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 ~~
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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 enviropment 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 insignificant 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 measurements 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 established 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 proximity 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 population may be systematically changing. Presently, pines are predominant on the summit of Mt. Kanobili at the U S . Environmental Protection Agency. *University of Washington. *Washington State University. A. I. Voeikov Main Geophysical Observatory. 0013-936X/83/0917-0389$01.50/0
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 production of aerosol particles (4);production of particles by OH radical reaction is suspected but has not been demonstrated. Several oxidized derivatives of a-pinene, e.g., pinonic acid, have been found in naturally occurring aerosol particles (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 Shenandoah 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 necessary. Conversely, Hull (11)has argued that under ambient conditions, terpenes react to form gas-phase products. To determine order-of-magnitude estimates of amounts of terpenes required to cause visible haze, we make a simple model calculation based on extinction coefficients for various aerosol distributions reported by Willeke and Brockman (12). First, we assume that an atmospberic concentration of 1ppb (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; consequently, 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
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 ambient aerosol particles measured at the Abastumani Observatory. Experimental Procedure (A) Gas Collections and Measurements. Nearly all samples were collected near a wooden experimental platform built from the rear of a small astronomical observatory 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 determine 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 detector 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 detedor gases were hydrogen (Airco, 99.999%) and air (Airco,
