Environ. Sci. Technol. 2000, 34, 2640-2648
Proton-Transfer Chemical-Ionization Mass Spectrometry Allows Real-Time Analysis of Volatile Organic Compounds Released from Cutting and Drying of Crops J O O S T A . D E G O U W * ,† A N D CARLETON J. HOWARD Aeronomy Laboratory, NOAA, Environmental Research Laboratories, Boulder, Colorado 80303 THOMAS G. CUSTER, BRADLY M. BAKER, AND RAY FALL† Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215
The wounding and drying of plant material during crop harvest could be a significant source of volatile organic compounds (VOCs) that enter the atmosphere. Here, we show that these primarily oxygenated VOCs can be measured using proton-transfer chemical-ionization mass spectrometry (PT-CIMS), a method that allows online and simultaneous monitoring of oxygenated VOC levels. For clover, alfalfa, and corn, leaf wounding and in particular drying were shown to lead to strongly enhanced emissions of a series of C6 aldehydes, alcohols, and esters derived from (Z)-3hexenal. Additionally, for the forage crops clover and alfalfa, enhanced emissions of methanol, acetaldehyde, acetone, and butanone were observed. The identities of the measured carbonyl compounds were confirmed using highpressure liquid chromatography. For clover, initial cutting led to a VOC release of about 175 µg of C (g dry wt)-1, while during drying the cut clover released >1000 µg of C (g dry wt)-1; qualitatively, similar amounts of VOCs were released from alfalfa, the major hay crop in the United States. The atmospheric implications of these findings may include effects on the local air quality in agricultural areas, contributions to long-range transport of pollutants, and effects on the formation of HOx (dOH + HO2) radicals in the upper troposphere.
Introduction The vegetation on the Earth is an important source of volatile organic compounds (VOCs) released to the atmosphere (1). These biogenic VOCs play an important role in the chemistry of the troposphere, as they are implicated in the formation of ozone and aerosols (2, 3). Ozone is an important oxidant in the troposphere and is the precursor of the hydroxyl radical (OH), which is chiefly responsible for the oxidation of hydrocarbons (4). Aerosols have direct and indirect effects on the Earth’s radiation budget: directly through the * Corresponding author present address: Institute for Marine and Atmospheric Research, University of Utrecht, Princetonplein 5, 3584 CC, Utrecht, The Netherlands; e-mail:
[email protected]. † Also at the Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309. 2640
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scattering and absorption of sunlight and indirectly through the formation of clouds (5, 6). Despite the importance of biogenic VOCs in atmospheric processes, the understanding of their sources is still incomplete. The emissions of isoprene and monoterpenes by forests are reasonably well understood (7). Much less is known however about the VOC emissions by grass and croplands, which have been shown to emit mostly oxygenated VOCs (8, 9). Leaf wounding is one of the mechanisms for VOC exit from a plant into the atmosphere (1). In response to the wounding of its tissue, most plants produce a series of C6 aldehydes, alcohols, and their derivatives arising from the enzymatic peroxidation of the fatty acids, linoleic acid, and linolenic acid (10). These C6 compounds may serve as antibiotics, inhibiting invasion of leaf microorganisms into the wound site(s). Leaf wound compounds are sufficiently volatile so that they are commonly observed in the atmosphere. For instance, Arey et al. measured the VOC emissions of 29 common crops and found the leaf wound compounds (Z)-3-hexenol and (Z)-3-hexenyl acetate among the major species (8). Kirstine et al. measured VOC emissions from pasture grass and clover and found the emissions of hexenyltype compounds to increase by several orders of magnitude when the vegetation was cut (9). Fall et al. (11) have shown that the emission of (Z)-3-hexenal from wounded aspen, beech, and clover leaves occurs within a few seconds after wounding but does not come from a preexisting pool; this suggests that hexenyl-type compounds are formed in response to wounding and that during the metabolism of (Z)3-hexenal to hexenols and hexenyl acetates these compounds can partition to the air surrounding the leaf. Our quantitative and even qualitative understanding of these emissions is still limited, partly as a result of the analytical difficulties in measuring oxygenated VOCs. Most current methods for analysis of VOCs in air require careful consideration of methods of preconcentration and chromatography (12). This is especially true for oxygenated VOCs such as methanol and acetone, which readily partition with water in sampling devices, and hexenals, which are relatively unstable in air or when preconcentrated with ozone. In addition, analysis of leaf wound VOCs such as hexenols and hexenyl acetates by gas chromatography methods typically require 1 h per sample (9, 13), seriously limiting the number of samples that can be analyzed during leaf wounding experiments. As described here, a new method for oxygenated VOC analysis using proton-transfer chemical-ionization mass spectrometry (PT-CIMS) avoids many of these analytical problems and allows real-time VOC analysis. Given the above findings, we have been interested in quantifying the release of wound compounds that arise during crop harvesting. Previously, we explored the use of PT-CIMS to measure leaf wound VOCs (14). PT-CIMS methods allow simultaneous measurements of many atmospheric VOCs (15), and since this type of instrument can be operated online, we were able to continuously measure VOC release that occurs immediately after cutting vegetation and a second, more intense and longer lasting release during the drying of excised leaves (14). In this paper, the PT-CIMS instrument used in this previous work is described in detail. New results are presented for the VOCs released from clover, alfalfa, and corn plants after leaf wounding and during the drying of cut crops. Some atmospheric implications of the present findings are briefly discussed. 10.1021/es991219k CCC: $19.00
2000 American Chemical Society Published on Web 05/13/2000
Experimental Section In the experiments, a flux chamber was used, which has been described elsewhere (16). The chamber was cubical, measured 30 cm on each side, and had an open bottom so that it could be placed over a plant or plants grown in plastic trays (40 × 56 × 14 cm). Three of the walls of the flux chamber consisted of a Teflon film; the two other walls were made of a polyethylene glovebag to facilitate cutting the plant without having to open the chamber. This setup avoids exposing the inside volume to laboratory air, which can contain large concentrations of VOCs such as methanol and acetone. Breathing air (General Air, Boulder, CO), low in VOCs, was passed through the flux chamber at a rate of 0.5-2.0 L min-1. Plants used in this work were grown in a greenhouse under well-defined conditions (17). The seeds were obtained from the following sources: Dutch white clover (Trifolium repens) from Arkansas Valley Seeds (Denver, CO); alfalfa (Medicago sativa) and dwarf corn (Zea mays d5/d5) from Carolina Biological Supply (Burlington, NC). Dwarf corn was used to provide small enough mature plants to fit inside the flux chamber. The corn was grown through 1 cm diameter holes in a PFA Teflon sheet to prevent VOC emissions from the underlying soil to interfere with the measurements. In some experiments, several leaves were cut off the plants, placed on a layer of Teflon film, and covered by the flux chamber to study the VOC emissions by drying plant material. An 80-W light bulb was placed immediately over the flux chamber to aid in drying the leaves. The VOC concentrations in the flux chamber were measured online using PT-CIMS. In PT-CIMS, the organic trace gases, R, in air are ionized by undergoing a protontransfer reaction with H3O+ ions in an ion drift tube:
H3O+ + R f RH+ + H2O
(1)
The primary and product ions are extracted from the drift tube and mass analyzed with a quadrupole mass spectrometer. Reaction 1 is exothermic and fast for those compounds that have a proton affinity higher than the proton affinity of water (166.5 kcal mol-1). This includes most of the atmospheric oxygenated VOCs (18). Chemical-ionization mass spectrometry has been used in the NOAA Aeronomy Laboratory for many years (19, 20). The new PT-CIMS apparatus has been extended with an ion drift tube, similar to the instrument described by Lindinger et al. (15). In the drift tube, an electric field is applied to increase the ion kinetic energy and inhibit the formation of cluster ions. A schematic diagram of the instrument is given in Figure 1. About 6 STP cm3 s-1 (STP ) 273 K and 1 atm) of sample air is pumped through an ion drift tube by a mechanical pump (1000 L min-1). The pressure in the drift tube ranges from 1 to 2.0 Torr. The ion drift tube consists of a stack of 50 stainless steel rings (0.8 cm × 3.8 cm o.d.) mounted inside a 70 cm × 5.1 cm o.d. glass tube. The rings, one ring per centimeter, are fastened on three strips of fiberglass electronic printed circuit board. Forty-nine chip resistors (1 MΩ per resistor) are soldered along one strip of the board ,and a DC voltage is applied over the stack of drift rings in order to produce a homogeneous electric field of around 35-70 V cm-1 inside the drift tube. At these fields and pressures, H3O+(H2O)n cluster ions are not stable and fragment into H3O+ ions, which dominate the mass spectrum. Ions are produced in a 25-cm side arm of the drift tube, which is similar in design to the main drift tube. Water vapor, leaked in from a glass bottle containing distilled water at room temperature, is diluted in 4.5 STP (cm of He)3 s-1 and passed through the side arm. Electrons from a resistively heated filament ionize the gas mixture. The electrons are accelerated toward a grid that is placed a few millimeters
FIGURE 1. Schematic diagram of the proton-transfer chemicalionization mass spectrometer. Air is pumped through a drift tube by a mechanical pump. H3O+ ions are produced in the side arm by electron-impact ionization of a He/H2O mixture. Organic trace gases are ionized in the drift tube by proton-transfer reaction with H3O+, mass selected with a quadrupole mass spectrometer (QMS) and detected with a conversion dynode electron multiplier (CDEM). downstream of the filament. A voltage difference of around 20 V was found to yield the highest H3O+ count rates. To stabilize the ion count rate, the electron emission current to the grid is measured and kept at a constant value of 10-15 µA by a feedback circuit controlling the current through the filament. Ion-molecule reactions downstream of the filament result in the production of mainly H3O+ ions. An electric field in the side arm prevents ion clustering and increases the ion yield by reducing the ion residence time in the side arm and thus the loss of ions due to diffusion and collisions with the wall. The He flow and H3O+ ions from the side arm are merged with the flow of air to be analyzed in the main drift tube. Proton-transfer reactions between the H3O+ ions and the trace gases R (reaction 1) take place over the entire length (50 cm) of the drift tube and convert a small fraction of the reagent H3O+ ions into product ions RH+. The number of product ions formed can be calculated from
∆[RH+] ) -∆[H3O+] ) k[H3O+]t[R]∆t
(2)
where k is the rate coefficient for the proton-transfer reaction 1 and ∆t is the reaction time. If only a small fraction of H3O+ ions reacts (-∆[H3O+] , [H3O+]t), then [H3O+]t can be assumed constant, and the number ∆[RH+] of product ions formed is proportional to the neutral concentration [R]. Reagent and product ions are extracted from the drift tube through an orifice, focused by a set of electrostatic lenses, and mass analyzed with a quadrupole mass filter. This part of the setup has been described by Huey et al. (21). The count rate of H3O+ ions in the present setup is typically between 5 × 105 and 1 × 106 cps. The response of the PT-CIMS with respect to the VOC concentration was calibrated using a standard gas mixture (Scott Specialty Gases, Longmont, CO). The composition of the mixture is given in Table 1. Of the eight compounds, only n-pentane cannot be measured with PT-CIMS, because its proton affinity is less than that of water. The concentrations of the VOCs contained in the cylinder were determined by gas chromatography (GC). The mixture was diluted using a system of several mass flow controllers (Tylan, San Diego, CA; Unit Instruments, Yorba Linda, CA) to precisely mix the standard gas with hydrocarbon-free air and produce a gas stream with VOC concentrations on the order of several parts per billion by volume. One liter of the diluted standard gas was sampled onto a glass bead trap cooled to liquid nitrogen temperature and then rapidly heated to inject the sample onto a 100-m DB-1 capillary column (J&W Scientific, Folsom, VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Composition of Standard Gas Mixture Used To Calibrate the PT-CIMS Instrumenta
compd
concn (ppmv)
product ion mass (amu)
n-pentane acetaldehyde acetone methacrolein methanol benzene 2-methyl-3-buten-2-ol
3.15 6.49 4.78 2.84 14.37 3.34 2.84
none 45 59 71 33 79 69 (81%); 87 (19%) 137 (24%); 81 (76%)
R-pinene
1.86
calibration factor (cps ppbv-1) 55 78 36 15 18 18 18
a The third column gives the masses at which the product ions are found, and the fourth column gives the calibration factors.
CA). The column was also cryo-cooled to -50 °C and then temperature ramped at a rate of 8 °C/min to 150 °C in a Hewlett-Packard (Palo Alto, CA) 5890 series II GC. The detector was a Hewlett-Packard 5921A atomic emission detector (AED) set to 496 nm, which is an emission line for the atomic carbon spectrum. This detector is ideal for calibrations due to the fact that its response is directly proportional to the number of carbon atoms a molecule contains and is not affected by the molecular environment of the carbon atom as is the case with other commonly used GC detectors (22). To calibrate the AED, a primary butane/ benzene standard was used (Scott-Marin, San Bernadino, CA) for which the concentration had already been wellestablished through intercomparisons with primary standard gases. A DB-1 column is not ideal for analyzing methanol and acetaldehyde due to extensive peak tailing. For these two compounds, a DB-624 (J&W Scientific, Folsom, CA) column was used with a flame ionization detector calibrated using primary gravimetric methanol and acetaldehyde standards. To determine the response of the PT-CIMS with respect to the VOC concentration, the standard gas mixture was diluted in the same manner as described above. Different concentrations were selected in a stepwise manner to test the linearity of the instrument’s response. The response of the PT-CIMS is shown in Figure 2, which shows the product ion count rates vs the VOC concentration. The response of the instrument was linear in the VOC concentration range from about 100 pptv to 30 ppbv. The calibration factors are defined as the number of product ions observed at a concentration of 1 ppbv, at a (typical) H3O+ count rate of 5 × 105 cps. The calibration factors are given in Table 1 and are in reasonable agreement with the values expected based on the residence time of the H3O+ ions in the drift tube and the rate coefficients for the proton-transfer reactions (eq 2). The different slopes for the compounds in Figure 2 are due to the difference in proton-transfer rate coefficients and also to the mass dependence of the ion detection efficiency. Measurements were done to determine at which masses the leaf wound compounds can be detected. Table 2 gives the results. For relatively small molecules, the proton-transfer reaction with H3O+ is usually not accompanied by further fragmentation of the product ions (Table 1). However, it is clear from Table 2 that the leaf wound compounds are, in most cases, not simply detected at their respective protonated masses. Protonated higher alcohols and aldehydes are known to fragment by giving up a water molecule (23). Indeed, (Z)3-hexenal is detected preferentially at mass 81, and the two isomers of hexenol are detected mostly at mass 83. (E)-2Hexenyl acetate fragments even more and is also mostly detected at mass 83. 2642
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FIGURE 2. Product ion signal measured for a number of compounds as a function of the volume mixing ratio. The details for independent calibration of this standard VOC mixture are presented in the Experimental Section.
TABLE 2. Masses of Ions Produced in Proton-Transfer Reactions between Several Leaf Wound Compounds and H3O+ Ions in the PT-CIMS Instrument compd (Z)-3-hexenal (E)-2-hexenal (Z)-3-hexenol (E)-2-hexenol (E)-2-hexenyl acetate n-hexanol
mass (amu)
product ion masses (amu)
98 98 100 100 142
81 (58%); 99 (41%); 71 (1%) 99 (70%); 81 (6%); 57 (23%) 83 (80%); 101 (4%); 59 (7%); 55 (9%) 83 (83%); 55 (17%) 83 (78%); 143 (1%); 55 (21%)
84
43 (70%); 85 (25%); 57 (4%)
Verification of carbonyl compound formation in these plant experiments was carried out using a chemical derivatization technique developed by Skaggs (24). Briefly, the technique involves reacting carbonyl compounds with 2,4dinitrophenyl hydrazine (DNPH) on the surface of controlledpore glass beads placed inside a modified guard-column cartridge; the resulting hydrazones are then transferred directly onto a reversed-phase HPLC (high-pressure liquid chromatography) column, separated, and detected. In these experiments, sample air containing unknown or standard carbonyls was passed through a cartridge for 10-to 20 min (total volumes of 400-1000 STP cm3), and the resulting hydrazones were separated using a gradient elution program and detected using an UV detector at 360 nm. A valve system connected to the PT-CIMS sampling line allowed cartridges to be added and removed for sampling while still monitoring flux chamber air using the PT-CIMS instrument.
Results and Discussion Wound VOCs from Clover. In Figure 3, the results of an experiment are shown in which the flux chamber was placed over clover plants, which were cut at t ≈ 60 min. Immediately after cutting, the production of positive ions at several different masses increased significantly. As described by Hatanaka (10), leaf wounding leads to the formation of (Z)3-hexenal by peroxidation of R-linolenic acid. (Z)-3-Hexenal can isomerize to form (E)-2-hexenal and can be converted into (Z)-3-hexenol by an alcohol dehydrogenase and sub-
FIGURE 3. VOC emissions from a patch of clover in the flux chamber after wounding (at t ≈ 60 min) as measured with the PT-CIMS instrument. sequently into (Z)-3-hexenyl acetate by an acetyltransferase. In accordance with this model, the measurements in Figure 3 show a strong increase immediately after cutting of the product ion signals at masses 81 and 99 amu, corresponding to a combination of (Z)-3-hexenal and (E)-2-hexenal. The signal at 83 amu rose more slowly after cutting, as expected for hexenol and hexenyl acetate, which can be detected at mass 83 amu (Table 2). No significant increase was observed at mass 143 amu, which corresponds to hexenyl acetate (but for only 1%, see Table 2). A minor response to leaf wounding is the release of n-hexanal by peroxidation of linoleic acid, followed by the formation of hexanol and hexyl acetate (10). Hexanol can be
detected with the PT-CIMS at 85 and 43 amu. The ion at 43 amu (CH3CO+ or C3H7+) is a common fragment ion for many compounds and not a unique indicator for hexanol. Hexanal is expected to be detected at mass 101 and 83 amu, and hexyl acetate is detected at 145 and 85 amu. The measurements in Figure 3 show only a small increase in the signal at mass 85 amu after leaf wounding, possibly due to the formation of hexanol or hexyl acetate, but no significant response was observed at masses 101 and 145 amu. Other product ions that were seen to respond to the leaf damage were at 69 and 71 amu, possibly due to C5 leaf wound compounds such as pentenol (69 amu) and pentanol (71 amu), but this assignment is uncertain. Recent work has indicated that the occurrence of an ion at mass 85 amu may also be related to formation of another C5 wound VOC, 1-penten-3-one (25). Figure 3 shows that the ion signals at 33, 45, 59, and 73 amu were enhanced by up to 1 order of magnitude as a result of leaf wounding. These signals are attributed to methanol, acetaldehyde, acetone, and butanone, respectively. These observations agree with the findings of Kirstine et al. (9), who studied VOC emissions from pasture grasses and found methanol, acetaldehyde, acetone, and butanone to be among the major compounds emitted by damaged clover in addition to (Z)-3-hexenal, (Z)-3-hexenol, and (E)-2-hexenal. A likely source of methanol in leaves is from pectin demethylation in cell walls (26). Young leaves, in particular, are known to emit significant quantities of methanol to the atmosphere (17). The wound-induced release of methanol, as observed in the present experiment, suggests that pools of methanol exist in the stem or transpiration stream of the plant, which can be released to the atmosphere by evaporation from the surface film that forms around the wound site. No biological explanation exists at present for the woundinduced release of acetaldehyde, acetone, and butanone by clover. The data in Figure 3 can be used to estimate the quantity of VOCs released by plants to the atmosphere as a result of leaf wounding. In the case of clover, this has been done for hexenal, methanol, acetaldehyde, acetone, and butanone. The count rate in Figure 3 can be converted into a concentration using the calibration factors for the PT-CIMS determined with the calibrated gas mixture. The calibration factors for methanol, acetaldehyde, and acetone are given in Table 1. For butanone, we used 36 cps ppbv-1, i.e., the same value as for methacrolein, which is detected at 71 amu; for hexenal, we used 18 cps ppbv-1 (from benzene, detected at 79 amu). The VOC concentrations as a result of the leaf wounding were integrated after subtracting the background concentration, determined as the concentration before the leaves were cut. The dry weight of the cutoff leaves was measured to be 3.67 g in this experiment. Using this value, we arrive at values of 60 µg of C gdw-1 (gram dry wt) of hexenal, 90 µg of C gdw-1 of methanol, 1.5 µg of C gdw-1 of acetaldehyde, 10 µg of C gdw-1 of acetone, and 14 µg of C gdw-1 of butanone, adding up to approximately 175 µg of C gdw-1. As the wound-induced VOC release lasted between 1 and 2 h, these values are in good agreement with the values found by Kirstine et al. (9), who measured the VOC release from clover to be between 33 and 830 µg of C gdw-1 h-1. The uncertainty in these values is high because of the different manners in which the leaves are wounded and from uncertainties in the recovery of oxygenated VOCs from the chamber (see below). Previously, we discovered that drying of cut vegetation leads to a large increase in VOC release (14). This effect can be seen in Figure 4, which shows the results of an experiment in which the VOC emissions by drying clover leaves were measured. Clover leaves were cut, and only the leaves were put in the flux chamber at t ≈ 50 min. An 80-W light bulb was placed over the flux chamber to aid in drying the leaves. VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. VOC emissions from freshly excised clover during the drying process as measured with the PT-CIMS instrument. As described in the text, a sample of clover stems and leaves was cut and immediately placed in the flux chamber for VOC analysis during subsequent drying. Two maxima in the VOC emissions were seen: one immediately after the leaf wounding and one a few hours later when the leaves were drying out. In particular, acetone and butanone were seen to be strongly increased during the drying phase. The integrated emissions as a result of drying were at least an order of magnitude higher than the VOC release as result of leaf wounding. We rationalize these results as follows. When a leaf dries out, all the cells deteriorate, whereas only a few are damaged in the case of leaf wounding. The more extensive deterioration associated with cell death and drying would lead to enhanced wound VOC release. 2644
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In the experiment with excised clover leaves only (Figure 4), a much higher signal at 143 amu (hexenyl acetate) was observed than in the experiment with a clover plant plus underlying soil (Figure 3). The only difference between the two experiments was the presence of the plant and the soil in the flux chamber. A possible explanation is that hexenyl acetate was taken up by the (humid) soil; uptake of VOCs on soil particles is a known process (27) and has been shown to be more efficient for polar compounds. Another explanation may be the uptake of hexenyl acetate by the plant surface, which is much larger in the live plant experiment than in the experiment with leaves only. Many VOCs are readily taken up by plant surfaces by partitioning into the waxy cuticle (28). Comparison between the two experiments shows that extreme care must be taken in extrapolating the results from the chamber studies to real atmospheric conditions. HPLC measurements were performed to confirm the identity of the carbonyls observed in the experiments with drying clover. DNPH-coated cartridges were used to sample air from the flux chamber at several different times after cutting. A typical set of chromatograms is shown in Figure 5. Formaldehyde, acetaldehyde, acetone, butanone, and hexenal compounds were clearly separated. (Z)-3- and (E)2-hexenal could not be distinguished from one another since pure standards of each compound yielded identical sets of peaks, possibly due to an interconversion on the solid packing material of the cartridges or due to impure standards. The combination of the two hexenal compounds could be separated from the other carbonyl compounds detected. In each chromatogram, the asymmetric carbonyls (except acetaldehyde) exhibited a double peak corresponding to a syn isomer and an anti isomer, as reported previously (29). Retention times for the major peaks in each plant sample matched those for standard compounds, confirming the identities of the carbonyls emitted by plant material. Figure 5 shows that peak areas for acetaldehyde, acetone, butanone, and hexenals grew above the blank signal immediately after cutting and then rose again as the leaves dry over time, just as observed in the PT-CIMS data. Both the HPLC and PTCIMS data show that hexenal emissions dominate after leaf wounding, whereas acetaldehyde, acetone, and butanone are the chief emissions during the drying phase. Further confirmation on the identity of the compounds observed could be obtained using PT-CIMS by analysis of the product ion isotopes. For example, detection of a signal at 59 amu (acetone) is accompanied by a parallel signal at 60 amu of 3.3% that of 59 amu, indicative of acetone containing natural abundance 13C. As described previously (14), the isotope ratios observed for the product ions corresponding to methanol, acetone and butanone from wounded clover indicate that these ions indeed contain 1, 3 and 4 carbon atoms, respectively. Wound VOCs from Alfalfa. Alfalfa is a major U.S. crop, planted in over 23 × 106 acres and subject to repeated cutting and drying in the field as a hay source (30). A patch of alfalfa was placed in the flux chamber, and several of the alfalfa branches were cut at t ≈ 200 min, leading to a strong burst of VOC emissions (Figure 6). Most of the damage was done to the stems rather than to the leaves. The emissions of leaf wound compounds at masses 81 and 99 amu (hexenal), 83 amu (hexenol and hexenyl acetate), and 143 amu (hexenyl acetate) were seen to be strong; qualitatively similar to the case of clover (Figure 3). The emissions of other oxygenated VOCs (acetaldehyde, acetone, and butanone) were much lower; however, only methanol was released in significant amounts after wounding. The methanol concentration in the flux chamber was still high in this experiment because of a previous experiment with the same alfalfa (of all the measured VOCs, methanol has the strongest “memory
FIGURE 5. HPLC chromatograms showing the carbonyl emissions of freshly cut clover during the drying process. As in the experiment described in Figure 4, a sample of clover stems and leaves was cut and immediately placed in the flux chamber for VOC analysis during subsequent drying, except that the air flow through the chamber was intermittently directed over DNPH-coated cartridges for carbonyl analysis. effects” in the chamber, possibly due to its relatively high solubility). Interestingly, product ions at mass 143 amu (hexenyl acetate) showed a clear increase in the case of alfalfa wounding (Figure 6) but were not observed in the case of clover (Figure 3). Figure 7 shows the result of an experiment in which alfalfa leaves and stems were removed immediately after cutting and then dried in the flux chamber. It was found that alfalfa is hard to dry out: it took about 500 min before drying alfalfa branches started to emit VOCs strongly, whereas this took only 250 min for clover (Figure 4). Qualitatively, the VOC emissions of drying alfalfa leaves were very similar to those of clover. Strong emissions of acetone, methanol, and acetaldehyde were observed during the drying of alfalfa, but butanone was much less pronounced. The emissions of hexenal were very similar to those for clover, whereas hexenyl acetate and mass 69 amu (possibly pentenol) seemed to be higher in the case of drying alfalfa. Wound VOCs from Corn. Since corn represents such a major U.S. crop species, we wanted to examine the effects of corn leaf wounding on VOC release. Figure 8 shows the results of a typical wounding experiment with corn. Four dwarf corn plants, grown through holes in a Teflon film, were placed inside the flux chamber. The use of dwarf corn allowed analysis of fully developed leaves in the flux chamber used, and the use of Teflon film avoided interference of the underlying soil in uptake of leaf wound VOCs. At t ≈ 185 min, five cuts of 3 cm each were applied to different corn leaves, leading to a strong increase in the ion signals at mass 81 and 99 amu (hexenal) and a smaller increase in the signal at 85 amu. Interestingly, there was no significant increase in the concentration of methanol, acetaldehyde, acetone, and butanone. At t ≈ 235 min, three of the corn stems were cut, and the observed pattern of VOC emissions was almost identical to the previous case in which the leaves were cut.
Methanol, however, showed a remarkably different behavior: there was a significant increase after cutting the stem but not after cutting the leaves. This experiment was repeated, and these findings appeared to be reproducible. An explanation may be that the stem of a corn plant consists of young, developing leaves. The synthesis of new cell wall in the stem may account for the presence of methanol as opposed to fully expanded mature corn leaves. As compared with the clover experiment, the observed concentration of methanol was much smaller. The experiments with corn need to be extended but do suggest that harvesting of fresh corn releases hexenyl-type compounds. In addition, it is likely, but not proven, that the natural drying of corn vegetation that occurs before most corn is harvested for grain will lead to the release of hexenyl and other oxygenated VOCs such as those seen with drying of clover and grass (14). Since corn is currently planted on about 77 × 106 acres in the United States (30), significant regional releases of VOCs may occur late in the harvest season in the U. S. Cornbelt.
Atmospheric Implications The experimental results presented in this paper demonstrate that leaf wounding and especially the drying of plant material can lead to a strongly enhanced VOC release from plants to the atmosphere. The results suggest that large amounts of leaf wound VOCs will be released from crops that are subject to the physical stress imposed by harvesting and from the drying that occurs with hay crops as well as with major crops such as corn and wheat that are allowed to dry in the field. The compounds observed in this work can be divided in two categories: short-lived VOCs with the ability to affect local air composition (acetaldehyde, C6 leaf wound compounds) and longer-lived VOCs that can be mixed with the VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. VOC emissions from a patch of alfalfa in the flux chamber after wounding as measured with the PT-CIMS instrument. higher atmosphere (methanol, acetone, and butanone). Acetaldehyde has a short atmospheric lifetime of about 1 day due to its fast reaction with OH and its efficient photolysis; both processes lead to the formation of acetyl peroxy radicals (CH3C(O)OO) that can sequester NOx from the atmosphere by the formation of peroxyacetyl nitrate (PAN; CH3C(O)OONO2). PAN is an important compound in photochemical smog and provides a mechanism for long-range transport of pollution because its major sink is thermal decomposition into CH3C(O)OO radicals and NO2. The atmospheric oxidation of the C6 leaf wound compounds (Z)-3-hexenol, (Z)-3hexenyl acetate, and (E)-2-hexenal was studied by Atkinson et al. (31). The tropospheric lifetimes of these compounds are a few hours, with propanal among the primary products. 2646
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FIGURE 7. VOC emissions from freshly cut alfalfa during the drying process as measured with the PT-CIMS instrument. As described in the text, a sample of alfalfa stems and leaves was cut and immediately placed in the flux chamber for VOC analysis during subsequent drying. Important secondary products include PAN and PPN (peroxypropionyl nitrate; C2H5C(O)OONO2), which possibly accounts for the observation of PPN over farmland in Indiana during the 1995 Southern Oxidants Study (32). Methanol and acetone have relatively long lifetimes in the lower troposphere and can be mixed with the upper troposphere, where they may contribute to the formation of HOx radicals (33). Methanol and acetone are released from biogenic and anthropogenic sources and can also be formed in the atmosphere. The global source of acetone is estimated to be 40-60 Tg yr-1 with a biogenic contribution of about
release of butanone may be a significant source of this compound to the atmosphere. It is likely that several of the wound VOCs observed here may be deposited on the plant surfaces in the flux chamber. In the atmosphere, the much better ventilation may limit the time the VOCs are exposed to the plant surface, suggesting that the true emissions can be higher than observed here. Alternatively, VOCs may be taken up on soil, leading to an overestimate of the fluxes in the present study. For this and other reasons, it is clear that extrapolating the results of these laboratory studies to the global atmosphere must be done with extreme caution.
Prospects There is continued interest in assembling biogenic VOC emission inventories for air quality models. In general, these inventories do not include leaf wound VOCs. Given the magnitude of the emissions seen here with crop species and the extensive croplands in many parts of the world, it seems likely that wound VOCs could play a role in oxidant formation during the harvest season. It is also possible that where intensive agricultural areas are near large urban areas (e.g., the San Joaquin Valley of California), there may be significant impacts of crop harvesting, via transport, on urban and regional air quality. Further analysis of the potential impacts of crop VOCs on air quality will require suitable field measurements. PT-CIMS instruments of the type described here have now been built in a field and have an aircraftportable design (37). As these instruments can be used to distinguish biogenic from certain anthropogenic VOCs (e.g., aromatic hydrocarbons), they should find wide application in a variety of field settings where oxygenated VOC analysis is needed. In addition, because they can be operated without interruption over extended periods of times (days and weeks) with sensitivities in the high parts per trillion by volume range, it will be possible to observe the formation and release of oxygenated VOCs during and after crop harvesting.
Acknowledgments This research was supported in part by NOAA’s Climate and Global Change program and by grants to R.F. from the National Science Foundation (ATM-9633285 and ATM9805191). T.G.C. gratefully acknowledges support at the University of Colorado from Veronica Bierbaum (NSF Grant CHE-9734867). The authors thank Werner Lindinger, Thomas Karl, and Armin Hansel (University of Innsbruck, Austria) for helpful discussions and for sharing unpublished data.
Literature Cited
FIGURE 8. VOC emissions from four corn plants after wounding several leaves (at t ≈ 190 min) and cutting three stems (at t ≈ 240 min). 21% (34). The global methanol source is estimated to be around 45 Tg yr-1 by Singh et al. (33), whereas Guenther et al. arrive at an even higher source from biogenic origins (35). The present work suggests that wounding and drying of plants may be an important mechanism to account for the exit of these VOCs to the atmosphere. Much less is known about the source strength, fate, and typical concentrations of butanone in the atmosphere. The reaction with OH is about a factor of 6 times higher for butanone than for acetone (36). The oxidation of butanone is therefore expected to take place mostly in the lower troposphere. The present experimental results demonstrate that butanone can be emitted in similar amounts from plants as acetone. This suggests that a biogenic
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Received for review October 26, 1999. Revised manuscript received March 6, 2000. Accepted March 21, 2000. ES991219K
