Emission rates of organics from vegetation in California's Central Valley

Atmospheric Environment Vol. 26A, No. 14, pp. 2647 2659, 1992. Printed in Great Britain.

000441981/92 $5.00+0.00 © 1992 Pergamon Press Ltd

EMISSION RATES OF ORGANICS FROM VEGETATION IN CALIFORNIA'S CENTRAL VALLEY ARTHUR M. WINER,*~" JANET AREY, ROGER ATKINSOr~, SARA M. ASCHMANN, WILLIAM D. LONG, C. LYNN MORRISON a n d DAVID M. OLSZYK~ Statewide Air Pollution Research Center, University of California, Riverside, CA 92521, U.S.A. (First received 22 February 1991 and in final forra 21 January 1992)

A~trac~Rates of emission of speciated hydrocarbons have been determined for more than 30 of the most dominant (based on acreage) agricultural and natural plant types found in California's Central Valley. These measurements employed flow-through Teflon chambers, sample collection on solid adsorbent and thermal desorption gas chromatography (GC) and GC-mass spectrometry analysis to identify more than 40 individual organic compounds. In addition to isoprene and the monoterpenes, we observed sesquiterpenes, alcohols, acetates, aldehydes, ketones, ethers, esters, alkanes, alkenes and aromatics as emissions from these plant species. Mean emission rates for total monoterpenes ranged from none detected in the case of beans, grapes, rice and wheat, to as high as 12-30/~g h- i g - 1for pistachio and tomato (normalized to dry leaf and total biomass, respectively). Other agricultural species exhibiting substantial rates of emission of monoterpenes included carrot, cotton, lemon, orange and walnut. All of the plant species studied showed total assigned compound emission rates in the range between 0.1 and 36 #g h - l g - 1. Key word index: Hydrocarbons, isoprene, monoterpene, agriculture, vegetation, NMHC, ozone production.

INTRODUCTION On a regional and global scale the emissions of nonmethane organic compounds from vegetation are estimated to be comparable with, or exceed, the emissions of non-methane organic compounds from anthropogenic sources (Zimmerman et al., 1978, 1988; Logan et al., 1981; Lamb et al., 1987). Under tropospheric conditions, isoprene and the monoterpenes have lifetimes due to reaction with OH and NO 3 radicals and 0 3 which are short compared to transport times (Corchnoy and Atkinson, 1990), and in the presence of NOx and sunlight these biogenic organic compounds can contribute to ozone formation. Thus, recent computer modeling studies, using isoprene as a surrogate for non-methane biogenic hydrocarbons, have shown that vegetative emissions can play important roles in the production of ozone in urban (Chameides et al., 1988) and rural (Trainer et al., 1987a) areas and in the chemistry of the lower troposphere (Trainer et al., 1987b; Jacob and Wofsy, 1988). While a number of studies have been carried out to determine the organic compounds emitted, and the corresponding emission rates, from a variety of vegetation types (see, for example, Rasmussen, 1972; * To whom correspondence should be addressed. I"Present address: Environmental Science and Engineerin.g Program, School of Public Health, University of California, Los Angeles, CA 90024, U.S.A. :~Present address: U.S. Environmental Protection Agency, Corvallis Environmental Laboratory, 200 SW 35th Street, Corvallis, OR 97333, U.S.A.

Zimmerman, 1979; Tingey et al., 1979, 1980; Evans et al., 1982; Winer et al., 1983; Isidorov et al., 1985, Lamb et al. 1985, 1986, 1987; Zimmerman et al., 1988), the number of plant species for which data are available is still small. In particular, prior to the present study essentially no organic compound emission rate data existed for the agricultural crops grown in California, which continues to have the most serious photochemical air pollution problem in the United States, including the highest ozone levels. The two major areas in California affected by adverse air quality are the Los Angeles Air Basin in southern California (South Coast Air Basin) and the Central Valley, which also receives polluted air masses transported from the San Francisco Bay area (Fig. 1). The Central Valley, which includes the San Joaquin Valley in the south and the Sacramento Valley in the north (Fig. 1), has the highest concentration of agricultural production in California, and also exhibits the topographical and meteorological characteristics conductive to the formation of photochemical air pollution. Indeed, the California Air Resources Board (ARB) estimates that if emission densities similar to those in the Southern California Air Basin were placed in the Central Valley, air quality could become worse than in the Los Angeles Air Basin (ARB, 1988). Moreover, by measures such as the number of days above the federal ozone standard, portions of the Central Valley (e.g. Fresno and Kings counties) already experience worse air quality than such major cities as New York, Houston, Philadelphia and Chicago (ARB, 1988). One of the most critical impacts of these adverse pollutant

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A.M. WINERet al.

Sa~

I JOAQUIN

~/ALLEY IR BASIN

San Diego

Fig. 1. Map of California showing the San Joaquin and Sacramento Valley air basins which encompass California's Central Valley.

levels is the reduction in yields of many of the state's important crops (ARB, 1987; Olszyk et al., 1988a, b; Winer et al., 1990). In order to develop and implement the most costeffective control strategies in the future for California's Central Valley, it is essential to continue to improve the databases upon which planning and computer modeling studies are based. Among the most important tools for this purpose is an accurate and comprehensive inventory of emissions of organic gases of anthropogenic and biogenic origin, as well as an emission inventory for oxides of nitrogen. To obtain a gridded emission inventory for biogenic compounds, it is essential to measure experimentally the emission rates of organic compounds from the dominant vegetation species, and to combine these data with plant species distribution or biomass assessments. The

objective of the present study was to determine experimentally the emission rates and chemical composition of organic gases from prominent vegetation sources that are likely to affect photochemical oxidant formation in California's Central Valley.

EXPERIMENTAL Selection of plant species

Since no experimental data were available in the literature for the emission of hydrocarbons from agricultural plant species found in California's Central Valley,we used acreage estimates developed by the ARB (see Winer et al., 1989,table III-1) and the availability in the literature of emission rates for certain members of the natural plant communities as the basis for eliminating certain plant types from approximately 60 candidate agricultural and natural species. For example,

Emission of organics from vegetation 12 agricultural species which appear on the California Agricultural Statistics Service maps for California, but which have small acreages in the Central Valley, were excluded from the study. In addition, we did not investigate the emissions of natural plant communities growing above approximately 4000 ft elevation, i.e. above the generally prevailing temperature inversion, although we recognize that under certain meteorological conditions down-slope flow may transport the organic emissions of higher elevation plant communities below the inversion. Thus, the acreages used in estimating the relative importance of the emissions from these plant.communities, particularly for the conifer communities, are expected to be much lower than the total acreages provided by the Department of Forestry for these vegetation categories. Moreover, literature data are available concerning the hydrocarbon emissions from a variety of pine and oak species (Bufalini and Arnts, 1981; Lamb et al., 1985, 1986), as well as for several types of sage (Dement et al., 1975; Winer et al., 1983). For these reasons, relatively little emphasis was given to the natural plant communities. On the basis of the foregoing considerations, the number of such plant species to be studied was reduced to a working list of approximately 40 species.

Locations of measurements and availability of plant specimens Since the meteorological conditions which normally prevail in Riverside, CA, during the summer months are not greatly different from those in the Central Valley, the study was conducted on the University of California, Riverside (UCR) campus. Emission measurements and associated analyses were conducted during the spring and summer of 1988 and 1989. The plant species for which emission rate measurements were made are listed in Table 1. Large, well-established specimens of the majority of the perennial plants were available on the UCR campus, including the native plant species which were available in the UCR Botanic Garden. The English walnut, French prune and Halford peach were nursery stock planted bare root and the herbaceous crops were started from seed. All species were grown in the field using either commercial cultural practices (crops) or near natural conditions (native plants). Additional details concerning the cultural practices followed are given elsewhere (Winer et al., 1989). Plant enclosure methods A flow-through plant enclosure system as employed previously by Winer et al. (1983) was utilized for emission measurements from fruit and nut trees and large agricultural crop plants (e.g. tomato and safflower). The enclosure chamber was constructed from a 2 ml Teflon film suspended from an external PVC frame measuring approximately 0.5 x 0.5 x 1 m, providing a chamber volume of ~ 150 f. This chamber was equipped with a stirring motor, a Teflon-coated blade, and inlet and outlet ports suitable for introduction of matrix air and withdrawal of analytical samples, respectively. Medical breathing air (Liquid Air; 99.6% stated purity level) contained a low background of organic species which eluted in the relevant retention time range covering the Cs-C~o hydrocarbons, and was used as the purge gas. A fraction of the air was flowed through distilled water, allowing the relative humidity of the air stream to be varied from zero to 100% depending on the measured humidity in theeenclosure during actual field measurements. All gas flows were monitored by calibrated rotameters. Carbon dioxide was added to the matrix air to achieve the ambient atmospheric CO 2 mixing ratio of 360 ppm in the air supplied to the plant enclosure. The temperature and humidity in the enclosure chamber was measured with a Vaisalia Model HMI 32 temperature and humidity sensor.

2649

After three air exchanges the concentration of the biogenic emissions in the plant enclosure chamber should be within 10% of the steady-state value. This was verified using carbon monoxide as a tracer. The typical flush time of 10 min at a flow rate of 45 I min-1 prior to sampling the hydrocarbon emissions in the chamber resulted in near steady-state concentrations of the biogenic emission and < 10% of the original ambient concentration of anthropogenic and biogenic hydrocarbons remaining. A second enclosure was constructed to measure emissions from ground species such as vegetables. This consisted of a PVC framework, approximately 1 m wide, 1.3 m long and 0.3 m high enclosed by 2 ml Teflon film. The enclosure was fitted with inlet and outlet ports suitable for introduction of matrix air and withdrawal of analytical samples, respectively. The skirts of the enclosure could be fitted tightly to the ground to create a chamber which was operated in the same manner as described above except that this enclosure did not contain a mixing fan.

Emission surveys and sampling protocols Initial surveys, with samples for gas chromatography/mass spectrometry (GC/MS) and GC with flame ionization detection (GC-FID) analyses simultaneously collected, were conducted for each plant species to determine by GC/MS the speciated hydorcarbon emissions for the plant type and to identify the GC-FID peaks observed. The standard sampling protocol consisted of a total of five measurements per species over a 6-h period (0900, 1030, 1200, 1300 and 1430 h PDT) for three different plant specimens, including two replications for one of the specimens. This protocol was chosen in an attempt to obtain information concerning the time dependence of emissions from the plant species of interest. Thus, the 0900 h sample was expected to be near the minima for isoprene and the monoterpene emissions and the noon and 1430 h samples near their maxima (Tingey et al., 1979, 1980; Juuti et al., 1990).

Analytical procedures Gas samples were collected from the plant enclosure chambers onto either Tenax-GC solid adsorbent or TenaxGC with a small amount of Carbosieve downstream of the Tenax-GC (to allow quantitative collection of volatile species such as isoprene). These Tenax-GC and Tenax-GC/Carbosieve cartridges were then thermally desorbed for compound identification by GC/MS and quantification by GC-FID. The gas samples for GC-FID analyses were of 0.5-2.6 f volume while those for GC/MS compound identification were generally larger (up to 10 E) to allow identifications to be made from full mass spectra (40-250 or 40-400 ainu). The Tenax-GC or Tenax-GC/Carbosieve samples for GC-FID analysis were thermally desorbed at 220 °C onto the head of a 15 m megabore DB-5 (5% phenylmethylsilicone phase) column, held at - 8 0 ° C for 5 rain and then temperature programmed at 8°C min- 1 to 200°C. The direction of flow through the Tenax-GC/Carbosieve cartridges during desorption was reversed from that during sample collection so that only the most volatile compounds (such as isoprene) for which breakthrough occurred on the Tenax-GC were collected on, and hence subsequently desorbed from, the Carbosieve. Only if very volatile species were observed in the GC-FID analyses was a small amount of Carbosieve added to the Tenax-GC cartridges used in the GC/MS analysis. For GC/MS analyses during the first summer, the adsorbent cartridges were desorbed in the split/splitless inlet of a Hewlett-Packard 5890 GC interfaced to a 5970 Mass Selective Detector (with the purge flow off, the cartridge was inserted and the inlet set to 225 °C). During the 10 min allowed for desorption, the column (50 m x 0.25 mm HP-5, 0.33 #m film thickness of 5% phenyl-methylsilicone phase) temperature was held at - 2 5 ° C and then programmed at 6°C min- 1. The desorption temperature was increased in the

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A. M. WINER et al.

Table 1. Plant species for which emission rate measurements were conducted Species Agricultural crops* Allium cepa

Common name onion

Beta vulgaris Carthamus tinctorius Citrus limon Citrus sinensis Citrus sinensis Daucus carota Gossypium hirsutum Irrigated pasture Juglans regia Lactuca sativa Lycopersicon lycopersicum Lycopersicon lycopersicum Medicago sativa Olea europea Oryza sativa Phaseolus vulgaris Pistacia vera Prunus armeniaca Prunus armeniaca Prunus avium Prunus domestica Prunus domestica Prunus dulcis vat. dulcis Prunus persica Prunus persica Prunus persica Sorghum bicolor Triticum aestivum Vitis vinifera Vitis vinifera

sugar beets safflower lemon orange orange carrot cotton multiplet walnut, English lettuce tomato tomato alfalfa olive rice~ bean, fresh pistachio apricot apricot cherry, sweet plum French prune almond peach nectarine nectarine sorghum wheat grape grape

Zea mays

field corn

Native plants** Adenostema fasciculatum H.& A. Arctostaphylos glauca Lindl. Avena spp., Bromus spp. Ceanothus leucodermis Greene Cercocarpus betuloides Nutt. Quercus lobata Nee.

Cultivar South Port White Globe UC H12 Lisbon Valencia Washington navel Imperator SJ2 multiple~/ Hartley Empire 6203 Sunny Pierce Manzanillo M202 Top Crop Kerman Blenheim II Royal¶ Bing Santa Rosa Marriana Nonpareil Halford Armking II Silver Lode¶ DK42Y Yecorro Rojo Thompson seedless French Columbard Pioneer 3183

chamise bigberry manzanita annual grassland whitethorn mountain mahogany Valley oak

* Species according to Hortus Third (Bailey and Bailey, 1976). t Orchard grass, perennial ryegrass, annual ryegrass, strawberry clover, birdsfoot trefoil. :~Unknown. § Started in greenhouse, then transplanted to pots in the field. II Formal sampling protocol. ¶ Survey. ** Species according to Munz and Keck (1975).

second summer of analyses to 250 °C when it became apparent that species of higher molecular weight than the monoterpenes, e.g. sesquiterpenes, were also present in some plant emissions. Additionally, the initial column oven temperature was changed to - 8 0 ° C (with programming after 10 rain at 8°C min -1) in an attempt to identify volatile,
columns), with those of the authentic compound (see Arey et al., 1991 a, b). p-Xylene (or initially 1, 2, 4,-trimethylbenzene) was added to the samples as a retention time marker for both the GC/MS and GC-FID analyses (the latter using the 15 m DB-5 megabore column in a Hewlett-Packard 5710A GC coupled to a 3390 recording integrator). An average calibration factor determined from several calibrations conducted during the study period for a variety of monoterpenes was used to quantify each compound as parts-per-billion of carbon (ppbC). Isoprene was quantified by GC-FID analysis on a 30 m GS-Q megabore column after calibration with an isoprene standard. For the C a ~ C s species other than isoprene observed on the GS-Q column, a factor (in terms of ppbC) which averaged the response of isoprene and of

Emission of organics from vegetation acetone was used, since it was expected in accord with the findings of Isidorov et al. (1985), that many of these species were likely to be oxygenated. Dry biomass determinations

Plants were harvested for dry weight measurements after all hydrocarbon emission samples had been taken for that sample. Woody plants were harvested by cutting off the entire branch that had been enclosed within the large Teflon bag. Herbaceous plants were cut off at ground level. The weight measurements focussed on dry leaf weights. Fruit, if present (for example on the tomatoes and certain citrus), was not included in the dry biomass weights. For some herbaceous plants (alfalfa, beans, annual grassland, pasture and wheat), leaves could not be separated from stems. As a result, only the total dry weight was obtained. For some woody plants (olive, chamise, mountain mahogany and whitehorn) leaves and stems were not originally separated. Thus, five extra samples of each of these species were collected to determine representative leaf/stem weight ratios. These ratios were then applied to the original total dry weight data to obtain representative dry leaf weights. The emission rates of the cotton and the two tomato varieties were determined both for total dry weight (excluding tomato fruit) and dry leaf weight.

2651 Table 2. (Contd.)

Acetates Bornylacetate Butylacetate (tentative) t cis-3-Hexenylacetate Aldehydes n-Hexanal trans-2-Hexenal Ketones 2-Heptanone 2-Methyl-6-methylene- 1, 7-octadien-3one (tentative) t Pinocarvone (tentative) # Verbenone (tentative) t Ethers 1,8-Cineole p-Dimethoxybenzene (tentative) t Estragole (tentative) t p-Methylanisole (tentative) t Esters Methylsalicylate (tentative) t n-AIkanes n-Hexane

C1o-'C1~ RESULTS AND DISCUSSION

Alkenes 1-Decene

Over 50 individual organic compounds were identified or tentatively identified as emissions from the agricultural and natural plant species studied, and these compounds are listed by chemical class in Table 2. In addition to isoprene (emitted only by the Valley oak) and the monoterpenes, we observed a

Table 2. Compounds identified* as emissions from the agricultural and natural plant species studied Isoprene Monoterpenes Camphene 2-Carene A3-Carene Limonene Myrcene cis-Ocimene trans-Ocimene ct-Phellandrene fl-Phellandrene ct-Pinene #-Pinene Sabinene ~t-Terpinene ~,-Terpinene Terpinolene Tricyclene or ~t-thujene (tentative)t Sesquiterpenes #-Caryophyllene Cyperene ~t-Humulene Other isomers :~ Alcohols p-Cymen-8-ol (tentative) t cis-3-Hexen-l-ol Linalool

1-Dodecene 1-Hexadecene (tentative) t

p-Mentha- 1,3,8-triene (tentative) t 1-Pentadecene (tentative) t 1-Tetradecene Aromatics p-Cymene * Unless labeled "Tentative," identifications were made on the basis of matching full mass spectra and retention times with authentic standards. t Tentative identifications were made on the basis of matching the mass spectra (and retention order when available) with published spectra (EPA/NIH Mass Spectral Data Base, and/or Adams, 1989). :~Several additional compounds were observed which can be assigned as C15H z4 sesquiterpenes based upon their mass spectra and apparent molecular ions at m/z 204.

number of sesquiterpenes, alcohols [including cis-3hexen-l-ol (leaf alcohol)], acetates and other esters, aldehydes, ketones, ethers, alkanes, alkenes and aromatic hydrocarbons. As discussed previously (Arey et al., 1991b), the range of monoterpenes observed includes 2-carene (a principal emission, along with /3phellandrene, from tomatoes), which to our knowledge has not previously been reported as a biogenic emission. In addition, sesquiterpenes were observed from a number of plant species and in some cases the emission rates of the sesquiterpenes exceeded the monoterpene emission rates (Arey et al., 1991b). As evident from Table 2, oxygenated compounds were also observed, with cis-3-hexen-l-ol and cis-3hexenylacetate being the most dominant (Arey et al., 1991a). The average emission rates of isoprene (only from the Valley oak), the monoterpenes, the sesquiterpenes

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and of the two oxygenates c i s - 3 - h e x e n - l - o l and cis-3hexenylacetate are given in Table 3, together with the average temperature (and the temperature range encountered) and the total assigned plant emission rate (TAPE). Apart from the pistachio, for which only four measurements were used, these average emission rates were based on the 5-sample standard protocol (see above). The emission rates are expressed as #g carbon emitted per hour per gram of dry weight (dry leaf weight, unless noted otherwise). The TAPE sums the monoterpene, sesquiterpene and oxygenated compound emission rates, which in general were the major emissions observed, together with the emission rates of any other compounds listed in Table 2 which were identified in a particular plant. The detailed speciated emissions observed for each plant investigated here are reported in Winer et al. (1989). In addition to the identified or tentatively identified organic compounds noted as emissions, and hence included in the TAPE, numerous generally small peaks were present in the G C - F I D traces of the plant emission samples. These GC peaks could be unidentified plant emissions and/or contaminants in the plant enclosure either due to background peaks from the matrix gases employed or from residual ambient air in the enclosure. F o r each emission measurement, the G C - F I D responses for all the observed peaks through the elution region of the sesquiterpenes were summed to calculate the total carbon (TC), an upper limit to the plant emissions. In general, the differences between the total carbon and the assigned plant emission were not significantly different from the TC calculated for blank matrix air samples in which no plant materials were placed in the chamber and, therefore, the TAPE given in Table 3 are believed to be a good measure of the total plant emissions. The two exceptions were irrigated pasture and the wheat samples, both of which showed numerous volatile (generally < C6) peaks in the G C - F I D analyses which could not be identified in the GC/MS analyses. The emission rates of wheat and irrigated pasture, as calculated from the total carbon observed in the G C - F I D analyses, were 1.1 and 3.2/~g h - 1 g - t, respectively, and these rates are probably more representative of the plant emissions than are the TAPE values given in Table 3. Mean emission rates for the monoterpenes ranged from none detected in the case of beans, grapes (both Thompson seedless and French Columbard), rice and wheat, to as high as > 3 0 # g h - l g - I for the two cultivars of tomato investigated (normalized to total dry biomass, excluding fruit). The Kerman pistachio also fell in the high emitter category with a monoterpene emission rate of about 12 # g h - : g - t . Other species exhibiting substantial rates of emission of monoterpenes included the agricultural crops carrot, cotton, lemon, orange and walnut and the natural plant species whitehorn. Crops which fell into a low monoterpene emitter category included alfalfa, almond, apricot, cherry, nectarine, olive, peach, plum, safflower and sorghum. The specific monoterpenes

emitted from each plant species have been detailed elsewhere (Arey et al., 1991b). For about a third of the agricultural crops studied, sesquiterpene emissions were below the detection limits of the analytical methods employed (note that sesquiterpenes were not quantified for the seven plant species studied in the summer of 1988). Alfalfa, cotton, and olive had sesquiterpene emission rates below 0.1 #g h - t g - t, while the remainder of the agricultural plant species exhibited total sesquiterpene emission rates which fell into a relatively narrow range (compared with monoterpene emissions) between 0.1 and 1 # g h - t g - ~. It is of particular interest that the sesquiterpene emissions from the cherry, French columbard grape, olive, peach and, in particular, the safflower exceeded the monoterpene emissions from these species. A qualitative grouping of the agricultural crops by order of magnitude ranges in the sum of the total monoterpene and sesquiterpene emissions rates is shown in Table 4. All of the agricultural crops for which full protocols were carried out exhibited total assigned plant emission (TAPE) rates above the detection limits of this study (see Table 5 for qualitative groupings). Crops with TAPE emission rates above 1 0 # g h - t g -~ included pistachio and tomato. Although rice also exhibited a mean TAPE emission rate above 10/~gh-t g-~, this result must be used with caution. Although two of the five protocol samples had much lower dry leaf weights than the specimen used in the 0900, 1200 and 1430 h samples, they all had similar measured TAPE. This resulted in two calculated emission rates approximately an order of magnitude larger than the average of the remaining three emission rates. If the two high values are removed, the mean emission rate for TAPE from rice would be 3 #g h - 1 g - 1 vs the value of 11 #g h - 1 g - ~ given in Table 3. The natural plant species whitethorn also had a TAPE emission rate above 10 # g h - 1 g - ~.

Table 4. Qualitative grouping of agricultural crops by rates ~g h- 1g- 1) of total monoterpene plus sesquiterpene emissions
O.l-I Alfalfa Apricot Cherry Cotton~; Grape~ Olive OrangeII Safflower

1-10 Carrot Cotton¶ Lemon Orange** Peach Walnut

* None detected. t Thompson seedless. Normalized to total dry weight. § French columbard. I)Washington navel. ¶ Normalized to dry leaf weight. ** Valencia. t t Sunny and canning.

>10 Pistachio Tomatott

Emission of organics from vegetation Table 5. Qualitative grouping of agricultural crops by rates (pg h- ~g- 1) of total assigned plant emission Low <1

Middle 1-10

High > 10

Bean Nectarine Olive Orange* Pasture Wheat

Alfalfa Almond Apricot Carrot Cherry Cotton Grapet Lemon Oranges Peach Plum Safflower Sorghum Walnut

Pistachio Rice§ Tomato PI

* Washington navel. t Thompson seedless and French columbard. :~Valencia. § See text. IISunny and canning.

Crops with TAPE emission rates between 1 and 10 ~ g h - 1 g - 1 included alfalfa, almond, apricot, carrot, cherry, cotton, grape, lemon, Valencia orange, peach, plum, safflower, sorghum and walnut. The abundant natural plant species chamise also had a TAPE emission rate above 1/~gh - l g - 1 , although this species had been reported to be a non-emitter in previous work (Winer et al., 1983). The remaining crops, i.e. beans, nectarine, olive, Washington navel orange, pasture and wheat, displayed TAPE emission rates below 1/~g h - 1 g - 1. Comparison with previous studies A#ricultural plant species. From the G C - F I D data obtained, it is possible to calculate an upper limit to the isoprene emissions from the agricultural species studied. While pentane and isoprene co-eluted on the GS-Q column employed for the quantifications, assuming that all of the observed C5 concentrations measured in the protocols were due to isoprene, then upper limits to the isoprene emissions for the agricultural crops were in the range 0.008-0.09 ttg h-1 g-1. This observed absence of significant emissions of isoprene from the agricultural crops is consistent with the previous work of Flyckt et al. (1980) and Evans et al. (1982). Evans et al. (1982) screened beans, alfalfa, field corn, wheat, sugar beets and cotton for isoprene emissions and found non-detectable levels in the sugar beets and cotton and "low" isoprene emissions for the remaining crops, while Flyckt et al. (1980) reported that isoprene was not emitted in significant amounts by corn, tobacco, alfalfa, clover and mixed forage, but noted that compounds eluting in the monoterpene region were the major emissions. From an evaluation of the literature data, Lamb et al. (1987) recommended as an average ct-pinene emis-

2655

sion rate from coniferous trees a value of ~3.5 /lg h - t g - 1 at ,,~ 30°C. Of the agricultural and natural species studied in this work, only the citrus (lemon and orange) and nut trees (pistachio and walnut) and the two tomato varieties emitted monoterpenes at comparable or higher levels than 3.5 /~g h-1 g-1. The two tomato cultivars, a fresh market tomato (Sunny) and a canning variety (6203), had very similar emission profiles, with the major emissions being identified as fl-phellandrene, 2-carene and limonene, fl-Phellandrene has been reported as the major monoterpene in the tomato leaf volatiles of several cultivars of Lycopersicon esculentum by Andersson et al. (1980), and limonene was the major monoterpene reported by Zimmerman (1979) for tomatoes grown in the Tampa/St Petersburg, FL, area. It should be noted that the grandular hairs on tomato plants (Kennedy and Sorenson, 1985) could make these plants unusually susceptible to enhanced emissions caused by touching the plant while locating it within the chamber, and hence our emission rate data for the tomato species may be upper limits to the emission rates occurring under "field" conditions. However, our tomato emission rates given in Table 1 are comparable to those reported by Zimmerman (1979), who used a technique in which background emissions (i.e. those in the chamber immediately after enclosing the plant) were subtracted from emissions measured over a designated time. Limonene was the major monoterpene emission we observed for the three citrus varieties examined. Our measured monoterpene emission values for the two varieties of orange (Table 3) can be compared to a total alkene emission value of 5.5 gg h - 1 g - 1 measured by Zimmerman (1979) in Florida, with an "unknown alkene" being the major emission. Recently, in a long-term study of the volatile emissions from a Valencia orange tree (Arey et al., 1991c), it was observed that the emissions increased by over an order of magnitude when blossoms were present on the tree. Linalool, an oxygenated terpene derivative, and the monoterpene, myrcene, were emitted from the blossoms and the other monoterpenes, which included limonene and sabinene, were only minor components of the total volatile emissions of the tree in blossom. This suggests that the absence or presence of blossoms on fruit trees is important with respect to both the compounds observed and the measured emission rates. Natural plant species. As seen in Table 3, the mean emission rates for the total assigned plant emissions from the natural plant communities studied ranged over two orders of magnitude, from ~ 0. I #g h - l g for grasslands and manzanita to ~ 10 #g h - 1 g - 1 for whitethorn. The Valley oak was the only confirmed isoprene emitter observed among either the agricultural or natural plant species investigated. The isoprene emission rate of 2.3 #g h - 1 g - 1 determined in the present study for Valley oak is at the low end of the range of emission rates reported in the literature for

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other oaks (Zimmerman, 1979; Flyckt et al., 1980; Tingey et al., 1980). As discussed above for the agricultural plant species, upper limits to the isoprene emission rates for the natural plant species other than the Valley oak can be calculated to range from less than 0.008 to less than 0.05/tgh -1 g-1. Chamise and whitethorn were observed to be significant monoterpene emitters. As mentioned above, previous work had indicated that chamise is not a monoterpene emitter (Winer et al., 1983). Importance of compound classes other than monoterpenes. The present study shows that emissions of compounds other than the commonly studied monoterpenes and isoprene do occur. These must be taken into account in addressing the contribution of biogenic emissions both to the formation of air pollution and to tropospheric chemical cycles. For example, oxygenated organics were observed, often with emission rates comparable to, or greater than, those of the monoterpenes (see Table 3). As discussed in detail elsewhere (Arey et al., 1991a), among the ultimate reaction products expected from these oxygenated compounds are peroxyacetyl nitrate and peroxypropionyl nitrate, compounds which may serve as temporary reservoirs of NOx (Singh and Salas, 1989). Additionally, at the temperatures typical of the Central Valley in summer, sesquiterpene emissions were determined to be significant from several plant species. In compiling a national inventory of biogenic hydrocarbon emissions, Lamb et al. (1987) noted that the role of oxygenated hydrocarbon emissions has not been documented to any significant extent. The emission of cis-3-hexen-l-ol (leaf alcohol) from higher plants has been reported by Ohta (1984) and both leaf alcohol and 3-hexenylacetate were reported by Isidorov et al.. (1985) to be among the volatile organic compounds produced by plants characteristic of Northern Hemisphere forests. For example, Isidorov et al. (1985) reported 3-hexenylacetate as one of the major emissions from bilberry shrubs. In addition, 3-hexenylacetate and leaf alcohol have been identified as major volatiles from the cereal grains wheat, oat and barley (Buttery et al., 1982, 1985), and from Sorghum bicolor (Lwande and Bently, 1987) (all of these plants having been cut near the base prior to sampling). While 3-hexenylacetate was the second most abundant species from cowpea (~-cedrene being the most abundant), 3-hexen-l-ol was not observed to be emitted from uprooted intact plants of this crop (Lwande et al., 1989). A more detailed discussion of the oxygenated emissions from agricultural crops is given by Arey et al. (1991a). All of the above studies in which the 3-hexenyl acetate and 3-hexen-l-ol were observed used TenaxGC adsorbent for collection of the volatile emissions. It is possible that these, and other, oxygenated compounds were not observed in previous studies which utilized stainless steel canisters for sample collection because of wall adsorption/desorption problems and poor recovery from such canisters.

Estimates of uncertainty and variation in emission rates Enclosure effects. While concerns have been expressed that emission measurements made using the enclosure technique can overestimate the actual plant emissions (Dimitriades, 1981), recent comparisons of enclosure emission measurements with atmospheric tracer and micrometeorological gradient techniques have shown reasonable to excellent agreement (Lamb et al., 1987). Our recent studies have shown, however, that reliable emission measurements require considerable care in using the enclosure technique, and that "rough handling" of a plant species when placing it within the enclosure dramatically increases the emission rates (Juuti et al., 1990; Arey et al., 1991a). In particular, although we initially thought that the presence of cis-3-hexen-l-ol and cis-3-hexenylacetate emissions were indicative of "rough handling", we observed that the emission rates of both the monoterpenes and these oxygenated species from agricultural crops were enhanced by "rough handling". Moreover, both cis-3-hexen-l-ol and cis-3-hexenylacetate were often observed from plants during the protocol samples when every effort was made to place the plant or plant limb gently within the enclosure, and these emissions were hence considered to be true representative plant emissions (Arey et al., 1991a). However, it is clear that ambient measurements of the alcohol and acetate above appropriate crops are needed to clarify the importance of the emissions of these compounds into the atmosphere under realistic conditions. Temperature effects and specimen variation. The emission sampling protocol, which called for five measurements for a given plant species over the course of a 6-h period from mid-morning to mid-afternoon, was designed in part to characterize, if possible, the temperature dependence of the emissions. Thus, the emissions from the same specimen were measured at 0900, 1200 and 1430 h and over a temperature range controlled by the ambient conditions encountered. The second and third specimens measured at 1030 and 1330 h showed the plant-to-plant variability in the emissions. An exponential temperature dependence for the monoterpene emissions was expected based on the work of Tingey et al. (1980) for slash pine and of Juuti et al. (1990) for a Monterey pine. Since a temperature variation of 10-17°C was commonly observed for the daily sampling protocols in this study, a variation in the emission rate of between a factor of two to four throughout the course of a given sampling protocol could be expected from the temperature variation. However, since the recent work of Juuti et al. (1990) with the Monterey pine suggests that at any given temperature the emission rate can vary by a factor of two, reliable temperature versus emission profiles with only three data points are unlikely. Of particular relevance to this study, Tingey et al. (1980) carried out emission rate measurements on slash pine for 14 individual plants. While each plant

Emission of organics from vegetation showed a similar temperature dependence of the monoterpene emission rate, the absolute emission rates at a given temperature varied by an order of magnitude. Thus, in the present study the specimento-specimen variability probably means that the addition of the 1030 and 1330 h samples may not better define the temperature dependence of the emissions. In practice, in a few cases the individual emission rates for monoterpenes within a protocol did appear to vary with temperature in a systematic manner. The whitethorn and cotton samples are two examples of this, where the logarithm of the emission rates for the 0900 h, noon and 1430 h samples gave a reasonable straight line when plotted against the sampling temperature. Furthermore, for the cotton sample the sesquiterpene emissions yielded a line parallel to the monoterpenes, suggesting that the temperature dependence of sesquiterpene emissions is similar to that of the monoterpenes. This observation is consistent with the recent finding of Guenther et al. (1991) that the temperature dependence of 1,8-cineole emission from individual leaves of Eucalyptus #lobulus is essentially identical to that for emission of ct-pinene. In accordance with previous data (Tingey et al., 1980; Juuti et al., 1990), the pattern of monoterpenes emitted for each plant species was characteristic of that species and the ratios of the monoterpenes to one another were generally constant, at least over the short time scale of the measurements described here (Arey et al., 1991b). However, the ratio of the monoterpenes to the oxygenates, especially cis-3-hexenylacetate, varied among measurements in a seemingly random fashion, indicating that temperature may not be the factor controlling the emission of the hexenylacetate and leaf alcohol. Since the oxygenated emissions were dominant for many of the agricultural species (see Table 3 and Arey et al., 1991 a), it is clearly important to verify these oxygenated emissions by ambient measurements and to investigate the factors controlling their emission rates. Tingey et al. (1980) suggested that the monoterpene vapor pressure (which over ambient temperature ranges is an exponential function of the temperature) and monoterpene pool size control emissions rates. For example, many pine needles are known to contain a large pool of monoterpenes available for volatilization into the atmosphere. For the agricultural species reported on here, in some cases (i.e. the carrot, lemon, nectarine and canning tomato) the first, and lowest temperature, emission rate was the highest monoterpene emission rate value of the day. In these instances one can speculate that the monoterpene pool size may have been limiting. Relevant to this, Dement et al. (1975) noted that the emission rate of camphor from California black sage was higher for samples obtained after a low night temperature. Additionally, Yokouchi and Ambe (1984) noted long-term effects of light levels, as well as of temperature, on the emission of monoterpenes from red pine. Yokouchi and Ambe (1984) suggested that the effect of light was

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the indirect result of changing the amount of monoterpene produced in the leaves, another indication (and consistent with the lower emission rate of the red pine compared to the slash pine) that monoterpene pool size, as well as temperature, influences the emission rates.

CONCLUSION

We have reported here mean emission rates. Generally, the mean sampling temperatures (Table 3) were above 30°C and these data can be viewed as being appropriate for the expected emissions in the California Central Valley during summertime conditions. Therefore, these mean emission rates, when combined with biomass data for the Central Valley, will be sufficient to determine which, if any, species should be evaluated in a more rigorous way with regard to their emissions at various temperatures. A further important qualification of the data obtained in the present study is that these results must be viewed as a "snapshot" of the emission rates from the various plant species investigated. Thus, as noted above, the volatile emissions for a Valencia orange tree increased by over an order of magnitude when the tree was blossoming. In each case, the data reported here are for a single day, and involve at most three different plant specimens for the given species. In a number of cases only two or even one plant specimen was involved, with emission rate measurements being obtained from different limbs or branches of this one specimen. These considerations must be borne in mind when the emission rate data reported here are employed in the construction of an emission inventory for vegetative emissions of organic compounds. Another qualification is that the plants were exposed to ambient Riverside ozone levels during growth. Related to this, Renwick and Potter (1981) showed enhanced emissions of terpenes from Balsam fir trees exposed to SO 2. However, the possible effects of exposure to ozone on monoterpene and isoprene emissions have not, to our knowledge, been studied, although the interaction of ozone with monoterpenes and isoprene has been suggested to be involved in plant injury, through a variety of proposed mechanisms (Franzen et al., 1989; Hewitt et al., 1990). In summary, taking into account the results obtained in the present study, and the work of Tingey et al. (1980) and Juuti et al. (1990), an uncertainty as large as a factor of five may apply to some or all of the mean emission rates reported in this study. This potential uncertainty should be reflected in the uncertainty of the emission inventories constructed from these data. Clearly, further additional studies for multiple plant specimens of a given species and over an entire growing season are necessary to narrow these uncertainties. However, the present database, in conjunction with a biomass database, will allow prioritization

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