Relaxed Eddy Accumulation Technique Research Articles

Dynamics of isotopic exchange of carbon dioxide in a Tennessee deciduous forest

The combination of isotopic measurements and micrometeorological flux measurements is a powerful new approach that will likely lead to new insight into the dynamics of CO2 exchange between terrestrial ecosystems and the atmosphere. Since the biological processes of photosynthesis and respiration modify the stable isotopic signature of atmospheric CO2 in different ways, measurements of the net fluxes of CO2, 13CO2, and C18OO can be used to investigate the relative contribution of each process to net ecosystem CO2 exchange. We used two independent approaches to measure isotopic fluxes of CO2 over a Tennessee oak‐maple‐hickory forest in summer 1998. These approaches involved (1) a combination of standard eddy covariance with intensive flask sampling, and (2) a modification to the relaxed eddy accumulation technique. Strong isotopic signals associated with photosynthesis and respiration were observed and persisted in forest air despite the potential for mixing due to atmospheric turbulence. Calm nights allowed a buildup of respiratory CO2 below the canopy and were associated with isotopically depleted forest air in the morning. Windy nights were followed by a relatively more enriched early‐morning isotopic signal. Entrainment of air from above the decaying nocturnal boundary layer during daytime mixed layer growth exerted strong control on isotopic composition of forest air, resulting in similar isotope ratios in the late afternoon despite different isotopic starting points following calm or windy nights. The influences of the convective boundary layer and turbulent mixing within the forest cannot be ignored when using isotopes of CO2 investigate biological processes.

Relaxed Eddy-Accumulation Technique for Measuring Ammonia Volatilization

Ammonia emissions from fertilized fields not only decrease the fertilizer value but also contribute to environmental pollution in the form of off-site deposition. Quantifying the magnitude of ammonia fluxes is therefore of great interest because of economical and environmental aspects. Two factors make ammonia flux measurements particularly challenging: (i) ammonium in particulate phase often interferes with gas phase ammonia and (ii) ammonia is easily adsorbed on surfaces. Due to the lack of a suitable fast-response ammonia sensor, we decided to explore the feasibility of using denuder tubes with the relaxed eddy-accumulation technique (REA) for measuring ammonia fluxes. In the spring of 1996, the REA technique was successfully used to measure ammonia fluxes over a corn field after urea application. The denuder tubes, coated with oxalic acid, were used at the inlet of the REA system to trap ammonia in air. During the 23 days of measurements, ammonia emission rates ranged from 1 to 1023 ng NH3 m-2 s-1, a...

Aircraft-based volatile organic compounds flux measurements with relaxed eddy accumulation

Regional estimates of fluxes of volatile organic compounds (VOCs) are required to improve our understanding of their role in the chemistry of the atmosphere. Flux measurements on such a scale can best be obtained using aircraft-based systems. These systems usually rely on the eddy covariance technique, which requires fast response gas sensors for flux measurement, but such sensors are not available for most organic compounds, therefore, the relaxed eddy-accumulation (REA) technique was selected. An aircraft-based REA sampling system was developed and used to measure isoprene emission over the boreal forest during the 1996 summer. Over a short period in July at the Boreal Ecosystem/Atmosphere Study (BOREAS) southern study area (SSA), the isoprene fluxes ranged from −0.06 to 1.79 μg m -2 s -1, with a mean of 0.59±0.34 μg m -2 s -1, while in August at the BOREAS northern study area (NSA) the isoprene fluxes ranged from 0.00 to 0.26 μg m -2 s -1, with a mean of 0.14±0.09 μg m -2 s -1. In the SSA, the isoprene fluxes over aspen ranged from 0.44 to 1.79 μg m -2 s -1, with a mean of 0.92±0.33 μg m -2 s -1, whereas over black spruce, isoprene fluxes ranged from −0.06 to 0.54 μg m -2 s -1, with a mean of 0.36±0.21 μg m -2 s -1. The isoprene fluxes were exponentially correlated with solar radiation and radiative surface temperature. High correlations between isoprene fluxes and the fluxes of CO 2 and latent heat were also observed. Carbon lost through isoprene emissions was about 0.7 and 0.8% of the CO 2 assimilation rate for aspen and black spruce, respectively. The results demonstrate that the aircraft-based relaxed eddy-accumulation technique is a promising approach for quantifying the atmosphere–surface exchange of VOCs on a regional scale.

Modification of the relaxed eddy accumulation technique to maximize measured scalar mixing ratio differences in updrafts and downdrafts

A modification to the relaxed eddy accumulation (REA) flux measurement technique is proposed which maximizes the scalar mixing ratio difference in updrafts and downdrafts. This technique was developed with the goal of measuring the stable isotope (13C/12C and 18O/16O) ratios of updraft and downdraft air and thus the net fluxes of 13C16O2 and 12C18O16O. Current mass spectrometer precision is small relative to measured isotopic gradients in CO2 in the Earth's boundary layer, and the conventional REA approach is likely to be ineffective. The new technique, which we refer to as hyperbolic relaxed eddy accumulation (HREA), uses the conditional sampling concept of hyperbolic hole analysis to control sampling of air during only those turbulent events which contribute most strongly to the flux. Instead of basing updraft/downdraft sampling decisions strictly on vertical wind velocity, CO2 mixing ratio ([CO2]) fluctuations or those of another scalar are also used. Simulations using 10‐Hz data show that a wind‐based/scalar‐based sampling threshold can achieve a factor of 2.7 increase in scalar updraft/downdraft [CO2] differences over simple REA. During midday periods with strong photosynthetic fluxes, up/down [CO2] differences with HREA of 8–10 ppm are possible, compared with 3–5 ppm for the best conventional REA case. Corresponding isotopic differences can likely be resolved with current mass spectrometers using this approach.

Aircraft Measurements of the Concentration and Flux of Agrochemicals

The use of agrochemicals, such as insecticides and herbicides, is of great environmental concern. To quantify and model the volatilization and deposition of these agrochemicals, measurements of the exchange rates between the atmosphere and different surfaces are required. An aircraft-based air sampling system using the relaxed eddy-accumulation technique for measuring concentrations and fluxes of agrochemicals was developed and tested in June 1993 and July 1994. Atrazine and metolachlor, which are used on corn and soybean crops, were detected in samples collected along flights from Ottawa to southern Ontario. Concentrations as high as 4.6 ng/m 3 for atrazine and 9.8 ng/m 3 for metolachlor were found. Fluxes ranged from -1.1 to 2.5 ng/m 2 .s for atrazine and from -0.01 to 0.56 ng/m 2 .s for metolachlor. These results demonstrate that the aircraft-based relaxed eddy-accumulation approach is a useful method for measuring concentrations and fluxes of agrochemicals at both local and regional scales. Combined with footprint calculations and air mass back-trajectories methods, the fluxes measured with this system should be useful to improve the understanding of the long-range transport of semivolatile organic compounds and their potential impact on various ecosystems.

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

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

Herbicides volatilization measured by the relaxed eddy-accumulation technique using two trapping media

Two relaxed eddy-accumulation (REA) systems were developed for measuring the flux of agrochemicals; one using polyurethane foam (PUF) plugs as a trapping medium with an air sampling flow rate of 60 1 min −1, and the other utilizing mini-tubes (MT) containing Tenax-TA resin and aspirated at 100–300 ml min −1. Analysis of agrochemicals trapped on PUF is time consuming in that Soxhlet extraction followed by extract concentration is required prior to quantification by gas chromatography (GC). In contrast, the MT technology provides enhanced sensitivity through direct thermal desorption of the entire trapped sample from the Tenax-TA resin onto a GC column. The objective of the study was to determine if the simpler-to-use MT technology was as effective as PUF, which has been traditionally used in the measurement of vapour losses of agrochemicals. The vapour fluxes of two soil-applied herbicides (170 mg active ingredient (a.i.) m −2 Z triallate and 115 mg a.i. m −2 trifluralin) were compared using PUF and Tenax-TA trapping media in the corresponding REA measuring systems over four days during autumn 1992. The aqueous emulsion of trifluralin and triallate was not soil incorporated as recommended, in order that a wider range of vapour fluxes could be studied. Vapour fluxes of both herbicides were highest on the day of application and were approximately 0.64 μg m −2 s −1 for trifluralin and 0.86 μg m −2 s −1 for triallate. On the following day, herbicide fluxes decreased by a factor of three. The temporal patterns of the fluxes obtained using the two trapping media were similar, but the magnitude of the flux maxima varied slightly. This was attributed to adsorption/desorption of the herbicides on the walls of the inlet tubing of the MT system, even though the inlet tubes were heated to minimize herbicide adsorption. The MT system was more sensitive to adsorption because the herbicide amounts trapped on the resin are much smaller. Cumulative fluxes from the two media agreed within 7% for triallate whereas a 22% higher flux was obtained with the MT system for trifluralin. Use of the REA system utilizing the MT technology with Tenax-TA resin is promising for measuring agrochemical fluxes. However, since the amounts of agrochemicals sampled are approximately 500 times less than those sampled using PUF, an improved system for controlling the temperature of the inlet tubing is required to minimize adsorption/desorption of the agrochemicals.

Accuracy of the relaxed eddy-accumulation technique, evaluated using CO2 flux measurements

A system capable of measuring the fluxes of trace gases was developed. It is based on a simpler version of the eddy-accumulation technique (EA), known as the relaxed eddy-accumulation technique (REA). It accumulates air samples associated with updrafts and downdrafts at a constant flow rate in two containers for later analysis of the trace gas mean concentration. The flux integration is based on the durations of updraft and downdraft events, rather than on the vertical wind velocity (W) as is the case for EA and eddy-correlation (EC) techniques. The flux, calculated by the REA technique, is equal to the difference in the mean concentration of the trace gas of interest between the upward and downward moving eddies, multiplied by the standard deviation of the vertical wind velocity and an empirical coefficient. CO2 fluxes measured for 162 half-hour periods over a soybean field by both EC and REA techniques showed excellent agreement (coefficient of determination,R2=0.92). The slope (0.985) and the intercept (−0.042 mg m−2 s−1) were not significantly different from 1 and 0, respectively, at the 5% level; and the standard error of estimate was 0.074 mg m−2 s−1. It is also shown that the empirical coefficient can be calculated from either latent or sensible heat fluxes. A model describing the effect on this empirical coefficient of not sampling aroundW equal to zero is proposed.

Impact of density fluctuations on flux measurements of trace gases: Implications for the relaxed eddy accumulation technique

A simple and fast approach to determine when density fluctuations are non-negligible in the calculation of the flux of trace gases (F c ) is proposed. The correction (F c − F c (raw)), when expressed as the percentage of the flux, is dependent on the ratio of background concentration of the trace gas over its flux (% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeikaiabeg% 8aYnaaBaaaleaacaWGJbaabeaakiaab+cacaWGgbWaaSbaaSqaaiaa% dogaaeqaaOGaaeykaaaa!3CBC!\[{\rm{(}}\rho _c {\rm{/}}F_c {\rm{)}}\], on the partitioning of available energy between sensible (F T ) and latent (F v ) heat fluxes, and on the flux measuring system. An increase from 100 to 200 W m-2 in available energy and from 0 to 20% in F T /(F T + F v ) led to a threefold reduction in the required value of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaacq% aHbpGCdaWgaaWcbaGaam4yaaqabaaaaOGaai4laiaadAeadaWgaaWc% baGaam4yaaqabaaaaa!3B6D!\[\overline {\rho _c } /F_c \] to have a density correction of 10%. A trace gas with a % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaceaaca% WGgbWaaSbaaSqaaiaadogaaeqaaaGccaGLhWUaayjcSdGaai4lamaa% naaabaGaeqyWdi3aaSbaaSqaaiaadogaaeqaaaaaaaa!3E91!\[\left| {F_c } \right|/\overline {\rho _c } \] value above 0.014 m s-1 has a density correction on flux of less than 10%, for even the worst case scenario. Values of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOramaaBa% aaleaacaWGJbaabeaakiaac+cadaqdaaqaaiabeg8aYnaaBaaaleaa% caWGJbaabeaaaaaaaa!3B6D!\[F_c /\overline {\rho _c } \] for several trace gases computed from typical situations show that the fluxes of N2O, NO, CO2, CH4 and O3 need to be corrected, while those of pesticides and volatile organic compounds, for example, do not. The corrections required with the newly developed relaxed eddy accumulation technique are discussed and equation development is shown for two sampling systems.