Micrometeorological Gradient Research Articles

Verification of micrometeorologically determined nitrous oxide fluxes following controlled release from pasture

We have developed a high-precision micrometeorological system capable of measuring emissions of nitrous oxide (N2O) from up to four adjacent pasture plots. The system can be used to compare the influence of environmental factors and management practice on N2O emissions at the paddock scale. The system is capable of determining a minimum detectable N2O difference of the order of 40 pmol/mol, with an ability to resolve flux differences among plots of ~26 µg (N2O-N)/m2.h. So as to independently verify the emission estimates of the micrometeorological system, we developed a calibrated N2O-release system and compared known release rates with the micrometeorological flux estimates. Adjustable release rates up to the equivalent average surface flux of ~500 µg (N2O-N)/m2.h were achieved using mass flow-controlled input of pure N2O in a compressed air stream over two 1.5-ha plots upwind of flux-measurement masts. The comparison of network release rate with measured emission rate was quite variable and complicated by a significant and varying background emissions of N2O from the soil. For optimal steady-wind cases, the ratio of uncorrected measured flux to known release, including the estimated background, was of the order of 0.4–0.5; this ratio is likely to be influenced by the turbulent Schmidt number. Flux estimates for uncorrected flux gradient and WindTrax backward Lagrangian Stochastic method (which includes Schmidt correction) agreed well with a ratio of 0.54. The experiment highlighted the need for accurate estimates of gas eddy diffusivity in the micrometeorological gradient or difference-based flux measurement of N2O.

A modified micrometeorological gradient method for estimating O<sub>3</sub> dry depositions over a forest canopy

Abstract. Small pollutant concentration gradients between levels above a plant canopy result in large uncertainties in estimated air–surface exchange fluxes when using existing micrometeorological gradient methods, including the aerodynamic gradient method (AGM) and the modified Bowen ratio method (MBR). A modified micrometeorological gradient method (MGM) is proposed in this study for estimating O3 dry deposition fluxes over a forest canopy using concentration gradients between a level above and a level below the canopy top, taking advantage of relatively large gradients between these levels due to significant pollutant uptake in the top layers of the canopy. The new method is compared with the AGM and MBR methods and is also evaluated using eddy-covariance (EC) flux measurements collected at the Harvard Forest Environmental Measurement Site, Massachusetts, during 1993–2000. All three gradient methods (AGM, MBR, and MGM) produced similar diurnal cycles of O3 dry deposition velocity (Vd(O3)) to the EC measurements, with the MGM method being the closest in magnitude to the EC measurements. The multi-year average Vd(O3) differed significantly between these methods, with the AGM, MBR, and MGM method being 2.28, 1.45, and 1.18 times that of the EC, respectively. Sensitivity experiments identified several input parameters for the MGM method as first-order parameters that affect the estimated Vd(O3). A 10% uncertainty in the wind speed attenuation coefficient or canopy displacement height can cause about 10% uncertainty in the estimated Vd(O3). An unrealistic leaf area density vertical profile can cause an uncertainty of a factor of 2.0 in the estimated Vd(O3). Other input parameters or formulas for stability functions only caused an uncertainly of a few percent. The new method provides an alternative approach to monitoring/estimating long-term deposition fluxes of similar pollutants over tall canopies.

Characterizing spatial and seasonal variability of carbon dioxide and water vapour fluxes above a tropical mixed mangrove forest canopy, India

The above canopy carbon dioxide and water vapour fluxes were measured by micrometeorological gradient technique at three distant stations, within the world’s largest mangrove ecosystem of Sundarban (Indian part), between April 2011 and March 2012. Quadrat analysis revealed that all the three study sites are characterized by a strong heterogeneity in the mangrove vegetation cover. At day time the forest was a sink for CO2, but its magnitude varied significantly from −0.39 to −1.33 mg m − 2 s − 1. The station named Jharkhali showed maximum annual fluxes followed by Henry Island and Bonnie Camp. Day time fluxes were higher during March and October, while in August and January the magnitudes were comparatively lower. The seasonal variation followed the same trend in all the sites. The spatial variation of CO2 flux above the canopy was mainly explained by the canopy density and photosynthetic efficiency of the mangrove species. The CO2 sink strength of the mangrove cover in different stations varied in the same way with the CO2 uptake potential of the species diversity in the respective sites. The relationship between the magnitude of day time CO2 uptake by the canopy and photosynthetic photon flux was defined by a non-linear exponential curve (R2 ranging from 0.51 to 0.60). Water vapour fluxes varied between 1.4 and 69.5 mg m − 2 s − 1. There were significant differences in magnitude between day and night time water vapour fluxes, but no spatial variation was observed.

Development and evaluation of a sampling system to determine gaseous Mercury fluxes using an aerodynamic micrometeorological gradient method

An aerodynamic gradient micrometeorological approach to the measurement of total gaseous mercury (TGM) flux has been developed. This method has been applied in many field studies for the characterization of TGM flux from various mercuriferous substrates. The resolution of the gradient method depends on the sampling systems characteristics and has been demonstrated to be on the order of 0.01 ± 0.01 ng Hg m−3 or better. The method is best suited to measuring high‐emitting sites such as studied here. The TGM flux resolution is based on the gradient resolution and depends on the site characteristics and the atmospheric condition. For a typical friction velocity u* of 0.1 m s−1 and gradient intake heights of 0.15 and 0.4 m the method can resolve a TGM flux on the order of 1.5 ng m−2 h−1. The system can be configured for two‐level or multilevel sampling, as needed. The method compares well with other micrometeorological methods as demonstrated during the Nevada storms intercomparison study. The micrometeorological method is shown to compare well with chamber techniques under comparable conditions.

Ammonia flux and dry deposition velocity from near-surface concentration gradient measurements over a grass surface in North Carolina

Atmospheric ammonia concentrations were measured during fall 2001, winter, spring, and summer 2002 over natural surfaces in North Carolina where animal farms and waste storage and treatment lagoons (which are known to emit ammonia) at two heights (2 and 6 m) above the surface employing an arrangement of two Thermo Environmental Instruments, Inc. Model 17C chemiluminescent nitrogen oxides (NOX)–ammonia (NH3) analyzers along with a solenoid for each analyzer to alternate measurements between the two elevations. Simultaneously, mean winds and temperatures are also measured at the same two heights. The micrometeorological gradient method is used in conjunction with the Monin–Obukhov similarity theory, to estimate the vertical flux and dry deposition velocity of ammonia under different meteorological conditions. Diurnal and seasonal variations of ammonia flux and dry deposition velocity were investigated under a wide range of wind and atmospheric stability conditions yielding hourly variation of NH3 fluxes and deposition during each seasonal campaign. Fall average NH3 concentrations were generally the highest with daytime concentrations of 7.60±6.54 μg m−3 at 2 m, while nighttime NH3 concentrations were 7.25±6.14 μg m−3 at 2 m. The winter season had the lowest overall concentrations collected during each seasonal campaign with averages of 1.73±2.00 μg m−3 (2 m) during daytime and 1.37±1.50 μg m−3 (2 m) during nighttime. Deposition and emission occur, both on daily and seasonal scales. Summer measurements yielded the largest average daytime deposition velocity of 3.94±2.79 cm s−1, whereas winter measurements gave the lowest daytime velocities with an average of 2.41±1.92 cm s−1. The largest nighttime deposition velocities were estimated during the summer season with an average of 0.76±1.69 cm s−1, whereas fall nighttime deposition velocities were considerably lower at 0.07±0.17 cm s−1. Regression relationships between the ammonia deposition velocity and the friction velocity are obtained for different stability conditions.

Measuring field-scale isotopic CO 2 fluxes with tunable diode laser absorption spectroscopy and micrometeorological techniques

The combination of micrometeorological and stable isotope techniques offers a relatively new approach for elucidating ecosystem-scale processes. Here we combined a micrometeorological gradient technique with tunable diode laser absorption spectroscopy (TDLAS) using the Trace Gas Analyzer (TGA100, Campbell Scientific, Inc., Utah, USA) to measure field-scale isotopic CO 2 mixing ratios and fluxes of 12 CO 2 and 13 CO 2 . The experiment was conducted in a recently harvested soybean ( Glycine max) field that had been in corn ( Zea mays) production the previous 4 years. Measurements were made over a period of 26 days from October 25 to November 19, 2002. Weather conditions were unusually cold and dry during the experiment. Isotopic gradients were small and averaged −0.153 and −0.0018 μmol mol −1 m −1 for 12 CO 2 and 13 CO 2 , respectively for u ∗>0.1 m s −1. The average 12 CO 2 and 13 CO 2 flux for the period was 1.0 and 0.012 μmol m −2 s −1, respectively. The isotope ratio of respired carbon ( δ 13 C R ) obtained from the linear intercept of a Keeling plot was −27.93‰ (±0.32‰) for the experimental period. The Keeling plot technique was compared to a new flux ratio methodology that estimates δ 13 C R from the slope of a linear plot of 13 CO 2 versus 12 CO 2 flux. This method eliminates a number of potential limitations associated with the Keeling plot and provides a δ 13 C R value that can be directly related to the flux footprint. In this initial comparison, our analysis showed that the flux ratio method produced a similar δ 13 C R value (−28.67‰), but with greater uncertainty (±2.1‰). Better results are expected during growing season conditions when fluxes are substantially larger and the signal to noise ratio is improved. The isotope ratio of respired carbon was consistent with C 3 agricultural systems indicating that soybean decomposition was the dominant substrate for respiration. The observed increase in ecosystem respiration ( R E) and decrease in δ 13 C R following tillage indicated that the incorporation of fresh soybean residue provided the major source for decomposition and further illustrates that the combination of micrometeorological and stable isotope techniques can be used to better interpret changes in carbon cycle processes. Long-term and continuous measurements of isotopic CO 2 exchange using tunable diode laser absorption spectroscopy and micrometeorological techniques offers a new opportunity to study carbon cycle processes at the field-scale.

Measurements of sulfur dioxide and ozone dry deposition over short vegetation in northern China—a preliminary study

We measured dry deposition fluxes of SO2 and O3 over a field covered mostly with short grass in a suburb of Beijing during late summer and early winter as an initiatory attempt to obtain information on dry deposition in northern China. The measurement was based on a micrometeorological gradient method, and only the data conforming to neutral atmospheric stability were analyzed because the data we collected were not adequate to extend the analysis to other stability cases. The data were further screened out according to a few selection criteria to ensure their credibility. For the selected periods Vd (deposition velocity) of SO2 showed a maximum in the morning and a minimum in the afternoon, with little seasonal dependence in this behavior. The mean (±standard deviation) Vd was 0.2 (±0.1) cm s−1 in late summer and 0.4 (±0.2) cm s−1 in early winter. The diurnal variation of Vd seemed to be more or less governed by that of Ra (aerodynamic resistance) +Rb (quasi-laminar boundary layer resistance), which was considerably larger than Rc (canopy resistance). The mean (±standard deviation) Vd of O3 was 0.2 (±0.2) cm s−1 in late summer and 0.4 (±0.3) cm s−1 in early winter, thus much the same as those for SO2. The variation of O3 Vd was qualitatively similar to that of SO2, but in this case Rc was comparable to or larger than Ra+Rb. Little distinct seasonal dependence was evident for O3 deposition, either. A tendency was discernible for Rc to increase in the afternoon, which was probably caused by a decline in stomatal conductance due to an increase in vapor pressure deficit and/or a rise in temperature.

Conditional sampling for measuring mercury vapor fluxes

Surface–atmosphere mercury fluxes are difficult to measure accurately. Current techniques include dynamic flux chambers and micrometeorological gradient and aerodynamic approaches, all of which have a number of intrinsic problems associated with them. We have adapted conditional sampling (relaxed eddy accumulation), a micrometeorological technique commonly used to measure other trace gas fluxes, to measure surface–air mercury fluxes. Our initial flux measurement campaign over an agricultural soil consisted of two 1-week measurement periods, and was longer in duration than previously reported mercury flux measurement periods. Fluxes during both measurement periods ranged between 190.5 (evolution) and –91.7 ng m −2 h −1 (deposition) with an average evolution of 9.67 ng m −2 h −1. The data showed significant diurnal trends, weakly correlated with shallow soil temperatures and solar radiation. This initial trial run indicates that conditional sampling has much promise for the accurate quantification of both short and long-term mercury fluxes.

Dry deposition of ammonia, nitric acid, ammonium, and nitrate to alpine tundra at Niwot Ridge, Colorado

Micrometeorological measurements and ambient air samples, analyzed for concentrations of NH 3, HNO 3, NH 4 +, and NO 3 −, were collected at an alpine tundra site on Niwot Ridge, Colorado. The measured concentrations were extremely low and ranged between 5 and 70 ng N m −3. Dry deposition fluxes of these atmospheric species were calculated using the micrometeorological gradient method. The calculated mean flux for NH 3 indicates a net deposition to the surface and indicates that NH 3 contributed significantly to the total N deposition to the tundra during the August–September measurement period. Our pre-measurement estimate of the compensation point for NH 3 in air above the tundra was 100–200 ng N m −3; thus, a net emission of NH 3 was expected given the low ambient concentrations of NH 3 observed. Based on our results, however, the NH 3 compensation point at this alpine tundra site appears to have been at or below about 20 ng N m −3. Large deposition velocities (>2 cm s −1) were determined for nitrate and ammonium and may result from reactions with surface-derived aerosols.

Isoprene emission inventory for the BOREAS southern study area.

An isoprene emission inventory for a section of boreal forest in central Saskatchewan was developed based on measured emission rates from the two dominant isoprene-emitting species, black spruce (Picea mariana (Mill.) BSP) and aspen (Populus tremuloides Michx.). The micrometeorological gradient technique was used to determine isoprene emission factors for establishing the inventory. Isoprene fluxes were measured during each of the three BOREAS intensive field campaigns (IFCs) during the 1994 growing season. Measured isoprene fluxes varied from 0.04 to 3.3 mg C m(-2) h(-1) over the black spruce canopy, and from 0.05 to 7.3 mg C m(-2) h(-1) above the aspen forest. Midsummer standard isoprene emission fluxes were 1.2 mg C m(-2) h(-1) and 2.3 mg C m(-2) h(-1) (at 20 degrees C and photosynthetically active radiation (PAR) of 1000 &mgr;mol m(-2) s(-1)) for black spruce and aspen, respectively. With light and temperature differences accounted for, there was an apparent seasonal effect on emissions with the highest rates in the summer months. The total amount of isoprene emitted from this section of the boreal forest was estimated to be 8.6 Gg C year(-1), which is about 1% of the net ecosystem carbon exchange for the study area. Aspen was the largest contributor, accounting for approximately 70% of the total. Branch enclosure and relaxed eddy accumulation measurements made at the black spruce site were used to define the uncertainty associated with flux measurements. Emission rates obtained by the gradient, enclosure and relaxed eddy accumulation methods showed good agreement when normalized to standard light and temperature conditions. The coefficient of variance between the three techniques was 12% for summer (IFC-2) measurements. The sensitivity of the annual isoprene emission total to the assignment of mean irradiance and temperature was also examined. If the hourly mean temperatures were increased by 1 degrees C throughout the growing season, annual carbon loss due to isoprene emission would increase by 14% from 8.6 to 9.8 Gg C.

The use of stable isotopes to study ecosystem gas exchange.

Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem research is increasing. In this review we introduce and discuss the relevant details underlying the use of carbon and oxygen isotopic compositions in ecosystem gas exchange research. The current use and potential developments of stable isotope measurements together with concentration and flux measurements of CO2 and water vapor are emphasized. For these applications it is critical to know the isotopic identity of specific ecosystem components such as the isotopic composition of CO2, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant- atmosphere system. Combining stable isotopes and concentration measurements is very effective through the use of "Keeling plots." This approach allows the identification of the isotopic composition and the contribution of ecosystem, or ecosystem components, to the exchange fluxes with the atmosphere. It also allows the estimation of net ecosystem discrimination and soil disequilibrium effects. Recent modifications of the Keeling plot approach permit examination of CO2 recycling in ecosystems. Combining stable isotopes with dynamic flux measurements requires precision in isotopic sampling and analysis, which is currently at the limit of detection. Combined with the micrometeorological gradient approach (applicable mostly in grasslands and crop fields), stable isotope measurements allow separation of net CO2 exchange into photosynthetic and soil respiration components, and the evapotranspiration flux into soil evaporation and leaf transpiration. Similar applications in conjunction with eddy correlation techniques (applicable to forests, in addition to grasslands and crop fields) are more demanding, but can potentially be applied in combination with the Keeling plot relationship. The advance and potential in using stable isotope measurements should make their use a standard component in the limited arsenal of ecosystem-scale research tools.

Canopy scale monoterpene emissions of Pinus sylvestris dominated forests

Canopy scale monoterpene emissions were measured using the micrometeorological gradient method at two sites, Huhus and Hyytiälä, in the European boreal zone. At both sites the dominant tree species was Pinus sylvestris. The dominant monoterpene emitted by the forests was α-pinene, followed by Δ 3-carene and β-pinene/myrcene. At the more homogenous Huhus site monoterpene emission correlated reasonably well with the IR-temperature of the canopy and the air temperature inside the canopy, and the common exponential equation could be used to describe the emissions. The β-coefficient obtained was 0.15°C −1, which is somewhat higher than the commonly used value of 0.09°C −1. The emission potential calculated using the measured canopy scale emissions and β=0.09°C −1 was 1.2 μg g −1 dw h −1 which is close to 1.5 μg g −1 dw h −1 used in recent emission inventories. The measured emissions at the more heterogeneous Hyytiälä site were not well correlated with the meteorological parameters. However, the coefficients obtained using the Huhus measurements predicted the magnitude of the fluxes measured at Hyytiälä reasonably well.

Increases in mercury emissions from desert soils in response to rainfall and irrigation

As part of an international Hg flux intercomparison at the Steamboat Springs, Nevada, geothermal area, several dynamic soil flux chambers and micrometeorological gradient systems were operated over desert soils in early September 1997. A series of unanticipated convective rain cells impacted the site with the first rainfall in ∼90 days, and the initial 4‐cm rainfall increased soil moisture from ∼0.01 to 0.06% (vol/vol). Several chambers were operating prior to the events, and two were deployed over wet soils following rainfall. Rainfall resulted in an immediate and steep rise in ambient air Hg concentrations and soil Hg emissions which persisted for 12–24 hours. Fluxes increased most quickly and to a greater degree over the wettest soils, and the rate of increase was related to chamber design and flushing rate. The flux response was also apparent in the micrometeorological data. In general, soil emissions increased by an order of magnitude following the rain, and reached levels ∼6 times above those at the same time the previous day. These fluxes were significantly correlated with temperature, radiation, humidity, wind speed, and soil moisture. After drying for ∼40 hours, selected soil plots were manually irrigated with low‐Hg‐distilled water. Mercury emissions responded similarly across the three treated sites, uniformly increasing from ∼60 ng m−2 h−1 pretreatment to ∼650 ng m−2 h−1 posttreatment, which was a factor of ∼6 higher than adjacent control soils. Possible causes of the increases in flux include soil gas displacement, desorption of Hg° by water molecules, and desorption of Hg(II) and subsequent reduction in solution. The kinetics of the flux response, combined with local soil and climatic conditions, suggest that Hg emissions were responding primarily to soil moisture and solar radiation. These data have interesting implications for the role of changing regional climates on biogeochemical cycling of Hg.

Wet and Dry Deposition of Sulphur at the Site Melpitz in East Germany. In memorium dedicated to Wolfgang Rolle

Sulphur concentrations in precipitation as sulphate and in air as sulphur dioxide were determined from October 1991 (May 1992), respectively, to December 1995. The total sulphur deposition was calculated from sum of the wet and dry deposition, using the concentration of sulphate and precipitation volume and the concentration and deposition velocity of sulphur dioxide, determined with the micrometeorological gradient method. The concentration of sulphur in precipitation decreased over the period by about 40% and in air about 70%. The wet deposition decreased over the period by about 30%. However there was no decrease in dry deposition despite the reduction in SO2 concentrations because the deposition velocity varied from 0.16 cm s-1 to 0.37 cm s-1 during the period. Therefore no trend was observed in the total sulphur deposition.

Vertical fluxes of monoterpenes above a Scots pine stand in the Boreal vegetation zone

The vertical fluxes of monoterpenes, measured above a boreal forest canopy by a micrometeorological gradient method during two days in summer 1997, are presented. The forest, composed mainly of Scots pine (Pinus sylvestris), emitted significant amounts of monoterpenes, α-pinene and δ3-carene being the major species. The correlations of measured monoterpene fluxes with several meteorological parameters are presented. The best correlation was found with the IR radiation temperature of the canopy. The temperature dependency of the monoterpene fluxes, obtained from the measurements, was β=0.20 °C−1. The mean total monoterpene emission rates, normalized to 30°C with temperature coefficients β=0.09 °C−1 and gb=0.20 °C−1, were 190 ng m−2s−1 and 570 ng m−2s−1, respectively.

Air/surface exchange of mercury vapor over forests—the need for a reassessment of continental biogenic emissions

Atmospheric sources are significant in the cycling of Hg in the biosphere, but there are few reliable data on air/surface exchange of Hg in terrestrial systems. We developed a tower-based micrometeorological gradient method for measuring gas-phase Hg° fluxes over soils and vegetation. We describe here results of the modified Bowen ratio approach from three separate flux sampling campaigns: over a mature deciduous forest at the Walker Branch Watershed in Tennessee, over a young pine plantation in Tennessee, and over the boreal forest floor at the Lake Gårdsjön watershed in Sweden. Our data show that Hg° exchange over these surfaces is bidirectional, but is primarily characterized by emissions from plants and soil. Dry deposition (foliar uptake) is less frequent, of generally lower magnitude, and may be enhanced by surface wetness. We measured emissions over tree canopies in Tennessee in the range of ∼ 10–300 ng m −2 h −1, and over the boreal forest floor in Sweden of ∼ 1–4 ng m −2 h −1. Fluxes were influenced by temperature, solar radiation, and atmospheric turbulence. The ability of trees to emit Hg° from soil pools has now been established. Others have proposed a significant biotic re-emission of Hg° from the oceans, and our data provide the first direct evidence of a similar process in terrestrial systems. These data have been combined with results from chamber studies to estimate the overall flux of gas-phase Hg° between the atmosphere and terrestrial systems. Transpiration of Hg° represents a previously unmeasured mobilization of Hg from the continents to the troposphere. Including this new source term could increase current estimates of so-called natural emissions by over 100%.

Atmospheric ammonia exchange on a heathland in Denmark

Passive flux samplers were used to determine the ammonia exchange on an inland heath in Denmark over the last 2 years. The samplers measured the horizontal ammonia flux directly. Data were sampled continuously for periods of 1–4 weeks. The micro-meteorological gradient method was used with passive flux samplers and cup anemometers at different heights above the vegetation in order to calculate the vertical fluxes of ammonia. First a fixed sampler system was used with tubes mounted in four orthogonal horizontal directions. This system has been successfully applied to measure the emission from fertilized crops. Adapting this type of sampler to measure the deposition to heathlands did not prove to be straightforward. The precision of the calculated ammonia concentrations was too poor to give an acceptable accuracy for the concentration gradient. The problems were (a) driving rain which entered the tubes, (b) too few measuring points in each concentration profile, and (c) too long sampling periods to allow for the low-concentration levels above the heath area and the detection limits. Therefore, a passive flux sampler mounted on a wind vane and fitted with a rain shelter was developed. Results from the first 5 weekly periods are very promising, yielding accurate concentration gradients. The advantages of the passive flux samplers on the wind vane are (a) the minimum measuring period can be approximately halved compared to the fixed samplers, (b) some of the directional correction terms used with the fixed passive flux samplers are dispensed with, and (c) the field and laboratory work is minimized.

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.

Deposition of Ammonia Over an Eucalyptus globulus sp. Forest During Winter: Preliminary Results

Ammonia is a very important air pollutant, playing an widespread role in acid pollution chemistry. The knowledge of the deposition processes (wet and dry) is of extreme importance for both long-range transport modelling and evaluation of critical loads for specific ecosystems.Dry deposition fluxes of NH4+/NH3 over an Eucalyptus globulus sp young plantation were determined during winter in a coastal environment, using the micrometeorological gradient method. The atmospheric concentrations were measured using filter-pack samplers at four heights above the canopy, between 6 and 16 m from the ground. Vertical gradients of temperature and wind speed were also measured allowing for the calculation of aerodynamic resistances Parallel measurements of throughfall were made to estimate global deposition.This paper presents preliminary results of the deposition fluxes and highlights the need for further research in order to model deposition under the studied conditions.

Dry deposition of ammonia and ammonium aerosols over heathland

The dry deposition velocities of ammonia (NH3) and particulate ammonium were determined using the micrometeorological gradient method. About 130 measurements of 1 hour duration were conducted in the Netherlands from 1984 to 1987 in different seasons, mainly over heather/purple moor grass vegetation. The results show that NH3 is deposited more rapidly than other trace gases, such as ozone, sulfur dioxide or nitrogen dioxide. An average deposition velocity of 1.9 cm/s was observed during the measurements (most of them carried out during the daytime) over all heather locations. The average flux of NH3 during these periods was equivalent to 16 kg NH3/ha/yr. The canopy resistance for NH3 varied between close to zero and 150 s/m, which is low relative to the stomatal resistance to water vapor. The effect of several variables on the canopy resistance was investigated, but only that of surface wetness and air temperature was established clearly. The results indicate that wet vegetation acts as an almost perfect sink for NH3. The low canopy resistance observed and the lack of any diurnal variation suggest that uptake at the leaf surface is relevant. No clear‐cut difference between the various locations was found. The observed variation in the measured deposition velocities could be explained to some 60% with a surface layer resistance model on the basis of fluctuations in meteorological variables, surface wetness, and air temperature. To obtain an estimate of the long‐term average deposition velocity of NH3, we used a surface‐layer model to account for the effect of nighttime meteorology. Measurements and calculations indicate an annual average 24‐hour deposition velocity to heathland of 1.4 cm/s. For particulate ammonium, only one in 10 measurements leads to flux values significantly higher than the detection limit of the method, owing to the low deposition velocity of particulate NH4+. The average flux measured is equivalent to 3.8 kg NH4+/ha/yr. The annual average 24‐hour dry deposition velocity for particulate NH4+ is 0.17 cm/s. Under the circumstances encountered in the Netherlands, only about 20% of NHx dry deposition was by way of dry deposition of particulate NH4+. For a typical site in the Netherlands the dry deposition was about 50% of the estimated input by wet deposition.