Enclosure Measurements Research Articles

Biogenic volatile organic compound emissions (BVOCs) II. Landscape flux potentials from three continental sites in the U.S.

Landscape flux potentials for biogenic volatile organic compounds (BVOCs) were derived for three ecosystems in the continental U.S. (Fernbank Forest, Atlanta, GA; Willow Creek, Rhinelander, WI; Temple Ridge, CO). Analytical data from branch enclosure measurements were combined with ecological survey data for plant species composition and biomass.Other quantitative flux measurements at the leaf and landscape level were incorporated to scale the results from the enclosure measurements to the landscape level.Flux estimates were derived by using a one week ambient temperature and light record (30 min time resolution) and adjusting all emission rates to these conditions with temperature and light correction algorithms.

Biogenic emissions of volatile organic compounds from gorse (Ulex europaeus): Diurnal emission fluxes at Kelling Heath, England

Volatile organic compound (VOC) emission fluxes from Gorse (Ulex europaeus) were measured during May 30–31, 1995 at Kelling Heath in eastern England by using bag enclosure and gradient methods simultaneously. The enclosure measurements were made from branches at different stages of physiological development (flowering, after flowering, and mixed). Isoprene was found to represent 90% of the total VOC emissions, and its emission rates fluctuated from 6 ng (g dwt)−1 h−1 in the early morning to about 9700 ng(g dwt)−1 h−1 at midday. Averaged emission rates standardized to 20°C were 1625, 2120, and 3700 ng (g dwt)−1 h−1 for the new grown, “mixed,” and flowering branch, respectively. Trans‐ocimene and α‐pinene were the main monoterpenes emitted and represented, on average, 47.6% and 36.9% of the total monoterpenes. Other monoterpenes, camphene, sabinene, β‐pinene, myrcene, limonene and γ‐terpinene, were positively identified but together represented less than 1.5% of the total VOC emissions from gorse. Maximum isoprene concentrations in air at the site were measured around midday at 2 m (174 parts per trillion by volume, or pptv) and 6 m (149 pptv), and minimum concentrations were measured during the night (8 pptv at both heights). Mean daytime α‐pinene air concentrations of 141 and 60 pptv at 2 and 6 m height were determined, but trans‐ocimene concentrations were less than the analytical detection limit (4 pptv), suggesting rapid chemical removal of this compound from air. The isoprene fluxes calculated by the micrometeorological gradient method showed a pattern similar to that of those calculated by the enclosure method, with isoprene emission rates maximum at midday (100 μg m−2 h−1) and not detectable during the nighttime. Assessment of the fraction of the site covered by gorse plants enabled an extrapolation of emission fluxes from the enclosure measurements. When averaged over the 2 day experiment, isoprene fluxes of 29.8 and 27.8 μg m−2 h−1 were obtained from the gradient and the enclosure extrapolation respectively. These isoprene fluxes to the atmosphere represented between 0.12% and 0.35% of the net assimilated carbon (as CO2) uptake rate for gorse.

Isoprene fluxes measured by enclosure, relaxed eddy accumulation, surface layer gradient, mixed layer gradient, and mixed layer mass balance techniques

Isoprene fluxes were estimated using eight different measurement techniques at a forested site near Oak Ridge, Tennessee, during July and August 1992. Fluxes from individual leaves and entire branches were estimated with four enclosure systems, including one system that controls leaf temperature and light. Variations in isoprene emission with changes in light, temperature, and canopy depth were investigated with leaf enclosure measurements. Representative emission rates for the dominant vegetation in the region were determined with branch enclosure measurements. Species from six tree genera had negligible isoprene emissions, while significant emissions were observed for Quercus, Liquidambar, and Nyssa species. Above‐canopy isoprene fluxes were estimated with surface layer gradients and relaxed eddy accumulation measurements from a 44‐m tower. Midday net emission fluxes from the canopy were typically 3 to 5 mg C m−2 h−1, although net isoprene deposition fluxes of −0.2 to −2 mg C m−2 h−1 were occasionally observed in early morning and late afternoon. Above‐canopy CO2 fluxes estimated by eddy correlation using either an open path sensor or a closed path sensor agreed within ±5%. Relaxed eddy accumulation estimates of CO2 fluxes were within 15% of the eddy correlation estimates. Daytime isoprene mixing ratios in the mixed layer were investigated with a tethered balloon sampling system and ranged from 0.2 to 5 ppbv, averaging 0.8 ppbv. The isoprene mixing ratios in the mixed layer above the forested landscape were used to estimate isoprene fluxes of 2 to 8 mg C m−2 h−1 with mixed layer gradient and mixed layer mass balance techniques. Total foliar density and dominant tree species composition for an approximately 8100 km2 region were estimated using high‐resolution (30 m) satellite data with classifications supervised by ground measurements. A biogenic isoprene emission model used to compare flux measurements, ranging from leaf scale (10 cm2) to landscape scale (102 km2), indicated agreement to within ±25%, the uncertainty associated with these measurement techniques. Existing biogenic emission models use isoprene emission rate capacities that range from 14.7 to 70 μg C g−1 h−1 (leaf temperature of 30°C and photosynthetically active radiation of 1000 μmol m−2 s−1) for oak foliage. An isoprene emission rate capacity of 100 μg C g−1 h−1 for oaks in this region is more realistic and is recommended, based on these measurements.

Landfill methane emissions measured by enclosure and atmospheric tracer methods

Methane (CH4) emissions were measured from the Nashua, New Hampshire municipal landfill using static enclosure and atmospheric tracer methods. The spatial variability of emissions was also examined using geostatistical methods. One hundred and thirty nine enclosure measurements were performed on a regular grid pattern over the emitting surface of the landfill resulting in an estimate of whole landfill emissions of 15,800 L CH4 min−1. Omnidirectional variograms displayed spatial correlation among CH4 fluxes below a separation distance of 7 m. Eleven tracer tests, using sulfur hexafluoride (SF6) as a tracer gas, resulted in a mean emissions estimate of 17,750 L CH4 min−1. The favorable agreement between the emission estimates was further refined using the observed relationship between atmospheric pressure and CH4 flux. This resulted in a pressure‐corrected tracer flux estimate of whole landfill emissions of 16,740 L CH4 min−1.

Leaf, branch, stand and landscape scale measurements of volatile organic compound fluxes from U.S. woodlands.

Natural volatile organic compound (VOC) fluxes were measured in three U.S. woodlands in summer 1993. Fluxes from individual leaves and branches were estimated with enclosure techniques and used to initialize and evaluate VOC emission model estimates. Ambient measurements were used to estimate above canopy fluxes for entire stands and landscapes. The branch enclosure experiments revealed 78 VOCs. Hexenol derivatives were the most commonly observed oxygenated compounds. The branch measurements also revealed high rates of isoprene emission from three genera of plants (Albizia, Chusqua and Mahonia) and high rates of monoterpene emission from three genera (Atriplex, Chrysthamnus and Sorbus) for which VOC emission rates have not been reported. Measurements on an additional 34 species confirmed previous results. Leaf enclosure measurements of isoprene emission rates from Quercus were substantially higher than the rates used in existing emission models. Model predictions of diurnal variations in isoprene fluxes were generally within +/- 35% of observed flux variations. Measurements with a fast response analyzer demonstrated that 60 min is a reasonable time resolution for biogenic emission models. Average daytime stand scale (hundreds of m) flux measurements ranged from about 1.3 mg C m(-2) h(-1) for a shrub oak stand to 1.5-2.5 mg C m(-2) h(-1) for a mixed forest stand. Morning, evening and nighttime fluxes were less than 0.1 mg C m(-2) h(-1). Average daytime landscape scale (tens of km) flux measurements ranged from about 3 mg C m(-2) h(-1) for a shrub oak-aspen and rangeland landscape to about 7 mg C m(-2) h(-1) for a deciduous forest landscape. Fluxes predicted by recent versions (BEIS2, BEIS2.1) of a biogenic emission model were within 10 to 50% of observed fluxes and about 300% higher than those predicted by a previous version of the model (BEIS).

Hydrocarbon emissions from spruce species using environmental chamber and branch enclosure methods

Isoprene emission rates from Engelmann ( Picea engelmannii), red ( Picea rubens), and black ( Picea mariana) spruce were measured from groups of seedlings in a controlled environmental chamber. Standard isoprene emission rate factors, obtained from measurements over an array of light and temperature conditions, ranged from 1 to 16 μgCg −1h −1 at 30°C and 1000 μmolm −2s −1 photosynthetically active radiation. Emission measurements were also collected using a dynamic enclosure technique from branches of a mature white spruce ( Picea glauca) through a complete growing season. These data indicate that isoprene emissions, normalized to standard conditions, are moderate in spring and fall (∼ 7 μgC g −1 h −1 at 30°C, 1000 μmol m −2 s −1) with a maximum in mid-summer (∼ 12 μgCg −1 h −1 at 30°C, 1000 μmolm −2s −1) Isoprene from white spruce represented more than 90% of the total identified hydrocarbons. Branch enclosure measurements from limited survey samples for blue spruce ( Picea pungens) yielded a standard emission rate of 12 μgC g −1 h −1 while samples for Norway spruce ( Picea abies) indicated very little isoprene emitted (< 0.5 μgC g −1 h −1). The wide range of isoprene emissions from these different spruce species suggest that improvements in hydrocarbon emission inventories may require detailed species distribution data. On a regional basis, this may be especially true in Europe where 22% of isoprene emissions are estimated to be from spruce and in boreal regions where spruce is a dominant part of the ecosystem.

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.

Measurements and estimation of CO2 fluxes from aircraft and satellite‐based systems

This study investigates the relationships between airborne observations of CO2 fluxes and satellite‐based vegetation indices using data from the First International Land Surface Climatology Project Field Experiment (FIFE) 1989. The CO2 flux data were obtained by flying a grid pattern at a mean altitude of 90 m over a 225 km2 grassland ecosystem, on six occasions during a 2‐week period. Two satellite‐based data sets were obtained from NOAA 10 advanced very high resolution radiometer (AVHRR) and one from Landsat 5 thematic mapper (TM). Linear relationships Fc = b SR* + a were found between CO2 flux data (Fc) and footprint‐weighted simple ratios (SR*) from the various satellites. These relationships improved significantly when the footprint was adjusted with the continuously changing wind conditions and with an empirical parameter that accounts for effects of surface roughness and stability on the diffusion pattern between the surface and the airborne sampling locations. The AVHRR data collected on July 28 accounted for 53 to 81% of the variations in CO2 flux measured during the six grid flights over the site and the Landsat TM data collected on August 4 accounted for 68 to 83% of the same CO2 variations. The regression coefficient b, which is related to the photosynthetic efficiency, appears to be an excellent way to characterize the dynamic response of complex ecosystems, and the flux (Fc) at SR* = 1.5 provides an estimate of soil respiration as shown through a comparison with enclosure measurements.

Application of static and dynamic enclosures for determining dimethyl sulfide and carbonyl sulfide exchange in Sphagnum peatlands: Implications for the magnitude and direction of flux

A static enclosure method was applied to determine the exchange of dimethyl sulfide (DMS) and carbonyl sulfide (OCS) between the surface of Sphagnum peatlands and the atmosphere. Measurements were performed concurrently with dynamic (flow through) enclosure measurements with sulfur‐free air used as sweep gas. This latter technique has been used to acquire the majority of available data on the exchange of S gases between the atmosphere and the continental surfaces and has been criticized because it is thought to overestimate the true flux of gases by disrupting natural S gas gradients. DMS emission rates determined by both methods were not statistically different between 4 and &gt;400 nmol m−2 h−1, indicating that previous data on emissions of at least DMS are probably valid. However, the increase in DMS in static enclosures was not linear, indicating the potential for a negative feedback of enclosure DMS concentrations on efflux. The dynamic enclosure method measured positive OCS flux rates (emission) at all sites, while data using static enclosures indicated that OCS was consumed from the atmosphere at these same sites at rates of 3.7 to 55 nmol m−2 h−1. Measurements using both enclosure techniques at a site devoid of vegetation showed that peat was a source of both DMS and OCS. However, the rate of OCS efflux from decomposing peat was more than counterbalanced by OCS consumption by vegetation, including Sphagnum mosses, and net OCS uptake occurred at all sites. We propose that all wetlands are net sinks for OCS.

Sources and sinks of formic, acetic, and pyruvic acids over central Amazonia: 2. Wet season

We have determined the gas phase concentrations of formic (HCOOH), acetic (CH3COOH), and pyruvic (CH3C(O)COOH) acids in the forest canopy, boundary layer, and free troposphere over the central Amazon Basin during the April–May segment of the 1987 wet season. At 150‐m altitude in the boundary layer the daytime average concentrations were 430±225, 340±155, and 25± 5 ppt for HCOOH, CH3COOH, and CH3C(O)COOH, respectively. These values were fivefold lower than those observed in the 1985 dry season. Concentrations measured near canopy top were not significantly different from boundary layer values (P = 0.10), while concentrations in the lower canopy were significantly less. Concentrations in the free troposphere (5 km) were lower than in the boundary layer and averaged 170±40, 210±40, and 15±15 ppt for HCOOH, CH3COOH, and CH3C(O)COOH, respectively. Fivefold enhancements of CH3C(O)COOH concentrations were observed in convective outflows at 5‐ to 6‐km altitudes. Aerosol carboxylate concentrations were usually below our detection limit of 5–10 ppt. Preliminary branch enclosure measurements indicated significant direct emission of carboxylic acids by vegetation. A one‐dimensional photochemical model for the canopy and the boundary layer was used to examine the contributions from various sources to the carboxylic acid budgets. Model results indicate that direct emissions from vegetation can account for most of the concentrations observed in the canopy. These emissions peak during the daytime hours, and 24‐hour average upward fluxes at canopy top are 4.4×109, 3.7×109, and 2.8×108 molecules cm−2 s−1 for HCOOH, CH3COOH, and CH3C(O)COOH, respectively. However, direct emissions from vegetation can account for only a small fraction of the observed carboxylic acid concentrations in the boundary layer, suggesting a large contribution from atmospheric sources. The atmospheric reactions previously suggested in the literature as sources of carboxylic acids (gas phase decomposition of isoprene, CH3CO3 + peroxy, aqueous phase oxidation of CH2O) appear to be too slow to explain the observed concentrations. Other atmospheric reactions, so far unidentified, could make a major contribution to the carboxylic acid budgets.

Experience with laser safety in the USA-a review

Following several research programs in the 1960's aimed at studying the adverse biological effects of lasers and other optical radiation sources, laser occupational exposure limits were set and general safety standards were developed. Today, the experience from laser accidents and the development of new lasers and new applications have altered the format of the exposure limits and the safety procedures. It is critically important to distinguish between different biological injury mechanisms. The biological effects of ultraviolet radiation upon the skin and eye are additive over a period of at least one workday, and require different safety procedures. The scattered UV irradiance from excimer lasers may be quite hazardous, depending upon wavelength and action spectra. Since laser technology is young, the exposure of an individual in natural sunlight must be studied to evaluate the potential for chronic effects. The safety measures necessary in the use of lasers depend upon a hazard evaluation. The appropriate control measures and alternate means of enclosure, baffling, and operational control measures are presented. Present laser safety standards are explained briefly. Eye protective techniques and eyewear are considered for a variety of sources. The optical properties of enclosure materials are also discussed.

Measurement of nitrogen oxide fluxes from soils: Intercomparison of enclosure and gradient measurement techniques

The present estimates of emissions of NOx from soils to the atmosphere are based on field studies that used enclosure techniques. While this technique is direct, convenient and precise, the technique may perturb the biogenic soil processes under study. In addition, the limited sample size may produce nonrepresentative results. In order to address these concerns, NO flux measurements made using an enclosure technique are intercompared with NO flux determinations made using a gradient technique. The measurements were made during the nighttime hours from August to October, 1985, at an unfertilized, uncultivated grassland site in Colorado, United States. The NO flux measured during this period ranged between 0.08 ng N m−2 s−1 and 21 ng N m−2 s−1. The flux measurements made by the two techniques, which are based on very different approaches as described herein, agree very well.

Photosynthesis and growth of spring barley: some effects of drought

SummaryDuring the dry summer of 1976, measurements were made of the photosynthesis, transpiration, respiration, and growth of irrigated (I) and non-irrigated (NI) spring barley growing in large field plots. Using a field enclosure, the photosynthesis of the irrigated barley was measured on 19 separate days during the latter two-thirds of the growing season when the plants were large enough to have measurable gas exchanges. The response of photosynthesis to water stress was determined from 3 days' comparative measurements on the I and NI crops, using both the field enclosure and, on 2 days only, a single-leaf photosynthesis chamber.Water stress in the NI crop caused large decreases in yield: the dry-matter yields of grain and straw were respectively reduced by 19 and 27%; number of grains (but not grain mass) was also reduced. Field enclosure measurements, which were in good agreement both with values for canopy net photosynthesis derived from the leaf chamber measurements and with estimates of dry-matter production derived from plant weighings, indicated that the net CO2 uptake per unit leaf area was little affected by water stress. Results from the plant weighings and mensurations showed that, during the growing season, the main effect of water stress, mediated by the survival of fewer tillers and the premature senescence of leaves, was a reduction of leaf area (by 40%).

The Hooded Antenna: An Approach to Meaningful Field Strength Measurements in Shielded Enclosures

A technique is described for significantly reducing the effect of multipath interference on field strength measurements made in conventional shielded enclosures by hooding the receiving antenna. Field strength measurements in a shielded enclosure and corresponding measurements in the open field were made at frequencies from 1 to 7 Gc/s with and without an antenna hood. It was found that when the appropriate insertion loss constants are applied to the hooded antenna data taken in a shielded enclosure, the agreement between these data and the data from the same experiments performed in the open field without a hood is excellent. Principal advantages of the hooded antenna over anechoic chambers include reduced cost, reduced inside space requirements for absorbing material, and applicability of use in most existing shielded enclosures.