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.

incorporate natural VOC emission estimates into ozone control strategies in both the United States and Europe. A more recent version of this model (BEIS2)includes improved methods for estimating landscape data and canopy environment [Geron et al., 1994]. BEIS2 also utilizes updated emission rate capacities , and experimentally verified relationships between emissions and environmental conditions [Guenther et al., 1993]. The VOC emission rates estimated by BEIS2 for most landscapes range from slightly less than BEIS to as much as a factor of 5 higher [ Geron et al., 1995]. Geron et al. [1994]  The surface layer (SL) flux region has a radius of 0.5 km and is centered on a 44-m walkup tower that provided a platform for leaf and branch enclosures and for above-canopy flux measurement systems (relaxed eddy accumulation and surface layer gradients). The mixed layer gradient (MLG) and mixed layer mass balance (MB) regions are centered on a clearing, about 400 m north of the walkup tower, where a tethered balloon sampling system was deployed. The MLG region has a 3-km radius and includes the landscape that influences mixed layer gradient flux estimates. The MB region has a 14-km radius and includes the landscape influencing the mixed layer mass .balance flux estimates. Previous studies have demonstrated that meaningful above-canopy surface layer flux estimates can be made with instruments on the 44-m tower even though it is located on a ridge in moderately complex terrain [Baldocchi and Harley, 1995;Verma et al., 1986]. The meteorological conditions during the study were typical of summer in the eastern United States. Maximum photosynthetically active radiation (PAR) fluxes were over 1700 gmol m -2 s -1. Above-canopy winds ranged from about 0.1 to 4.6 m s -1, predominately from the southwest (54%) and northeast (22%) quadrants that correspond to flow along the ridge. Temperatures during sampling periods ranged from 18 ø to 31 øC (mean = 24.5øC), and relative humidity ranged from 40% to 87% (mean = 77%). The hot and humid aRemoons were often accompanied by thunderstorms.

Land Cover Characterization
Thirty-one circular 30 m diameter (707 m2) plots were established along three transects located in representative vegetation types within the MLG and MB region. Each plot was sampled for species composition, diameter at breast height (dbh, ---1.5 m), tree height, seedling and sapling count, leaf area index, and percent cover of the dominant growth forms (evergreen scrub, deciduous shrub, ericaceous shrub, forb, graminoid, vine, lichen, pteridophyte, and moss). Individual trees were assigned to one of three classes: trees, > 10 cm dbh; saplings, dbh = 4 to 10 cm and height > 1.5 rn; and seedlings, dbh < 4 cm or height < 1.5 m In addition to the transects, an area of 3500 m 2 within the SL region was sampled for tree species composition and diameter at breast height information.
A land cover database for the Oak Ridge region was developed using a Landsat satellite Thematic Mapper (TM) classification image created from multidate imagery and supervised classification techniques, referred to here as the Thematic Mapper Land Cover (TMLC) database. The classification procedures and results will be described in detail elsewhere. The TMLC database has 30 m spatial resolution and covers an area of approximately 90 km X 90 km. The database contains 1 1 land cover classes: loblolly pine, mixed pine, regrowing (young) pine, high oak deciduous, medium oak deciduous, low oak deciduous, shrubs and grasses, agriculture, water, bare soil and urban. Four general categories are shown in Plate 1. Total foliar density and the fraction contributed by each tree species were estimated for each land cover class from forest statistics data collected in the 30-m circular plots.

Flux Measurement Techniques
Isoprene fluxes were estimated using the eight measurement systems listed in Table 1. The four systems used to estimate fluxes from individual leaves or branches included a portable leaf cuvette with environmental control (LEC), portable leaf cuvettes with no environmental control (LNC), a towermounted branch enclosure for investigating branches at different canopy depths (BCS) and a tripod-mounted branch enclosure for investigating lower branches of various tree species throughout the region (BRS). Two systems were used to estimate fluxes in the surface layer above the forest canopy: a relaxed eddy accumulation (REA) system and a system that estimated fluxes from isoprene gradient profiles and an eddy diffusivity based on eddy correlation measurements of water vapor (GPEC). Data collected with a tethered balloon system were used to estimate landscape scale fluxes using both a mixed layer gradient (MLG) technique and a mass balance (MB) technique.
Enclosure methods. Three of the enclosure systems (LEC, LNC, and BCS) are described in detail by P. Harley et al. (1996b). The LEC system consisted of an open path gas exchange system that provided control of temperature, light intensity, water vapor, and CO2 concentration within the enclosure. The LNC system included three separate leaf where f is the flow rate (cubic meters per hour) into the enclosure, Co is the isoprene concentration (micrograms carbon per cubic meters) of the outlet airstream, C i is the isoprene concentration (micrograms carbon per cubic meter)of the inlet airstream and b •s the foliar mass (grams dry weight) within the enclosure.
The 30-L cylindrical enclosure used for the BRS system was constructed of Teflon film over a stainless steel support frame. The enclosure was supported by an external PVC pipe frame mounted on a camera tripod. Sweep air was supplied from compressed gas cylinders at flow rates of 10 to 12 L min-1, measured with a mass flow meter. Ambient humidity and CO 2 levels were approximated in the sweep air by mixing hydrocarbon and CO2 free air with hydrocarbon flee air containing 2% CO2 and then passing this air through distilled water. Ancillary measurements included leaf temperature, relative humidity, PAR, and CO 2 concentration. The entire apparatus was battery powered and mounted on a cart for mobility. Whole air samples from the enclosure were collected in Sumadeactivated stainless steel canisters and analyzed by GC with flame ionization detector (FID) described in section 2.3. Surface layer methods. Isoprene fluxes in the surface layer above the forest canopy were measured by relaxed eddy accumulation and gradient profile methods. The data presented in this paper are the first measurements of isoprene fluxes with an REA system. Tracer and gradient profile methods have previously been used to measure isoprene fluxes [Lamb et al., 1985;Lamb et al., 1986]. Previous applications of the gradient profile method used indirect estimates of the relationship between scalar fluxes and gradients. The REA system was deployed at a height of 30.5-m AGL on the walkup tower. Isoprene and CO2 fluxes were estimated using the relationship described by Businger and Oncley [1990] :15 (Cu -ca) an (2) where 13 is a nondimensional coefficient and Cu and Ca are the mean concentrations associated with updrafts and downdrafts, respectively. A 4-min running mean for vertical wind speed, Wo (meter per second), and standard deviation of vertical wind velocity, (•w (meter per second), were calculated in real time and used to calculate the vertical wind speed threshold (wr). This value was used to separate whole air samples into updraft (w'> w/,), downdraft (w' <-w/,) and near zero (-w/, < w' < w/,) zomponents, where w' is the difference between the instantaneous vertical wind speed and Wo. The threshold velocity, w r = 0.5 (•w, was selected to maximize the signal-to-noise ratio of (Cu -Ca). Oncley et al. [1993] discuss the choice of w7, in some detail and conclude that a value of around 0.6 (•w is optimal.
Estimates of [5 and (•w were calculated for each 27-min sampling period using wind and temperature data stored in digital format at 10 Hz. Coordinate transformations were performed on wind velocity data to set the mean vertical wind speed to zero.

Estimates of •3 were calculated by assuming similarity between
isoprene and sensible heat fluxes. Eddy covariance estimates of sensible heat flux, w'T', and the mean temperatures associated with updrafts and downdrafts, Tu and Ta, were used to estimate 13 by rearranging (2) and substituting w'T' for F, T u for Cu and Tcl for Ca. Mixed layer. The tethered balloon profiling system consists of a commercial helium-filled tethered balloon and meteorological sounding system (AIR, Boulder, Colorado) and a custom made whole air sampling unit that attaches to any point on the tether line and pumps air into 15-L Teflon bags. Automatic timers were used to collect 30-min samples simultaneously at two to four heights between 50 and 800 m AGL. Whole air samples were analyzed for isoprene by the GC-FID method described in section 2.3. Isoprene fluxes were calculated using the mixed layer mass balance (MB) and mixed layer gradient (MLG) techniques.
The MB method assumptions and associated uncertainties are discussed by Guenther et al. [1996]. MB fluxes are calculated as

F = zi Cm V-1 (3)
where zi is the mixed layer height (m AGL), z is the estimated lifetime of isoprene (s), and Cm (milligrams carbon per cubic meter) is the mean mixed layer isoprene concentration. Estimates of zi were obtained using airsondes (AIR, Boulder, Colorado) that measure temperature and humidity profiles up to heights of 5 km AGL. The mixed layer height was identified by an inversion layer that appears as a region of increasing potential temperature with height. To estimate the lifetime of isoprene, z, we used the OH and ozone reaction rate coefficients reported by [,4tkinson, 1990], the measured ozone concentration, the OH diurnal variation described by

Isoprene Analysis
The GC-RGD and GC-FID systems deployed at the field site were intercalibrated using a compressed gas standard containing 71-ppbv isoprene referenced to a National Institute of Standards and Technology (NIST)propane standard on the GC-FID. Compound identification was accomplished by retention time comparison with known standards and by GC with mass spectrometer (MS) analysis of samples transported in stainless steel canisters to Boulder, Colorado. The isothermal GC-RGD system has a 2-mL sample loop, a stainless steel column (1.3m long x 2mm ID) packed with Unibeads 3S, 60/80 mesh (Alltech Assoc., Deerfield, Illinois), and an RGD2 (Trace Analytical, Menlo Park, California) detector. This system does not require any preconcentration for isoprene mixing ratios above 1 ppbv and is described in detail elsewhere [Greenberg et al., 1993].
The Hewlett-Packard 5890 GC-FID system was equipped with a 30-m DB-! fused silica capillary column (J&W Scientific). A 100-500 mL whole air sample was preconcentrated on a cryogenically (liquid 02)cooled' stainless steel loop containing 60-80 mesh silanized glass beads. The concentrated samples were transferred to the GC column by immersing the loop in an 80ø-90øC water bath. The GC oven was temperature programmed from-50øC to 80øC at 4øC min -1.

Isoprene Emission Model
Isoprene emission rates, E (gg C g-1 h-l), for individual leaves or branches are modeled as

E=• ),/5 (4)
where • is an isoprene emission capacity (gg C g-1 h-l) that represents the emission rate expected for a particular plant species at specified conditions (e.g., sun leaf during peak growing season, leaf temperature of 30øC and PAR of 1000 gmol m -2 s-1). The nondimensional emission activity factor, T, accounts for variations in emissions due to changes in leaf temperature and PAR and is estimated using the equations of Guenther et al. [1993]. Leaf level estimates of T can be estimated directly from leaf level PAR and leaf temperature. Branch level estimates are divided by a factor of 1.75 to account for decreases in PAR due to self shading . where D is total foliar density (g dry weight m -2) and •,T, and/5 are all area-averaged estimates of corresponding leaf level parameters. The area-averaged • represents the weighted average of all plant species within the area.

• = (•OAK Dom•:) + (•Pe4[• D?INE) + (•MAPI•[• DM•J,oeE) +... (5b)
where DOA K is the ratio of oak foliage to total foliage. Since oak trees are estimated to be responsible for over 95% of all isoprene emissions in each of the regions shown in Plate 1, we can simplify the following discussion by multiplying F by a factor A, equal to the fraction of total isoprene emissions contributed by oaks, and then neglecting isoprene emissions from vegetation other than oaks: We can compare the results from each measurement technique by inverting (6a) and solving for eOAK.

oeOAK = FA / [7 5 D DOAK] (6b)
Total foliar density, D, and the fraction of oaks, DOAK, for the SL, MB, and MLG regions were estimated using the techniques described in section 2.1. Canopy-averaged 7 and /5 were estimated by dividing the canopy into "sun" and "shade" leaf components and using the leaf level procedures described above. Estimates of PAR for sun leaves and for shade leaves were based on measured leaf area index (LAI) and above-canopy PAR and calculated sun angle using the sunfleck radiative transfer model of Norman [1982]. Relative humidity, wind speed, and ambient temperature near sun and shade leaves were estimated from above-canopy relative humidity, wind speed, and ambient temperature using vertical profiles similar to those of Lamb et al. [1993]. Leaf temperatures for the shade and sun leaves were then calculated from estimates of total radiation, ambient temperature, wind speed, and relative humidity using a leaf energy balance model [Lamb et al., 1993]. To convert between the LAI values used in the radiative transfer model and the foliar density estimates used in the emission model, we assume that shade leaves have a specific leaf weight that is 70% [Geron et al., 1994;P. Harley et al., 1996b] of the value used for sun leaves. Table 1 and described in section 2 are reported in this section. The results are compared and used to evaluate emission model procedures in section 4.

Leaf and Branch Fluxes
Isoprene fluxes from tree species of nine different genera were investigated with the BRS enclosure system. All of the sampled trees were growing within the MB region (Plate 1). Negligible isoprene emission rates were observed for six species: Liriodendron tulipifera, Oxydendrum arboreum, Carya tomentosa, Sassafras albidum, Comus florida, and Prunus serotina. Significant isoprene emission rates were observed for species of Quercus, Nyssa, and Liquidambar. These results agree with the emission database compiled by Guenther et al. [1994]. An average emission rate of 1 gg C g-1 h -1 (n=10) determined for Nyssa sylvatica was representative of very low light levels (mean PAR=85 gmol m -2 s -1) and a mean temperature of 24.5øC. Considerably higher mean isoprene emission rates were measured for Liquidambar styracifiua (9.5 gg g-1 h-l, n=14), Q. alba (14.1 gg C g-1 h-l, n=16), Q. prinus (29.4 gg C g-1 h-l, n=15) and Q. velutina (49.0 gg C g-1 h-l, n=l 0). The mean PAR for each set of measurements ranged from 180 to 462 gmol m -2 s -1, while mean leaf temperatures ranged from 24.2 ø to 30.0øC. The oak emission capacities listed in Table 2  The LEC, LNC, and BCS systems were used to measure emissions from a single mature Q. alba tree located adjacent t o the walkup tower. The BCS system averaged a large number of leaves (15 to 30) with each measurement and was used to compare leaf and branch measurements. Individual leaf emission rates were estimated for a large number of leaves with the LNC system. The LEC system was used to investigate emission rate variations associated with changes in PAR and leaf temperature and to investigate leaf-to-leaf variation at constant PAR and temperature. Leaves near the top of the canopy typically had isoprene emission rates of less than 0.1 gg C g-1 h-1 before 700 local standard time (LST) and after 1700 LST with peak emission rates of over 150 gg C g-1 h-1 in early afternoon, associated with PAR fluxes over 1000 gmol m -2 s -1 and leaf temperatures over 33øC. Peak emissions for leaves lower in the canopy were as high as 100 gg C g-1 h-1 and occurred at times ranging from morning to late afternoon depending on their position relative to sunfleck gaps in the canopy.

Surface Layer Fluxes
Isoprene fluxes in the surface layer above the forest canopy were measui'ed by relaxed eddy accumulation and a gradient profile method. The objectives of these surface layer flux measurements included (1) testing the REA system, (2) comparing REA and GPEC estimates of isoprene fluxes, and (3) evaluating the results of isoprene emission models with abovecanopy flux measurements.

Isoprene flux units include a gg C g-• h -• and b mg C m -2 h -1. N is the number of flux measurements. Emission model components are described by equations 5 and 6 and include oak (Quercus) emission rate capacity (eo^Io gg C g-l h-l), emission activity factors (T and/5), total foliar density (D, g dry weight m-2), and the oak fraction of total
foliar density (Do^i0.

Mixed Layer Fluxes
Isoprene mixing ratios in the atmospheric boundary layer above the Walker Branch field site range from less than 0.2 to greater than 5 ppbv isoprene. These data ( Figure 5)   Isoprene mixing ratio profiles were used to estimate fluxes using the mixed layer gradient technique described in section 2.2. Eight of the 29 sampling periods were suitable for calculating fluxes with the MLG method (i.e., significant surface heat flux and minimal cumulus cloud cover). Isoprene fluxes for these eight sampling periods were estimated to be 5.0+1.7 mg C m-2 h-1. The observed profiles and fits are shown in

Emission Model Evaluation
The isoprene emission models described in section 2.4 require accurate estimates of oak emission capacity (CoAt,), emission activity factors (3' and ;5), total foliar density (D) and the fraction of foliage that is oak leaves (Do^r0. Table 2 includes estimates of each of these variables and the actual fluxes estimated with each of the eight techniques (section 2, Table 1). The 30-m resolution Landsat TM land cover (TMLC) database indicates that 98% of the SL region, 82% of the MLG landscape region and 70% of the MB landscape region are covered by one of six forest land cover types. The TMLC estimates of total foliar density shown in Table 2 range from 420 g m-2 in the SL region to 400 g m-2 in the MLG region and 380 g m-2 in the MB region. The total foliar density predicted by the BEIS2 model for the three counties surrounding the field site is about 40% less than the TMLC estimate for the MB region. Table 2 shows that the TMLC estimates of DOA K include 0.20 in the MB region, 0.24 in the MLG region and 0.40 in the SL region. The BEIS2 estimates of DOA K for the three-county area (0.42) is a factor of 2 higher than the TMLC estimate for the MB region.

Foliar Density, D, and Fraction of Oak
The TMLC estimate of oak foliar density in the SL region is 168 g m-2. The various estimates of Lamb et al. [1996] are within about +50% of this value. Considerably lower oak foliar densities are estimated by the TMLC database for the MLG (96 g m-2) and MB (76 g m-2)regions. The TMLC estimate for the MB region is within about 25% of the BEIS2 model estimate for the three-county area. These results indicate that estimates of oak foliar densities are associated with uncertainties of_+50% or less.

Emission Activity Factors, T and $
The enclosure measurement systems were used to investigate emission activity factors. The results summarized in this section are described in detail by P. Harley et al., (1996b A tree branch with a PAR flux of 1000 gmol m -2 s -t at the top of the branch has PAR fluxes on individual leaves that range from less than 100 to 1000 gmol m-2 s-1 due to leaf angle orientation and shading by upper leaves. In addition, the temperature of various leaves on a branch may differ by several degrees. Guenther et al. [1994] recognized that the emission activity factor applied to a branch must be adjusted to account for these effects. They noted that field comparisons of the ratio between leaf and branch level emission rates range from 1.46 to 2.03 [Guenther et at., 1996] and recommended that ¾ be divided by a factor of 1.75 for branch measurements. We evaluated this ratio by measuring isoprene emission rates for whole branches and individual leaves within the canopy of a mature oak (Q. atba) tree with the BCS and LNC systems. The ratio of leaf-to-branch emission rates was 1.92 near the top of the canopy and decreased to 1.69 in the middle of the canopy. The radiative transfer model and emission algorithms described in section 2.4 predict that this ratio will vary between 1.3 and 1.9 depending on branch LAI, mean leaf orientation angle, and solar elevation. This ratio increases with decreasing mean leaf angle, increasing LAI and decreasing solar elevation. These results support the Guenther et at.
[1994] recommendation to estimate ¾ based on above branch PAR and temperature and then divide by 1.75 to calculate a branch average ¾. An overall uncertainty of about +30% is associated with estimates of ¾ for branch enclosure measurements.
Uncertainties in estimating ¾ for an entire forest canopy are greater than those associated with branch measurements. Lamb et al. [ 1996] evaluated the canopy average estimates of ¾ predicted by five different canopy environment models for the REA and GPEC estimates. The complexity of these canopy environment models range from treating the canopy as a single leaf to a detailed numerical model that accounts for leaf-sun geometry, leaf energy balance, photosynthesis, transpiration, respiration, and gas transport within the canopy. The results indicate that the various models predict fluxes that are within approximately +20%. Additional uncertainties in the leaf level relationships (about _+15%) and in estimating ambient temperature and above-canopy PAR (about _+25%) result in an overall uncertainty of about _+35% for ¾.
The diurnal patterns of predicted and observed fluxes for the entire study period (Figure 4) show positive fluxes between 700 and 1800 LST with a maximum occurring in early afternoon. The observed diurnal patterns are consistent with emission model predictions.

Emission Capacity, •
Emission capacities tbr individual leaves or for an entire branch are calculated from (6) using the measured emission rate and an estimate of ¾ calculated from the algorithms described by Guenther et al. [1993]. Measurements of individual leaves at conditions where ¾ = 1 result in estimates of e that have relatively low uncertainties (_+10%). Uncertainties in e are considerably higher when ¾ must be estimated from leaf temperature and PAR conditions that deviate substantially from standard conditions. Uncertainties in extrapolating e obtained for a few leaves to all leaves in a forest belonging to a particular plant genus are often about +50%

g-t h-t for Liquidambar species and 12_+6 gg C g-t h -• for
Nyssa species. Estimates of e for the three oak species measured with the BRS system range from 75 gg C g-t h -I for Q. alba to 114 gg C g-t h-t for Q. velutina. Emission capacities estimated with the LEC, LNC, and BCS systems for a single Q. alba tree next to the walkup tower range from 91 to 111 gg C g-1 h-l. The lowest uncertainty is associated with the isoprene emission capacity of 99 gg C g-1 h-l estimated from measurements on sun leaves with a leaf temperature of 30øC and PAR of 1000 gmol m-2 s-l. Eight of the nine leaf and branch estimates of e listed in Table 2 fall within the relatively narrow range of 100•-_11 gg C g-1 h-l.
Estimates of area-averaged oak emission capacities were estimated from above-canopy surface layer and mixed layer measurements and (6). The isoprene emission capacities estimated for oaks in the SL flux region include 86 gg C g-1 h-1 with the GPEC system and 102 gg C g-l h -t with the REA system. The MLG estimate of 148 gg C g-• h-• is the highest oak emission capacity listed in Table 2. The MB estimate for the same eight sampling periods used for the MLG estimates i s 122 gg C g-1 h-1 and is about 20% higher than the mean emission capacity calculated for all 29 sampling periods. The surface layer estimates of oak isoprene oe (94+8 I. tg C g-1 h-l) are about 25% less than the mixed layer estimates of oak isoprene oe (124+22 gg C g-1 h-l).
The mean oak isoprene emission capacities for each measurement scale shown in Table 2 include 102 gg C g-1 h-1 for leaf enclosure measurements, 100 I. tg C g-1 h-1 for branch enclosure measurements, 94 I. tg C g-1 h-1 for above-canopy surface layer measurements, and 124 I. tg C g-1 h-1 for above-canopy mixed layer measurements. These results demonstrate that the emission model techniques, which enable us to estimate the oak isoprene emission capacity associated with each measurement, produce results within about +25% which is within the uncertainties associated with these flux measurements. Significant deposition losses within the forest canopy would result in lower emission capacities estimated by the abovecanopy flux measurement techniques. While one of the surface layer techniques (GPEC) had a somewhat lower estimated oe, the other techniques (REA, MB, MLG) resulted in oak isoprene emission capacities that were about equal to or higher than the emission capacities estimated from enclosure techniques. This result suggests that although it is likely that isoprene deposition losses occur, as indicated by the morning and evening surface layer flux measurements, they are a relatively minor component of the net flux. Thirteen of the fourteen emission capacities estimated in Table 2 are within +25% of 100 gg C g-1 h-1 and the emission capacity estimated using the mixed layer gradient technique (148 gg C g-1 h-l) is even greater. These results demonstrate that the isoprene emission capacity for oak trees, in at least this region, is higher than the value used in existing models (70 gg C g-1 h-1 for BEIS2, 14.7 gg C g-1 h-1 for BEIS).

Summary
Eight flux measurement techniques were used to estimate isoprene fluxes from a temperate forest. Fluxes from individual leaves and branches were measured with enclosure systems, above-canopy fluxes were measured from a tower by relaxed eddy accumulation and surface layer gradients, and fluxes from landscapes covering an area of up to a few hundred square kilometers were estimated using a tethered balloon sampling system. Each measurement system provided specific advantages for investigating different aspects of biogenic emission models. The results from all measurement techniques demonstrate that existing emission models underestimate the isoprene emission capacity for oaks in this region and a value of 100 I. tg C g-1 h-1 is recommended. The results also demonstrate that reasonable estimates of isoprene fluxes and boundary layer isoprene mixing ratios can be predicted with existing models if accurate data (e.g., emission capacities and land characteristics data)are available for initializing the models. It is clear that considerable uncertainties exist in field flux measurements and in each of the main components of emission models. Research directed at narrowing these uncertainties and acquiring data for initializing models in other regions remains a priority for biogenic emission studies.