Gene Allwine

Estimates of regional natural volatile organic compound fluxes from enclosure and ambient measurements

Natural volatile organic compound (VOC) emissions were investigated at two forested sites in the southeastern United States. A variety of VOC compounds including methanol, 2-methyl-3-buten-2-ol, 6-methyl-5-hepten-2-one, isoprene and 15 monoterpenes were emitted from vegetation at these sites. Diurnal variations in VOC emissions were observed and related to light and temperature. Variations in isoprene emission from individual branches are well correlated with light intensity and leaf temperature while variations in monoterpene emissions can be explained by variations in leaf temperature alone. Isoprene emission rates for individual leaves tend to be about 75% higher than branch average emission rates due to shading on the lower leaves of a branch. Average daytime mixing ratios of 13.8 and 6.6 ppbv C isoprene and 5.0 and 4.5 ppbv C monoterpenes were observed at heights between 40 m and 1 km above ground level the two sites. Isoprene and monoterpenes account for 30% to 40% of the total carbon in the ambient non-methane VOC quantified in the mixed layer at these sites and over 90% of the VOC reactivity with OH. Ambient mixing ratios were used to estimate isoprene and monoterpene fluxes by applying box model and mixed-layer gradient techniques. Although the two techniques estimate fluxes averaged over different spatial scales, the average fluxes calculated by the two techniques agree within a factor of two. The ambient mixing ratios were used to evaluate a biogenic VOC emission model that uses field measurements of plant species composition, remotely sensed vegetation distributions, leaf level emission potentials determined from vegetation enclosures, and light and temperature dependent emission activity factors. Emissions estimated for a temperature of 30°C and above canopy photosynthetically active radiation flux of 1000 μmol m−2 s−1 are around 4 mg C m−2 h−1 of isoprene and 0.7 mg C m−2 h−1 of monoterpenes at the ROSE site in western Alabama and 3 mg C m−2 h−1 of isoprene and 0.5 mg C m−2 h−1 of monoterpenes at the SOS-M site in eastern Georgia. Isoprene and monoterpene emissions based on land characteristics data and emission enclosure measurements are within a factor of two of estimates based on ambient measurements in most cases. This represents reasonable agreement due to the large uncertainties associated with these models and because the observed differences are at least partially due to differences in the size and location of the source region (“flux footprint”) associated with each flux estimate.

Isoprene emission fluxes determined by an atmospheric tracer technique

Abstract Sulfur hexafluoride tracer was used in a series of experiments to simulate isoprene emissions from an isolated oak grove. The measured tracer release rate and ambient concentrations of isoprene and SF 6 observed along downwind sample lines were combined to determine the mass flux of isoprene from the forest. The fluxes determined from the tracer data increased exponentially with temperature and were in close agreement with estimates determined from emission rates measured in a series of branch enclosure samples. The results of this field study demonstrate the feasibility and usefulness of simulating forest emissions as a tool for investigating turbulent transport in forested areas. Isoprene emission fluxes that can be applied in regional models are reported.

Measurement of Biogenic Sulfur Emissions from Soils and Vegetation: Application of Dynamic Enclosure Methods with Natusch Filter and GC/FPD Analysis

Emission rates of reduced sulfur gases from vegetation and soils were measured in various regions of the United States during the summer of 1985. The predominant sulfur gases emitted were hydrogen sulfide, carbonyl sulfide and dimethylsulfide. Typically, vegetative (forests, crops, etc.) emission fluxes varied between approximately 10 and 60 ng S m-2 min-1. Biogenic sulfur fluxes from mollisol and histisol soils averaged 15 and 217 ng S m-2 min-1, respectively. Salt water marsh fluxes with a geometric mean of 293 ng S m-2 min-1 were the highest measured. These biogenic sulfur fluxes are somewhat lower than those measured during the SURE study at some of the same sites. The natural sulfur emission fluxes reported herein together with those data included in the two accompanying manuscripts provide the basis for developing a national inventory of reduced sulfur emissions from soils, crops and trees. When combined these data also will provide a foundation for deriving uncertainty limits associated with these flux estimates.

Modeling and measuring the nocturnal drainage flow in a high‐elevation, subalpine forest with complex terrain

[1] The nocturnal drainage flow of air causes significant uncertainty in ecosystem CO2, H2O, and energy budgets determined with the eddy covariance measurement approach. In this study, we examined the magnitude, nature, and dynamics of the nocturnal drainage flow in a subalpine forest ecosystem with complex terrain. We used an experimental approach involving four towers, each with vertical profiling of wind speed to measure the magnitude of drainage flows and dynamics in their occurrence. We developed an analytical drainage flow model, constrained with measurements of canopy structure and SF6 diffusion, to help us interpret the tower profile results. Model predictions were in good agreement with observed profiles of wind speed, leaf area density, and wind drag coefficient. Using theory, we showed that this one-dimensional model is reduced to the widely used exponential wind profile model under conditions where vertical leaf area density and drag coefficient are uniformly distributed. We used the model for stability analysis, which predicted the presence of a very stable layer near the height of maximum leaf area density. This stable layer acts as a flow impediment, minimizing vertical dispersion between the subcanopy air space and the atmosphere above the canopy. The prediction is consistent with the results of SF6 diffusion observations that showed minimal vertical dispersion of nighttime, subcanopy drainage flows. The stable within-canopy air layer coincided with the height of maximum wake-to-shear production ratio. We concluded that nighttime drainage flows are restricted to a relatively shallow layer of air beneath the canopy, with little vertical mixing across a relatively long horizontal fetch. Insight into the horizontal and vertical structure of the drainage flow is crucial for understanding the magnitude and dynamics of the mean advective CO2 flux that becomes significant during stable nighttime conditions and are typically missed during measurement of the turbulent CO2 flux. The model and interpretation provided in this study should lead to research strategies for the measurement of these advective fluxes and their inclusion in the overall mass balance for CO2 at this site with complex terrain.

Evaluation of PM10 emission rates from paved and unpaved roads using tracer techniques

Spokane, WA, is a nonattainment area for airborne particulate matter smaller than 10μm (PM10), so that a detailed emission inventory for PM10 is needed to evaluate various control strategies. It is thought that emissions from paved and unpaved roads in Spokane contribute three-fourths of the anthropogenic PM10 (neglecting wind-blown dust from agricultural areas). A study was conducted in the summer and fall of 1992 and again in the spring and summer of 1994 to measure PM10 emission rates from paved and unpaved roads in Eastern Washington state using a novel tracer technique. A known amount of an inert tracer (SF6) was released and concentrations of PM10 and SF6 downwind of the road, along with meteorological parameters and traffic volume, were measured. The results of the tracer experiments showed that within experimental uncertainties the PM10 and the tracer gas disperse in the same manner, suggesting that the use of a tracer in a line source to simulate roadway PM10 emissions can provide a tool for improving the existing emission inventories from roads. The emission factors obtained from two unpaved road experiments (136 g per vehicle per kilometer traveled, or g VKT−1, and 336 g VKT−1) were similar in magnitude to those predicted using currently accepted empirical algorithms. The factors determined from six paved road experiments were approximately 80% higher than that predicted using current formulae (6.7 ± 3.7 g VKT−1 compared to 3.7 g VKT−1) for two-lane roads with daily traffic less than 10,000 vehicles. For major highways (4 + lanes and traffic in excess of 10,000 vehicles per day) the emission factors obtained from the tracer experiments were, on average, 44% lower than those predicted using standard formulae (1.0 ± 0.5 g VKT−1 compared to 1.8 g VKT−1). The calculated emission factors for paved roads exhibited a wide range of variability, suggesting that in order to quantify PM10 emission rates from paved roads, more investigation is warranted.

A Simple Model to Predict Scalar Dispersion within a Successively Thinned Loblolly Pine Canopy

Abstract Bark beetles kill millions of acres of trees in the United States annually by using chemical signaling to attack host trees en masse. As an attempt to control infestations, forest managers use synthetic semiochemical sources to attract beetles to traps and/or repel beetles from high-value resources such as trees and stands. The purpose of this study was to develop a simple numerical technique that may be used by forest managers as a guide in the placement of synthetic semiochemicals. The authors used a one-dimensional, one-equation turbulence model (k–lm) to drive a three-dimensional transport and dispersion model. Predictions were compared with observations from a unique tracer gas experiment conducted in a successively thinned loblolly pine canopy. Predictions of wind speed and turbulent kinetic energy compared well with observations. Scalar concentration was predicted well and trends of maximum observed concentration versus leaf area index were captured within 30 m of the release location. A h...

Application of Atmospheric Tracer Techniques for Determining Biogenic Hydrocarbon Fluxes from an Oak Forest

Sulfur hexafluoride tracer was used in a series of experiments to simulate isoprene emissions from an isolated oak forest. The measured tracer release rate and ambient concentrations of isoprene and SF6 observed along a downwind sample line were combined to determine the mass flux of isoprene from the forest. Results based only upon the maximum isoprene and SF6 concentrations observed along the downwind sample line were in good agreement with fluxes calculated from the crosswind concentration integrals of isoprene and SF6. The fluxes determined from the tracer data increased exponentially with temperature and were in close agreement with estimates determined from emission rates measured in a series of branch enclosure samples collected in the forest and a biomass factor taken at the midpoint of a range of available biomass factors. The results of this field study demonstrate the feasibility and usefulness of simulating forest emissions as a tool for investigating turbulent transport in forested areas.

Pheromone Movement in Four Stand Thinning Scenarios: High Frequency Plume Observations

An atmospheric tracer experiment using SF6 was designed to assess changes in the dispersive environment in the trunk space of a southern pine forest through four thinning regimes. The experimental plot was thinned from dense boles and thick understory (>140 ft2 (13 m2) basal area) in four stages with the final basal area being 70 ft2 (6.5 m2 ). Observations indicate that thinned stands are less susceptible to bark beetle attack and one possible reason could be that the plumes of pheromone the insects use for signaling cannot achieve the intended purpose in the thinned stand. The pheromone mechanism could be altered through direct dilution of the gaseous plume, through a reduction in spatial coherence of a plume making it more difficult to follow back to a source or through loss of the plume vertically from the stand environment through lofting due to surface heating and loss of overhead containment. This work is part of a larger program to improve the success of forest insect management strategies using pheromone. Data analysis is on-going but the high frequency tracer plumes examined in this study show that the plumes are more dilute and much less coherent spatially in the thinned stands. The plumes are filamentous in nature in all cases but wander (whip) across a larger volume of space in the thinned stands.

Mass-Balance of Pheromone Surrogate Plumes in the Canopy Trunk Space

A large, multi-year study has been completed to evaluate the dispersion of insect pheromone in forest canopy trunk spaces. The study was undertaken to guide operational pest managers in the placement of pheromone sources in forest canopies. A very dense network of tracer sampling devices was deployed and over 200 average (half-hourly) plumes consisting of over 13000 chemical samples are available for analysis. This spatially dense near-field array provides an important opportunity to conduct a mass balance exercise. The subset of tests analyzed here indicates that mass recovery in this situation is possible within a factor of two and often close to 100%, though even in this dense sampler field there is substantial uncertainty and a tendency to over recover (>100%).

Near-Field Trunk Space Dispersion

A near-field tracer study was conducted to study the movement and dispersion of gas in the lower forest canopy in an attempt to improve guidance for forest managers deploying anti-aggregation pheromone sources to protect high value forest stands. Data are shown from three forest canopies and include over 13000 chemical tracer samples compiled into half hour dispersion fields around a point source. A high frequency sampler was also deployed to ascertain the structure of the gas plumes at 1 Hz. The plumes showed strong Gaussian tendencies in many cases and very high peak-to-mean ratios. Average maximum ÷/Q values were relatively consistent over the canopies studied though high variance in the maximum ÷/Q values was observed.