical oxidant concentrations of 0.12 ppm or higher
in Tokyo. (Bureau of Environment, Tokyo Metropolitan
1984. In September 2005, Saitama Prefecture
Government 2005)
issued a similar serious warning and somedamage was reported from the smog
conditions. Possible causes of these increases
in photochemical oxidant concentrations include (1) meteorological changes, including changes intemperature and solar radiation, (2) an increase in the ratio of VOCs to NOx, and (3) an increase oftransboundary air pollution. However, it is not yet clear which factors are actually at work and how they
are doing so. Quantitative analysis is therefore urgently needed in order to develop effective
countermeasures.
References
Bureau of Environment, Tokyo Metropolitan Government, Report of the Committee on Measures about
Photochemical Oxidants, 2005.
Ohara , T. and T. Sakata, Journal of Japan Society for Atmospheric Environment (in Japanese), 38, 47-54, 2003.

 Tropospheric Ozone: A Growing Threat

■How is tropospheric ozone formed?

HO2
CH4
Tropospheric ozone (O3) is principally generated
RO2
CO
through photochemical reactions involving nitrogen
RCO3
VOC
dioxide (NO2) (Figure 1-6). It is formed by a reaction
between oxygen molecules (O2) and oxygen atoms
OH,RO(O) released when NO2 reverts to nitrogen monoxide
HNO3
NO + O3
NO2(NO) through photolysis under the influence ofultraviolet light from the sun.
In a chamber containing clean air and nitrogenhνRCO3oxides (NOx), after O3 decomposition (O3 + NO → O2 +PAN
NO
2→NO+ONO2), an equilibrium of generation and decomposition
O+O2+M→O3+M（M:N2 or O2）
reactions will be reached when the ozone concentration
reaches 10 to 20 ppb at most. However, if volatile Figure 1-6. Generation and transformation process oforganic compounds (VOCs) are added to that chamber, ozone and nitrogen compounds.
O3 is formed through the photolysis of NO2. The
VOCs react with atoms of oxygen (O), O3 and OH production of NO2 is caused by O3 and peroxide
radicals in the oxidation process under sunlight, to radicals. NO2, reacts again to become HNO3 and PAN.
generate oxide radicals (RO) and peroxide radicals Note: R indicates alkyl group, aryl group, etc. PAN
indicates peroxyacetylnitrate.
(HO2, RO2, RCO3), which play a role in re-producing
NO2 from NO. Among various types of VOCs, olefins(one group of hydrocarbons) generally have strong
photochemical activity, followed by alkyl benzene andparaffin (another group of hydrocarbons). The two latter
substances contribute significantly to the regenerationof NO2, since they exist in high concentrations in theatmosphere.
The formation of NO through photolysis and the（Time）
regeneration of NO2 from the newly produced NO byperoxide radicals constitute one cycle, in which one
molecule of ozone is produced. The peroxide radical
functions as a kind of catalyst, since it reverts to its Figure 1-7. Behavior of ozone and its precursors in a polluted
air mass over a long distance. (Carmichael et al. 1994).
original form quickly after reduction.
Ozone concentrations increase in proportion to differences in the rates of formation anddecomposition (i.e., the net rate of formation). One study monitored polluted air masses moving from theTokyo metropolitan area to the Chubu mountain area in Japan on a sunny day in summer (Figure 1-7).
In the early morning, NO was in high concentrations in a polluted air mass above Tokyo Bay and thecoastal area. It began to be transformed to NO2 at sunrise. After further photochemical reactions, theNOx concentrations (the total of both substances) declined rapidly after about nine o clock in themorning, and they were transformed into HNOx, nitrate aerosols and PAN. At that time, the ratio of
ethylene/acethylene concentrations (an indicator of photochemical activity) also declined rapidly whileozone concentrations increased sharply. At around 2 o clock in the afternoon, when the air massreached Urawa and Fukaya, dozens of kilometers downwind from the emission sources, ozone
concentrations peaked and the air mass continued traveling to the Chubu mountain area with those highozone concentrations.
Initial concentrations and the ratio of NOx and VOCs near the emission sources have a great
influence on the amount of photochemical ozone production as well as its geographical distribution.
When the NOx/VOC ratio is higher than around 0.1 ppb/ppbC, the amount of ozone produced depends
largely on VOC concentrations, and not so much on NOx concentrations (and vice versa). This meansthat measures to reduce NOx and VOC emissions will have different results, depending on the
NOx/VOC ratio in the vicinity of emissions sources. In addition, when the proportion of VOCs is large,ozone concentrations will increase in early hours of the day, with the result that high ozone
concentrations will be observed near the emission sources.

Tropospheric Ozone: A Growing Threat2. NOx and VOCs Increasing in Asia
■Rising NO2 concentrations(evidence by satellite data)
It is thought that emissions of NOx, whichlead to the formation of ozone, are increasing
dramatically in Asia as the region experiences
rapid industrialization. The Global Ozone
Monitoring Experiment (GOME), whichobserved the distribution of NO2 column
concentrations in the troposphere, provided
evidence of this trend (Richter et al. 2005).
Figure 2-1 shows the distribution of NO2
concentrations in the troposphere of East Asia
in January 1996 and 2002 (Irie et al. 2005;
JAMSTEC 2005). It reveals that NO 2
concentrations are extremely high above thenorthern plain in eastern China and that theregion of high-concentrations expanded
between 1996 and 2002. Figure 2-2 shows thechanges in winter concentrations (November toJanuary) from 1996 to 2002 (difference
obtained by subtracting values in 1996 fromvalues in 2002). Areas where the increaseduring that period is most noticeable include
Beijing, Shanghai, and Henan Province.
Analysis of changes in average NO2
concentrations by season inside the squaredrawn around the northern plain shows that
NO2 concentrations are highest in winter, andare increasing by eight percent per year. The
same trend is evident in other seasons, with anannual rate of increase of 7 percent (±1percent). Such increases of NO2 concentrationsin China result from increases in NOx
emissions, which not only raise ozone
concentrations at the ground level in China but
also lead to an increase in ozone
concentrations over Japan through longdistance transport of air pollution.

Figure 2-1. Distribution of NO2 concentrations in the
troposphere of East Asia, measured by the Global Ozone
Monitoring Experiment. Shown here are average January
concentrations in 1996 (top) and 2002 (bottom). Gray zones
indicate lack of data. (JAMSTEC 2005)

Figure 2-2. Changes in NO2 concentrations in the
troposphere in winter (November-January) between 1996
and 2002. (JAMSTEC 2005)

References
Irie, H. et al., Geophysical Research Letters, 32, L11810, 2005.
Japan Agency for Marine-Earth Science and Technology (JAMSTEC),<http://www.jamstec.go.jp/frsgc/jp/press/050617/index.html>, 2005.
Richter, A. et al., Nature, 32, 129-132, 2005.

 Tropospheric Ozone: A Growing Threat

■Rising NOx and VOC emissions
In this section, we discuss trends in NOx and VOC emissions, precursors for the formation of ozone,
based on estimates of emissions in Asia. Figure 2-3 shows the trend of NOx and VOC emissions in
Asia, estimated by the Frontier Research Center for Global Change (FRCGC) and the ResearchInstitute for Humanity and Nature (RIHN) (Ohara et al. 2005). NOx emissions in Asia increased by about
2.5 times over the twenty years from 1980 to
2000. China showed a dramatic increase of
3.1 times, a trend that has likely continued
after 2000. Figure 2-4 compares distributionsof NOx emissions in 1980 and 2000, showingthat increases are large in the northern plain
and the southern coastal area of China,
similar to the findings of the GOME study
mentioned above. Increases over Indochina
and India are also dramatic. A large part ofthese increases in emissions is due to therising combustion of coal in power plants andthe growing popularity of the automobile.
Similarly, the increase of VOC emissions
is large. VOC emissions in Asia increased 1.9
times in Asia and 2.4 times in China from1980 to 2000. The main causes of theseincreases are increasing emissions fromautomobiles and evaporation from the use ofsolvents.
As mentioned above, both NOx and VOCemissions, which lead to ozone formation,
Figure 2-3. NOx (top) and VOC (bottom) emissions in Asia
have been increasing dramatically in Asia
from 1980 to 2000. Units: Kilotons per year. (Ohara et al.
over the past quarter of a century. Unless2005)
drastic measures are taken to addressenergy and environmental issues, NOx and VOC emissions will continue increasing. Such a scenario
will mean the elevation of tropospheric ozone concentrations over a wide area, with serious
consequences. Urgent action is therefore needed to reduce emissions, through international
cooperation in the Asian region.

1980

2000

Figure 2-4. Distribution of NOx emissions in Asia in 1980 (left) and 2000 (right). Units: Tons year-1 per grid cell.
Cells are 0.5 degrees longitude by 0.5 degrees latitude. (Ohara et al. 2005)

References
Ohara, T. et al., Proceedings of the 46th Annual Meeting of Japan Society for Atmospheric Environment(in Japanese), 511, 2005.

Tropospheric Ozone: A Growing Threat3. Tropospheric Ozone: A powerful Greenhouse Gas■Greenhouse effect by ozone

Altitude (km)Ozone has a powerful capacity to absorb infrared light with a wavelength of around 10 microns.
Since these wavelengths do not overlap those of water vapor (H2O) and carbon dioxide (CO2), ozone in
the atmosphere efficiently absorbs the infrared rays radiated from the Earth and thereby has powerfulgreenhouse effect. Because ozone has a relatively short lifetime in the atmosphere―from about a week
to a few months, depending on the season, altitude and region―the spatial distribution of ozone isextremely uneven compared to greenhouse gases such as CO2 that have a long lifetime. As a result,when considering the greenhouse effect caused byozone, it is important to know how it differs
depending on its location in the atmosphere.
First, let us examine the relationship between
altitude and the greenhouse effect efficiency of
REGION À
ozone. Figure 3-1 shows the rate of change of(Ozone increases cause
cooling)ground-level temperature caused by increased
ozone concentrations at different altitudes. As
REGION ¿(Ozone increases cause
shown by the figure, the effect of warming is small
warming)
at ground level. As we approach the upper
troposphere, the effect becomes stronger, reaching
the maximum around the tropopause, which divides
the stratosphere and the troposphere, about ten
kilometers above the Earth. In the lower layer of theDegree/DU Ozone change (°Ｃ／m atm-cm)
stratosphere as well, ozone has a positive
greenhouse effect. However, at an altitude above
Figure 3-1. Relation between altitudes and changes of groundabout 30 kilometers, an increase in ozone―level temperature caused by increase in ozone column
concentrations lowers the temperature at thedensity (DU, Dobson Units) (Based on Lacis et al. 1990)
ground level.
The reason why the effect at ground level is nearly zero is that the ozone s absorption andradiation of infrared rays (from the Earth) is balanced, since the air temperature is almost the same asthat of the Earth s surface and there is no net absorption. At higher altitudes, where the temperature islower, the net of infrared absorption by ozone is larger, making the greenhouse effect more efficient.
Similarly, near the tropopause where the temperature of the atmosphere is at its lowest, the greenhouseeffect of ozone is at its highest. In contrast, at higher altitudes in the stratosphere where the temperatureis also higher, the net absorption of infrared rays by ozone is reduced, making the greenhouse effectnegative above the middle layer of the stratosphere.
In short, ozone in the troposphere and the lower layer of the stratosphere has a positive
greenhouse effect. Ozone has its strongest greenhouse effect in the upper troposphere.

■Geographical distribution of global warming effect by ozone
Here, we examine the global distribution of the effects of ozone on global warming. Figure 3-2 is a
scenario simulated by a chemical-climate model showing the global distribution of the effects oftropospheric ozone concentrations that increased between the Industrial Revolution and 1990. It showsthat the effects on global warming are the largest in the southern region of the Mediterranean Sea, thenorthern part of Africa and the Middle East. These are followed by the southern parts of North America
and Europe and the southeastern part of Asia. The effects of tropospheric ozone on global warming
extend to Japan as well, from the main island of Honshu southwards. The reason why the regions
feeling the greatest effects are shifted south from the areas where ozone is highest in concentration(Figures 1-1 and 1-3) is that greenhouse effect of ozone is largest in the regions with more solar
radiation and fewer clouds.

 Tropospheric Ozone: A Growing ThreatSincetheincreaseoftropospheric ozone in the northernhemisphere is larger than in thesouthern hemisphere due to thehigher intensity of human activities
in the North, in the past the global
warming effects of ozone have beenmuch greater in the northernhemisphere. In addition, even within
the northern hemisphere, there arelarge differences in the size of theeffects depending on location.
These are special features of thegreenhouse effect of troposphericozone.

Figure 3-2. Annual average climate forcings in 1990 caused by theincrease of tropospheric ozone since 1850, with normalized relativevalues by annual average maximum value in 1990. (Bernsten et al.
2000)■Ozone, the third most powerful greenhouse gas
We have shown that the greenhouse effect of ozone differs largely by region. In this section, we
compare the average global warming effect of ozone on the entire Earth with those of other greenhousegases. For the purpose of comparing effects on global warming, an indicator called climate forcing is
generally used. This index (in Watts per square meter, or W/m2) is obtained by assessing to what extent
infrared light passing through the tropopause is reduced as the concentration of a given greenhouse gas
increases in the atmosphere. Figure 3-3 describes the effects on global warming that several kinds ofgreenhouse gases have caused since immediately before the Industrial Revolution to the present. CO2
has had the largest effect on global warming, followed by methane and tropospheric ozone. This is why
the IPCC calls tropospheric ozone the third greenhouse gas. It should be noted that the figurecompares the average global warming effects on the entire planet. As shown above, the increase of
tropospheric ozone in the northern hemisphere is larger than in the southern hemisphere, and the pasteffects on global warming are also much larger in the northern hemisphere. In addition, because of thelocalization of effects even in the northern hemisphere, the effects of ozone on global warming may bemuch larger in some regions than inothers. Figure 3-3 also shows that the
Aerosols・Clouds
climate forcings of aerosols, which likeozone are also air pollutants with short
lifetimes, are very large (both positive andTrop.
negative), and differ greatly depending onozone
the chemical components.

Greenhouse gases with long lifetimes

Air pollutants with short lifetimes

Figure 3-3. Global climate forcings of different greenhouse gases
from 1850 to 2000. (Based on Hansen and Sato 2001)

References
Berntsen, T. K. et al., Journal of Geophysical Research, 105, 8915-8930, 2000.
Hansen, J. and M. Sato, Proceedings of National Academy of Science, 98, 14778-14783, 2001.
Lacis, A. et al., Journal of Geophysical Research, 95, 9971-9981, 1990.

Tropospheric Ozone: A Growing Threat4. New Concerns about Health Effects of Ozone■Health effects of ozone
Photochemical oxidants are substances such as ozone and peroxyacetylnitrate (PAN), formed
secondarily through photochemical reactions. They cause what is known as photochemical smog.
Approximately 90 percent of the components are ozone (O3), which is the main cause for the healtheffects of photochemical oxidants. Since ozone is not very soluble in water, if ozone is inhaled, it goes
through the trachea unabsorbed before going deeply into the lungs and reaching the alveoli. The strongoxidizing action causes a variety of damage to living cells. Most common symptoms are acute effects,
including stinging of the eyes, tears, sore throat and coughing, due to irritation of the mucous
membranes of eyes, nose and throat.
In animal experiments, with concentrations more than about 0.12 ppm, type I alveoli cells aredestroyed quickly, while type II cells, alveolar macrophages and polynuclear white blood cells increase.
Next, cilia drop out or disappear. As a result, hypertrophy of the epidermis is observed in the bronchialtube and alveolar walls. As for biochemical changes, it is reported that, due to oxidation stress, products
from the decomposition of lipid peroxide increase and the anti-oxidation activity of enzymes is amplified.
With regard to changes of respiratory functions, at concentrations of 0.3 to 0.4 ppm or higher, studies
have reported increases of tracheal resistance, decreases of dynamic lung compliance, and, in guinea
pigs exposed to low concentrations for long periods, hypersensitiveness of the trachea. In humanasthmatics exposed to concentrations of 0.08 ppm or higher the forced expiratory volume (FEV) is
reduced during physical exercise.

■Collective health damage from photochemical oxidants in JapanIn 1970 for the first time in Japan, a group of people suffered negative health impacts apparently
caused by photochemical oxidants. The incident took place at a senior high school in the Suginami
Ward of Tokyo. Female students exercising in the schoolyard and people living in the neighborhoodcomplained of stinging eyes and throats as well as other non-respiratory symptoms like nausea,
dizziness and fainting. Some students were hospitalized.
Respiratory impacts had been considered the main health effects of air pollutants before thatincident. The atmospheric ozone concentrations at that time were estimated to be about 0.2 ppm―notconsidered to be high enough to cause acute respiratory symptoms. The mechanisms causing
neurological symptoms like nausea and dizziness are not entirely clear. However, subsequent animal
experiments suggested that they could be caused by ozone stimulation leading to irritation ofparasympathetic nerves, and suppression of sympathetic nerves, which cause decreases in heart rate
and blood pressure, and irregular pulse (Figure 4-1).
As a result of such incidents of negative health impacts, the government established an air quality
standard for photochemical oxidants in 1973. Unlike the otherair pollutants, it was set at a level to prevent acute rather than
Blood pressure
Clean airchronic effects, at 0.06 ppm or lower as an hourly value.

■Attainment of environmental standards,
complaints of health damage

Cardiogram

In 2004, the attainment rate of the air quality standard forBlood pressure
Average hourly heart rate (/minute)photochemical oxidants was extremely low in Japan. Only two
stations attained the daytime standard (5 a.m. to 8 p.m.) andnever exceeded the hourly maximum of 0.12 ppm. This
Cardiogram
P-waverepresents only 0.2 percent of all the stations in Japan,
including ambient and roadside monitoring stations. Although
the total number of days having photochemical oxidant
warnings had been decreasing since a peak in 1973, this Figure 4-1. Blood pressure decrease andnumber has been fluctuating since the 1980s. In 2004, irregular pulse in rats exposed to 1 ppm-ozone
photochemical oxidant warnings were issued in 22 for three hours. (Uchiyama 2003)

 Tropospheric Ozone: A Growing Threat

Average hourly heart rate (/minute)prefectures, with a cumulative total 189 daysof warnings in all prefectures, and 393
reported victims apparently affected byphotochemical oxidants.
Although incidences of health impacts areless common than in the past, they are stillreported periodically. Once a photochemical
Clean airoxidant warning is issued, people are advised
to avoid outdoor exercise, and any physicaleducation being conducted outdoors at
schools must be moved indoors.
Under repeated ozone exposure,
Figure 4-2. Changes in heart rate when exposed to ozone forreactions such as the decline of respiratory
three consecutive days. (Arito et al. 1990)
functions in humans and decreased heart rate
in animal experiments are most evident on the first day, but in about three days, they no longer occur
under the same atmospheric concentrations (Figure 4-2). Likewise, health damage caused byphotochemical oxidants is more likely to occur when the concentration of oxidants rises suddenly, forexample, on a sunny day in Japan s rainy season, rather than when the concentration of oxidants has
risen gradually with a series of hot days in midsummer.
People sometimes complain or question the need to change their behavior after an warning isissued, for example when they are told they must stop swimming in an outdoor pool. However, it isimportant to follow the actions required when warnings are issued, as high oxidant levels may induce
the acute symptoms mentioned above, as well as other impacts such as asthma attacks in children
suffering from asthma.

■Standards in Europe and the United States
The United States Environmental Protection Agency (USEPA), during a standards review process
in 1996, considered that (1) even with hourly concentrations of 0.12 ppm or lower, acute health effects
can be observed if people are exposed to ozone for a long period while doing a medium level of
exercise; (2) pulmonary damage observed as chronic effects in animal experiments with ozone arealmost the same regardless of species; and (3) repeated inflammation by ozone exposure during life
may damage pulmonary tissues and reduce the quality of life in old age. Since most regions in the
United States had attained existing standards for hourly concentrations, the USEPA felt that an eighthour value would now be more appropriate than a one-hour value, for properly controlling exposurelevels and preventing harmful effects. Thus, an eight-hour mean value of 0.08 ppm was judged as
appropriate, based on risk assessments, especially on the proportion of children who suffer frommedium or high level decline in pulmonary functions through outdoor activities, and the proportion ofchildren who experience chest pains when breathing deeply.
In the United Kingdom, the provisional target of ozone concentrations to be achieved by the end of2005 was set at 100μg/m3 (0.05 ppm) as a daily maximum running eight-hour mean not to be exceeded
more than ten times a year. Similarly, the European Union set the eight-hour value (calculated fourtimes a day) of 110μg/m3 (0.055ppm) as a target to be achieved by 2010. Both were established as apermissible range of concentrations, from the public health perspective, based on risk assessments ofhealth effects such as aggravation of symptoms and respiratory functions of asthmatics and theincreased hospitalization of patients with respiratory disease. Other thresholds, such as a yearly meanvalue, have not yet been established, however, since chronic effects have not yet been adequately
determined in epidemiological studies.
References
Arito, H., I. Uchiyama et al., Toxicology Letters, 52. 169-178, 1990.
Uchiyama, I., Journal of Japan Society for Atmospheric Environment (in Japanese), 38, 347-357, 2003.

Tropospheric Ozone: A Growing Threat5. Ozone Damage to Agricultural Crops■Discoveries of ozone damage in plants
In 1944, photochemical smog damage was observed for the first time on plants near Los Angeles in
the United States. Later, ozone was identified as the primary causal agent of the damage. Since then,many studies conducted on the effects of ozone on plants have shown that ozone is damaging plants invarious regions across Japan, Europe and North America. When a high-concentration of ozone diffuses
into a plant, the strong oxidizing power injures the plant tissues, resulting in visible damage, such as
white, yellow, or reddish spots in the leaves.
The leaf damage in leafy vegetables likespinach reduces the commercial value
significantly (Photo 5-1). Even if the ozone
concentration is less than a level that causes
visible damage, the elevated level of ozone
concentrations reduces the photosynthesis
function and accelerates aging in plantleaves. Plant growth is hence retarded, and,in agricultural crops, the harvest is reduced.

Photo 5-1. Ozone damage to spinach leaves. (Photo courtesy by
Dr. Isamu Nouchi, National Institute for Agro-Environmental
Sciences)■Agricultural production losses caused by ozone
To estimate the amount of crop harvest lost to ozone, researchers have studied crop plants grownin greenhouses where ozone concentrations were raised compared to the ambient air. The observedchanges in the crop yield were related to average ozone concentrations during the daytime for major
crop species (Figure 5-1). The relationships for corn, winter wheat, cotton and soybean have been
derived from the experiments in the United States (Lesser et al. 1990), for spring wheat fromexperiments in Europe (Skarby et al. 1993), and for rice from an experiment in Japan (Kobayashi et al.
1995). The relationship between ozone and yield loss differs with crop species: rice, corn and winterwheat are less sensitive to ozone than cotton and soybean, whereas spring wheat in Europe is,interestingly, as sensitive as soybean in the United States. In addition, even the same species can show
different sensitivity among different varieties and under different environments. Therefore, it would be
more realistic to consider the relationships in Figure 5-1 as a collection of wide bands rather than aslines. We do not yet know exactly how the width of the bands is determined, however.
Using the relationships shown in Figure 5-1, crop yield losses due to ozone were estimated forsoybean and winter wheat across the United States, except for the middle and western regions. The
result showed a 14 percent loss on average over the six years from 1982 to 1987 (Tingey et al. 1994).
As for Japanese rice, the yield loss in the Kanto region was estimated to be around three percent on
average over the five years from 1981 to 1985 with the highest loss of up to seven percent in the middleof the region (Figure 5-2). Takagi and Ohara (2003) also estimated a yield loss of rice about 3.5 percent
in the Kanto region using more recent data of ozone from 1996.

¡ Tropospheric Ozone: A Growing Threat

Yieldloss(%)

No rice

Figure 5-1. Relationships between ozone
concentrations and yield losses in major
agricultural crops. (Lesser et al. 1990; Skarby et al.
1993; Kobayashi et al. 1995)

Figure 5-2. Yield loss of rice caused by ozone in
the Kanto region of Japan. Relative yield loss (%)
was compared to yields at a reference ozone
concentration (20 ppb), and averaged over five
years from 1981 to 1985. (Kobayashi 1999)■Rising impacts of ozone across Asia
As in other industrialized regions of the world, surface ozone concentrations are increasing in Asia,and their impacts on agricultural crops and natural vegetation have become a matter of concern amongscientists. Wang et al. (2005) estimated that the yield of winter wheat was reduced by about 25 percent
in the lower Yangtze River basin in 2000, with the mean daytime ozone concentration from April to June
being around 60 ppb. For rice, Wang et al. (2005) estimated the average daytime ozone concentration
to be 45 to 50 ppb during the growing season (June to October). The yield loss of rice could therefore beabout seven percent using the relationship shown in Figure 5-1. With rapid economic growth occurringin China, there is strong reason to expect further increases in ozone concentrations in the future. Wang
and Mauzerall (2004) predicted that daytime surface ozone concentrations in July will exceed 55 ppb inmost parts of China in 2020, using a global-scale model of atmospheric chemistry and transport. They
also predicted that the total production of soybean, corn and winter wheat in China will be reduced byabout 40 to 60 percent in 2020. Since Chinese crop production is the world s largest for wheat, secondlargest for corn, and fourth-largest for soybean, the Wang-Mauzerall predictions on the crop losses toozone have very strong implications for the global food supply in the future. Admittedly, there are many
uncertainties in their predictions, and future studies may lead to more moderate estimates of crop
losses. Nevertheless, considering that we now have less than 15 years remaining before 2020, and thatthe implications of ozone impacts are so significant, it will be important to launch further studies as soon
as possible to minimize the uncertainties in crop loss predictions.
References
Kobayashi K., Journal of Japan Society for Atmospheric Environment (in Japanese), 34, 162-175, 1999.
Kobayashi, K. et al., Agriculture Ecosystems and Environment, 53, 109-122, 1995.
Lesser, V.M. et al., Crop Science, 30, 148-155, 1990.
Skarby, L. et al., Effects of Air Pollution on Agricultural Crops in Europe, 241-259, Commission of the European
Communities, Brussels, 1993.
Takagi, T. and T. Ohara, Journal of Japan Society for Atmospheric Environment (in Japanese), 38, 205-216,
2003.
Tingey, D. T. et al., Tropospheric Ozone: Human Health and Agricultural Impacts, 175-206, Lewis Publishers,
Boca Raton, 1994.
Wang H. et al., Atmospheric Environment 39, 3843-3850, 2005.
Wang X. and D. L. Mauzerall, Atmospheric Environment 38, 4383-4402, 2004.

Tropospheric Ozone: A Growing Threat¢6. Ozone Threat to Japan s Forests■Tree damage in Japan
Forests act as a life-support system on the Earth.
Through photosynthesis, trees, a main component offorest ecosystems, supply the oxygen that is
indispensable to support life. Forests also preserve ourenvironment by fixing carbon dioxide, a major cause of
the global warming, and by absorbing air pollutants.
Unfortunately, forest decline and tree dieback are being
observed in many areas of Japan (Izuta 2001).
In the Tanzawa Mountains of Kanagawa Prefecture,
the decline and dieback of Japanese beech (Fagus
crenata) have been observed at Mt. Hinokiboramaru and
Mt. Hirugatake (Photo 6-1 top), and decline of Japanesefir (Abies firma) has been observed at Mt. Ohyama. In the
Oku-Nikko area of Tochigi Prefecture, Veitch s fir (Abies
veitchii), Maries fir (Abies mariesii) and Erman's birch(Betula ermanii) are in a state of decline (Photo 6-1
bottom). In the Sanyo Region, which includes Hiroshima
Prefecture, the decline of Japanese red pine (Pinus
densiflora) has been observed. In some areas along the
Sea of Japan, including Ishikawa, Tottori and Shimane
Prefectures, the decline and dieback of konara oakmizunara oak (Quercus(Quercus serrata) andmongolica) have also been observed.

Photo 6-1. Decline of Japanese beech at Mt.
Hirugatake, Kanagawa Prefecture in 1995 (topphoto). Veitch s fir and Maries fir in Oku-Nikko,
Tochigi Prefecture in 1994 (bottom photo).
Photos: Dr. Yoshihisa Kohno.

■Suspected causes of forest damage elsewhere
Several hypotheses have been presented on the causes of forest decline or tree dieback in Europe
and North America. Possible causes differ with each site. Ozone, soil acidification due to acid depositionand excess nitrogen deposition from the atmosphere have been suggested as causes in northern
Europe; ozone, acid deposition such as acid mist and fog and sulfur dioxide have been suggested inwestern Europe; and sulfur dioxide, nitrogen dioxide, ozone and acid deposition such as acid mist/fog
have been suggested in eastern Europe. In North America, meanwhile, ozone is thought to be closely
related to the forest decline and tree dieback. For example, in the Sierra Nevada-San Bernardino
Mountains, ozone is regarded as the main cause of the decline of pine species such as ponderosa pine
(Pinus ponderosa) and Jeffrey Pine (Pinus jeffreyi). In the northern Appalachian Mountains, where thedecline of red spruce (Picea rubens) has been observed, relatively high concentrations of ozone havebeen recorded. In the southeastern part of the United States, it has also been suggested that ozone is afactor in the decline of eastern white pine (Pinus strobus).
In Japan, tropospheric ozone concentrations have been increasing in recent years, andconcentrations of ozone high enough to cause harmful effects on forest tree species have beenrecorded. Based on these facts, ozone is considered to be one of the main factors relating to forest
decline and tree dieback in Japan. In fact, relatively high concentrations of ozone over 100 ppb havebeen recorded in the Oku-Nikko area and at Mt. Hinokiboramaru in the Tanzawa Mountains, where thedecline and dieback of Japanese beech (Fagus crenata) have been observed. Thus, adverse impacts ofambient ozone on growth and physiological functions such as photosynthesis of forest tree species arematters of concern.

£ Tropospheric Ozone: A Growing Threat

Figure 6-1. Relationship between
the whole-plant dry mass forest tree
species and AOT40 of ozone (Izuta
and Matsumura 1997). The
horizontal axis shows AOT40 ofozone calculated for six months,
and the vertical axis indicates
relative value of the whole-plant dry
mass of the seedlings exposed toozone to that of the seedlings
exposed to charcoal-filtered air.

Legend for species: Pd, Pinus densiflora (Japanese red pine); Pt, Pinus thunbergii (Japanese black pine); Ps, Pinus
strobus (eastern white pine); Lk, Larix kaempferi (Japanese larch); Pa, Picea abies (Norway spruce); Ah, Abies
homolepis (Nikko fir); Co, Chamaecyparis obtusa (Japanese cypress); Cj, Cryptomeria japonica (Japanese cedar); Pm,
Populus maximowiczii (Japanese poplar); Bp, Betula platyphylla (Japanese white birch); Qs, Quercus serrata (konaraoak); Qm, Quercus mongolica (mizunara oak); Fc, Fagus crenata (Japanese beech); Lt, Liriodendron tulipifera (yellow
poplar); Ab, Acer buergerianum (trident maple); Fj, Fraxinus japonica (Japanese ash).

■Effects of ozone on Japanese tree species
Ozone in the atmosphere is absorbed into the leaf tissues of trees through the stomata, and causes
visible damage on the leaves, as well as a reduction of growth and suppression of physiological
functions such as photosynthesis. Based on the results obtained from several studies conducted inJapan, there are great differences in ozone sensitivity among Japanese forest tree species, in terms ofgrowth and net photosynthesis. To clarify the sensitivity of growth of forest tree species to ozone, in one
study, seedlings of 16 tree species were exposed to either charcoal-filtered air or ozone for several
years (Figure 6-1). The study ranked various species for ozone sensitivity of the whole-plant dry mass
with accumulated ozone exposure over a threshold of 40 ppb (AOT40; see page 17 for details) at 20
ppm･h [from high to low sensitivity: Japanese poplar (Populus maximowiczii) > trident maple (Acerbuergerianum) > Japanese beech (Fagus crenata) and eastern white pine (Pinus strobus) > Japaneseash (Fraxinus japonica) > Japanese red pine (Pinus densiflora) > Nikko fir (Abies homolepis) > yellow
poplar (Liriodendron tulipifera) > Japanese larch (Larix kaempferi) > Japanese white birch (Betula
platyphylla var. japonica) and mizunara oak (Quercus mongolica) > konara oak (Quercus serrata) >Japanese cedar (Cryptomeria japonica) and Norway spruce (Picea abies) > Japanese black pine (Pinus
thunbergii) > Japanese cypress (Chamaecyparis obtuse)]. The AOT40 values corresponding to a 10
percent reduction in the whole-plant dry mass were approximately 8 ppm･h for Japanese popular, themost sensitive species, and 12-21 ppm･h for Japanese red pine, eastern white pine, Japanese beech,
trident maple and Japanese ash.
In Maebashi City of Gunma Prefecture, the site of the study described above, AOT40 values for six
months were 10-24 ppm･h. At Inukoeji in the Tanzawa Mountains of Kanagawa Prefecture, relatively
high concentrations of ozone over 0.12 ppm were observed, and the AOT40 value from March to Junein 1997 was approximately 30 ppm･h (Aso 1999). Taking these observations and results ofexperimental studies into account, it can be deduced that ozone causes harmful effects on relatively
sensitive tree species such as Japanese beech. Since tropospheric ozone concentrations are expected
to increase steadily in Japan, ozone has the potential to adversely affect many Japanese forest tree
species in the future.
References
Aso, T., Annual Report of the Center for Environmental Science (in Japanese), Kanagawa Prefecture, 31, pp. 32, 1999.
Izuta, T., Plant Responses to Atmospheric Change (in Japanese) (edited and written by Nouchi I.), pp. 168-208,
Youken-dou, 2001.
Izuta, T. and Matsumura H., Journal of Japan Society for Atmospheric Environment (in Japanese), 32(6), A73-A81,
1997.

Tropospheric Ozone: A Growing Threat¤

7. Environmental Standards and Other Indexes for Ozone Exposure
Terms such as standards and criteria are commonly used to describe indicators. The World
Health Organization (WHO) uses the term guidelines. The values of environmental standards for air
pollutants are expressed in terms of concentrations or doses. Several expressions are used asunits of measure for concentrations, such as hourly average, daily average, daily maximum of hourly
values, monthly average, annual average. The environmental standard of photochemical oxidants inJapan is stated, for example, as the one-hour value must be 0.06 ppm or lower. Units of dose suchas ppm･h express concentrations multiplied by time. The same concentration will result in a different
dose if the period of exposure is different. Conversely, it is important to note that, for example, a 20-hour
exposure to 0.05 ppm amounts to the same 1 ppm･h dose that would result from a 1-hour exposure to 1
ppm. Even if the dose is the same, long-term exposure to low concentrations, or short-term exposure tohigh concentrations, may cause different reactions in organisms. These examples show that actual
exposure conditions should be fully considered when evaluating the results of experiments.
Standards in concentration or dose are decided based on reviews of the latest scientific knowledge
from monitoring results, epidemiological studies, and exposure experiments for human beings, animals,
and plants. Since the methods of setting standards differ with each country, region, and affected subject,
no common standard is used worldwide (see table on pages 25 and 26).

■Standards for plants (crops, forests, and ecosystems, etc.)
In general, national environmental standards are set mainly for the protection of human health ineach country, and most of them are based on concentrations. However, European and other countries
have begun to apply the concept of critical levels that incorporate the idea of doses.
A critical level is defined as the concentrations of pollutants in the atmosphere above which directadverse effects on receptors (such as human beings, plants, ecosystems or materials) may occur,
based on current knowledge. A related term, critical load, is defined as a quantitative estimate ofan exposure to one or more pollutants below which significant harmful effects on specified sensitiveelements of the environment do not occur, based on current knowledge.

■AOT40 and SUM60
Europe originally used concentration-based
standards for ozone, but it has been replaced by a
new index called AOT40, which refers to cumulativeozone exposure above a threshold of 40 ppb (Fuhrer
et al. 1997) (Figure 7-1). By definition, AOT40 drops
to zero when ozone concentrations stay below 40
ppb for a certain period.
In Europe, 3 ppm･h has been proposed as the
AOT40 for three months corresponding to a 5
percent yield reduction of crops, based on open-top
Figure 7-1. Calculation of AOT40. (Redrawn fromchamber experiments with wheat cultivars for 10
Fuhrer et al. 1997)
growing seasons in six countries. The AOT40 of 3
Cumulative dose over 40 ppb (concentration × time)
ppm･h was also suggested as the critical level (Level
I standard) for 10 percent growth reduction on grassland and pasture (natural and semi-naturalvegetation). As for forests, the AOT40 of 10 ppm･h was suggested as the critical level (Level I standard)for 10 percent growth reduction based on experiments of ozone exposure to beech trees. Since growth
conditions, such as temperature and soil moisture, have not been reflected in those critical levels,
discussions on Level II critical levels were necessary to consider effects of those factors. In this context,¥ Tropospheric Ozone: A Growing Threat

discussions were started on flux-based critical levels, with the result that a flux threshold of 1.6
nmol/m2/s (about 0.077μg/m2/s) was proposed as a preliminary value for forests (Karlsson et al. 2003).
While European countries discuss the AOT40 as the critical level of ozone, the United States andCanada are discussing another index, known as SUM60. This is the sum of hourly ozone concentrations
equal to or greater than 60 ppb over a given period. As with AOT40, ppm･h is also used as the units
for SUM60. While the AOT40 is the sum of the differences between hourly ozone concentrations and 40
ppb for each hour, SUM60 is the sum of concentrations that exceed 60 ppb.

■Mapping of areas exceeding critical levels
Mapping of monitoring data on the atmosphere in
relation to those critical levels makes it possible to study
where the ozone concentrations exceed the critical level, towhat extent, and which areas have high risk of ozone
effects. Yield of agricultural crops can also be evaluated anddiscussed based on the mapping. Figure 7-2 is a map
available on the Internet showing the spatial distribution of
SUM60 in the United States.
In Japan, estimates were also made in recent years
regarding the effects on rice yield and growth, but the areastudied was limited to the Kanto region (Takagi and Ohara
2003; Yonekura et al. 2005).
Japan's nation-wide monitoring system observes
photochemical oxidants, rather than ozone concentrations.
Published data include the number of hours whenconcentrations exceed 0.06 ppm and exceed 0.12 ppm ateach monitoring station, only during the daytime, so 24-hour
statistical data are not available except for limited sites.
Thus, in one study, the AOT40s at each station were
estimated in detail based on the relationship between thenumber of hours exceeding the standards and hourly
concentrations. This analysis showed that the AOT40 inJapan was approximately 5 ppm･h in the early 1980s, thengradually increased year by year, to the point that it
exceeded 10 ppm･h after the year 2000 (Figure 7-3).
Analysis on the nation-wide spatial distribution shows that
the AOT40 is high in suburban areas of large cities, forexample, in the Kanto region, Osaka, and Hiroshima. In the
Kanto region, the AOT40 is extremely high in Saitama andGunma Prefectures (Figure 7-4).

Figure 7-2. Five-year average (1999-2003) ofambient ozone exposures using SUM60 in the
United States. (USFS-FIA)
Period: June 1-August 31
Time: 8 a.m.-8 p.m.
Contour lines: Increments of 5 ppm･h. (Areanear California exceeds 30 ppm･h.)Figure 7-3. Trend of mean AOT40 forphotochemical oxidants (Ox) in Japan.
AOT40 was estimated by calculating from sumof numbers of hours exceeding 0.06 and 0.12
ppm. (Kohno 2005)

Tropospheric Ozone: A Growing Threat¦

Figure 7-4. Mean AOT40 for photochemical oxidants (Ox) in Japan.
AOT40 was estimated and mapped by calculating from sum of numbers of hours exceeding 0.06 and 0.12 ppm. Units:
ppb･h. (Kohno 2005)

Monitoring stations for ozone and oxidants of Japan are located mainly in densely populated urban
areas, and only a few stations are in mountainous areas. For the conservation of the naturalenvironment, it will be important to take measures to strengthen monitoring in such areas that now
provide insufficient data. When we consider that, except for Japan, stations equipped with automaticozone monitors are very limited in number in East Asia, it is clear that this region is far behind Europe
and North America in monitoring and research systems on ozone and oxidants.

References
Ashmore, M. R. and R. B. Wilson, Critical levels of air pollutants for Europe, background papers for UNECE
workshop on critical levels, 1992.
Environment Canada, Summary of Science Assessment Document, 1999.
..
Fuhrer, J., L. Skaby and M. R. Ashmore, Critical levels for ozone effects on vegetation in Europe, Environmental
Pollution 97(1-2):91-106,1997.
Karlsson, P.-E., G. Sellden and H. Pleijel (eds.), Establishing ozone critical levels II-UNECE Workshop Report.
IVL Report B 1523, 2003.
Kohno Y., Global Environment Research Fund, Ministry of the Environment, C-7 Study for assessment ofvegetation effects of acid and oxidizing substances and establishment of critical levels in East Asia (inJapanese) 2005.
Kohno, Y., H. Matsumura, T. Ishii and T. Izuta, Establishing critical levels of air pollutants for protecting East
Asian vegetation-A challenge, p. 243-250. In K. Omasa, I. Nouchi, and L. J. De Kok (eds.), Plant
Responses to Air Pollution and Global Change, Springer-Verlag, Tokyo, 2005.
Yonekura, T., T. Shimada, M. Miwa, A. Arzate and K. Ogawa, J. Agric. Meteorol. 60(5):1045-1048, 2005.
Takagi K. and T. Ohara, Estimation of Ozone Impact on Plants by Damage Functions in the Kanto area (inJapanese), Journal of the Japan Society for Atmospheric Environment, 38(4):205-206, 2003.

§ Tropospheric Ozone: A Growing Threat

8. The Ozone Threat in East Asia
■Models of growing ozone pollution in East Asia
The first chapter showed that tropospheric ozone concentrations are increasing in the northernhemisphere in large regions of intense human activity. In this chapter, we examine the situation of ozone
pollution in East Asia. While the current status is not necessarily clear, due to the lack of an extensive
monitoring network on the ground, recent developments in modeling enable relatively accurate
descriptions of the geographical distribution of ozone pollution.
Figure 8-1 is the result of a regional-scale chemical transport model, showing the distribution ofmonthly average ozone concentrations from spring to summer in East Asia.
The figure reveals high ground-level ozone concentrations in spring and summer in large parts ofEast Asia, and a band of high-concentration ozone extending from Central Asia to the Pacific Ocean.
The concentrations reached a peak in May in Japan, and in June in the middle of the Asian continent.
The peak in June is thought to be due to the highest intensity of ultraviolet light from the sun in thatmonth. Japan s peak in May, rather than June, is thought to be due to the fact that June is generally therainy season there. Although July and August are the months when high ozone concentrations are most
likely to occur in major cities in Japan, theaverage concentration all over Japan insummer is lower than the period fromspring to early summer, due to the
influences of the monsoon resulting fromhigh atmospheric pressure over the
Pacific Ocean. In the figure, the worst
April
May
ozone pollution, with a monthly averageof more than 70 ppb, appears to occur inthe eastern region of the Asian continent
where the emissions of air pollutants may
be the highest in East Asia.

June

July

Figure 8-1. Ozone distribution at the ground level in East Asia in thespring and summer of 1996. Calculated by a regional-scale modelwith 80 km resolution. Dark red and light red indicate more than 70
and 60 ppb, respectively (monthly average). Orange and yellow
indicate more than 50 and 40 ppb, respectively. (Zhu et al. 2005)

What are the impacts on Japancaused by ozone pollution covering all of
East Asia? The results of monitoring on
the Oki islands in the Sea of Japan, may
be a useful illustration of the impacts on
Japan of ozone from the continent.
Figure 8-2 shows the variation of ozone
concentrations (hourly values) at the Oki
monitoring station of the Acid Deposition
Monitoring Network in East Asia
(EANET) (see page 24) in May 1994.

O3 concentration（ppb）■Evidence of the threat ofozone pollution in East Asia

2 May

7 May

12 May17 May22 May27 May1 JuneFigure 8-2. Hourly values of ozone concentrations monitored on
Oki islands (Sea of Japan) in May 1994. (Akimoto 2003)

Tropospheric Ozone: A Growing Threat¨Pollutants emitted from Japan have almost no impact at this remote site, so one can assume that the
Asian continent is the source. Ozone concentrations often exceeded the Japanese environmental
standard of 60 ppb, and peaked at 80 ppb.
Figure 8-3 classifies the ozone concentrations monitored in Oki by type of air mass, in an effort toclarify the influence of the long-range transport of pollutants. As shown by the graph, ozone
concentrations were highest in air masses indicated by black square (■), which had passed throughregions of intensive human activity on the Asian continent; they were lowest in air masses indicated bywhite circles (○), which came directly from Siberia (also part of the Asian continent). The ozone
concentrations of the former air mass reached 55 ppb (monthly average) from spring to autumn. The
difference of concentrations between the air masses shown by ■ and ○ is thought to represent the
influence of ozone produced on the Asian continent and transported to Japan. It is about 20 ppb(monthly average) in summer and 10 to 15 ppb in spring and autumn, as shown.
Emissions of pollutants are likely to increase with future economic development in East Asia. If that
increase contributes 10 ppb to ozone concentrations in Japan, even remote or rural areas in Japan will
be exposed to ozone concentrations exceeding 60 ppb during spring and summer. Concentrations ofphotochemical oxidants in major cities will also rise further and result in more days exceeding thresholds
that trigger pollution warnings. This model also suggests that China and Korea will also experience
increasingly serious environmental impacts from ozone pollution.

Figure 8-3. Ozone concentrations (monthly averages)
on Oki islands (Sea of Japan), monitored in the air
masses from the Asian continent in 1994 and 1995.
■ Polluted air mass that passed over areas ofintense human activity in East Asia
○ Relatively clean continental air mass from
Siberia (did not pass through polluted areas in East
Asia)
Solid line at bottom indicates the difference in ozone
concentrations between the polluted air masses andclean air masses. (Pochanart et al. 1999)

References
Akimoto, H., Measures for the Resources and Environment (in Japanese), 39, 90-96, 2003.
Pochanart, P. et al., Journal of Geophysical Research, 104, 3621-3631, 1999.
Zhu, B. et al., Geophysical Research Letters, 31, L17104, 2004.

© Tropospheric Ozone: A Growing Threat9. Future Research and Monitoring Needs
Past observations and experiments and recently-developed atmospheric chemistry models haverevealed the impacts on human health and plants caused by tropospheric ozone, an air pollutant formedby reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs).
Ozone concentrations are particularly high in the middle and high latitudes of the northernhemisphere, where the background ozone concentrations have more than doubled during the lastcentury. In Japan as well, ozone concentrations have been increasing across the country.
Emissions of ozone precursors have more than doubled (for NOx) or doubled (for VOCs) in Asia
over the twenty years from 1980 to 2000. NOx concentrations are particularly high in China s northern
plain, where they continue to increase by about 7 percent per year. Tropospheric ozone is also a strong
greenhouse gas, considered to be the third most important after carbon dioxide and methane.
Ozone is known to have negative effects on human health, such as on respiratory functions. Paying
special attention to the health effects of long-term ozone exposure, countries of Europe and North
America have established environmental standards of eight-hour value to protect human health. As forthe effects of ozone on vegetation, it is thought that sensitive species such as the beech tree havealready been negatively affected in Japan, and that other species will also be affected in the coming
years. As crop yields are also reduced by ozone exposure, crop production in 2020 is expected to see
further declines due to increases in ozone concentrations around the world.
Factors that have contributed to the increased photochemical oxidant concentrations in Japan
include changes in climatic variables such as temperature and solar radiation, changes in the ratio ofNOx to VOCs in the atmosphere, and increased transboundary air pollution. Long-term increases inbackground ozone levels suggest that tropospheric ozone has become a major issue affecting not only
all of East Asia, but also the entire northern hemisphere.

■Future research needs
As mentioned above, tropospheric ozone has become increasingly important as a form oftransboundary air pollution. In order to properly address this problem, further studies are needed in thefollowing areas.

Monitoring anddata analysis

At EANET monitoring sites in East Asia, continuous measurements of ozone, NOx, and VOCs,
should be given a higher priority, and an effort should be made to introduce well-calibrated
instruments. Information technology should be exploited further for data acquisition and distribution
via the Internet.
To verify the intercontinental transport of pollutants, it is also important to consider the
expansion of monitoring networks to include South and Central Asia, and the Middle East. Efforts
should also be made to establish the capacity for satellite-based monitoring of air pollution in Asia.

Emission
inventories

To provide information for modeling and to consider measures for rational reduction, it isnecessary to prepare and improve emission inventories of both anthropogenic and natural sourcesof NOx and VOCs, the precursors of ozone in the atmosphere. (An emission inventory is a list ofemission sources and amounts in a nation or a region, etc.) Systems for sharing, publishing,reporting, utilizing and updating these inventories should also be established.

Modeling

The accuracy of projections in models should be enhanced through greater sophistication ofmodels dealing with the transport, diffusion, reactions and deposition processes related to ozone,
and verification using monitoring data. It is also important to create integrated models dealing withdifferent scales, from the city to the global scale, which will allow estimations of interactions
between large and small scales. It is particularly important to do projections based on futurescenarios of economic activities, with quantitative analysis of transboundary pollution in East Asia.
Cooperation with the United Nations Economic Commission for Europe (UNECE) and theUnited States Environment Protection Agency should be promoted in order to deal with theintercontinental transport of pollutants in the northern hemisphere, as well as to take measures todeal with the pollutants.

Tropospheric Ozone: A Growing ThreatªHuman healthimpacts

An hourly value (less than 0.06 ppm) was established as the only environmental quality
standard relating to photochemical oxidants in Japan, based on consideration of acute healthimpacts. As an hourly value, this may be the most stringent standard in the world. Countries thathave performed risk assessments based on epidemiological surveys, however, have establishedeight-hour values as environmental standards, in addition to the hourly values. The government ofJapan may need to reexamine its own environmental standards based on such information. For
that purpose, it is necessary to conduct epidemiological research on the relationships between
photochemical oxidant concentrations and increases in asthma attacks, the number of persons
tested for respiratory ailments, and hospitalization. Also important are studies of compound effectsof photochemical oxidants and suspended particulate matter, and literature reviews of documentsthat formed the basis for current environmental quality standards in Europe and North America.

Effects onvegetation

Critical ozone levels must be determined for various types of vegetation in East Asia with a
special attention to the sensitivity of representative plant species in the region.
In the face of the large variability in climatic conditions across East Asia, studies must be
conducted in various representative ecosystems in the region for observation and modeling ofozone fluxes affecting vegetation. Such studies should reveal the extent of the ozone deposition onvegetation in this region, in interaction with temperature, wind velocity, humidity and CO2
concentrations. These findings should be incorporated into dynamic vegetation models, which, in
combination with atmospheric chemistry and transport models, will facilitate assessments of thepresent status of ozone impacts on vegetation and projections of future impacts.
This scientific information should underpin policy-making efforts to set up environmental quality
standards relating to ozone across East Asia, in order to prevent the worst projections of damage
to vegetation from becoming reality.

■Future steps
Measures for reducing NOx and VOC emissions have been promoted and achieved some degreeof success in Japan. Despite this success, concentrations of photochemical oxidants have been
increasing since around 1980. As shown by the various scientific results described in this booklet, theproblem of tropospheric ozone needs to be tackled collectively in East Asia and the entire northernhemisphere.
At present, the Acid Deposition Monitoring Network in East Asia (see next page), with thirteen
participating countries, is working with a view to promoting international cooperation to protect theatmosphere in East Asia. As soon as possible, the Network needs to begin careful monitoring of ozone,
which as an oxidizing agent is closely associated with acid deposition. In addition to monitoring, aframework including emission inventories and modeling is also needed in order to facilitate integrated
assessments. Policy targets also need to be established in order to ensure the protection of theatmospheric environment in the entire region.

« Tropospheric Ozone: A Growing Threat

Acid Deposition Monitoring Network in East Asia (EANET)The Acid Deposition Monitoring Network in East Asia (EANET), established in 1998, is aninternational network for which the main aims are to create a common understanding of thestate of the acid deposition problems in East Asia, and to provide useful inputs for decisionmaking that can prevent or reduce adverse impacts on the environment caused by aciddeposition. The Network s Secretariat was established within the United Nations Environment
Programme (UNEP). The Acid Deposition and Oxidant Research Center (ADORC),
designated as the Network Center for EANET, provides technical support for participating
countries.
After a preparatory phase that ended in 2000, EANET started its regular phase activities
in 2001. There are thirteen participating countries at present: Cambodia, China, Indonesia,
Japan, Lao PDR, Malaysia, Mongolia, Myanmar, Philippines, Republic of Korea, Russia,
Thailand and Vietnam. In addition to the implementation of the monitoring of acid deposition
and its effects on ecosystems, the Network Center compiles monitoring data and releases it toparticipating countries and beyond. The first periodic report on the state of acid deposition in
East Asia will be completed in 2006. To support participating countries, the Network Centerhas also conducted joint research projects as well as activities to enhance the reliability ofmonitoring data.
In addition, to promote regional air quality management, including acid deposition, in East
Asia and to provide a sound basis for financial contributions and extending the activities of
EANET, discussions are now in progress towards establishment of a regional instrument andfurther development of the network.

Figure 9-1. Organizational chart of EANET.

Figure 9-2. EANET monitoring sites (47 sites
monitoring wet acid deposition).

Tropospheric Ozone: A Growing Threat¬

Air Quality Standards and other Indexes on Ozone
WHO（1998）
Guideline

EU-EEA（1998）
Criteria

UNECE（2004） New Zealand（2002）
Guideline
Critical level

120μg/ã
100〜200μg/ã
0.06ppm（0.05〜0.1 ppm） （0.06ppm）（1-hour value）（1-hour value） （8-hour value）

110μg/ã
（0.055ppm）（8-hour value）

120μg/ã
（0.06ppm）（8-hour value）

150μg/ã
（0.075ppm）（1-hour value）

Warning：0.12ppm（1-hour value）

Information：180μg/ã（0.09ppm）（1-hour value）

AOT60＝0(1 year）

100μg/ã
（0.050ppm）（8-hour value）

Serious Warning：
0.24ppm（1-hour value）

Warning：360μg/ã（0.18ppm）（1-hour value）

Japan
Environmental Standard
Human Health

EHC 7（1978）
Guideline

Emergency：0.40ppm（1-hour value）

200μg/ã（0.10ppm）（1-hour value）

65μg/ã（0.033ppm）（24-hour value）

PlantsAgriculture(visible damage)

Saturation deficit isless than 1.5ｋPa

428μg/ã-h
AOT40=0.2ppm・h（0.214ppm・h）（5 days）（5 days）

Saturation deficit ismore than 1.5ｋPa

1,070μg/ã-h
AOT40=0.5ppm・h（0.535ppm・ｈ）（5 days）（5 days）

Agriculture (yield)

AOT40=3ppm・h(May - July）（5％ decrease of yield）

Natural/Seminatural Vegetation

AOT40=3ppm・h(May - July）
AOT40=10ppm・h（April - September）

Forest/Trees

Sensitive species

AOT40=3ppm・h
6,420μg/ã-h（3 months）（3.21ppm・h）（5％ decrease of yield） （3 months）
6,420μg/ã-h
AOT40=3ppm・h（3.21ppm・h）(May - July）(10% decrease of growth） （3 months）
AOT40=10ppm・h 21,400μg/ã-h（April - September） （10.7ppm・h）(10% decrease of growth） （6 months）

In case saturation
deficit is small

AOT40=5ppm・hFlux base

1.6n mol/ß/s（１ growth period）

Saturation deficit isless than 1.5ｋPa
Saturation deficit ismore than 1.5ｋPa

Ecological Risk
Assessment

AOT40=0.2ppm・h
AOT40=0.5ppm・h
AOT40
Value proper to each receptor

The standards or indicators that have no category like plants and forest are in the row of human health.
Japan: Notification 25 of the Environment Agency（8 May 1973）. Japan's standard is for photochemical oxidants. Other countries/organizations commonly base theirs on
ozone. Serious Warnings in Japan are provided by prefectural governments, based on their own guidelines.
EHC 7(1978): Photochemical oxidants. In: Environmental Health Criteria 7 (1978). International Programme on Chemical Safety (IPCS).
AOT40: Accumulated exposure over a threshold ozone concentration of 40 ppb, when solar radiation 50Wm2 or more.
SUM60: AOT40 is calculated excluding parts less than 40ppb. SUM60 is calculated including the whole values during the periods more than 60 ppb. The period for the
calculation is 12 hours from 8 a.m. through 8 p.m. For assessing acute effects, the total values for three days are used. For long lasting chronic effects, the total
values for three months are used. SUM0 is equivalent to the total of all the hourly values.
EU-EEA(1998): Tropospheric ozone in EU - The consolidated report, Topic report No. 8/1998, European Environment Agency.
UNECE: UNECE(2004): Section IX, 1999 Protocol to abate acidification, eutrophication and ground-level ozone. In: Handbook for the 1979 convention on long-range
transboundary air pollution and its protocols.
Karlsson et al. (2003): Establishing ozone critical levels II - UNECE Workshop Report. IVL Report B 1523.
Fuhrer et al.(1997): Critical levels for ozone effects on vegetation in Europe. Environ. Pollut. 97(1-2): 91-106.
New Zealand (2002): Ambient air quality guidelines, 2002 Update, Air quality report No.32, Ministry of the Environment. May, 2002.

­ Tropospheric Ozone: A Growing Threat

（Kohno）
USA（1979）
Standard

USA（1997）
New standard

0.12ppm（1-hour value）

0.08ppm（8-hour value）

0.12ppm（1-hour value）

0.08ppm（8-hour value）

R. of Korea
China (1996)
Standard
Standard
Class I Standard
Maximum Desirable：
100μg/ã
0.06ppm
0.09ppm
0.12mg/ã
51ppb（0.05ppm）（8-hour value）（1-hour value）（0.06ppm）（8-hour value）（1-hour value）（1-hour value）
Class II Standard
Maximum Desirable：
0.1ppm
0.070ppm
0.16mg/ã
15ppb（1-hour value）（8-hour value）（0.08ppm）（24-hour value)（1-hour value）
Class III Standard
Maximum Acceptable：
0.20mg/ã
82ppb（0.10ppm）（1-hour value）（1-hour value）
Maximum Acceptable：
25ppb（24-hour value)
Maximum Acceptable：
15ppb(Yearly Average）
Maximum Tolerable
Level: 153 ppb（1-hour value）
Class I Standard
0.12mg/ã（0.06ppm）（1-hour value）
Class II Standard
0.16mg/ã（0.08ppm）（1-hour value）
Class III Standard
0.20mg/ã（0.10ppm）（1-hour value）
California（2004）
Standard

CanadaGuideline

UKStandardThailandStandard
0.10ppm（1-hour value）

0.04ppm（yearly value）

SUM60=0.5-0.7ppm・h(12 hours，
3 months）

SUM60=5.9-7.4ppm・h(12 hours，
3 months）

SUM60=4.4-6.6ppm・h(12 hours，
3 months）

USA: Although the primary and secondary air quality standards have the same value, the primary standard targets human health and the secondary standard targets
visibility, plants, animals, and buildings. In this table, the latter is in the row of plants, because agricultural products are not clearly distinguished in the standard. The
standards revised in 1997 are applied for the three-year average of the fourth largest value in a year of maximum daily 8-hour average values, which should be lessthan 0.08 ppm. However, for sites where consecutive three-year data do not attain the hourly value, the previous hourly value is applied.
California, USA (2004）: Recommendation for an ambient air quality standard for ozone. Office of Environmental Health Hazard Assessment, California Environmental
Protection Agency. June 2004.
Canada: LOAEL (Lowest Observable Adverse Effect Level) values are used as the recommended values for plants. National ambient air quality objectives for groundlevel ozone - Summary Science Assessment Document, Environment Canada (1999).
UK: Daily maximum of 8-hour average should not exceed the standard 10 times a year. At the present, ozone is not targeted in regulation.
China: Ambient air quality standard. GB 3095-1996. Class I standard targets natural reserves, places of scenic beauty and special protection areas. Class II standard
targets residential areas, commercial and traffic areas, cultural areas, industrial areas and rural areas. Class III standard targets specified industrial areas.
Ecological risk assessment: Five-year average AOT40 value during a period specific to each targeted item.
Critical level for flux base: This was discussed at UNECE workshop in 2002 and adopted as provisional Critical Level I.

Tropospheric Ozone: A Growing Threat®

Editorial Board
Chairman: Hajime Akimoto, Japan Agency for Marine-Earth Science and Technology
Members: Takeshi Izuta, Tokyo University of Agriculture and Technology
Hiromasa Ueda, Acid Deposition and Oxidant Research Center
Iwao Uchiyama, Kyoto University
Toshimasa Ohara, National Institute for Environmental Studies
Yoshihisa Kohno, Central Research Institute of Electric Power Industry
Kazuhiko Kobayashi, The University of Tokyo
Shinji Wakamatsu, National Institute for Environmental Studies
Secretariat
Chief:
Yoshiyuki Yoichi
Members: Jiro Sato
Tsuyoshi Ohizumi
Hiroyuki Sase
Kozue Kasahara
Acid Deposition and Oxidant Research Center, 1182 Sowa, Niigata-City, Niigata,950-2144, Japan
Tel. 81-25-263-0556 Fax. 81-25-263-0567 http://www.adorc.gr.jp/

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