&EPA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development EPA-600/D-83-047 July 1983
ENVIRONMENTAL
RESEARCH BRIEF
Assessment of Natural Volatile Organic Substances
and Their Effect on Air Quality in the United States
A. P. Altshuller, Senior Science Advisor
Environmental Sciences Research Laboratory
EPA Office of Research and Development, Research Triangle Park, NC 27711
Abstract
This research brief is a summary of the extensive review
and critical analysis of the literature on natural volatile
organic substances, sometimes referred to as biogenic
hydrocarbons, and an assessment of that body of scientific
information. The review is reported separately (Natural
Volatile Organic Substances and Their Effect on Air Quality,
submitted to Atmospheric Environment). The conclusion
reached in this assessment is that there is a lack of
evidence that natural hydrocarbons contribute
substantially to the formation of ambient air concentrations
of ozone.
Introduction
This assessment is based on a review document entitled
"Natural Volatile Organic Substances and Their Effect on
Air Quality" accepted for publication in Atmospheric
Environment. The review considers sources of natural
volatile organic substances, environmental effects on their
emission rates, emission inventories, ambient air
concentrations, lifetimes and their ambient air reaction
products.
Six scientific issues were identified. A detailed discussion
of each of these issues is contained in the review. In this
research brief, each issue will be stated, its background
briefly discussed, and the conclusions summarized. In
addition, specific recommendations will be presented.
(Additional recommendations are given in the Review.)
Issue 1. What are the relationships between
anthropogenic and biogenic emission inventories
and air quality measurements?
The biogenic emission inventory developed by Zimmerman
(1979) predicts that 15 million metric tons of isoprene and
50 million tons of monoterpenes are emitted from the
contiguous United States. The anthropogenic emissions of
hydrocarbons for the United States are estimated to total at
27 million metric tons (U.S. Environmental Protection
Agency, 1974). Therefore, it might be expected that
isoprene and several of the monoterpenes would be
measured in the ambient atmosphere at large excesses to
individual anthropogenic hydrocarbons such as pupene
and acetylene. Based on the biogenic emission inventory
for isoprene and the fraction on anthropogenic hydrocar-
bons as propene and acetylene, emission ratios of isoprene
to propene or acetylene ranged from 27:1 to 70:1. In
contrast, the ratios of ambient air concentrations at various
study sites ranged from 1 •! to 9:1. The emission inventory
results overpredict these ratios by factors of 7 to 35
compared to ambient air measurements. Various field
studies also indicate low ambient air concentrations for a-
pinene and other monoterpenes.
Among the possible causes for the large differences
discussed above a^ the following: (1) The biogenic
emission inventory is much too high. (2) The anthropogenic
emission inventory is much too low. (3) The biogenic
compounds react so rapidly in the atmosphere as to
disappear before collection. (4) The biogenic compounds
react rapidly with ozone in the sampling container. (5) Most
of the biogenic compounds are lost to the walls of the
containers used during the storage period before analysis.
Of the various possibilities discussed above, the most likely
ones to explain the larger part of the discrepancies are
those associated with the emission inventories. The other
possibilities appear capable of only accounting for only a
small portion of the differences in ratios between source
inventories and ambient air concentrations. A combination
of substantial under-estimates in the anthropogenic
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emission inventory, along with a substantial overestimate
in the biogenic emissions, appears necessary to account for
these differences in ratios Existence of a substantial
underestimate in the anthropogenic inventories for volatile
organic compounds would be of general concern with
respect to implementation of ambient air standards for
ozone. Therefore, any and all independent means of
verifying anthropogenic inventories should be utilized.
Since air quality simulation models used for
implementation of standards start with emission
inventories and not with ambient air concentrations of
hydrocarbons, any substantial inventory errors could
interfere with utilization of the air quality simulation
models in developing control strategies.
Issue 2. Have biogenic species been adequately
identified and quantitated?
There has been a concentration of effort on measurements
of emissions from forest species. Only limited biogenic
emission measurements are available on grasses and field
crops. Tingey (1981) recently investigated emissions from
beans, soybeans, corn and clovers and found these to have
smaller emissions than trees. There also has been little
work done on the emissions from the sclerophyll scrub
species in California. Most investigators have not
attempted to identify biogenic oxygenated species so
results are limited for these types of compounds. Biogenic
species have been adequately identified and perhaps
quantitated for a number of hydrocarbons from a number of
important forest species, but substantial gaps in
information exist with respect to emissions from grasses,
agricultural crops and soils.
Issue 3. How reliable is the bag enclosure tech-
nique for estimating the composition of emitted
products?
Substantial differences in composition have been reported
between investigators for the same species. In some
instances, the differences may be associated with the
completeness of the analytical measurements.
Composition also may vary because of variations in
emissions from different ecotypes and genotypes of the
same species (Tingey, 1981). However, ambient air
measurements in pine forests also appear to give different
compositional results than do bag enclosure
measurements (Arnts, et al, 1981). Therefore, the adequacy
of compositional measurements from the bag enclosure
techniques appears to be unclear.
Issue 4. Ho w effective are biogenic hydrocarbons
in contributing to the formation of ozone in the
atmosphere and how does this potential compare
with that of anthropogenic hydrocarbons?
The ozone-producing potential of isoprene and various
monoterpenes do not exceed, and usually are less than, the
potential associated with most anthropogenic hydrocar-
bons. Smog chamber results suggest that the ozone
production from atmospheric mixtures of anthropogenic
hydrocarbons should be in the range of 10 to 30 ppb of
ozone produced per 100 ppbC of hydrocarbon consumed.
Based on average concentrations of isoprene and
monoterpenes measured at several rural sites, these
biogenic hydrocarbons are estimated to be capable of
producing from 0.1 to 0.7 ppb of ozone. The highest
isoprene concentrations measured during a stagnation
(August 12, 1975) constituted from 5 to 20 percent of the
total hydrocarbon measured at the rural site, Glasgow, IL
(Rasmussen, et al, 1977). The a-pinene concentrations
were low, 0.3 ppbC. Since «-pinene is less effective than
isoprene in ozone production, its contribution can be
ignored. The concentrations of isoprene present during this
stagnation based on its smog chamber ozone production
potential, might contribute from 2 to 10 ppb of ozone. The
ozone peaked at 1900 at slightly over 100 ppb. Therefore,
the biogenic hydrocarbons identified did not contribute a
substantial part of the ozone observed.
It should be mentioned that higher concentrations of
biogenic hydrocarbons occasionally have been measured.
For example, Ferman (1980) measured over 150 ppb of
isoprene as a maximum concentration at a site near
Keysville, VA. The concentrations of ozone, nitrogen oxides
or other hydrocarbons concurrently present were not
reported. Rasmussen and Went (1965) observed unusually
high concentrations at a rural site during leaf drops of forest
species. It is possible under such circumstances, that
biogenic hydrocarbons may make a moresignificant contri-
bution to ozone formation.
Issue 5. What is the potential ofparaffinic hydro-
carbons from seepage losses through soil to
participate in the formation of ozone?
Seepage losses through soil of paraffinic hydrocarbons
may contribute ambient air concentrations capable of
generating ozone concentrations in excess of those
attributed to isoprene and monoterpenes. Because of the
lack of sufficient measurements in the literature, it is not
now possible to determine whether or not hydrocarbons
from seepage through soil could account for a substantial
part of the ozone observed downwind of gas- and
petroleum-bearing areas.
Issue 6. What is the potential of biogenic
hydrocarbons compared to anthropogenic
emissions to participate in the formation of
aerosols in the atmosphere?
Aromatic hydrocarbons have moderate aerosol-forming
capabilities (Grosjean, 1977). In the review, based on the
study by Miller and Joseph, 1976, it was estimated that
aromatic hydrocarbons can generate on the average about
0.02 g rrr3 of aerosol per ppmC of aromatic hydrocarbon
Monoterpenes appear to be very effective in producing
organic aerosols. Laboratory studies show that the
monoterpene, «-pinene, has an organic aerosol-forming
potential about a factor of 5 to 10 times higher than
aromatic hydrocarbons.
At a site in the Smoky Mountains, TN, (Arnts and Meeks,
1980) 20 ppbC of aromatics and 1 ppbC of a-pinene were
measured. Applying the aerosol-forming potentials just
discussed, about 0.4 fjg nr3 of organic aerosol can be
attributed to reactions involving aromatic hydrocarbons
and 0.1 to 0.2 /ug rrr3 to reactions involving a-pmene. An
estimate based on an independent approach in the review
results in an estimate of 0 1 5 /ug rrr3 of organic aerosol
produced from a-pinene No other monoterpene was
detected. The sulfate concentrations have been measured
at the Smoky Mountain site (Stevens, et al, 1980a) at a
rural site in the Shenandoah Valley of Virginia (Stevens, et
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a/. 1980b) and at the Allegheny Mountain site in
southwestern Pennsylvania (Pierson, et ai, 1980). At these
rural sites along the Appalachian Mountain chain the
sulfates (as acid sulfates) were inthe 14to 16/ug rrr3range.
Clearly, the estimates of organic aerosol generated by
observed «-pinene concentrations are insignificant
compared to these sulfate concentrations. Based on these
estimation procedures, even if the a-pinene concentration
had been averaged 10ppbC rather than 1 ppbC, the organic
aerosol produced would only have been about 10 percent of
the sulfate aerosol concentration
It is concluded in the review that in the eastern United
States the aerosols formed from monoterpenes are not
likely to constitute a substantial fraction of the fine particle
aerosol concentrations.
Conclusions
There is a lack of evidence in the literature that natural
hydrocarbons contribute substantially to the formation of
ambient air concentrations of ozone and aerosols. There is
some evidence that very small amounts of ozone, less than
10 ppb, might form from atmospheric reactions of isoprene
from natural sources with nitrogen oxides from
anthropogenic sources. Within forest canopies where
natural hydrocarbons are the most abundant, natural
hydrocarbons are most likely to consume rather than form
ozone. Even those investigators who contend that natural
hydrocarbon emission rates are substantial have not
provided evidence that substantial amounts of ozone are
formed from the reactions of these hydrocarbons with
nitrogen oxides.
It is possible that future work might substantiate high
natural hydrocarbon emission rates, particularly for short
intervals of time at specific locations. Even if such results
should be obtained, it is not evident that inclusion of such
results would be critical to the development of practical air
quality models for ozone.
It is concluded that adequate justification does not exist at
present to support the considerable effort needed to
develop natural emission inventories for all areas of the
United States.
Recommendations
1. The Zimmerman bag enclosure technique is still the
only practical approach to obtaining emissions in the
field for a variety of species. Since this technique has
been shown to give substantially different emission
rates than other techniques and to be inconsistent with
ambient air measurements, its applicability in
developing acceptable biogenic emission inventories is
in question. If the results of the bag enclosure
technique can be considered usable onare/af/Vebasis,
a large upward gradient is predicted in emissions from
higher to lower latitudes The southern United States,
including the Gulf Coast Region, is likely to have the
highest emissions of biogenic hydrocarbon emissions.
If a biogenic emission inventory is to be included in air
quality models for ozone, it would be more appropriate
to consider doing so in the southern United States
rather than elsewhere.
2 A fundamental difficulty in developing a biogenic
emission inventory by any technique is the wide
variability in these emissions. The emissions rates are
functions of temperature and light intensity. There may
be growing season effects for field crops. Lead drop
from trees also has been reported to increase
emissions. Therefore, the possibility exists for
excursions to high emission levels for short time
periods. This problem also limits the usefulness of
seasonal biogenic emission inventories. Therefore, it
does not appear useful todevelop annual inventories of
natural hydrocarbons for the U.S. Because a
substantial fraction of U.S. emissions of natural
hydrocarbons is estimated to occur in the warmer
months of the year in the southeastern United States,
an improved emission inventory should be developed
for this area.
3. The magnitude of these short time biogenic emission
effects has received little attention. A number of the
aspects involved could be investigated in local forested
areas
4. Because of the small amount of effort expended on
study of biogenic oxygenated hydrocarbons, it is
possible that their contributions to the total biogenic
emissions are being underestimated. Some additional
measurements on these types of compounds in the
southern United States would be appropriate.
5. One of the explanations for the much lower ratios of
biogenic to anthropogenic hydrocarbons from ambient
air measurements compared to biogenic emission
measurements is a serious underestimation of anthro-
pogenic emissions. The techniques for estimating
anthropogenic emissions should be carefully reviewed.
6. One additional source of hydrocarbons not included in
anthropogenic emission inventories nor in current
biogenic inventories is seepage losses of paraffinic
hydrocarbons. Only a few measurements related to
seepage losses are available. Since gas- and
petroleum-bearing formation cover substantial on-
shore and off-shore areas of the United States,
additional ambient air measurements would be de-
sirable in selected locations.
Literature Cited
Arnts, R. R. and Meeks, S. A. (1981}. Biogenic Hydrocarbon
Contribution to the Ambient Air in Selected Areas. Atmos.
Environ. 15, 1643-1651.
Arnts, R. R., Petersen, W. B., Seila, R. L, Gay, B. W., Jr.
(1981). Pine Forest Hydrocarbon Emissions: Source
Strength vs Ambient Concentrations. Paper 81 -27.4. 74th
Annual Air Pollution Control Meeting, Philadelphia. PA,
June 21-26
Ferman, M. A. (1980) Rural Non-Methane Hydrocarbon
Concentration and Composition. In- Proceedings of the
Symposium on Atmospheric Biogenic Hydrocarbons:
Emission Rates, Concentrations, and Fates. (J.J. Bufalini
and R R. Arnts, eds.) U. S. Environmental Protection
Agency, Environmental Sciences Research Laboratory,
Research Triangle Park, NC, January 8-9
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Grosjean, D. (1977). Aerosols. In: Ozone and Other
Photochemical Oxidants National Academy of Sciences
(ISBN-)-309-02531-1), Washington, DC.
Miller, D. F. and Joseph, D. W. (1976). Smog Chamber
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Agency, Environmental Sciences Research Laboratory,
Research Triangle Park, NC.
Pierson, W. R., Brachaczek, W. W., Truex, T. J., Butler, J. W.
and Korniski, T. J. (1980). Ambient Sulfate Measurements
on Allegheny Mountain and the Question of Atmospheric
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Stevens, R. K., Dzubay, T. G., Shaw, R. W., Jr., McClenny,
W. A., Lewis, C. W. and Wilson, W. E. (1980a).
Characterization of the Aerosol in the Great Smoky
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Stevens, R. K., McClenny, W A., Dzubay, T. G. (1980b).
Analytical Methods to Measure Carbonaceous Content of
Aerosols In: Symposium on Paniculate Carbon. G. M.
Research Lab., Warren, Ml, October 12-14.
Tingey, D. T. Private communication, September 23,1981.
U. S. Environmental Protection Agency (1974). 1972
National Emissions Report. EPA-450/2-74-012. Office of
Air Quality Planning and Standards, Research Triangle
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Zimmerman, P. R. (1979). Testing for Hydrocarbon
Emissions from Vegetation, Leaf Litter and Aquatic
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Environmental Protection Agency, Office of Air Quality
Planning and Standards, Research Triangle Park, NC.
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