Robert R. Arnts

A review and synthesis of monoterpene speciation from forests in the United States

Abstract The monoterpene composition (emission and tissue internal concentration) of major forest tree species in the United States is discussed. Of the 14 most commonly occurring compounds ( α -pinene, β -pinene, Δ 3 -carene, d -limonene, camphene, myrcene, α -terpinene, β -phellandrene, sabinene, ρ -cymene, ocimene, α -thujene, terpinolene, and γ -terpinene), the first six are usually found to be most abundant. Expected regional variability based on the monoterpene composition fingerprints and corresponding tree species distribution and abundance is examined. In the southeast, α -pinene and β -pinene seem to dominate monoterpene emissions, while in the northern forests emissions are distributed more evenly among the six major compounds. In some parts of western forests, β -pinene and Δ 3 -carene can be more abundant than α -pinene. Among the other eight compounds, β -phellandrene and sabinene occasionally are significant percentages of expected local monoterpene emissions. Ocimene and ρ -cymene are estimated to be more common in regions dominated by deciduous broadleaf forests, although total emission rates are generally lower for these forests relative to those dominated by conifers. These percentages are compared with monoterpene composition measured in ambient air at various sites. Estimated monoterpene emission composition based on local forest species composition agrees fairly well with ambient measurements for the six major compounds. The past assumption that α -pinene composes approximately 50% of total monoterpene emissions appears reasonable for many areas, except for possibly the northern coniferous forests and some areas in the west dominated by true firs, spruce, and western pines (lodgepole and ponderosa pines). The oxygenated monoterpenes such as camphor, bornyl acetate, and cineole often compose high percentages of the monoterpenes within plant tissues, but are much less abundant in emission samples. Even after adjusting for lower vapor pressures of these compounds, emission rates relative to the hydrocarbon monoterpenes are often lower than would be expected from their internal concentrations. More study is warranted on monoterpene emission rates and composition, especially from the spruces, true firs, hemlocks, cedars, and some deciduous species such as the maples. Non-invasive canopy level and whole ecosystem flux studies are also needed to establish uncertainty estimates for monoterpene emission models.

Biogenic isoprene emission: Model evaluation in a southeastern United States bottomland deciduous forest

Isoprene is usually the dominant natural volatile organic compound emission from forest ecosystems, especially those with a major broadleaf deciduous component. Here we report isoprene emission model performance versus leaf and canopy level isoprene emission measurements made at the Duke University Research Forest near Chapel Hill, North Carolina. Emission factors, light and temperature response, canopy environment models, foliar mass, leaf area, and canopy level isoprene emission were evaluated in the field and compared with model estimates. Model components performed reasonably well and generally yielded estimates within 20% of values measured at the site. However, measured emission factors were much higher in early summer following an unusually dry spring. These decreased later in the summer but remained higher than values currently used in emission models. There was also a pronounced decline in basal emission rates in lower portions of the canopy which could not be entirely explained by decreasing specific leaf weight. Foliar biomass estimates by genera using basal area ratios adjusted for crown form were in excellent agreement with values measured by litterfall. Overall, the stand level isoprene emissions determined by relaxed eddy accumulation techniques agreed reasonably well with those predicted by the model, although there is some evidence for underprediction at ambient temperatures approaching 30°C, and overprediction during October as the canopy foliage senesced. A Big Leaf model considers the canopy as a single multispecies layer and expresses isoprene emission as a function of leaf area rather than mass. This simple model performs nearly as well as the other biomass-based models. We speculate that seasonal water balance may impact isoprene emission. Possible improvements to the canopy environment model and other components are discussed.

Biogenic hydrocarbon contribution to the ambient air of selected areas

Abstract In response to suggestions that biogenic emissions are responsible for high hydrocarbon concentrations described in several reports, a short-term sampling program was initiated in the reported areas to test this hypothesis. Limited numbers of whole-air samples were collected in Tedlar bags and analyzed by gas chromatography (GC) with flame ionization detection. Tulsa air was found to contain an average of 0.2% isoprene of the total nonmethane hydrocarbon (TNMHC) load. Rio Blanco County, Colorado, and Smoky Mountain air, respectively, averaged about 2 % and 4 % biogenic hydrocarbon of the total nonmethane hydrocarbon loads. Isoprene appears to be a dominant olefin in rural and remote areas. Although the tests were of short duration, results suggest monoterpenes and isoprene constitute only minor components in these areas relative to anthropogenic hydrocarbons.

Estimates of α-pinene emissions from a loblolly pine forest using an atmospheric diffusion model

Abstract A field study was conducted to estimate the emissions of α-pinene from a pine forest without disturbing the vegetation. A tracer, sulfur hexafluoride, was released from the forest and collected downwind in order to define the dispersive characteristics of the atmosphere. Simultaneously, α-pinene concentrations were measured both upwind and downwind of the forest. A Gaussian plume dispersion algorithm incorporating hydrocarbon lose due to O3 and OH reaction was used to estimate α-pinene emissions. Emission rates ranged from 11 to 19 μg m−2 min−1 at 37 °C.

The design and testing of a relaxed eddy accumulation system

A flux measuring system has been developed based on relaxed eddy accumulation (REA, or conditional sampling) methodology. The system uses sampling tubes packed with solid substrates to collect air samples for determining the average concentration of compounds of interest. It has been designed for measuring fluxes of various trace gas species by using appropriate solid substrates in the tubes. The chief advantages to this design are the following : (1) the placement of pump and flow control apparatus are located downstream of the sampling tubes to eliminate sample heating and reduce contamination of the sample stream ; (2) the addition of a zero gas channel available to the sampling tubes improves the flow stability without diluting the sample ; (3) the design permits the use of multiple sample tube magazines to allow sequential runs without interruption ; and (4) the use of compact, easily transportable sampling tubes permits analyses to be conducted at an off-site laboratory where analytical conditions are better controlled. The REA system was tested by measuring water vapor flux under field conditions and the resulting measurements are compared with those made using a Bowen ratio system. The two methods gave similar results with fluxes values that agreed within 12%, although the measurements from the REA system were generally higher than those from the Bowen ratio method.

Canopy level emissions of 2-methyl-3-buten-2-ol, monoterpenes, and sesquiterpenes from an experimental Pinus taeda plantation

Emissions of Biogenic Volatile Organic Compounds (BVOCs) observed during 2007 from an experimental Pinus taeda plantation in Central North Carolina are compared with model estimates from the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 2.1. Relaxed eddy accumulation (REA) estimates of 2-methyl-3-buten-2-ol (MBO) fluxes are a factor of 3-4 higher than MEGAN estimates. MEGAN monoterpene emission estimates were a factor of approximately two higher than REA flux measurements. MEGAN β-caryophyllene emission estimates were within 60% of growing season REA flux estimates but were several times higher than REA fluxes during cooler, dormant season periods. The sum of other sesquiterpene emissions estimated by MEGAN was several times higher than REA estimates throughout the year. Model components are examined to understand these discrepancies. Measured summertime leaf area index (LAI) (and therefore foliar biomass) is a factor of two higher than assumed in MEGAN for the P. taeda default. Increasing the canopy mean MBO emission factor from 0.35 to 1.0mgm(-2)h(-1) also reduces MEGAN vs. REA flux differences. This increase is within current MBO emission factor uncertainties. The algorithm within MEGAN which adjusts isoprene emission estimates as a function of the temperature and light of the previous 24h seems also to improve the seasonal MEGAN MBO correlation with REA fluxes. Including the effects of the previous 240h, however, seems to degrade temporal model correlation with fluxes. Monoterpene and sesquiterpene composition data from the REA are compared with MEGAN2.1 estimates and also branch enclosure and needle extract data collected at this site. To our knowledge, the flux data presented here are the first reported for MBO and sesquiterpenes from a P. taeda ecosystem.

Gas Chromatographic Techniques for the Measurement of Isoprene in Air

Isoprene emission from plants was discovered by Sanadze in 1966 (reviewed in Sanadze 1991) while studying plant-insect interactions. Isoprene, 2-methyl-1,3butadiene, appears to have no role in plant-insect interactions but the amount of isoprene emitted is so great that it affects atmospheric chemistry and may be important to the carbon balance of some plants. Rasmussen and Went (1964) independently discovered isoprene emission from plants while studying plant hydrocarbon emissions (Went 1960). Isoprene is the root member of a large class of compounds, many of which are important in plant fragrance. Isoprene itself is not universally appealing when in high enough concentration to be detected by smell.