Envlron. Sci. Technol. 1983, 17, 389-395

Ambient Concentratlons of Hydrocarbons from Conifers in Atmospheric Gases and Aerosol Particles Measured in Soviet Georgia

Robert W. Shaw, Jr.,"? Alden L. Crlttenden,t Robert K. Stevens,+ Dagmar Rals Cronn,§ and Vltall S. Tltovl

Environmental Sclences Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1, Department of Chemistry, Universlty of Washington, Seattle, Washington 98195, Air Pollution Research Section, Washington State Universlty, Pullman, Washington 99 164, and A. I. Voeikov Main Geophysical Observatory, Leningrad, USSR

~~ ~

w A l-month field study was performed in the mountains of the Georgian Republic of the USSR in July 1979 to study the properties of aerosols in a relatively clean en- viropment containing naturally emitted hydrocarbons, in this case, terpenes from evergreen forests. We performed gas chromatographic analysis of gaseous hydrocarbons and mass spectrometric analysis of aerosol particles. Terpenes were present in the gas phase at an estimated average concentration of 40 ppbC. The estimated upper limits of terpenes and their reaction products found in aerosol particles were of the order of 1% of the corresponding gas-phase terpene concentrations. The amounts of natural organic materials were much smaller than amounts of sulfate in the aerosol particles and were relatively insig- nificant with respect to visibility.

Introduction A field study to examine the chemistry and physics of

natural aerosols was performed in the Georgian Republic of the USSR in July 1979. During this study, investigators from the United States and the Soviet Union made mea- surements of the properties of ambient atmospheric aerosol particles and gases and meteorological observations. This paper reports the results of analyses for gaseous and particulate hydrocarbons.

The experimental site was located on the grounds of the Abastumani Astrophysical Observatory on the peak of Mt. Kanobili, 1700 m above sea level on the southern slopes of the Adjar-Imeretian range. Abastumani, which lies in the valley to the eqst of Mt. Kanobili, is a resort village that has several sanatoriums. The Observatory was es- tablished near Abaqtumani because of the remarkable clarity of the region's atmosphere. Its remote location from pollution sources, its ability to sustain a large working group and equipment for a 5-week period, and its prox- imity to the extensive coniferous forests that cover Mt. Kanobili and neighboring mountains and valleys made the Observatory an ideal site for the natural aerosol study.

The Abastumani Forest is composed principally of pine (Pinus syluestris L.) and spruce (Picea orientalis (L.) Link). The mature pines are branched only near the top of the tree, have long needles, and prefer sunny, southern slopes. They are normally found on slopes characterized by open areas and considerable ground vegetation. The spruce have dense, short needles and branch from the ground. They populate mostly shady, northern slopes, grow very closely together, and hence exclude other trees and ground vegetation. Because the pines are desirable for lumber and the spruce are not, the pines are cut and the spruce tend to replace them. Thus, the forest popu- lation may be systematically changing. Presently, pines are predominant on the summit of Mt. Kanobili a t the

US. Environmental Protection Agency. *University of Washington. *Washington State University.

A. I. Voeikov Main Geophysical Observatory.

Observatory site and on the slope running down from the instrument platform where most of the ambient samples were collected.

A variety of monoterpenes are emitted by coniferous forests, including a-pinene, @-pinene, limonene, A3-carene, and others. The predominant species emitted from pines is generally considered to be a-pinepe. Monoterpenes are known to react rapidly with hydroxyl (OH) radicals and ozone and/or nitrogen oxides (1-3). Reactions with ozone, when carried out in the laboratory, lead to rapid produc- tion of aerosol particles (4); production of particles by OH radical reaction is suspected but has not been demon- strated.

Several oxidized derivatives of a-pinene, e.g., pinonic acid, have been found in naturally occurring aerosol par- ticles (5), and one might expect that oxidized terpene compounds could constitute a large, nonanthropogenic fraction of aerosol particles in rural, forested areas. Went (6) has suggested that such compoynds are responsible for the blue haze found in mountain areas of the eastern United States. Recent experiments by Stevens et al. in the Great Smoky Mountains (7), Weiss et al. in the She- nandoah Valley (8), and Pierson et al. (9) in the Allegheny Mountains, however, show that natural hydrocarbons were not significant contributors to the haze in these mountain areas during the periods of observatipn. These results do not indicate that Went's conjecture was incorrect but that sulfate aerosol particles transported into the region now dominate atmospheric light scattering.

The extent of gas-to-particle conversion of terpenes is a subject of controversy. Duce (10) concluded that the bulk of gaseous terpenes from vegetation are rapidly converted to particles although he recognized that some contradictory evidence existed and remarked that more work was nec- essary. Conversely, Hull (11) has argued that under am- bient conditions, terpenes react to form gas-phase prod- ucts.

To determine order-of-magnitude estimates of amounts of terpenes required to cause visible haze, we make a sim- ple model calculation based on extinction coefficients for various aerosol distributions reported by Willeke and Brockman (12). First, we assume that an atmospberic concentration of 1 ppb (lo4 v/v) of monoterpene (5 pg/m3 of carbon) is changed entirely into particles. Theh we assume a typical fine-mode distribution with a maximum at 0.3-pm aerodynamic diameter, width of 2, refractive index of 1.5-0.02i, and density of 2 g/cm3. Using these assumptions and Willecke and Brockman's results (their figyre 21, we find that the scattering coefficient associated with the aerosol particles formed from 1 ppb of terpene is Pert (A = 0.55 pm) N 1.4 X 10" m-l. This value i s close to the Rayleigh scattering coefficient from air; conse- quently, the visible range will be reduced by the particles to about half that in clear air. This result suggests that even low atmospheric concentrations of gaseous terpenes, if converted to particles, can contribute significantly to atmospheric extinction and cause visible haze. However, the results of our experiment suggest that, although natural

0013-936X/83/0917-0389$01.50/0 0 1983 Amerlcan Chemical Society Environ. Scl. Technol., Vol. 17, No. 7, 1983 389

hydrocarbons were present in significant amounts in the gas phase, they contributed only a small fraction to the am- bient aerosol particles measured at the Abastumani Ob- servatory.

Experimental Procedure (A) Gas Collections and Measurements. Nearly all

samples were collected near a wooden experimental plat- form built from the rear of a small astronomical observa- tory that was no longer in use. The forest floor sloped away from the rear of the observatory building so that the edge of the platform was about 5 m from the ground. A total of 46 samples were taken near the platform; samples were taken every day except July 12 during the period July 10-27. Of those samples, 14 were taken between 0600 and 1200,24 between 1200 and 1800, and 8 between 1800 and 2400.

Samples were collected in two ways: 200-mL syringe samples were taken from the ambient atmosphere, and 50-mL samples were taken from a living branch that was partially enclosed for a short time by the syringe. Almost all the ambient samples were taken in the forest about 10 m from the experimental platform and injected into the gas chromatograph (GC) within 5 min. Three ambient samples were taken in the valley near the village of Abastumani and injected into the chromatograph within 30 min. Three additional ambient samples were collected from a meteorology tower at about 20 m, just above the pine canopy, near the experimental platform.

The two types of collection syringes used in the study were glass barrel syringes and Telfon plungers and all-glass syringes. Samples from living branches were collected by removing the plunger from an all-glass syringe and gently inserting a branch end, with as little handling as possible, into the barrel. The syringe remained over the branch for 5 min. These branch samples were taken not in order to measure emission rates quantitatively but rather to de- termine whether chromatographic peaks observed from ambient air samples were also present in branch samples.

A Varian GC (Model 2440-10) with a support-coated, open-tubular capillary column and flame ionization de- tector was used for analysis of hydrocarbons. The column was 200 ft long and 0.020 in. i.d.; the stationary phase was OV-101 plus 5% Igepal CO-880 on silica. Syringe samples were injected through a silicone rubber septum into a 12-in. stainless steel cold trap (1/16 in. o.d., 0.040 in. i.d.) that was cooled by liquid oxygen (bp -183 "C). All column and cold-trap hardware were stainless steel. The condensed sample was subsequently released to the column by warming the trap to 100 OC. The column temperature was increased from 30 to 100 "C during the run at a rate of 6 "C/min. For ambient samples the electrometer input range was set to A/mV.

The carrier gas was nitrogen (Airco, 99.995%); the de- tedor gases were hydrogen (Airco, 99.999%) and air (Airco, <1 ppm total HC). Carrier gas flow rate was checked at least daily. The flow rate (5 mL/min) was set at a column temperature of 30 OC with an observed reproducibility of 3% or better; this level of imprecision was due possibly to temperature changes in the column.

Three standard gas mixtures all contained in stainless steel vessels were used for system response calibration. Standard 1 consisted of 14 hydrocarbons, ranging from C2 to C9, and 1,1,2-trichloroethane in concentrations ranging from 20 to 100 ppbC in nitrogen at 30 psig. Standard 2 was isoprene and d-limonene; standard 3 was benzene and a-pinene. Standards 2 and 3 were at concentrations of approximately 10 ppmC in air at 160 and 300 psig, re- spectively. Subsequent identification of ambient hydro-

390 Envlron. Sci. Technol., Vol. 17, No. 7, 1983

carbons as CZ, etc., was based solely on retention time. The retention time for a-pinene was 26 min. After the ex- periment all standards were returned to EPA laboratories. Five months after their preparation, reanalysis showed that the concentrations of the components of standard 1 were stable. The terpene standards showed no change in con- centration for isoprene and losses of 50% and 40% for a-pinene and d-limonene, respectively.

Mixing ratios (concentrations) were calculated on the basis of instrument responses (peak heights adjusted for carbon number) to the standards. The response factor used for a-pinene was based on isopropylbenzene, a standard compound with retention times close to that of a-pinene. We chose to use the more stable compounds in standard 1 for response factor calibration and the terpenes in standards 2 and 3 for retention time checks. During the field study, the isopropylbenzene response varied with a range of 19% of the mean value. The response factor used for isoprene was that of 2,2-dimethylbutane; this response varied with a range of 32% of the mean. The sensitivity variations are rather large, due to various difficulties in operating for an extended period in a remote area. The performance was, nevertheless, sufficient for approximate measurements and to satisfy the goals of the study. The estimated minimum detection limit (MDL) was approxi- mately 1 ppbC. Blank values (samples from an Aadco generator) were below the MDL for all measured com- pounds, and all reported carbon compounds were below the MDL in at least one ambient sample taken during the study.

A syringe contamination peak occurred in the terpene region of the chromatograms. Although the peaks for a-pinene and d-limonene were not affected, part of the region between those peaks was obscured. On the basis of observations of samples collected directly from branches with use of a clean syringe, however, we found that the ratios of the sum of concentrations of all peaks in the Clo region to the concentration of a-pinene have a mean of 3.6 and vary by about 25%. In order to estimate total natural hydrocarbon (TNHC), we used [TNHC] = [isoprene] + 3.6[a-pinene]. It is important to note that, although the values for isoprene and a-pinene are experimentally de- termined, the values for TNHC are estimated.

(B) Aerosol Particle Collection and Measurements. Aerosol particles were collected on previously ignited glass fiber filters of 25-mm diameter by using an apparatus located on the experimental platform. The filters were preceded by a single-stage impactor to remove particles larger than about l-pm diameter. Samples contacted no organic materials ahead of the filters. A total of 21 sam- ples, each representing 18 m3 of air, were taken over 24-h periods, beginning at 8 a.m. local time during the period July 5-27, 1979. Of these, the eight samples having the highest concentrations of terpene products will be dis- cussed. The exposed filters were wrapped in aluminum foil, enclosed in plastic bags, and stored at dry ice tem- peratures until analysis.

Electron impact mass spectra were taken by using an Associated Electronics Industries Type MS/9 mass spec- trometer at a resolving power of 7000-8000. Three disks of 7-mm diameter were punched from a sample filter and inserted into the spectrometer via a heated probe (5). The probe was heated from 25 "C at 20 "C/min to 300 "C and then kept isothermal for the remainder of the observations. Mass spectra of the evolved gases were recorded every 50 s to a total of 30 spectra per sample. Known amounts of p-terphenyl were added to the disks as an internal standard with the mass 230.1094 peak. During scanning

Table I. Relative Amounts of Gaseous Hydrocarbons from Pine and Fir Boughs, Abastumani, July 1979

date time tree isoprene a-pinene A B C D d-limonene 18 2040 pine <0.01 1 0.2 0.5 0.6 0.8 <0.1 19 0720 pine 0.03 1 0.1 0.5 0.7 0.8 0.1

1225 pine <0.01 1530 pine <0.01 1 0.1 0.6 0.9 1.4 0.2

20 1045 spruce > 300 1 1315 pine <0.01 1 0.1 0.5 0.3 0.9 0.2

24 1050 pine <0.01 1 0.1 0.6 0.9 1.7 <0.1

Table 11. Relative Retention Times a-pinene A B C D d-limonene

field sample 0 0.14 0.40 0.49 0.77 1.00 f 0.03

a -pinene camphene @-pinene myrcene A3-carene d-limonene

lab standards 0 0.13 0.38 0.47 0.74 1.00 I 0.01

Table 111. Mean and Maximum Ambient Gaseous C,-C, Hydrocarbon Concentrations observatory site valley floor above tree canopy

mean mean mean

PPbC % max,ppbC ppbC % max,ppbC ppbC % max,ppbC c, NDa 0 8 c, 5 9 20

6 10 20 5 9 30

30 60 100 C6 3 6 10 5 9 20

c, ND 0 10

c* c9

E: b

sum 54 100 no. 46

4 2 5 2

20 9 2 1

100 60 40 20 20 10

ND 0 207 100

3

10 8

40 5

300 50 30

ND

ND 0 3 3 5 6 4 4

60 7 0 5 6

10 10 ND 0 91 100

3

ND 8 14

7 120 6

20 ND

a Not detected. Not including isoprene.

of mass spectra, perfluorokerosene vapor was added to provide calibration points for mass calculation and for checking the calculations. The average error in mass for peaks not used as calibration points for any one sample varied from 1.6 to 2.0 millimass units.

Mass spectra were searched for the presence of any of 400 ion masses including both parent (unfragmented) and fragment ions. The list contained masses of ions previously observed in aerosols (5) and of ions expected from com- pounds possibly present, from either literature data or spectra of known compounds. An ion was regarded as detected if it appeared in a t least three of five consecutive mass spectra from a sample and had a computed mass within 5 millimass units of the reference value.

Quantities of compounds found were calculated from the ratio of the total ion abundances in the series spectra to that of the internal standard ion, provided that the relative sensitivities were known. These sensitivity factors were measured from known amounts of pure compounds for pentanedioic acid, hexanedioic acid, benzoic acid, pinonic acid, pinonaldehyde, and sulfuric acid. Sensitivity factors were calculated from literature data or were estimated.

Results (A) Hydrocarbon Concentrations in the Gas Phase.

The emissions from pine boughs produced a characteristic fingerprint chromatogram of seven peaks corresponding to seven compounds with retention times equal to or greater than a-pinene. The absolute amounts emitted varied considerably with time. The relative amounts of individual compounds are given in Table I and vary by no more than a factor of 2. Standards run during the field study showed that two of the compounds had retention

Table IV. Mean and Maximum Ambient Gaseous Natural Hydrocarbons (ppbC)

observa- above tree tory site valley floor canopy

mean max mean max mean max

a-pinene 8 20 30 50 2 6 isoprene 7 30 NDa ND ND ND TNHC (estimate) 40 100 100 200 7 20 no. 50 3 3

a Not detected.

times indistinguishable from a-pinene and d-limonene. Standards run in the laboratory, but duplicating field study procedures and instrument parameters, permitted the tentative identification of the remaining compounds.

Table I1 shows the retention times, relative to a-pinene, of a field sample measured in Abastumani with initially unidentified compounds labeled A-D and a number of standard compounds run subsequently in the laboratory. On the basis of this tentative identification (which would require GC/MS analysis of a field sample for confirma- tion), the pine bough fingerprint compounds were assigned in rough order of abundance: A3-carene, a-pinene, myrc- ene, @-pinene, d-limonene, and camphene. As shown in Table I, isoprene levels were about 100 times lower than terpene levels from pine. For spruce, however, isoprene levels were more than 100 times higher than terpene levels.

The results of the ambient gaseous organic compound measurements are presented in Tables I11 and IV as mean and maximum values for the entire study. Because of the measurement precision, results are given to one significant

Environ. Sci. Technol., Voi. 17, No. 7, 1983 391

Table V. Aerosol Particle Analysis:a Adsorbed Terpenes, Terpene Reaction Products, and Sulfate (fig/m3)

reaction adsorbed products terpenes (upper limit)

date from from limo- (July) C,H,,+ C,H,+ pinene nene sulfate

9 0 0.006 0 0.36 1.6 1 0 0.014 0,010 0.03 0.60 6.2 14 0.005 0.004 0.02 0 .31 4.2 16 0 0.004 0.02 0.28 7.5 20 0 0.010 0.03 0.53 7.0 22 0 0.006 0 0 .41 9.9 24 0 0.006 0 .01 0.49 5.0 25 0 0.005 0 0.35 0.9

a For the 8 of 2 1 samples highest in estimated terpene reaction products.

figure only. The C2-Cg compounds are suspected to be anthropogenic; their concentrations were highest when the wind was from the Abastumani village in the valley to the east of the observatory. When the wind came from the unoccupied forests to the west, the Cz-Cg compound con- centrations often dropped below 20 ppbC. The observation that average C2-Cg concentrations are higher above the pine canopy than at the experimental platform may be due to the very small number of measurements taken above the canopy but may also reflect the trajectory of upslope flow from the valley up the very steep mountainside. Note that the identification of the compounds referred to as C2-C, was based only on the similarity of their retention times to those in standard 1; in this sense they would, perhaps, be more properly identified as “Cz-like”, etc.

The estimated TNHC concentrations based on mea- surements of isoprene, a-pinene, and other terpenes ranged from nearly 0 to almost 200 ppbC. The maximum values were observed during relatively warm, sunny days. Con- centrations above the tree canopy were lower than those from subsequent samples collected at the observatory.

(B) Organic Compounds in Aerosol Particles. Non-Terpene Compounds. In general, the samples collected contained little organic material. Very small amounts of hydrocarbons, hexanedioic acid, pentanedioic acid, and phthalates were found in each sample. Of these, the phthalates were of greatest abundance, but in no case did they exceed our estimate of terpene oxidation products. By far the most abundant material found in every sample was sulfate. Ambient concentrations of sulfate based on mass spectrometer measurements for the eight samples with highest hydrocarbon concentrations are given in Table V. Those sulfate concentrations above 2 rcg/m3 agree to within 25% of the independent measurements of Dzubay (13), who used dichotomous aerosol samplers and X-ray fluorescence analysis.

Monoterpenes. The isomeric monoterpenes have sim- ilar mass spectra. These compounds are fairly volatile and hard to introduce into the spectrometer without loss of material. The monoterpene parent ions appear with low intensity in electron impact spectra. The C7Hg+ ion is the most abundant ion from the pinenes, A3-carene, and ter- pinolene and is the second most abundant ion from li- monene. Unfortunately, the low molecular weight of the C7Hg+ ion makes it an ambiguous identifier for terpenes. The C&,+ is only about 20% of the intensity of the C7Hg+ ion but is more specific for terpenes. Columns 2 and 3 of Table V show estimates of total monoterpenes based on intensities of these two ions and sensitivity factors from the literature.

” ZJ prnonoldehyde norpinonoidehyde pinonic acid norpinonic acid

HOOC .OK%

HOOC norp in ic acid

(a-pin en e I

HOOC ’9 pinic ocid

i I1

( l i m o n e n e )

0 eo I V

I I i ( t e r p i n o l e n e l

Figure 1. Terpene-ozone reaction products: X, Y, and Z represent functional groups (from ref 4 and 13).

In interpreting Table V, notice that 0.005 pg/m3 of particulate carbon corresponds to 0.008 ppbC. This is 3-4 orders of magnitude lower than the range of values given in Tables I11 and IV for TNHC’s in the gas phase. Therefore, it appears that direct adsorption or dissolution of terpenes or terpene polymers in particles is not likely to be an important sink for terpenes. In fact, the results in Table V are upper limits for terpene adsorption because it is possible that the identifier ions were produced from non-terpene material or that terpenes were adsorbed di- rectly from the gas phase onto the aerosol filters.

Oxidized Monoterpenes. The mass spectra of com- pounds formed by the reaction of monoterpenes with ozone (and sometimes nitrogen oxides) in closed laboratory containers have been reported. Schwartz (14) identified six compounds produced for a-pinene; these are shown in Figure 1 where Z is either CHO or COOH. Schuetzle and Rasmussen (4) reported similar results with limonene and terpinolene. They observed oxidation of the methylene group of limonene and the isopropylidene group of ter- pinolene in addition to cleavage of the ring double bond. The result was a series of compounds of the structures shown in Figure 1, where Y was either CHz or 0 and X was CHO, CHzOH, COOH, or COOOH. For limonene, in the presence of NO,, X could also be a variety of nitrogen- containing acyl groups.

Because of the complex and often ambiguous nature of mass spectra, our identification scheme is described in detail. The best identifier ions for terpene ozidation products are the parent ions of the above species; unfor- tunately, they are of low intensity. For example, the parent ion of pinonic acid is only about 4% of the intensity of the most abundant fragment ion. No parent ions of any of the above compounds were found in the samples studied here, although they have been detected in previous work (5).

Because of the failure to detect parent ions, the more abundant fragment ions containing oxygen and possibly nitrogen were chosen for use. Small fragment ions can be formed from a variety of nonterpene compounds and would normally be of little value. However, in the samples studied here, the amounts of organic compounds, partic- ularly oxygen-containing species, were found to be quite low, thus reducing the chance of misidentification and permitting an upper limit to be set on the amounts of terpenes present. A list of these 51 most abundant oxy- gen-containing fragment ions to be expected from terpene products was prepared by using the data of Schuetzle and

392 Envlron. Sci. Technol., Vol. 17, No. 7, 1983

Table VI. Relative Abundances of Oxygen-Containing Ions Possibly Originating from Terpene-Ozone Reactions

date (July) sample 9 10 14 16 20 22 24 25

C,H,O,+ 1 .1 0 0.7 0 2.0 0 1.5 1.1 C,H,O,+ 1 .4 3.3 1 .3 1.0 3.2 2 .2 1.0 0 C,H,O,+ 6.5 11.9 4.7 7 .1 10.4 9.1 7 .2 6.7 C,H,O,+ 8.6 9.7 4.7 8 . 2 11.7 5.7 11.7 6.0 C,H,O,: 4.8 10.6 4.6 4.1 8 .9 10.0 8.1 7.1 C,H,O, 3.2 0 1.9 0 1.7 0 3.1 1.8 C,H,,O+ 0 3.1 1.4 1 .1 0 0 3.2 0.8 C,H,,O+ 0.8 2.1 1 .0 0 0 0 0 0.9 C,H,O+ 5.2 6.2 2.5 2.3 3.9 3.0 4.5 2.1 C,H,,O* 0 3.0 1.7 2.0 2 .3 0 1.2 0 C,H,O+ 3.1 6.5 2.5 4.2 5.0 4.2 3.7 1.0 C,H,O+ 2.0 4.8 1 .8 1 .2 3.4 0 1.5 1.0 C,H,O+ 8.0 11.6 4 .1 5 .5 9.0 9.3 9.5 7 .6

Rasmussen, the known spectra of pinonic acid and pino- naldehyde, and the expected fragmentation patterns of other likely compounds. Of course, many of these ions are not specific to one terpene; for example, the major ions expected from oxidized A3-carene are generally the same as those expected from other terpenes, although known spectra are not available. Of the 51 expected ions, 26 appeared in a t least one sample and 13 appeared in six of the samples. Most of these ions were close to the limit of detectability, and no fragments containing nitrogen were found.

The principal oxygen-containing non-terpene com- pounds found in the aerosols were pentan-dioic acid and hexanedioic acid. By use of the known spectra of these compounds observed in our laboratory, the spectra from the aerosol particles were corrected to eliminate their contributions. Two ions (C&g02+ and C4H702+) could be attributed solely to the diacids and were not considered further. Corrections to the other ions were usually zero and never exceeded 10% of the observed abundance. Two oxygen-containing ions (C5H702' and C5H70+) occurred in all 21 aerosol particle samples taken. Thirteen ions occurred in a t least six samples. The relative abundances of these 13 ions are shown in Table VI for the eight sam- ples having the highest filter loadings.

All the ions in Table VI have plausible origins in terp- ene-ozone reactions. The six dioxygen ions correspond to fragmentation of the limonene daughter structures I and I1 (Figure l), where Y = 0 and X = CHO; the fragmen- tation is shown in Figure 2. Further, these six fragments are the most intense fragments reported by Schuetzel and Rasmussen (4) for limonene. The aldehyde fragments can, in addition to the structures shown above, lose an addi- tional hydrogen atom to produce an ion of similar intensity. Small amounts of the carboxylic acids analogous to the above three reactions were found in a few samples. Fragments from the cleavages reported by Schuetzle and Rasmussen and shown in Figure 2 were found in a few samples.

Table VI also indicates fragmentation of the limonene daughter structures I and I1 in Figure 1, where Y is CH2 and X is CHO. The C6HgO+ ion corresponds to reaction A (Figure 2), C5H70+ to reaction B, and C7H11O+ and C7H,oO+ to reaction C. The products of reaction C are of lower abundance. These observations are in agreement with the findings of Schuetzle and Rasmussen. The C5HgO+ ion corresponds to reaction B, where Y is CH2 and X is CH20H rather than CHO.

Table VI contains some evidence for a-pinene oxidation. The C5H70+ ion is the most abundant ion (base peak) in the pinonic acid mass spectrum; however, it is also formed

" " " OHC OHC

Flgure 2. Fragmentation reactions of limonene daughter structures I and I 1 shown in Flgure 1.

from limonene as discussed above. The C6HloO+ ion ap- pears in most of the samples in Table VI, is the second most abundant ion in the spectrum of pinonic acid (47% of base peak), and has not been reported in the mass spectrum of the other terpene derivatives. The C7H90+ ion also occurs in the pinonic acid mass spectrum, but at lower intensity (11% of base peak), and thus formation from pinonic acid cannot account for the large relative abundances of this ion as reported in Table VI. Other sources for this ion in terpene derivatives have not been reported.

From the above, it appears that ozone-limonene reaction products were indeed present in the aerosol particles in low and variable amounts, with aldehyde predominating and lesser amounts of carboxylic acid and alcohol. These observations are in agreement with the experiments of Schuetzle and Rasmussen (4). There is also some evidence of the presence of pinonic acid or pinonaldehyde. It is possible that other oxidized terpenes were present; for example, the products of A3-carene would be expected to be very similar to those of cy-pinene. No evidence of ni- trogen-containing terpene products was found.

Estimation of Amounts of Terpene-Ozone Reaction Products. Because of the failure to detect parent ions, it was impossible to calculate actual concentration of terpene products. Upper limits on the amounts of oxidized terpenes present in the aerosol particle samples, however, were calculated. We assumed, for example, that the C6H1oOf ion comes entirely from oxidized pinenes (pinonic acid or pinonaldehyde), used sensitivities determined in this laboratory from known compounds, and calculated the amounts of oxidized pinene shown in column 4 of Table V.

With the exception of the C7H90+ ion, which is of un- known origin, the C6HloO' ion, which is assigned to pinene, and the C5H70+ ion, which may come from pinene or li- monene, the remaining 10 ions shown in Table VI can all be attributed to oxidized limonene compounds. The in- tensity of the C5H70+ ion can be corrected for contribu- tions from pinene by using our estimated upper limits for oxidized pinene. An additional 11 oxygen-containing ions, all attributable to limonene, were observed in a few sam- ples a t low intensities. The sums of the ion abundances over these 22 ions were taken as estimates of total limonene compounds present. Sensitivity factors for these ions are not known but should be fairly high, because other more abundant ions were not found. Assuming a rather low sensitivity factor of 0.10 vs. p-terphenyl (half that of the most intense ion from pinonic acid) and assuming that all the 22 ions originated entirely from limonene compounds,

Environ. Scl. Technol., Vol. 17, No. 7, 1983 393

we calculated upper bounds for limonene products as shown in column 5 of Table V. Use of higher sensitivity factors would have produced lower concentrations. Of course, if multiple ions had resulted from a single com- pound, the sensitivity factor should have been decreased; in that case, however, only one ion, and not the sum, should have been used in calculating concentrations, and our method is not subject to large errors from multiple fragmentation.

Discussion Arnts and Meeks (15) have recently published a com-

pilation of results showing total hydrocarbon concentra- tions in the range 10-100 ppbC in a variety of rural areas. Arnts and Meeks also report their measurements made during a study of the aerosol in the Great Smoky Moun- tains National Park (7). They measured about 100 ppbC of gas-phase hydrocarbons, of which only 1-6% was veg- etative. Our measurements in Abastumani show about the same total amount of gas-phase hydrocarbon; however, the average vegetative contribution was about 40%.

Weather during the period reported in this paper was unsettled-four cold fronts passed through the site during July 10-27-consequently, our samples were collected under a wide variety of conditions. Because of the limited number of ambient gaseous hydrocarbon measurements, we have reported mean concentration values for the entire period rather than dividing the data into smaller groups according to temperature, etc. The concentrations of vegetative hydrocarbons do, however, show a temporal trend. Beginning a t 2400 and dividing the day into four quarters, we observed that mean concentrations are highest during the fourth quarter, intermediate during the third, and lowest during the second (no measurements were made in the first quarter). Mean concentrations in the fourth quarter are twice those in the second quarter. These ob- servations suggest that our estimates of vegetative hy- drocarbon mean concentrations, which are based on short-term samples, are not strongly biased because most of the measurements were taken during the third quarter and because the differences from quarter to quarter are not large. We believe, therefore, that effects of the dif- ferences in our sampling times for gases and aerosols do not affect the conclusions of this paper.

Measurements in Abastumani of other atmospheric species, halocarbons (16), sulfur dioxide, nitrogen oxides, carbon monoxide, and aerosols, will be reported in detail elsewhere. All these measurements indicated that the levels of all these species were much lower in Abastumani than in the Great Smoky Mountains (7). Ozone concen- trations in Abastumani ranged from 20 to 50 ppb (13). The aerosols collected in both locations, however, have sulfate as their most abundant constituent. Concentrations of sulfur dioxide gas (1 7) were correlated with particle sulfate to a significance level of 99% but were a factor of 40 lower than sulfate throughout the experiment. This observation suggests that the sulfate is not of local origin.

Other measurements from samples collected during the study at Abastumani are consistent with our observations that the aerosol particles contained only small amounts of carbon compounds and that most of these compounds were terpene reaction products. Measurements of particle scattering and absorption coefficients by Weiss et al. (18) show that the mean ratio of scatter to extinction was 0.89. This relatively high value shows that the graphitic carbon associated with anthropogenic activity is present only in small amounts in the Abastumani aerosol, which resembles that found in other remote areas. Radiocarbon analyses of aerosol particles by Currie and Klouda (19) show that,

394 Environ. Sci. Technol., Vol. 17, No. 7, 1983

of the total particulate carbon, modern carbon ranged from 53% to97%.

Our observations of terpenes from pine are consistent with those of Zimmerman (20), who reports emissions of a- and P-pinene, limonene, A3-carene, and myrcene from pine. Squillace and Wells (21) have found these and camphene in pine cortical oleoresin. Our observations of very low percentage isoprene from Pinus sylvestris and high percentage isoprene from Picea orientalis are con- sistent with a compilation by Rasmussen (22) of species grouped into emitters and nonemitters of isoprene.

We now consider the significance of direct measurements of emissions from a few trees in a forest. Measurements of the relative amounts of terpenes in pine oleoresin (21, 23,24) show that very large differences can occur among trees of the same species in different geographical areas. (For a cross-referenced index to the literature of mono- terpene composition of conifers see Squillace (24)). Squillace and Wells (21) report differences of a factor of 5 in percent a-pinene and factors of 20 or more in percent /%pinene, myrcene, and limonene for loblolly pine from five states in the Southeastern United States. Significant differences in ratios of monoterpenes exist among indi- vidual trees of the same species, among different tissues from a given tree, and at different tree heights. The largest differences, however, are associated with geographical distribution. In fact, Gansel and Squillace (23) suggest that terpene composition can be used to determine geographic origin of seed. For these reasons, our measurements of gas-phase hydrocarbons from a few trees were assumed to be, in an approximate sense, characteristic of the forest emissions in the geographical area of the field study.

The estimates of terpene compounds shown in Table V should not be taken as evidence that more limonene com- pounds than pinene compounds were actually present in the aerosols. It is likely that the mass spectra contained contributions from other terpenes for which the ozone reaction products and their mass spectra are unknown. Products from A+-carene should be similar to those from a-pinene. The C7H90+ ion appearing in fair abundance in most samples could arise from the oxidation of the propylidene group in myrcene, followed by the copmonly observed loss of a hydrogen from the aldehyde on ioniza- tion. If these other terpenes did form ozone reaction products that contributed to the mass spectra, the results in Table V would not represent limonene and a-pinene alone; nevertheless, these values should be approximate estimates of the upper limits of the total oxidized terpene products.

The ambiguity of our identification of limonene products should be emphasized because a-pinene was usually identified in gas-phase measurements and limonene was not and because our crude terpene emission measurements indicated that the ratio of emissions of a-pinene to li- monene was 10:l. These results appear to contradict the aerosol particle analysis, which indicates a ratio of a-pinene products to limonene products of about 1:20. A part of this discrepancy may be explained by the faster reaction rate of limonene with ozone; the ratio of the rate constant of a-pinene to that of limonene is 1:5 (2). It may be, however, that some of the compounds that were tentatively identified as limonene products are, in fact, products of myrcene or A3-carene, both of which were emitted at higher rates than limonene.

An additional complication in the terpene product analysis may have occurred due to the reaction of terpenes with OH radicals. Using the rate constants compiled by Arnts and Gay (2) and assuming that in Abastumani [OH]

= lo6 molecules cm3, one finds that the rates of reaction of terpenes with ozone and OH radicals may have been comparable. The products that may result from terp- ene-OH radical reactions are unknown; however, it seems likely that the typical ozone addition to double bonds and the OH radical addition or abstraction of primary hydro- gens will lead to different initial product compounds.

Conclusions Although we cannot make strict identifications of the

parent compounds of hydrocarbons found in the aerosol particles, our estimated upper limits on terpene-derived compounds are only about 1 % of the estimated vegetative hydrocarbons measured in the gas phase. There is evi- dence that terpene-ozone reactions occurred and that the aerosol particles contained small amounts of these prod- ucts. The amounts of particle hydrocarbons are far too small to account for significant removal of terpenes from the air, and these amounts are not a large portion of the measured aerosol particle mass. The particles sampled contained very small amounts of free monoterpenes. Direct adsorption or dissolution of terpenes in aerosol particles did not play a significant role in their removal from the air.

It may be that terpenes were rapidly degraded to small molecules that were not detected. In order to determine the fate of gas-phase terpenes, additional measurements of small fragment products, e.g., carbon monoxide and formaldehyde, may be necessary. Our results show that, in a relatively clean, natural area, with significant amounts of gas-phase terpenes and ozone, the contribution of veg- etative hydrocarbons to aerosol concentration, and con- sequently to visibility degradation, was small.

The fine fraction aerosol particles consisted mostly of sulfate. A detailed analysis of measured atmospheric scattering coefficients and inorganic constitutents of the aerosols will be published elsewhere (13). All data are consistent with the interpretation that the major part of the aerosol-sulfate-was transported into the experi- mental site from distant sources.

Acknowledgments

We thank M. Thornton of the University of Washington for assistance in performing mass spectral observations, W. Lonneman and A. Coleman of EPA for providing GC standards and advice, and R. Varco of Northrop Services for developing the GC instrumentation. We thank Y. Mateshvilli of the Abastumani Observatory for providing assistance and information during the experiments. We also thank W. Wilson of EPA for his aid in initial exper- imental planning and research support for university personnel. These experiments would not have been pos- sible without the cooperation of V. D. Stepanenko (head of the Soviet Delegation), G. B. Rosenberg (Chief Soviet Scientist), and E. K. Kharadze (Director of the Abastu- mani Observatory). We are indebted to H. Weiser (EPA,

Chairman of the U.S. Delegation on Air Pollution), who provided the planning and negotiation necessary to con- duct this field study.

Registry No. Isoprene, 78-79-5; a-pinene, 80-56-8; camphene, 79-92-5; &pinene, 127-91-3; myrcene, 123-35-3; As-carene, 13466-78-9; d-limonene, 5989-27-5.

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Received for review April 1,1982. Revised manuscript received November 22, 1982. Accepted February 22,1983.

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