Emission of 2-methyl-3-buten-2-ol by pines: A potentially large natural source of reactive carbon to the atmosphere

. High rates of emission of 2-methyl-3-buten-2-ol (MBO) were measured from needles of several pine species. Emissions of MBO in the light were 1 to 2 orders of magnitude higher than emissions of monoterpenes and, in contrast to monoterpene emissions from pines, were absent in the dark. MBO emissions were strongly dependent on incident light, behaving similarly to net photosynthesis. Emission rates of MBO increased exponentially with temperature up to approximately 35øC. Above approximately 42øC, emission rates declined rapidly. Emissions could be modeled using existing algorithms for isoprene emission. We propose that emissions of MBO from lodgepole and ponderosa pine are the primary source of high concentrations of this compound, averaging 1-3 ppbv, found in ambient air samples collected in Colorado at an isolated mountain site approximately 3050 rn above sea level. Subsequent field studies in a ponderosa pine plantation in California ...... 1 -1 confirmed high MBO emissions, which averaged 25 pg C g h for 1-year-old needles, corrected to 30øC and photon flux of 1000 pmol m -2 s -r. A total of 34 pine species growing were investigated, of which 11 exhibited high emissions of MBO (>5 [xg C g-(cid:127) h-(cid:127)), and 6 emitted small but detectable amounts. All the emitting species are of North American origin, and most are restricted to western North America. These results indicate that MBO emissions from pines may constitute a significant source of reactive carbon and a significant source of acetone, to the atmosphere, particularly United States.


Introduction
On a global basis, the biosphere contributes more volatile hydrocarbons to the atmosphere than does human activity [Singh and Zimmerman, 1992;Guenther et al., 1995], and the majority of these biogenic emissions arise from forests. Many of the volatile organic compounds (VOC) emitted by vegetation are reactive constituents in tropospheric chemistry, affecting regional air quality and contributing to the photochemical production of ozone in both urban and rural landscapes [Trainer et al., 1987;Chameides et al., 1988]. For trees with high rates of VOC emission, between 0.5 and 2% of the carbon fixed by leaves in photosynthesis may be volatilized immediately, and under certain conditions, volatile losses may exceed 20% of fixed carbon over short periods of time [Monson and Fall, 1989;Sharkey and Loreto, 1993]. Over 1000 VOC are now known to be emitted by vegetation [Graedel, 1979;Knudsen et al., 1993], but until recently attention has focused largely on isoprene (2-methyl-l,3butadiene), generally the dominant hydrocarbon emitted by vegetation [Guenther et al., 1995], and the monoterpenes. Recently, increased attention has been directed toward emission of oxygenated hydrocarbons by vegetation [K6nig et al., 1995] Goldan et al. [1993] characterized the trace gas composition of ambient air at a remote mountain site (3050 m elevation) in Colorado in June 1991 and found that a C 5 alcohol, identified as 2-methyl-3-buten-2-ol (hereinafter referred to as methylbutenol or MBO), was the dominant trace compound, with concentrations 4 to 7 times those of isoprene. Based on the fact that diurnal changes in ambient MBO concentrations were very similar to those of isoprene, with known biogenic sources, and on the fact that MBO Williams, Oregon. Plants were grown in commercial potting soil and maintained in the Frost Phytotron plant growth facility at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. Further laboratory measurements on ponderosa pine were made on branches cut from trees growing in a natural landscape adjacent to NCAR's Mesa Laboratory. Measurements of net photosynthesis and stomatal conductance were made using a commercially available openpath gas exchange system (MPH-1000, Campbell Scientific, Logan, Utah) which consists of a temperature-controlled, fanstirred leaf chamber, tethered to a measurement and control system. Air of specified water vapor and CO 2 concentration was generated using mass flow controllers (Model 825, Edwards High Vacuum International, Wilmington, Massachusetts) and passed to the cuvette. The flow rate of gas entering the cuvette was measured using a mass flow meter (Model 831, Edwards). The difference in water vapor before and after the cuvette was measured using two dew point mirrors (General Eastern, Watertown, Massachusetts) and the difference in CO 2 concentration determined by infrared gas analysis (Model 225 Mk3, Analytical Development Corp., Hoddleston, England). Artificial light was provided using a 1000-W high intensity discharge lamp (Philips MS 1000/BU), and neutral density filters of blackened window screen were inserted in the light path to vary the irradiance. When a new set of pine needles was placed in the cuvette, a minimum of 30 min was allowed for equilibration, and all reported measurements were made after steady state conditions were achieved, as indicated by continuous realtime monitoring of CO 2 and H20 fluxes.
To quantify the emissions from the enclosed needles, a carefully measured volume of gas, between 400 and 500 cm 3, was slowly withdrawn from the leaf cuvette and cryogenically Air exiting the cuvette was drawn through the 1 cm 3 sample loop of a portable, isothermal (45øC) gas chromatograph. MBO was separated on a capillary column (DB-1; Alltech Assoc., Deerfield, Illinois) and measured using a reduction gas detector (RGD2, Trace Analytical, Menlo Park, California). Peak integration was accomplished using a commercial integrator (Model 3390, Hewlett-Packard, Avondale, Pennsylvania). The system was calibrated several times daily against a standard cylinder containing 39.2 ppbv MBO. In order to quantify the sensitivity of the GC/FID to MBO, two gas phase standards of MBO, one humidified and one dry, were analyzed using GC/FID and GC with an atomic emission detector (AED). Whereas FID sensitivity varies with the addition of functional groups, AED response depends only on the number of carbon atoms (J.P. Greenberg et al., Tethered balloon measurements of biogenic VOCs in the atmospheric boundary layer, submitted to Journal of Geophysical Research, 1998). In analyzing the dry MBO standard, the response of the FID was 10% less than that of the AED; the response of the FID was 3% less than the AED when analyzing the humidified standard. Since all emission samples were humidified, a 3% correction was applied to FID values in calculating emission rates using GC/FID data.
Quantitative analysis of MBO presents several challenges. Goldan et al. [1993] indicate that a large fraction of MBO is lost if the sample stream is cooled below -50øC to remove water. In addition, we have observed over 50% conversion of MBO to isoprene through dehydration when a dry MBO standard was transferred through heated stainless steel capillary tubing to the cryogenic preconcentration trap. Humidification of the MBO standards eliminated the dehydration, and emission samples from the cuvette contained enough moisture that there was no conversion of MBO to isoprene.
Although none of the pine emission results reported here were obtained using solid adsorbents, standards were collected on solid adsorbents for introduction into the GC/MS and GC/AED. Solid adsorbent cartridges of several compositions (Carbotrap 200 (80 mg glass beads, 170 mg, 350 mg Carbosieve s-m), Carbotrap 300 (300 mg Carbotrap C, 200 mg Carbotrap B, 125 mg Carbosieve s-m), and Tenax TA (all from Supelco, Inc., Bellefonte, Pennsylvania)) were tested for MBO collection efficiency and quantitative desorption. High percentages of MBO conversion to isoprene were observed when cartridges were dry purged prior to desorption (to remove water vapor collected during sampling) and when cartridges were heated slowly from room temperature to 275øC for desorption (water vapor elutes from the cartridge before MBO, which is then desorbed in a dry gas stream). When cartridges were heated quickly to 275øC by clamping on a desorption heater maintained at 275øC, only slight conversion of MBO to isoprene was observed. There was considerable cartridge to cartridge variability in MBOisoprene conversion, although Carbotrap 300 formulations gave the highest rates of conversion, while new adsorbents used to formulate Carbotrap 200 multistage adsorbents typically gave approximately 10% conversion. Less than 5% conversion, in most cases, was observed when Tenax TA was desorbed rapidly; although Tenax TA is hydrophobic, the MBO is presumably weakly adsorbed and eluted quickly from the heated cartridge before significant conversion to isoprene occurs. Given these potential analytical difficulties, literature reports of significant isoprene emission from pine species should be re-evaluated in the context of the analytical techniques used. Whether this difference is due to variation in tree age or growth environment or is an artifact of the excision procedure itself is unknown, although field experiments described below tend to preclude the latter.

Laboratory Study
An anonymous reviewer suggested that the high rates of MBO emission reported in this study might be an artifact of needle damage during insertion into the cuvette, analogous to the situation observed for monoterpene emissions from pines [Juuti et al., 1990]. Pines contain specialized storage structures for monoterpenes, which, if mechanically disrupted, empty their contents rapidly, leading to a burst of monoterpene emission, followed by a decline as pools are depleted. In one laboratory experiment, however, we left a set of ponderosa pine needles in the leaf cuvette for 3 days, turning off the light each night, and making measurements periodically during the day under constant conditions (PAR=1000 gmol m -2 s -• and leaf temperature=30øC). Except during the first half hour following illumination each day, measured rates over the 3-day period remained essentially constant at 9.3 + 0.8 •g C g-• h '•, demonstrating that MBO was not escaping from a damaged storage pool. Furthermore, the fact that MBO emissions rapidly fall to near zero in darkness (see below) strongly suggests that MBO is being emitted immediately upon production, rather than stored in specialized structures.

Ponderosa Pine Plantation Study
Measurements were made on 13 sets of 1-year-old needles from intact branches of ponderosa pine to determine the mean MBO emission potential (determined at 30øC and PAR between 1200 and 1500 gmol m -2 s-•). In many situations '(including the Eddy Arboretum measurements described below) it is impractical to make measurements on intact branches. To determine whether cutting branches was an acceptable technique, measurements were also made on five sets of 1-year-old needles from cut branches and the results compared to data from intact branches. In an attempt to maintain the transpiration stream during cut branch measurements, branches were cut, immediately placed in a bucket of needles (less than 50% of full length) were shown to emit MBO but at substantially lower rates. Two-year-old needles also continued to emit MBO at reduced rates, though whether due to needle age or to more shaded growth environment is unknown.
The influence of incident PAR and needle temperature on both net photosynthesis and MBO emission was determined for field-grown ponderosa pine (Figure 2). MBO emission and photosynthesis responded similarly to irradiance, increasing nearly linearly at low PAR, then leveling off. However, whereas photosynthesis became light saturated at approximately 1000 gmol m -2 s -], MBO emissions continued to increase above 1500 gmol m -2 s -•. With respect to varying temperature, photosynthesis and MBO emisson behaved very differently. Photosynthesis was maximal at or below 20øC and declined rapidly as temperature increased. MBO emissions, on the other hand, increased rapidly between 20 ø and 35øC, reached a maximum at approximately 40øC and declined precipitously above about 42øC.

Modeling Methylbutenol Emissions
Emission of MBO in significant amounts has been demonstrated for several species in the genus Pinus, but has not been found (or looked for extensively) among other coniferous or nonconiferous trees. For those species in Table   1 found to emit MBO in detectable but low amounts, emission rates are similar to typical emission rates of monoterpenes from pines. Those species placed in the high emission category emit MBO at rates 1 to 2 orders of magnitude greater than typical monoterpene emissions, and in some cases at rates comparable to isoprene emissions from high isoprene emitting tree species. Coupled with high values of foliar mass per unit ground area in pine forests (Geron et al. [1994] use a value of 700 g m -2 compared with 375 g m -2 for deciduous forests), these high emission capacities suggest that MBO could be the major source of reactive carbon in extensive ecosystems dominated by emitting pine species.
In order to fit the temperature data in Figure 3 (bottom), we used a temperature algorithm which is equivalent to that of Guenther et al. [1993]

Implications for Atmospheric Chemistry
The major photochemical sink for MBO during daylight hours is assumed to be reaction with OH. The rate coefficient for the OH reaction with MBO was reported by Rudich et al.
[1995] to be k= ( where s is a landscape average emission potential (mg C h-l), D is the total foliar density (g m-2), and ¾ is an emission activity factor that accounts for sensitivity of emissions to variations in PAR and temperature, described by (1) and (2). An emission potential is the emission rate of a specific VOC expected for a particular plant species at a temperature of 30øC and PAR of 1000 gmol m '2 s -I The landscape average emission potential • is the weighted average, that is, E(ei x D3, of emission potentials representative of various tree species (i) where D• is the fraction of the total foliar density composed of each species. Forest biomass and species composition data were derived using the method of Geron et al. [1994]. Forests cover 75% of the region and the average total foliar density of forests in this region (734 g m -2) results in a total foliar density of 550 g m '2, dominated by lodgepole pine (336 g m-2). Ponderosa pine (62 g m-2), subalpine fir (45 g m-2), Douglas fir (45 g m-2), aspen (39 g m-2), Englemann spruce (16 g m-2), and blue spruce (6 g m -2) also comprise a significant fraction of the total biomass. These foliar densities were used to weight the MBO emission capacities determined in this study (25 gg C g-I h-I for lodgepole and ponderosa pine and 0 for all other species) and the isoprene and monoterpene emission capacities reported by Guenther et al. [ 1994] for each species. The resulting landscape average emission capacity (at 30øC and PAR=1000 gmol m '2 s ~•) of 14.5 mg C m -2 h 'l is comprised of 69% MBO, 21% isoprene, and 10% monoterpenes. Using the canopy environment model described by Guenther et al. [1995] and the light and temperature relationships described above, for a typical summer day in this region (temperature=22 øC, PAR=1500, Sun angle = 56 ø) we calculate a total VOC emission rate of 4.6 mg C m -2 h (64% MBO, 20% isoprene, and 16% monoterpenes).
Using this calculated VOC flux, we estimated above canopy hydrocarbon concentrations using a simple box model [Guenther et al., 1996]. The model is based on a mixed-layer scalar conservation equation and assumes that turbulent horizontal fluxes and mean vertical advection are negligible, the vertical flux profile is linear, the mean concentration has reached steady state and is homogeneous in space, and that entrainment flux is negligible. We used the OH and 03 reaction rate coefficients reported by Atkinson [1990] for isoprene and monoterpenes (assuming that the monoterpene flux is 50% ct-pinene and 50%  Guenther et al. [1996] estimate the uncertainty associated with attempting to estimate a surface flux based on ambient concentration measurements in the mixed layer and arrive at an uncertainty of about _+50%, due primarily to uncertainty in the OH concentration. Here we attempt to estimate mixed layer isoprene concentrations from modeled surface fluxes, and the uncertainty in the surface flux model adds considerably to the overall uncertainty in mixed layer predicitions. Although the predicted ratio of MBO to isoprene is almost 3, isoprene, due to its greater reactivity, contributes about 29% and MBO contributes 58% of the total OH sink due to these biogenic hydrocarbons. Using the ratios between surface layer and mixed-layer concentrations observed by Guenther et al. [1996], we estimate surface layer concentrations of about 1.5 ppb MBO and 0.5 ppb isoprene. Given the expected large variations in surface concentrations due to local sources, these estimates, though quite uncertain, are in reasonable agreement with the ambient concentrations observed by Goldan et al. [1993] (1-3 ppb MBO and 200-500 ppt isoprene) at a site, dominated by lodgepole pine, within our model domain.
The reported rate constant of the MBO-OH reaction is quite high, comparable to that of the isoprene-OH reaction (k=9.88 x 10 -11 cm 3 molecule -1 s -1 at 300 K [Atkinson, 1990]) and the contribution of MBO oxidation to tropospheric 03 formation is likely to be substantial, possibly dominating 03 production in pine forests of western North America. Secondary 03 production from oxidation of the products of the MBO-OH reactions is likely to be less than in the case of isoprene, due to the relatively long lifetime of acetone (10-30 days in summertime [Singh et al., 1994]), a major stable product of MBO oxidation.
In addition to contributing to regional tropospheric 03, oxidation of MBO may be a significant source of acetone to the atmosphere, at least on a regional scale. Singh et al. [1995] have pointed out the potential importance of acetone photolysis as a source of HOx radicals in the free troposphere. Ferronato et al. [1998] indicated that acetone is a major product not only of MBO oxidation but probably of the oxidation of 2-hydroxy-2-methylpropanal as well. Assuming 100% conversion of the latter to acetone, the total acetone yield from MBO oxidation by OH could approach 85%, with the production of 0.6 g acetone per gram of MBO destroyed. Whether the amount of acetone produced by the oxidation of MBO from pines comprises a significant fraction of the global source strength, estimated at 40-60 Tg annually [Singh et al., 1995], depends largely on the global distribution of pine species which emit significant amounts of MBO.

Phylogenetic and Geographical Patterns of Emissions
There are clear phylogenetic and geographical biases in the data collected at Eddy Arboretum (Table 1). Of 34 species examined, only six are among the white or soft pines in subgenus Strobus. None of these was found to emit MBO, however, and until there is evidence to the contrary, it seems reasonable to assume that all MBO-emitting species are found in subgenus Pinus. Within that subgenus, all emitting species were found in five subsections of section Pinus, but more data are necessary to prove whether this is uniformly the case. Of •t •p•c,i•q •arninect nnlxt qeven are F,•raqian and nf the 27 North American species, 21 are restricted to western North America. Despite the sampling bias, it is striking that all species of pine found to emit MBO are of North American origin. All emitting species were found in section Pinus, containing six subsections. Subsection Sylvestres is almost entirely Eurasian, the single significant exception being P. resinosa, the Red Pine of eastern North America (not sampled). Subsection Australes consists entirely of eastern North American species, of which several were found to emit MBO, but only one, P. palustris, emitted in large amounts.
The final four subsections of section Pinus are restricted to western North America and Mexico, and the large majority of those species emitted MBO in significant quantities. More data are necessary to verify these generalizations, but unless new evidence to the contrary arises, it seems likely that MBO is a significant emission only in North America and primarily in western North America (including Mexico, which though not well represented in the sample, contains many species in the emitting groups). Ponderosa pine, in terms of abundance and range, is a dominant tree of western North America, growing from northern Mexico to southern British Columbia, and from sea level to 2800 m [Elias, 1980]. Lodgepole pine, often a colonizer of burned-over sites and found in pure stands, also ranges from Baja California to Alaska, increasing in abundance in the northern Rocky Mountains and Pacific Coast regions [Elias, 1980]. Other MBO emitting species occupy much more restricted geographical rangesø Based on Forest Inventory and Analysis data from the U.S. Department of Agriculture Forest Service and using methods of Geron et al. [1994], the percentage of forest crown area occupied by high MBO emitting species (primarily ponderosa and lodgepole pine, with a smaller contribution from Jeffrey pine) ranges from 10.5% in Utah to over 37% in Wyoming, averaging slightly under 24% for the 13 westernmost United States (excluding Nevada). Using these fractions and forest density estimates based on satellite data [Zhu and Evans, 1994], MBO emitting pine species cover approximately 0.18 x 106 km 2. Assuming foliage density of 700 g m '2, an average MBO emission capacity of 25 gg C g-1 h-•, and incorporating canopy effects and effects of seasonal variation in PAR and temperature using the current Biogenic Emissions Inventory System [Pierce and Waldruff, 1991;Geron et al., 1994], we calculate annual emissions of 2.2 Tg MBO. If we further assume 0.6 g acetone produced for each gram of MBO destroyed [Ferronato et al., 1998], pines in the western United States may represent an annual source of acetone of the order 1.3 Tg. Inclusion of western Canada and Mexico in the analysis would raise this estimate somewhat. However, given the apparently limited geographical extent of MBO emissions from pines, and despite the importance of MBO emitting pines in western North America, it is unlikely that production of acetone by MBO oxidation constitutes a large percentage of the global source.