EPA-AA-IMS/AQ-80-2
Natural Sources of Ozone: Their Origin
and Their Effect on Air Quality
March, 1980
NOTICE
Technical Reports do not necessarily represent final EPA decisions or posi-
tions. They are intended to present technical analysis of issues using data
which are currently available. The purpose in the release of such reports
is to facilitate the exchange of technical information and to inform the
public of technical developments which may form the basis for a final EPA
decision, position, or regulatory action.
Inspection/Maintenance Staff
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental Protection Agency
-------
INTRODUCTION
The National Ambient Air Quality Standard for ozone is being violated in
many urban and rural areas of the U.S. Cities and states which violate the
standard must submit plans for meeting it. Since controlling emissions for
ozone reduction is a costly endeavor, any possibility that violations occur
due to natural causes is a very important consideration.
This report deals with the major natural sources of ozone in the atmosphere,
and tries to explain how they compare and interact with anthropogenic,
(man-made) sources. The two main natural sources of ozone are: 1) the
stratosphere, which provides ozone to the troposphere through various physi-
cal transport mechanisms and 2) biogenic hydrocarbons, (from plants and
trees) which react in sunlight with nitrogen oxides to form ozone. In
comparison to these sources, other natural sources, such as lightning, of
atmospheric ozone are negligible, so this report will focus on these two
sources only.
On the average, natural sources of ozone contribute an ambient background
(non-man-made) level of about 0.02 - 0.05 ppm (parts per million). The
actual level for a particular site depends on many factors, such as geogra-
phy and climate.
The National. Ambient Air Ouality Standard for ozone is a one-hour standard
of 0.12 ppm (multiple exceedances in a single day count as one, however), so
it is clear that man-made sources must contribute most of the ozone in
places where the standard is being violated. The concern in this paper,
however, is whether occasional peaks of naturally produced ozone could cause
ozone standard violations that might not have occurred otherwise.
Most of the background ozone concentration derives from the stratosphere.
Therefore, the first natural source of ozone to be discussed will be the
stratosphere. The issue of natural hydrocarbons' effect on ozone levels
will be analyzed next, and finally the total effect will be examined in
light of its impact on air quality.
STRATOSPHERIC INTRUSION
Stratospheric Ozone: Background
The stratosphere is the second layer of the atmosphere. It is separated
from the troposphere (the layer nearest the earth) by a boundary area called
the tropopause, which is about 5-11 miles high (5 miles at the poles, 11
miles above the equator). Warm air from the earth's surface rises in the
troposphere, and then gradually sinks again after cooling and contracting.
The stratosphere is very stable, and does not contain much air circulation,
whereas the troposphere is more active and contains most of the earth's
winds.
Ozone is present in both the stratosphere and the troposphere, however, the
mechanisms for ozone's formation in these two layers differs. In the stra-
tosphere, ultraviolet light from the sun provides energy to dissociate, or
-------
break apart, molecular oxygen (CL) into atomic oxygen (0). Atomic oxygen
then reacts very quickly with either CL to form ozone (0 ), or with more
atomic oxygen to reform 0 , as diagrammed below:
0 sunlight 0 + 0
00 sunlight 20
2
inlight
0 + 0 v 02
Since 02 and 0 are constantly being dissociated, the net reaction produces
a steady state concentration of 0. in the stratosphere.
Stratospheric ozone absorbs ultraviolet light and thus acts as a shield
protecting the biosphere below. Life as we know it would not exist without
this shield. Ozone from the stratosphere is constantly diffusing into the
troposphere, resulting in background levels from this source of about 0.02-
0.04 ppm. In addition, meteorological factors can cause infrequent, decen-
tralized, larger scale intrusions of ozone from the stratosphere into the
troposphere.
Stratospheric ozone in the troposphere is a source of some concern to air
quality planners, because of its fluctuations and because it is sometimes
difficult to identify as such. The main source of concern is sporadic,
larger-scale intrusions of stratospheric ozone (which might contribute to
violations of the ozone standard), rather than the more usual, gradual
diffusion.
Mechanisms for Ozone Intrusion from the Stratosphere
There are four key mechanisms by which stratospheric ozone enters the tropo-
sphere. Two of these processes take place on a world-wide, relatively
continuous basis. The other two occur sporadically, and their contribution
to ozone concentrations in any given time or place is difficult to ascer-
tain.
The gradual diffusion mechanisms account for almost all of the ambient
natural background ozone of stratospheric origin, about 0.02-0.04 ppm . The
most important of these processes is called mean meridional circulation.
This refers to the mixing of stratospheric and tropospheric air along the
polar front, and subsequent horizontal transport to the mid-latitudes. This
atmospheric process, though it occurs year-round, is most intense during the
winter when storm activity is greatest. The second mechanism, seasonal
tropopause adjustment, involves the mixing that results from the seasonal
changes in the height of the tropopause boundary. The height of the tropo-
pause varies seasonally due to temperature changes that cause the tropo-
sphere to expand and contract. During the spring, as the tropopause rises
with the warming of the atmosphere, this process is of most importance to
ozone intrusion.
-------
There are also two sporadic intrusion mechanisms, which are by definition
short and localized in nature. However, they can result in high ambient
ozone concentrations of short duration. Fortunately they occur very rarely,
so their impact on average ozone background levels is minimal. The more
important of these mechanisms results from the folding over of the tropo-
pause that can occur when weather phenomena arise which involve large ex-
changes of air between high and low altitudes (i.e., a thunderstorm). They
occur mainly at mid-latitudes where polar and tropical air masses meet, and
are most frequent during the spring. The other sporadic intrusion mechan-
ism, small-scale eddy transport, is essentially a smaller version of tropo-
pause folding. Although most tropopause folding contributes an insignifi-
cant amount to background ozone concentrations, some sporadic intrusions can
yield ambient ozone concentrations at ground level of 0.08 ppm or more.
Such strong intrusions have been .estimated to occur about once a year at
particular mid-latitude locations.
Relationships Between Stratospheric Ozone and Ozone of
Anthropogenic Origin
Although ozone from these two sources is physically indistinguishable it is
possible in most cases to attribute high ambient ozone concentrations to
their proper source. This is due to the different conditions under which
the two sources prevail. First, stratospheric ozone intrusions of a spora-
dic nature (which are the only ones likely to result in high ozone levels)
occur predominantly during the spring, a season when there is not usually
large-scale formation of ozone in the troposphere from anthropogenic pollu-
tants. Ozone formation from tropospheric sources tends to peak in the
summer because of its photochemical nature.
Second, high ozone concentrations of stratospheric origin observed at ground
level are usually associated with meteorological phenomena such as storms
and rapid temperature changes. High concentrations of ozone of anthropo-
genic origin, on the other hand, are associated with stagnant air and con-
tinuous warm temperatures.
Thus, high levels of ozone from both sources usually do not occur simultan-
eously. Where an ozone violation occurs without a significant increase in
anthropogenic pollution, or sunny weather and warm temperatures, one can
probably attribute it to stratospheric intrusion, and vice versa. In the
unlikely event that these processes are both present, however, it would be
difficult to accurately account for the contributions of each process unless
sophisticated meteorological or tracer analyses are performed.
The Effect of Stratospheric Intrusion on Ozone Non-Attainment Areas
The possibility of stratospheric intrusion contributing in a major way to an
ozone violation is minimal. First, as discussed previously, intrusions
resulting in elevated ground-level ozone concentrations are estimated to
occur only about once a year, and second, a combination of high anthropo-
genic and stratospheric ozone levels is even less likely to occur. However,
should a situation occur where unusual stratospheric impact is indicated,
the EPA allows it to be disregarded for regulatory purposes as long as the
-------
4
stratospheric intrusion event is documented and supportable. In addition,
one exceedance of the daily standard per year is allowed, so the chance of a
non-attainment designation due mainly to stratospheric ozone intrusion is
almost nil. Recent studies by Singh, et. al. indicate that at lower U.S.
latitudes (below 30°) the threat of summer intrusions affecting ground level
ozone concentrations is non-existent.
In summary, although stratospheric ozone is present in the air we breathe,
it is not generally a health hazard in and of itself because it is usually
present at very low background levels. The main source of ozone levels
which are high enough to be unhealthy is anthropogenic in nature. Thus, it
is appropriate that air quality planners need to concentrate on the control
of man-made ozone precursors, namely hydrocarbons and oxides of nitrogen.
NATURAL HYDROCARBONS;
Ozone Formation
In the lower layer of the atmosphere ozone (0 ) is formed in a series of
photochemical reactions where oxygen, non-methane hydrocarbons (NMHC's) and
oxides of nitrogen (NOx) combine, as shown in the general equation below:
NMHC + NOx SUnUsh> 0.
°2
The rate of formation of ozone in the atmosphere is affected by the relative
concentrations of reactants present, the amount of sunlight, the prevailing
temperature, and other meteorological conditions. These factors are dis-
cussed below.
The relative concentrations of NMHC and NOx are most important. Where the
NMHC/NOx ratio is very low or very high, the reaction is inhibited. At very
high ratios (greater than 40:1), there is an excess of HC's, and most NXHC's
tend to scavenge 0 , NOx, and radicals, thus ending the photochemical reac-
tion chain. At low ratios (less than 4:1), the availability of NMHC becomes
the limiting factor. At all times, unreacted NOx can react with 0 , thus
acting as an ozone sink (as well as precursor). Smog chamber studies have
shown that for the natural hydrocarbons, the maximum ozone is observed at
HC/NOx ratios between 10:1 and 20:1; for anthropogenic hydrocarbons the
maximum ozone HC/NOx ratio varies widely with hydrocarbon structure.
Sunlight is necessary for ozone formation to take place on any appreciable
scale, since the reaction is photochemical in nature. Higher temperatures
also facilitate the reaction.
Meteorological conditions (cloud cover, wind, mixing height, humidity, etc.)
play a role in determining the concentration of the reactants as well as in
influencing temperature and sunlight. If NMHC's and NOx are dispersed
quickly by strong winds, ozone production will be slight. On the other
hand, stagnant weather conditions can exacerbate the ozone formation problem
by holding pollutants in an area and producing high concentrations of ozone
precursors (NMHC and NOx).
-------
Thus, meteorological conditions influence diurnal (day/night) and seasonal
patterns in 0_ concentrations. Generally, photochemical ozone levels peak
in the early afternoon on a daily basis and during the summer months over
the course of the year.
The types of NMHC's present also affect ozone formation. Hydrocarbons
differ in their weight and molecular structure, which affect their reactiv-
ity. This is discussed in detail later.
Hydrocarbon Sources
Hydrocarbons are produced by many sources. The largest share (possibly 80%
by mass of all emissions in the U.S.) , is emitted by vegetation, especially
forests. Anthropogenic (man-made) sources include combustion by industry,
evaporation from refineries, and, of course, the automobile. However, even
though total natural NMHC emissions may be much greater than anthropogenic
ones, they are spread out thinly over wide areas, while industrial and
automobile emissions tend to be concentrated in urbanized areas, where a
large percentage of the population is located.
Even close to densely wooded areas, the concentration of natural HC'S is
generally very low, both in actual terms and in relation to total NMHC.
Rural sites typically show early morning (peak) NMHC levels of 0.05-0.20
parts per million carbon (ppmC) of which most is due to anthropogenic ori-
gin. In urban areas, observations are often five to tweJve times higher,
with anthropogenic sources dominating the hydrocarbon mix.
Hydrocarbon Reactivities
Natural hydrocarbons are very reactive, however, most natural HC's are ter-
penes, which react with C- to form aerosols in areas with little NOx (i.e.,
rural areas). Therefore they act as an ozone sink as well as a source.
Anthropogenic HC's do not have as great a potential ^foj: aerosol formation.
These conclusions are supported by smog chamber data. '
Rural and Urban Ozone Production
In rural areas the ratio of HC to NOx can be as high as 200:1. In urban
areas with ozone problems this figure is usually about 10:1, close to the
optimum ratio for producing ozone. This difference in ratios is due prin-
cipally to the fact that NOx sources are low in rural areas. Therefore, in
rural sites, there is insufficient NOx to drive the photochemical reactions
necessary for significant ozone production. The small amount of 0 which is
formed reacts quickly with the excess natural hydrocarbons. Actual ozone
measurements for rural areas, typically average 0.02-0.05 ppm during the
photochemical oxidant season.
The production of 0. in rural areas is further inhibited by the fact that
most of the HC's emitted are produced by natural sources. As described
earlier, natural HC's, when present in large excess relative to NOx, react
with ozone rapidly, thus preventing accumulation of ozone to problem levels.
-------
Transport of NMHC's Between Rural and Urban Areas
The high NMHC concentrations of urban areas can affect sites downwind, but
naturally produced NMHC's do not seem to be transported to urban areas. The
high reactivity of natural hydrocarbons results in a short average lifetime.
Thus, only a few species of naturally produced HC's last long enough to
travel to urban areas.
Downwind from urban areas, however, both HC and ozone levels can be very
high. This is because the anthropogenic HC's are not used up as fast in
ozone formation, (they have longer lifetimes), and there are higher HC
concentrations than in rural areas to start with.
Natural HC Contribution to the 0 Problem
The properties of biogenic HC's that have been discussed tend to discount
their effects on urban areas. Unlike the man-made hydrocarbons, natural HCs
tend to form photochemically unreactive products, such as aerosols. Their
short lifetimes mean they are unlikely to contribute to the transport pro-
blem. Known ambient hydrocarbon measurements in urban areas bear this out.
Natural HC's are never found in significant concentrations in these areas.
It is possible that they are not found because they have already reacted to
form ozone, but in such a case one would also expect to find significant
quantities of aerosols, another product of natural hydrocarbon reactions.
This has not been found. It is also possible that the measurements of
natural HC emissions are erroneously high. These possibilities have not
been resolved yet.
Due to the complex nature of the photochemical reactions involved in ozone
formation, and the variables that affect them, the effect described here
cannot yet be firmly quantified. However, the tendencies are clear and
available data suggest that it is highly unlikely that air quality is sig-
nificantly affected by emission from vegetation.
Recent Controversial Reports
Two recently publicized reports have ignited controversy on the issue of
increased ozone due to natural hydrocarbon production. One, a study by P.
R. Zimmerman of Washington State University, measured the NHHC's produced in
the Tampa/St. Petersburg area by biogenic and anthropogenic sources. Since
the ambient concentrations of NMHC are low, the resulting figure of 68%
biogenic hydrocarbon emissions is somewhat questionable.
Another study, by J.S. Sandberg et. al. of the San Francisco Bay Air Pollu-
tion Control District, seemed to show a relationship between levels of
winter rain and summer ozone, suggesting that increased vegetative growth
resulted in increased ozone production. Subsequent studies at University
of California/ Riverside (Pitts) did not support such correlations; further-
more Sandberg has not shown a cause and effect relationship to exist, and,
in fact, evidence exists to the contrary. Levels of ambient biogenic HC
measured in the San Francisco area in the past decade are negligible.
-------
Natural Hydrocarbon Sources and the Ozone Standard
In conclusion, it seems that natural hydrocarbons do not contribute enough
to the ozone problem in non-attainment areas to make the ozone standard
difficult to meet. In urban areas their concentrations are low due to the
relative lack of greenery, and due to insignificant transport from rural
•areas. These low concentrations do not lead to significant ozone produc-
tion. In fact, smog chamber experiments indicate that even at .an optimum
terpene/NOx ratio, about 20 ppbC would produce only 1 ppb ozone. In rural
areas where natural hydrocarbon concentrations can be higher there are
generally not ozone problems, and where there are, the evidence shows
anthropogenic/transport origin.
CONCLUSION:
For urban areas, relative to ozone produced from man-made sources, ozone
produced by natural hydrocarbons and stratospheric ozone is of little signi-
ficance on average. However, in certain instances, stratospheric ozone can
have a significant impact on peak levels of ozone. The development of
better means of detecting and separating the effects of anthropogenic and
biogenic ozone, can resolve the conflict surrounding them. Even without
this development, however, it is clear that in order to meet ozone air
quality standards, anthropogenic sources of ozone precursors must be reduced.
-------
References
1. Air Quality Criteria for Ozone and Other Photochemical Oxidants. EPA,
Office of Research and Development, Washington n.C., Publication NO. EPA-
600/8-78-004. April 1978.
2. Reiter, E.R. and Mohnen, V.A. Int. Conf. on Oxidants, 1976 — Analysis
of Evidence and Viewpoints. Part III. The Issue of Stratospheric Ozone
Intrusion. U.S. EPA, ORD, Research Triangle Park, N.C. Publication No.
EPA-600/3-77-115. December, 1977.
3. Procedures for Ouantifying Relationships Between Photochemical Oxidants
and Precursors: Supporting Documentation. U.S. EPA, Office of Air Quality
Planning and Standards. Research Triangle Park, N.C. Publication No. EPA-
450/2-77-02Ib. February 1978.
4. Revisions to the National Ambient Air Quality Standards for Photochemi-
cal Oxidants, Federal Register Vol. 44 No. 28, February 8, 1979.
5. Arnts, R.R., Gay, B.W., Jr., Photochemistry of Naturally Emitted Hydro-
carbons: Isoprene, P-cymene and Selected Monoterpenes, EPA-ESRL Draft
Report, April 1979.
6. Rasmussen, R.A. What do the hydrocarbons from trees contribute to air
pollution? J. Air Pollution Control Association. ^:537, 1972.
7. Bufalini, Joseph J., The Issue of Natural Hydrocarbons and their Role in
the Production of Rural and Urban Ozone. Unpublished report. Research
Triangle Park, N.C.
8. Zimmerman, P.R., Tampa Bay Area Photochemical Oxidant Study. U.S. EPA
Region IV, Publication No. EPA 904/9-77-028, February 1979.
9. Sandberg, J.S., Basso, M.J., Okin, B.A., Winter Rain and Summer Ozone:
A Predictive Relationship. Science 2£0:1051, 1978.
10. Bufalini, J.J., Factors in Summer Ozone Production in the San Francisco
Air Basin. Science 2O3:81, 1979.
11. Coffey, P.E. and Westberg, H. Int. Conf. on Oxidants, 1976 - Analysis
of Evidence and Viewpoints. Part IV. The Issue of Natural Organic Emis-
sions. U.S. EPA, ORD, Research Triangle Park, N.C. Publication No. EPA-
600/3-77-116. October, 1977.
12. Singh, B.S., Viezee, W., Johnson, W.E., and Ludwig, F.L., Proceedings
of APCA Specialty Conference on Ozone/Oxidants, Houston, Texas, October
. To he published..
* UJ. GOVERNMENT WUNTINO OFFICE: 1980- 651-112/0191
------- |