v-xEPA
United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/M-84-004 Jan. 1985
ENVIRONMENTAL
RESEARCH BRIEF
Assessment of the Role of Nitrogen Oxides in
Nonurban Ozone Formation
A. P. Altshuller1
Introduction
This is the fourth and final assessment of technical issues
related to ozone and other photochemically generated
products formed in the atmosphere. It was requested by the
Office of Air Quality Planning and Standards. An earlier
survey of information available on the role of NOX in
nonurban 63 formation (Martinez and Singh, 1979) was
conducted to examine the hypothesis that NOX was the
limiting precursor species with respect to photochemical
formation of 03 over nonurban areas. If proved, this
hypothesis would indicate that increases in N0« concentra-
tions could lead to higher concentration levels of nonurban
O3, but because of fragmentary knowledge of the inter-
actions between NOX and 03 in nonurban areas, the authors
of the survey could not arrive at any definite conclusions
Nevertheless, this survey also included an interesting
analysis of 03 and NO, measurements obtained from the
Sulfate Regional Experiment (SURE) sites in the eastern
United States. Additional experimental measurements,
laboratory smog chamber results, and modeling studies
have become available during the last five years. These
results have been discussed in a review (Altshuller 1984b)
prepared on this subject. Based on these recent studies and
earlier results, a reevaluation of the factors influencing
rural O3 formation destruction and the role of NOX and O3
formation is provided in the present assessment.
'Atmospheric Sciences Research Laboratory, USEPA, Research Triangle
Park, NC 27711
Discussion of Issues
Three issues have been identified for consideration in this
assessment. These issues concern both the characteristics
of Oaformation and transport over nonurban areas and the
role of N0« in processes leading to the O3 formation that
contributes to the observed 03 concentrations.
Issue 1. Why are ozone concentrations exceeding 120 ppbv
observed at some types of nonurban locations, but not at
other nonurban locations in the United States?
Ozone concentrations exceeding 120 ppbv have been
reported at nonurban locations in the northeastern, mid-
western, and southeastern United States and m California
Some of these nonurban locations are within the interior of
the continent, whereas others are at coastal sites. Ozone
concentrations in excess of 120 ppbv have not been
reported at nonurban locations scattered over large areas of
the western United States. What conditions are consistent
with these observations?
In a number of episodes, the elevated ozone concentrations
observed were associated with the passage of urban
plumes through the nonurban locations well downwind of
the urban areas (Altshuller, 1984b). Elevated ozone con-
centrations have been observed during summer days after
several hours of transport downwind in power plant plumes
aloft within the eastern United States. However, evidence
appears to be lacking on the ability of such plumes to
fumigate ground level nonurban locations. Although some
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elevation of ozone has been observed immediately down-
wind of large isolated industrial sources, there appears to
be no evidence that such plumes can cause ozone
concentrations in excess of 120 ppbv at nonurban locations
well downwind.
There is evidence from both ground level measurements
and aircraft measurements within urban plumes that ozone
concentrations downwind on the same day as the plume
left an urban area can exceed 120 ppbv at nonurban
locations (Altshuller, 1 984b). Smog chamber studies and
photochemical model simulations also predict that such
elevated ozone concentrations can occur downwind in
urban plumes. The ground level observations of elevation of
ozone in an urban plume fumigating a specific nonurban
location in some episodes have been based on air quality
measurements as well as back trajectory analyses.
Elevated ozone concentrations at nonurban locations
associated with more than one day of transport over land
are difficult to demonstrate except based on circumstantial
evidence. It is very plausible that ozone demonstrated to be
trapped aloft in the evening hours of one day and then
subjected to high velocity nocturnal jet winds aloft should
be transported overnight substantial distances downwind.
The next morning breakup of the nocturnal surface
inversion layer with strong vertical mixing during summer
morning hours brings the ozone from aloft to the surface.
There also are experimental results consistent with the
survival of most of the ozone trapped aloft the previous
night into the next morning. However, it has been imposs-
ible to associate clearly the elevated ozone at ground level
the second day with a specific source area upwind.
Back trajectory analyses usually provide what evidence is
available about the history of the air parcels reaching
nonurban locations. However, the transport distances and
directions predicted over multiday periods by backtrajectory
analyses are subject to substantial uncertainties. Within
the eastern United States, many adjacent source areas and
combinations of source areas exist. Small errors in back
trajectory analyses could lead to misidentifications among
such sources. Where flows coming from relatively un-
polluted areas need to be distinguished from those coming
from highly polluted areas and the quadrants from which
the flows come are well separated, backtrajectory analyses
are less ambiguous. Back trajectories to unpolluted land
areas upwind usually are not associated with elevated
ozone concentrations.
Results from several smog chamber studies and modeling
simulations of transport from urban areas are consistent
with the observation of ozone concentrations above 120
ppbv downwind over land on the second day of transport.
However, such modeling exercises usually have not
attempted to simulate the details of dilution and deposition
over multiday periods of transport over land areas.
Often elevated ozone concentrations within highly popu-
lated areas of the eastern United States are associated with
the presence of a slowly moving high pressure system in
the same region. Such summer high pressure systems are
associated with periods of higti temperature, nearly max-
imum available solar radiation, and low wind speeds. These
conditions favor ozone formation within air parcels moving
through the high pressure systems. At times the clockwise
flow can cause the accumulated precursors and ozone from
upwind to flow across a large urban area before reaching a
nonurban location farther downwind. Such conditions are
particularly favorable to high ozone concentrations being
observed at such nonurban locations.
Aircraft traverses of urban plumes over nonurban areas in
the eastern United States indicate that the ozone concen-
trations outside of the plumes are one-half to one-third the
concentrations within the plumes (Altshuller, 1984b).
Therefore, the ozone background concentration rarely
exceeds 1 20 ppbv, although it can exceed 80 ppbv. Long-
range aircraft traverses over the eastern United States on
summer days during episodes of elevated ozone concentra-
tions associated with high pressure systems indicate that
these ozone concentrations ordinarily are below 120 ppbv.
When the concentrations exceed 120 ppbv, they usually are
associated with aircraft traverses across specific plumes
from urban areas upwind. Such aircraft results are
consistent with the relatively low frequency of ozone
concentrations above 120 ppbv being observed at nonurban
sites even within heavily populated regions of the eastern
United States.
However, it should be noted that the frequency with which
elevated ozone concentrations occur in the presence of
high pressure systems can vary from one summer season
to another. A critical factor appears to be the occurrence of
macroscale high pressure systems having a period greater
than 20 days.
Episodic periods occur during which elevated ozone
concentrations, at times exceeding 120 ppbv, have been
observed at sites with offshore winds on the east and west
coasts of the U nited States. Aircraft flights over areas of the
Atlantic Ocean and Gulf of Mexico on several occasions
have resulted in observed ozone concentrations exceeding
120 ppbv (Altshuller, 1 984b). Elevated ozone concentra-
tions also have been observed at nonurban coastal locations
in western Europe. On a few occasions the upwind
transport has been tracked by aircraft. More usually the
upwind transport is defined by back trajectory analyses.
Such results indicate flow from heavily populated areas
off-shore with transport over open water for 24 to 48 hours
followed by on-shore flow in the vicinity of the monitoring
location.
The persistence of ozone over water is consistent with the
very low dry deposition rates for ozone over water. Ozone
formation can continue at reduced rates with slow ozone
depletion by chemical processes and with dilution by
cleaner air from aloft. This combination of processes, when
modeled for multiday periods over water, results in a slow
depletion of ozone (Altshuller, 1984b). As a result, ozone
concentrations after 24 to 48 hours of transport over water
are only moderately lower than when the air parcels left
land.
The ozone concentrations reported at monitoring locations
in many remote areas of the western United States have not
been reported as exceeding 1 20 ppbv and rarely exceed 80
ppbv (Altshuller, 1984b). Aircraft measurements of ozone
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in the western United States usually result in the observa-
tion of low ozone concentrations. Therefore, evidence is
lacking for fumigation of these western monitoring sites
after multiday transport from distant urban areas. Such
results are consistent with the following circumstances: (a)
the small number of urban areas within 500 km of such
monitoring sites, (b) the low population densities in the
immediate areas of such sites, (c) the more rapid movement
of high pressure systems in the western United States,
reducing what accumulation of ozone and precurors may
occur, (d) the substantial rate of dry deposition of ozone
likely to occur during the summer months over such land
areas, and (e) the spreading and dilution of isolated urban
plumes under stable conditions experienced during multi-
day transport.
Issue 2. What processes for ozone formation contribute
significantly to the longer time averaged ozone concentra-
tions in nonurban areas?
Episodic peak-hour ozone concentrations and longer-time
averaged Oa levels are of some consequence to health and
agriculture Longer time exposures to ozone can result in
yield reductions of field crops (Heck et a/., 1 984; Skarby and
Sellden, 1984) and may affect growth in forests (Skarby and
Sellden, 1984). The most appropriate exposure statistics
are still under evaluation (Skarby and Sellden, 1984;
Larson and Heck, 1984). The mean O3 concentrations
associated with natural processes for Oa formation can be
of significance in the mathematical models used to
estimate yield reductions for crops such as soybeans and
cotton (Larson and Heck, 1984).
Asdiscussed mthe reviewpaper(Altshuller, 1 984b)and in
issue 1 of this assessment, plumes from cities, towns, and
industrial sources can frequently impact nonurban areas
containing field crops or forests in the more heavily
populated regions of eastern United States, and ozone
formed within such plumes can contribute significantly to
upper-percentile Oa concentrations in these areas. How-
ever, even in the eastern United States the mean seasonal
concentrations reported often are not much higher than in
more lightly populated areas of the western United States
An example is provided by the monitoring results of Decker
and coworkers (1976). During the summer of 1975 the
mean Oa concentrations at two eastern locations, Bradford,
PA, and Lewisburg, WV, were 41 and 38 ppbv, whereas at
sites west of the Mississippi River, Creston, IA, and Wolf
Point, MT, the mean 03 concentrations were 35 and 29
ppbv. However, the 03 concentrations above 70 ppbv were
exceeded 7 to 8% of the time at Bradford, PA, and
Lewisburg, WV; 2.3% of the time at Creston, IA; and 0.0% of
the time at Wolf Point, MT. The much higher frequency of
occurrence of somewhat elevated concentrations at the
two eastern sites was influenced not only by higher
population densities and more sources producing 03-
containing plumes but by the greater tendency for stagna-
tion of high pressure systems in this region of the eastern
United States (Korshover, 1 975). Such slowly moving high
pressure systems allow time for many sources to super-
impose precursor emissions to Oa formation in air parcels
moving across the high pressure systems
A very small spread in mean Oa concentrations has been
reported for a group of three nonurban monitoring locations
in Minnesota and North Dakota (Pratt eta/., 1983). The sites
ranged from 42 km out to 486 km from the Minneapohs-St.
Paul metropolitan area. The mean Oa concentrations over
the 1 977-1 981 study period were 32.26 ppbv, 32.09 ppbv,
and 32.42 ppbv However, the site nearestthe metropolitan
area was morefrequentlyfumigated bythe urban plume, so
that Os concentrations exceeding 70 ppbv were measured
on a total of 368 hr. At the most remote site Oa concentra-
tions above 70 ppbv were measured for a total of 275 hr.
This is not a large difference in frequency of occurrence. In
such western areas only one or a few plumes are likely to
fumigate a nonurban site, and stagnating high pressure
systems are infrequent in western areas of the United
States (Korshover, 1975).
Such results as those discussed above for mean Oa
concentrations can lead to the hypothesis that mean Oa
concentrations are predominately determined by natural
sources of Oa formation. This hypothesis would appear to
have particular applicability in large, lightly populated
agricultural areas west of the Mississippi River.
Two natural processes are frequently considered as to their
significance in contributing to Oa concentrations at ground
level. One of these is transport of ozone from in lower
stratosphere through the troposphere to ground level In a
recent assessment on this subject (Altshuller, 1984a) use
of both the 7Be to 0 a and 90Sr to 03 ratio techniques, as well
as tropospheric modeling results lead to the estimate that
15 ppbvor less of the Oa measured at ground level locations
would be of stratospheric origin during the summer
months. Since measured mean concentrations during the
summer range from 29 to 45 ppbv, no more than one-third
to one-ha If of the mean non urban 0 a concentrations can be
attributed to stratospheric O3. The other natural process
often considered is 03 formation from biogenic hydro-
carbons, isoprene and monoterpenes, reacting with NO,. In
general these reactions do not appear to account for a
substantial part of the nonurban 03concentrations. Such a
conclusion is based on estimates derived from the 03-
forming potential of the ambient air concentrations of
biogenic hydrocarbons (Altshuller, 1983a) and on recent
model results (Altshuller, 1 984b). In particular it is the
heavily forested areas, not the sparsely forested agricultural
areas west of the Mississippi River, to which biogenic
hydrocarbons are attributed. Finally, it should be noted that
NOX is crucial to O3 formation from biogenic hydrocarbons,
and the NO, is predominately from anthropogenic sources
throughout the United States (Altshuller, 1984a)
If the two natural processes discussed above are not likely
to account for the major part of the mean 03 concentrations
measured, what process is responsible? The process most
likely to account for a major part of the longer-time
averaged 03 concentrations in nonurban areas during the
warmer months of the year is photochemical 03 formation
within the free troposphere (Altshuller, 1984a,b). Anthro-
pogenically, emissions of carbon monoxide, non-methane
hydrocarbons and NOX constitute a major part of the
precursors to 03 formation in the free troposphere
(Altshuller, 1984a). Predictions of Oa concentrations by
tropospheric models are strongly dependent on the vertical
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NOx concentration gradient assumed through the free
troposphere. Comparisons of 03 concentrations at mid-
latitudes in the northern hemisphere compared to the
southern hemisphere indicate that anthropogenic emis-
sions may be associated with 20 to 25 ppbv of the 03formed
in the free troposphere (Altshuller, 1984a). Since this
estimate was based on modeling results obtained before
anthropogenic non-methane hydrocarbons were included
as precursors in a tropospheric model, the estimates may
need to be increased moderately. Of course, in considering
the contribution of free troposphere 03 to ground level O3, it
is assumed that transport of 03 between the free tropo-
sphere and the boundary layer readily occurs on a time
averaged basis.
Substantial year to year variations in mean O3 concentra-
tions and in the frequency distribution of 03 concentrations
have been observed at a number of nonurban locations
(Decker et a/., 1976; Pratt et a/., 1983; Mohnen, 1982).
Such year to year variations can be greater than site to site
variations within the same region during the same year
(Pratt et a/., 1 983). Variations m the frequency and duration
of stagnating high pressure systems may explain such year
to year variations in some of these locations (Decker et a/.,
1976). However, such year to year variations also are
observed in relatively remote areas not strongly influenced
by stagnating high pressure systems (Pratt et a/., 1983).
Therefore, year to year variations in the 03 production i n the
free troposphere or in the 03 transport from the lower
stratosphere also may be in part responsible for such
ground level O3 concentration levels.
Issue 3. Is /VOx the precursor limiting ozone formation over
nonurban areas of the United States?
Both hydrocarbons and nitrogen oxides serve as precursors
to ozone production in nonurban areas. This is so both for
ozone formation within the planetary boundary layer and in
the free troposphere. Most hydrocarbons are consumed
predominately by reactions with hydroxyl radicals. Olefmic
hydrocarbons are consumed by reactions with both hydroxyl
radicals and ozone. Nitrogen dioxide is converted reversibly
to PANs by reactions with acyl peroxy radicals and
irreversibly to nitric acid by reaction with hydroxyl radicals.
Since the concentrations of all of the radical species vary
with available solar light intensity, the lifetimes of hydro-
carbons and nitrogen oxides should vary with season,
latitude, and altitude. The reactivity of individual hydro-
carbons with hydroxyl radicals or ozone varies greatly,
resulting in lifetimes ranging from fractions of an hour for
certain olefins to months for ethane and acetylene. The
lifetime of NO, has been estimated in smog chamber
studies and from modeling simulations. Of particular
interest are the estimates of NOX lifetime obtained by
measurements within urban plumes.
The lifetime of N02 under sunny summertime, moderately
polluted conditions has been obtained by measurements
within the plume of Boston as it was transported over the
Atlantic Ocean (Altshuller, 1984b). Dilution was corrected
for by use of one of several tracers which were inert over
the experimental periods involved. These tracers were
acetylene, carbon monoxide, and F-11. Based on these
measurements the lifetimes (1/K) range from 4.2 to 7.1
hours and average 5.5 hours Subsequent measurements
over land in the Philadelphia plume calculated upper limits
for NO, lifetime of five and eight hours. Therefore, an
average lifetime under the conditions of these experiments
is near six hours. Since these measurements were made
after the plume had left the city, the NO to N02 conversion
time of one to three hours needs to be added to obtain NOX
lifetimes. The plume measurements over the ocean are
particularly useful because dry deposition is minimized.
The technique used cannot distinguish between losses of
NOX resulting and chemical reaction compared to deposi-
tion. In an urban plume simulation study, NOX lifetimes
consistent with these experimental results were obtained
(Altshuller, 1984b).
Lifetimes of hydrocarbons, excepting ethene, were not
estimated from the same field experiments. Hydrocarbon
lifetimes can be estimated using ozone concentrations and
estimates of hydroxyl radical concentrations (Altshuller,
1983c, 1984b). Alternatively, hydrocarbon lifetimes for the
more reactive hydrocarbons can be obtained from smog
chamber studies. Based on such estimates, the lifetimes of
olef ins other than ethene and of dialkyl and trialkylbenzenes
are equal to or shorter than that of NOX. From available
estimates, the lifetime of ethene ranges from about that of
NOX to moderately longer than NO,. The lifetimes of
monoalkylbenzenes and of the most reactive alkanes
during summer days are two to three times longer than that
of NOx. Hydrocarbons such as benzene, propane, butanes
and pentanes have lifetimes of several days and longer.
There will be a continuing change in the composition of
hydrocarbons and their ratios to NOX as an urban plume
travels downwind over water or lightly populated land
areas. Over heavily populated land areas, these precursors
may be somewhat depleted, but significant replenishment
can occur from fresh emissions. Therefore, the life histories
of plumes with respect to hydrocarbon concentrations and
composition and ratios to NOX within each plume should
vary with plume transport time and direction. It should be
noted that the most reactive hydrocarbon species are not
necessarily the most abundant species, so their consump-
tion may not reduce NMHC concentrations substantially. In
addition, the oxygenated hydrocarbon products formed
from the more reactive hydrocarbons will continue to
contribute to the chemical reactivity of the plume for a
number of additional hours. It is consistent with these
changes in composition that as plumes age, NOxeventually
will become the limiting species with respect to ozone
formation. Other information also must be considered to
better determine when NO, actually should become the
limiting species with respect to ozone production.
The results of several smog chamber studies simulating
first day irradiations indicate that ozone formation is not
NO, limited but, instead, ozone formation increases with
decreases in NOX concentration (Altshuller, 1 983b, 1984b).
The results obtained from chemical kinetic modeling and
from statistical treatments of ambient air measurements
are consistent with these smog chamber studies.
In a model ing simulation of urban plumes, NOx also was not
found to be limiting during the daylight hours of the first
day. A fourfold increase in the HC/NOX ratio substantially
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increased the maximum ozone formed. Similarly a negative
correlation was obtained between ozone and NO,from local
sources in statistical studies involved with analyzing the
effects of precursor emissions into air parcels traveling
with transport layer winds in the 300 m to 200 m altitudes.
For multiday transport the key factor is the short lifetime of
NOX. Both experimental plume studies and plume and air
parcel model simulations indicate very little NOX available
by the evening of the first day. However, modeling studies
indicate that PAN would serve as a reservoir for NO, into the
second day. Substantial ozone production is predicted the
second day from both multiday smog chamber experiments
and modeling studies. Subsequently, both NOX and PAN
concentrations decrease to very low levels unless fresh
emissions are available. Although the background NOX
concentrations may be sufficient to continue low rates of
ozone production during subsequent days, dilution and
deposition over land result in substantial reductions in
ozone concentrations. Over water, the depletion of ozone is
slower owing to negligible deposition of ozone. A positive
correlation was obtained in the statistical studies men-
tioned above between ozone and previous day NOX emis-
sions. Therefore, a number of types of studies indicate that
NO, should be the limiting species on the second and
subsequent days.
In modeling studies of ozone production in the "clean"
troposphere, a background of about 40 ppbv of ozone is
predicted at 1 km. The ozone production in "clean"
troposphere is NOX limited. The concentration of PAN can
exceed that of NO, in the free troposphere; thus, PAN as a
reservoir of NO, is important in the "clean" atmosphere as
well as in the aged urban plume.
5. Mean Oa concentrations do not show large variations
among nonurban locations across the United States,
and especially small variations are reported among
locations in the same region.
6. Although natural processes for 03 formation appear
likely candidates to account for mean 03 concentra-
tions especially in more lightly populated agricultural
or forecasted areas, the available assessments do not
confirm natural processes as the major sources for
the 03.
7. Photochemical 03 formation in the free troposphere
from precursors which, to a substantial extent, are of
anthropogenic origin appears to account for a major
part of the mean 03 concentrations measured in
nonurban areas during the warmer months of the
year. Ozone formation by this process is sensitive to
the vertical NO, distributions and the contribution
from anthropogenic NO, emissions.
8. Field experiments, model studies, statistical and
smog chamber experiments do not indicate that
ozone formation is NO, limited during the first day of
transport downwind from urban areas.
9. For multiday transport without added emissions of
precursors, NO, does appear to be the limiting
precursor. Experimental and modeling results indi-
cate that NO, is largely depleted before the second
day of transport. The PANs and NO, reservoir should
be largely depleted also during the second day of
transport in the absence of fresh NO, emissions.
Conclusions
1. Concentrations of ozone exceeding 120 ppbv are
observed at nonurban locations in the northeastern,
midwestern, and southeastern United States but not
at locations remote from urban areas in the western
United States.
2. The highest ozone concentrations observed at non-
urban locations have often been reported when
fumigation of the site by an urban plume occurs
during passage of a high pressure system through the
same region of the country.
3. Concentrations of ozone exceeding 120 ppbv have
been reported at coastal locations when back tra-
jectory analyses indicate transport starting from a
populated area with subsequent passage for 24 to 48
hours over the ocean. Slow depletion of ozone is
reasonable in such circumstances because of the
negligible deposition of ozone over water.
4. The best direct evidence for multiday transport of
ozone involves transport for 24 to 48 hour periods
over the ocean. Evidence for substantial survival of
ozone into the second day of transport is available
overland, but multiday transport of ozone over land is
largely based on circumstantial evidence.
Recommendations
1. The treatment of some of the experimental results in
the SURE data base (Martinez and Singh, 1979)
should be extended to include the entire data base as
well as other data bases ava liable m St. Lou is and the
Ohio valley.
2. Multiday transport modeling studies involving air
parcel movements over land and ocean areas have
been conducted in Europe. Similar simplified model-
ing approaches can be applied now and can be used
over large areas of the United States and adjacent
ocean areas where there are no plans to employ
Eulenan regional scale ozone models.
3. A number of modeling studies consistently indicate a
significant background of ozone throughout the
troposphere as a result of in situ ozone formation in
the free troposphere. Anthropogenic emissions of
NO,, CO, and NMHC appear to make substantial
contributions to ozone formation. However, none of
the models have been exercised using a range of
scenarios with respect to anthropogenic and biogenic
emissions at midlatitudes in the northern hemi-
sphere. Such modeling scenarios would better eval-
uate the portion of the ozone background potentially
controllable by reductions in anthropogenic NO,
emissions.
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