Relaxed Eddy Accumulation Flux Research Articles

Choosing an optimal <i>β</i> factor for relaxed eddy accumulation applications across vegetated and non-vegetated surfaces

Abstract. Accurately measuring the turbulent transport of reactive and conservative greenhouse gases, heat, and organic compounds between the surface and the atmosphere is critical for understanding trace gas exchange and its response to changes in climate and anthropogenic activities. The relaxed eddy accumulation (REA) method enables measuring the land surface exchange when fast-response sensors are not available, broadening the suite of trace gases that can be investigated. The β factor scales the concentration differences to the flux, and its choice is central to successfully using REA. Deadbands are used to select only certain turbulent motions to compute the flux. This study evaluates a variety of different REA approaches with the goal of formulating recommendations applicable over a wide range of surfaces and meteorological conditions for an optimal choice of the β factor in combination with a suitable deadband. Observations were collected across three contrasting ecosystems offering stark differences in scalar transport and dynamics: a mid-latitude grassland ecosystem in Europe, a loose gravel surface of the Dry Valleys of Antarctica, and a spruce forest site in the European mid-range mountains. We tested a total of four different REA models for the β factor: the first two methods, referred to as model 1 and model 2, derive βp based on a proxy p for which high-frequency observations are available (sensible heat Ts). In the first case, a linear deadband is applied, while in the second case, we are using a hyperbolic deadband. The third method, model 3, employs the approach first published by Baker et al. (1992), which computes βw solely based upon the vertical wind statistics. The fourth method, model 4, uses a constant βp, const derived from long-term averaging of the proxy-based βp factor. Each β model was optimized with respect to deadband size before intercomparison. To our best knowledge, this is the first study intercomparing these different approaches over a range of different sites. With respect to overall REA performance, we found that the βw and constant βp, const performed more robustly than the dynamic proxy-dependent approaches. The latter models still performed well when scalar similarity between the proxy (here Ts) and the scalar of interest (here water vapor) showed strong statistical correlation, i.e., during periods when the distribution and temporal behavior of sources and sinks were similar. Concerning the sensitivity of the different β factors to atmospheric stability, we observed that βT slightly increased with increasing stability parameter z/L when no deadband is applied, but this trend vanished with increasing deadband size. βw was unrelated to dynamic stability and displayed a generally low variability across all sites, suggesting that βw can be considered a site-independent constant. To explain why the βw approach seems to be insensitive towards changes in atmospheric stability, we separated the contribution of w′ kurtosis to the flux uncertainty. For REA applications without deeper site-specific knowledge of the turbulent transport and degree of scalar similarity, we recommend using either the βp, const or βw models when the uncertainty of the REA flux quantification is not limited by the detection limit of the instrument. For conditions when REA sampling differences are close to the instrument's detection limit, the βp models using a hyperbolic deadband are the recommended choice.

Canopy level emissions of 2-methyl-3-buten-2-ol, monoterpenes, and sesquiterpenes from an experimental Pinus taeda plantation

Emissions of Biogenic Volatile Organic Compounds (BVOCs) observed during 2007 from an experimental Pinus taeda plantation in Central North Carolina are compared with model estimates from the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 2.1. Relaxed eddy accumulation (REA) estimates of 2-methyl-3-buten-2-ol (MBO) fluxes are a factor of 3–4 higher than MEGAN estimates. MEGAN monoterpene emission estimates were a factor of approximately two higher than REA flux measurements. MEGAN β-caryophyllene emission estimates were within 60% of growing season REA flux estimates but were several times higher than REA fluxes during cooler, dormant season periods. The sum of other sesquiterpene emissions estimated by MEGAN was several times higher than REA estimates throughout the year. Model components are examined to understand these discrepancies. Measured summertime leaf area index (LAI) (and therefore foliar biomass) is a factor of two higher than assumed in MEGAN for the P. taeda default. Increasing the canopy mean MBO emission factor from 0.35 to 1.0mgm−2h−1 also reduces MEGAN vs. REA flux differences. This increase is within current MBO emission factor uncertainties. The algorithm within MEGAN which adjusts isoprene emission estimates as a function of the temperature and light of the previous 24h seems also to improve the seasonal MEGAN MBO correlation with REA fluxes. Including the effects of the previous 240h, however, seems to degrade temporal model correlation with fluxes. Monoterpene and sesquiterpene composition data from the REA are compared with MEGAN2.1 estimates and also branch enclosure and needle extract data collected at this site. To our knowledge, the flux data presented here are the first reported for MBO and sesquiterpenes from a P. taeda ecosystem.

First measurements of H 2O 2 and organic peroxides surface fluxes by the relaxed eddy-accumulation technique

The relaxed eddy-accumulation (REA) technique was specially adapted to a high-performance liquid chromatographer (enzymatic method) and scrubbing coils to measure concentrations and fluxes of hydrogen peroxide (H 2O 2) and organic peroxides with a carbon chain ⩽C 4, of which only methylhydroperoxide (MHP) and hydroxymethylhydroperoxide (HMHP) were detected. Flux measurements were carried out above the canopy of a Norway spruce forest in Germany (775 m a.s.l.) in NE Bavaria, Germany, during the BEWA2000 research cluster in summer 2001. This period was characterised by H 2O 2 maximum mixing ratios of 1 ppb and mixing ratios of organic peroxides below 200 ppt. Daily mean H 2O 2 deposition fluxes of −0.8±0.3 nmol m −2 s −1, MHP of −0.03±0.03 nmol m −2 s −1 and HMHP of −0.7±0.5 nmol m −2 s −1 were obtained. Profile measurements were performed as a qualitative comparison of the controlling mechanism of the surface exchanges. The REA as well as the profile measurements show that during daylight the surface exchanges of H 2O 2 and MHP over coniferous forest are mainly controlled by dry deposition. The high H 2O 2 dry-deposition velocity suggests a negligible surface uptake resistance for H 2O 2, whereas the significantly smaller MHP-deposition velocity indicates a significant surface uptake resistance. However, nighttime surface exchanges of these compounds should be further investigated since the in-canopy ozonolysis reaction is expected to affect exchanges. HMHP REA flux measurements show mainly deposition, whereas the ones based on the profile method suggest in-canopy chemical production.

Canopy and leaf level 2-methyl-3-buten-2-ol fluxes from a ponderosa pine plantation

Canopy and leaf level 2-methyl-3-butene-2-ol (methylbutenol, MBO) emissions were measured from a ponderosa pine plantation in the Sierra Nevada mountains from July to October 1998. Canopy scale fluxes were measured using a gradient approach, leaf level fluxes used a flow-through chamber. Emissions were dependent on ambient light and temperature levels and showed a diurnal cycle very similar to isoprene in deciduous forests. Maximum fluxes occurred between 1000 and 1700 h with an average of 2 mg C m −2 h −1 in July and August, which equaled approximately 0.3–0.5% of the simultaneously measured gross carbon uptake. MBO mixing ratios and fluxes over the pine plantation were also measured with a relaxed eddy accumulation (REA) system operated during part of our measurement campaign ( Baker et al., 1999 Journal of Geophysical Research, in press). Mixing ratios measured by both systems were in good agreement but the gradient approach estimated MBO fluxes twice as high than the REA approach. Leaf level investigations revealed a seasonal cycle in basal emission rate (emissions at 1000 μmol m −2 s −1 PAR and 30°C) with a maximum in August, decreasing towards the end of the season. We developed an emission model to scale MBO fluxes from the leaf level to the ecosystem level based on the well-known isoprene algorithm. The model estimates are substantially lower than our gradient flux measurements, and match better with the REA flux measurements, and we conclude that the gradient approach overestimated MBO fluxes at this site. Comparing seasonal changes of measured with modeled fluxes showed a pattern of basal emission rates similar to those observed at the leaf level, and the basal emission rate was related to daytime air temperatures. While MBO contributes substantially to local photochemistry and its oxidation contributes significantly to the regional acetone budget, the latter probably does not represent a significant global source of atmospheric acetone.

The use of relaxed eddy accumulation to measure biosphere-atmosphere exchange of isoprene and other biological trace gases.

The micrometeorological flux measurement technique known as relaxed eddy accumulation (REA) holds promise as a powerful new tool for ecologists. The more popular eddy covariance (eddy correlation) technique requires the use of sensors that can respond at fast rates (10 Hz), and these are unavailable for many ecologically relevant compounds. In contrast, the use of REA allows flux measurement with sensors that have much slower response time, such as gas chromatography and mass spectrometry. In this review, relevant micrometeorological details underlying REA are presented, and critical analytical and system design details are discussed, with the goal of introducing the technique and its potential applications to ecologists. The validity of REA for measuring fluxes of isoprene, a photochemically reactive hydrocarbon emitted by several plant species, was tested with measurements over an oak-hickory forest in the Walker Branch Watershed in eastern Tennessee. Concurrent eddy covariance measurements of isoprene flux were made using a newly available chemiluminesence instrument. Excellent agreement was obtained between the two techniques (r 2 = 0.974, n = 62), providing the first direct comparison between REA and eddy covariance for measuring the flux rate of a reactive compound. The influence of a bias in vertical wind velocity on the accuracy of REA was examined. This bias has been thought to be a source of significant error in the past. Measurements of normalized bias ([Formula: see text]) alone would lead us to think that a large potential error exists at this site. However, with our isoprene data and through simulations of REA with fast-response H2O and CO2 data, we conclude that accurate REA flux measurements can be made even in the presence of a bias in w.