CORRESPONDENCE
SIR: Let me offer a few comments on the recent note by Lonneman, Seila, and Bufalini ( I ) concerning the importance of terpene emissions from vegetation in the generation of ambient ozone levels. Air samples were collected in two Florida orange groves at about noon in May 1976. While citrus trees have been shown to emit various monoterpenes (C10H16) a t this time of year (2-4), GC analyses conducted 4-5 days after sample collection failed to detect any C ~terpenes. O The authors have thus concluded that “the amount of naturally emitted hydrocarbon is low” and that “these low levels cannot possibly contribute to the production of significant levels of ozone” in the area studied. The data presented by these authors do not provide strong support for either of these two conclusions. When dealing with a photochemically generated pollutant such as ozone, ambient precursor concentrations do not provide a good index of either mass emissions or relative importance among precursor sources. This fact is easy to overlook, since most smog chamber studies have been batch reaction systems. In such systems, the amounts of reactants can be expressed on either a concentration or a mass basis. It is the mass of reactant injected into these systems which determines the initial concentration. The basic nature of photochemical ozone generation adds further insight. In the critical reaction process, organic compounds function as NO oxidizing agents through a chain reaction sequence of hydrocarbon oxidation and free-radical formation, while NO is involved in a cyclic oxidation-photodissociation sequence. Thus, the practice of describing batch reaction smog chamber studies in terms of initial reactant concentrations obscures the essential significance of hydrocarbon mass emissions. Real world smog systems involve essentially continuous reactant injection. In such real atmospheres, low ambient concentrations of a precursor compound could indicate either well-diffused emissions from isolated, high concentration point sources; cumulatively high mass emissions from widespread but individually minor emission sources; or high reaction rates regardless of source concentration. Thus, the study described by Lonneman et al. must be evaluated in terms of two interrelated but distinct issues: whether a significant ambient Clo terpene concentration should have been expected in the first place, and whether natural terpene compounds play a significant role in atmospheric photochemical reactions. I have approached the issue of expected ambient terpene concentrations by using the following simple spherical volume, steady-state model derived by analogy to the traditional simple box model: terpene emission rate X canopy diam canopy vol X wind speed In May 1977, Zimmerman ( 2 , 3 )measured a terpene emission rate for Florida orange trees of 3.1 pg/(g-h) on a foliage dryweight basis. Since no data on tree size were reported by Lonneman et al., I have used data from some California citrus trees ( 5 ) .A 29-year-old orange tree (4.72 m tall, with a trunk diameter of 27.2 cm) had a crown circumference of 15.7 m and a foliage biomass of about 450 kg (fresh weight). Data from three 10-year-old orange trees indicated that foliage dry weight averaged 50% of foliage fresh weight. Figure 1 illustrates expected steady-state canopy terpene concentrations for this 29-year-old orange tree as a function of wind speed. Inherent model assumptions are a uniform laminar wind flow, free air movement through the canopy, “clean” air unaffected by upwind trees, and no chemical recanopy concn =
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actions. A terpene molecular weight of 136.24 and a temperature of 25 “C are also assumed. Figure 1 indicates that wind speeds over 2.7 m per s (5.2 miles per h) would produce a canopy terpene concentration of less than 1ppb [11.34 ppbC (parts per billion of carbon)]. Lonneman et al. do not report any wind speed measurements. They do note that samples were taken outside the tree canopy (a few inches to 10 ft from the trees) at noon on a day when the mixing depth was greater than 2000 m. All of the key assumptions inherent in my simple model are violated by real world conditions. I t seems most reasonable to me that the model overestimates the ambient terpene concentration expected in the collected air samples. The authors note that ambient ozone levels at the time of sample collection were about 45 ppb. They also note that 50% pump losses of ozone were experienced. Tests with known terpene samples (5-25 ppbC) indicated no pump losses and excellent storage characteristics. Test conditions are not explained in much detail. It would have been quite informative if additional terpene storage tests had been conducted using an air plus 20-25 ppb03 mixture to simulate actual sample storage conditions. As recognized by the authors, monoterpenes react rapidly with ozone under both light and dark conditions (6-9). Rate constants for terpene-ozone reactions have been reported in some studies (8, 9 ) . Those presented in ref 9 were used by Lonneman et al. to estimate potential terpene storage losses from reaction with ozone. These rate constants were determined from flow system experiments having reaction times ranging from 4 s to 4.5 min. Reaction rates determined in both ref 8 and 9 were based on systems having terpene concentrations significantly in excess of ozone concentrations. This is certainly not the expected condition for the air samples analyzed by Lonneman et al. I t is also noteworthy that, where determinations could be made by Grimsrud et al. ( 9 ) ,terpene to ozone loss ratios ranged from 0.47 to 1.5; loss ratios of 0.7 to 1.4 were reported for myrcene, limonene, and a-pinene. Thus, with an initial ozone concentration of 20 times the expected terpene concentration, I am not surprised by the failure of Lonneman et al. to detect any Clo terpenes after 4-5 days of sample storage. The significance of terpenes to photochemical smog reactions can best be judged by their contribution to the reactive organic emission inventory for an appropriate geographic area. Zimmerman (2, 3) has estimated that terpenes from natural vegetation account for 68% of the total reactive organic emissions in the St. Petersburg-Tampa area. While available emission inventory data for that area need significant refinement, this estimate clearly indicates that terpenes are a major hydrocarbon source in the area studied by Lonneman et al. Terpenes are highly reactive with ozone, as previously mentioned. These chain and cyclic olefins are also highly reactive in the typical photochemical system (7-15), being some of the most reactive compounds yet tested. It should also be noted that the ozone-terpene reaction can lead to free-radical formation, either directly or by photodissociation of resultant aldehydes and ketones (16). Thus, even the daytime scavenging of ozone by terpenes can lead to subsequent downwind ozone generation. All things considered, Zimmerman’s terpene emission estimates seem easy to reconcile with low ambient terpene concentrations. One final factor needs to be recognized in assessing the significance of terpenes to photochemical smog reactions. These reactions will not proceed unless NO or NO2 sources are also present. While I have not fully investigated the lit-
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(9) Grimsrud, E. P., Westberg, H. H., Rasmussen, R. A,, Int. J . Chem. Kinet., Symp. No. 1,183-95 (1975). (10) Went, F. W., Proc. Natl. Acad. Sci. U.S.A., 51, 1259-67 (1964). i l l ) Went. F. W.. Slemmons. D. B.. Mozineo. H. N.. Proc. Natl. Acad. Sci. U.S.A., 58,69-74 (1967). (12) Rimerton, L. A., Jeffries, H. E., Worth, J. B., Enuzron. Sci Technil., 5,246-8 (1971). (13) Lillian, D., Adu. Chern. Ser., No. 113,211-18 (1972). (14) Stephens, E. R., Price, M. A., in “Aerosols and Atmospheric Chemistry”, G. M. Hidy, Ed., pp 167-81, Academic Press, New York, N.Y., 1972. (15) Pitts, J. N., Winer, A. M., Darnall, K. R., Lloyd, A. C., Doyle, G. J., in “International Conference on Photochemical Oxidant Pollution and Its Control, Proceedings”, B. Dimitriades, Ed., Vol. 11, pp 687-704, EPA 600/3-77-001b,1977. (16) Haagen-Smit, A. J., Wayne, L. G., in “Air Pollution”, A. C. Stern, Ed., 3rd ed, Vol. I, pp 235-88, Academic Press, New York, N.Y., 1976.
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erature on natural NO and NO2 sources, I remain skeptical of their significance for two simple reasons. Natural background ozone levels, while subject to argument as to sources, are clearly low. There is also abundant evidence of visible injury to vegetation produced by relatively low ozone levels. Thus, I can only conclude that photochemical smog is clearly a product of modern technology. In this regard, the NO and NO:! produced by combustion processes may be a t least as important as hydrocarbon emissions from anthropogenic sources. I would also conclude that while terpene emissions can be a significant hydrocarbon source (and need to be recognized when formulating smog control strategies), this does not in any way mean they are “the cause” of observed smog levels.
Literature Cited (1) Lonneman, U’.A., Seila, R. L., Bufalini, J. J., Enuiron. Sci. Technol., 12,459-63 (1978). (2) Zimmerman, P., “Procedures for Conducting Hydrocarbon Emission Inventories of Biogenic Sources and Some Results of Recent Investigations”, presented at EPA Emission Inventory/ Factor Workshop, Raleigh, N.C., Sept 1977. (3) Zimmerman, P., “The Determination of Biogenic Hydrocarbon Emissions”, presented a t Pacific Northwestern International Section, Air Pollution Control Association Meeting, Nov 1977. (4) Zimmerman, P., Air Pollution Research Section, Washington State University, Pullman, Wash., private communication. (5) Turrell, F. M., Garber, M. J., Jones, W. W., Cooper, W. C., Young, R. H., Hilgardia, 39,428-45 (1969). (6) Went, F. W., Proc. Natl. Acad. Sci. U.S.A., 46,212-21 (1969). (7) Groblicki, P. cJ., Nebel, G. J., in “Chemical Reactions in Urban Atmospheres”, C!. S. Tuesday, Ed., p p 241-67, American Elsevier, New York. N.Y.. 1971. (8) Ripperton, L. A.,Jeffries, H. E., White, O., Adu. Chem. Ser., No. 113,219-31 (1972). 0013-936X/79/09 13-0235$01.OO/O
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SIR: Lonneman et al. ( 1 ) recently presented data concerning the abundance of natural hydrocarbons in the Florida atmosphere and advanced the conclusion that natural sources are not significant. Our review of this paper indicates that because of limitations in their sampling, storage, and analytical systems, this conclusion is not justified and further work is required to determine the true abundance of natural emissions. Contrary to the authors’ reasoning as to why terpenes and other natural organic compounds were not detected in the field samples, our calculations indicate that because these compounds are not stable under the conditions imposed by the sampling and storage system, substantial concentrations could have existed in the ambient atmosphere. The samples which best indicate such storage loss are those taken in the orange grove, samples G-2 and G-3. Based on the authors’ assumption that the emissions in the orange grove should include the volatile constituents of orange oil [composition >97% d-limonene, 1-3% myrcene, and a trace of a-pinene (2, 3)]one would expect to find d-limonene and possibly myrcene in the samples. However, calculations of the d-limonene and myrcene storage loss in the Tedlar bags in the same manner as used by the authors for a-pinene show that in the 5 h allotted for reaction with ozone, 98.7% of the d-limonene and 99.98% of the myrcene would have been destroyed. These calculations are based on ozonolysis rate constants of 0.016 and 0.031 ppm-l s-l for d-limonene and myrcene, respectively (4). Obviously, there is a very high probability that these important terpenes were completely destroyed by storage in the Tedlar bags and were never delivered to the GLC for analysis. Our calculations also show that the initial bag concentrations of these terpenes could have been as high as 77 ppbC (parts per billion of carbon) d-limonene and 90 ppbC myrcene. Considering that substantial losses of these terpenes would also be expected during sampling when ozone was also present, it is evident that very high ambient concentrations of these terpenes could have existed. Another problem with the storage loss position taken by the authors is the fact that no data were presented for the terpene laboratory storage test. Based on the ozonolysis rate given, it is assumed that a-pinene was used in this test. Ripperton, Jeffries, and White ( 5 ) have reported that a-pinene reacts much faster with ozone than calculated by the first-order kinetics used by the authors. Ripperton and co-workers postuVolume 13, Number 2, February 1979 235
