Measuring eddy covariance fluxes of ozone with a slow-response analyser

Ozone (O₃) fluxes above a temperate mountain grassland were measured by means of the eddy covariance (EC) method using a slow-response O₃ analyser. The resultant flux loss was corrected for by a series of transfer functions which model the various sources of high- and, in particular, low-pass filtering. The resulting correction factors varied on average between 1.7 and 3.5 during night and daytime, respectively. A cospectral analysis confirmed the accuracy of this approach. O₃ fluxes were characterised by a comparatively large random uncertainty, which during daytime typically amounted to 60%. EC O₃ fluxes were compared against O₃ flux measurements made concurrently with the flux-gradient (FG) method. The two methods generally agreed well, except for a period between sunrise and early afternoon, when the FG method was suspected of being affected by the presence of photochemical sources/sinks. O₃ flux magnitudes and deposition velocities determined with the EC method compared nicely with the available literature from grassland studies. We conclude that our understanding of the causes and consequences of various sources of flux loss (associated with any EC system) has sufficiently matured so that also less-than-ideal instrumentation may be used in EC flux applications, albeit at the cost of relatively large empirical corrections.     

Particle fluxes above forests: observations, methodological considerations and method comparisons

This paper reports a study designed to test, evaluate and compare micro-meteorological methods for determining the particle number flux above forest canopies. Half-hour average particle number fluxes above a representative broad-leaved forest in Denmark derived using eddy covariance range from -7x10(7) m(-2) s(-1) (1st percentile) to 5x10(7) m(-2) s(-1) (99th percentile), and have a median value of -1.6x10(6) m(-2) s(-1). The statistical uncertainties associated with the particle number flux estimates are larger than those for momentum fluxes and imply that in this data set approximately half of the particle number fluxes are not statistically different to zero. Particle number fluxes from relaxed eddy accumulation (REA) and eddy covariance are highly correlated and of almost identical magnitude. Flux estimates from the co-spectral and dissipation methods are also correlated with those from eddy covariance but exhibit higher absolute magnitude of fluxes.     

Aerosol properties,in-canopy gradients,turbulent fluxes and VOC concentrations at a pristine forest site in Amazonia

Aerosol physical and chemical properties were measured in a forest site in central Amazonia (Cuieiras reservation, 2.61S; 60.21W) during the dry season of 2004 (Aug–Oct). Aerosol light scattering and absorption, mass concentration, elemental composition and size distributions were measured at three tower levels (Ground: 2 m; Canopy: 28 m, and Top: 40 m). For the first time, simultaneous eddy covariance fluxes of fine mode particles and volatile organic compounds (VOC) were measured above the Amazonian forest canopy. Aerosol fluxes were measured by eddy covariance using a Condensation Particle Counter (CPC) and a sonic anemometer. VOC fluxes were measured by disjunct eddy covariance using a Proton Transfer Reaction Mass Spectrometer (PTR-MS). At nighttime, a strong vertical gradient of phosphorus and potassium in the aerosol coarse mode was observed, with higher concentrations at Ground level. This suggests a source of primary biogenic particles below the canopy. Equivalent black carbon measurements indicate the presence of light-absorbing aerosols from biogenic origin. Aerosol number size distributions typically consisted of superimposed Aitken (76 nm) and accumulation modes (144 nm), without clear events of new particle formation. Isoprene and monoterpene fluxes reached respectively 7.4 and 0.82 mg m^?2 s^?1 around noon. An average fine particle flux of 0.05 ± 0.10 10⁶ m^?2 s^?1 was calculated, denoting an equilibrium between emission and deposition fluxes of fine mode particles at daytime. No significant correlations were found between VOC and fine mode aerosol concentrations or fluxes.     

Dealing with disjunct concentration measurements in eddy covariance applications: A comparison of available approaches

Using proton transfer reaction mass spectrometry equipped with a quadrupol mass analyser to quantify the biosphere-atmosphere exchange of volatile organic compounds (VOC), concentrations of different VOC are measured sequentially. Depending on how many VOC species are targeted and their respective integration times, each VOC is measured at repeat rates on the order of a few seconds. This represents an order of magnitude longer sample interval compared to the standard eddy covariance (EC) method (5–20 Hz sampling rates). Here we simulate the effect of disjunct sampling on EC flux estimates by decreasing the time resolution of CO₂ and H₂O concentrations measured at 20 Hz above a temperate mountain grassland in the Austrian Alps. Fluxes for one month are calculated with the standard EC method and compared to fluxes calculated based on the disjunct data (1, 3 and 5 s sampling rates) using the following approaches: i) imputation of missing concentrations based on the nearest neighbouring samples (iDEC_nn), ii) imputation by linear interpolation (iDEC_li), and iii) virtual disjunct EC (vDEC), i.e. flux calculation based solely on the disjunct concentrations. It is shown that the two imputation methods result in additional low-pass filtering, longer lag times (as determined with the maximum cross-correlation method) and a flux loss of 3–30% as compared to the standard EC method. A novel procedure, based on a transfer function approach, which specifically corrects for the effect of data treatment, was developed, resulting in improved correspondence (to within 2%). The vDEC method yields fluxes which approximate the true (20 Hz) fluxes to within 3–7% and it is this approach we recommend because it involves no additional empirical corrections. The only drawback of the vDEC method is the noisy nature of the cross-correlations, which poses problems with lag determination – practical approaches to overcome this limitation are discussed.     

Deliquescence induces eddy covariance and estimable dry deposition errors

The effects of deliquescent processes on eddy covariance estimates of dry deposition are considered, both theoretically and using some representative data. Turbulent and latent heat fluxes near the Earth's surface imply a vertical “saturation ratio flux” (Fairall, C.W., Atmospheric Environment, 18 (1984) 1329). Deliquescence, instantaneous relative to turbulent time-scales, responds to saturation ratio (or relative humidity) fluctuations and induces covariance between vertical winds and size-resolved number concentrations of hygroscopic aerosols. The induced covariance represents an error in the estimation of surface exchange by direct application of the eddy covariance technique. Under deliquescent conditions (high relative humidity), resulting errors in dry deposition estimates are shown to be often as large as typical deposition velocities reported for small particles, depending on the shape of the number distribution and the magnitudes of heat and vapor fluxes in the boundary layer.     

Modeling CO2 exchange and meteorological factors of an apple orchard using partial least square regression

Environmental Science and Pollution Research - The eddy covariance (EC) technique was used to measure variations of orchard-atmosphere CO2 exchange, as a function of meteorological variables in an...     

Application and test of a simple tool for operational footprint evaluations

We present a user-friendly tool for footprint calculations of flux measurements in the surface layer. The calculations are based on the analytical footprint model by Kormann, R. and Meixner, F.X. [2001. An analytical footprint model for Non-neutral Stratification. Boundary-Layer Meteorology 99, 207-224]. The footprint density function of a flux sensor is determined using readily available data from standard eddy covariance measurements. This footprint density function is integrated over defined surface areas given as quadrangular polygons representing e.g. agricultural fields. We illustrate the use and performance of the tool by applying it to CO2 flux measurements with three eddy covariance system at the Swiss CarboEurope grassland site. Two flux towers were positioned in the centre of two neighbouring fields, respectively, that showed a very different CO2 flux during the study period. The third tower was located near the border of the two fields and was frequently influenced by both fields to a similar degree. The calculated footprint fractions were used to simulate the latter flux from the other two systems. The measured and simulated fluxes showed a good agreement and thus support the reliability of the footprint calculation. The presented simple footprint tool can be used as a routine quality check for flux monitoring stations influenced by surface areas with varying vegetation covers and/or land-use.     

Comparison of seasonal variations of ozone exposure and fluxes in a Mediterranean Holm oak forest between the exceptionally dry 2003 and the following year

Ozone and energy fluxes have been measured using the eddy covariance technique, from June to December 2004 in Castelporziano near Rome (Italy), and compared to similar measurements made in the previous year. The studied ecosystem consisted in a typical Mediterranean Holm oak forest. Stomatal fluxes have been calculated using the resistance analogy and by inverting the Penmann-Monteith equation. Results showed that the average stomatal contribution accounts for 42.6% of the total fluxes. Non-stomatal deposition proved to be enhanced by increasing leaf wetness and air humidity during the autumnal months. From a comparison of the two years, it can be inferred that water supply is the most important limiting factor for ozone uptake and that prolonged droughts alter significantly the stomatal conductance, even 2 months after the soil water content is replenished. Ozone exposure, expressed as AOT40, behaves similarly to the cumulated stomatal flux in dry conditions whereas a different behaviour for the two indices appears in wet autumnal conditions. A difference also occurs between the two years.     

Combining sap flow and eddy covariance approaches to derive stomatal and non-stomatal O3 fluxes in a forest stand

Stomatal O₃ fluxes to a mixed beech/spruce stand (Fagus sylvatica/Picea abies) in Central Europe were determined using two different approaches. The sap flow technique yielded the tree-level transpiration, whereas the eddy covariance method provided the stand-level evapotranspiration. Both data were then converted into stomatal ozone fluxes, exemplifying this novel concept for July 2007. Sap flow-based stomatal O₃ flux was 33% of the total O₃ flux, whereas derivation from evapotranspiration rates in combination with the Penman-Monteith algorithm amounted to 47%. In addition to this proportional difference, the sap flow-based assessment yielded lower levels of stomatal O₃ flux and reflected stomatal regulation rather than O₃ exposure, paralleling the daily courses of canopy conductance for water vapor and eddy covariance-based total stand-level O₃ flux. The demonstrated combination of sap flow and eddy covariance approaches supports the development of O₃ risk assessment in forests from O₃ exposure towards flux-based concepts.     

Quantifying simultaneous fluxes of ozone, carbon dioxide and water vapor above a subalpine forest ecosystem

Assessing the long-term exchange of trace gases and energy between terrestrial ecosystems and the atmosphere is an important priority of the current climate change research. In this regard, it is particularly significant to provide valid data on simultaneous fluxes of carbon, water vapor and pollutants over representative ecosystems. Eddy covariance measurements and model analyses of such combined fluxes over a subalpine coniferous forest in southern Wyoming (USA) are presented. While the exchange of water vapor and ozone are successfully measured by the eddy covariance system, fluxes of carbon dioxide (CO(2)) are uncertain. This is established by comparing measured fluxes with simulations produced by a detailed biophysical model (FORFLUX). The bias in CO(2) flux measurements is partially attributed to below-canopy advection caused by a complex terrain. We emphasize the difficulty of obtaining continuous long-term flux data in mountainous areas by direct measurements. Instrumental records are combined with simulation models as a feasible approach to assess seasonal and annual ecosystem exchange of carbon, water and ozone in alpine environments. The viability of this approach is demonstrated by: (1) showing the ability of the FORFLUX model to predict observed fluxes over a 9-day period in the summer of 1996; and (2) applying the model to estimate seasonal dynamics and annual totals of ozone deposition and carbon, and water vapor exchange at our study site. Estimated fluxes above this subalpine ecosystem in 1996 are: 195 g C m(-2) year(-1) net ecosystem production, 277 g C m(-2) year(-1) net primary production, 535 mm year(-1) total evapo-transpiration, 174 mm year(-1) canopy transpiration, 2.9 g m(-2) year(-1) total ozone deposition, and 1.72 g O(3) m(-2) year(-1) plant ozone uptake via leaf stomata. Given the large portion of non-stomatal ozone uptake (i.e. 41% of the total annual flux) predicted for this site, we suggest that future research of pollution-vegetation interactions should relate plant response to actively assimilated ozone by foliage rather than to total deposition. In this regard, we propose the Physiological Ozone Uptake Per Unit of Leaf Area (POUPULA) as a practical index for quantifying vegetation vulnerability to ozone damage. We estimate POUPULA to be 0.614 g O(3) m(-2) leaf area year(-1) at our subalpine site in 1996.     

Local-scale fluxes of carbon dioxide in urban environments: methodological challenges and results from Chicago

Much attention is being directed to the measurement and modeling of surface-atmosphere exchanges of CO2 for different surface types. However, as yet, few measurements have been conducted in cities, even though these environments are widely acknowledged to be major sources of anthropogenic CO2. This paper highlights some of the challenges facing micrometeorologists attempting to use eddy covariance techniques to directly monitor CO2 fluxes in urban environments, focusing on the inherent variability within and between urban areas, and the importance of scale and the appropriate height of measurements. Results from a very short-term study of CO2 fluxes, undertaken in Chicago, Illinois in the summer of 1995, are presented. Mid-afternoon minimum CO2 concentrations and negative fluxes are attributed to the strength of biospheric photosynthesis and strong mixing of local anthropogenic sources in a deep mixed layer. Poor night-time atmospheric mixing, lower mixed layer depths, biospheric respiration, and continued missions from mobile and fixed anthropogenic sources, account for the night-time maxima in CO2 concentrations. The need for more, longer-term, continuous eddy covariance measurements is stressed.     

Eddy flux measurements of methane over the fen “Murnauer Moos”, 11°11′E, 47°39′N,using a fast tunable diode laser spectrometer

Eddy covariance measurements of methane were carried out over the fen “Murnauer Moos” in the south of Germany in order to evaluate the performance of a newly developed eddy covariance measurement system, based on a frequency-modulated tunable diode laser spectrometer as a fast chemical sensor. During a six-day period, an average daytime methane emission of (5.4±1.8) mg CH₄ m⁻² h⁻¹ was measured. We find this value moderate, considering the favorable meteorological and soil conditions for methane emission. Diurnal cycles of the fluxes of methane and carbon dioxide as well as of sensible and latent heat are presented. Results are discussed in terms of relevant micrometeorological quantities, and quality control procedures based on Allan variance and spectral analysis are discussed.     

Computing surface concentration fluxes of trace gases from a variational method using measured variance and single-level concentration data

Surface fluxes of O₃, CO₂ and SO₂ were estimated from a variational method by using measured concentrations and variances of these trace gases. The measurements were taken over a deciduous forest when it was fully leafed during the summer of 1988 and when it was leafless during the winter of 1990. A flux–variance relation and a flux–gradient relation were employed as constraints in a cost function which is minimized to find the optimal estimate of concentration fluxes of the gases under study. Fluxes of O₃, CO₂ and SO₂ from the variational method were compared with fluxes estimated by the flux–variance relation and measured using an eddy correlation technique. Results show that the variational method improves the estimates of fluxes.     

Flux measurements of volatile organic compounds by the relaxed eddy accumulation method combined with a GC-FID system in urban Houston,Texas

A tall tower flux measurement setup was established in metropolitan Houston, Texas, to measure trace gas fluxes from emission sources in the urban surface layer. We describe a new relaxed eddy accumulation (REA) system combined with a dual-channel GC-FID used for VOC flux measurements, focusing on benzene, toluene, ethylbenzene and xylenes (BTEX) results. Ambient air sampled from 60 m above the ground next to a sonic anemometer was subsampled by a membrane pump and pushed into an REA valve system with two Teflon bag reservoirs, then transferred to two preconcentration units for thermal desorption. We discuss the performance of our system and the selected BTEX measurement results using approximately 8 weeks of data (May 22–July 22, 2008), presenting diurnal variations of concentrations and fluxes of these traffic tracers. The measured values exhibited diurnal cycles with dominant morning and midday peaks during weekdays related to rush hour traffic and additional weekday daytime toluene and xylenes emissions. Local evaporative emissions, likely from solvent usage, significantly contributed to the measured fluxes. We upscaled measured emissions to the county level using a high resolution land cover data set and compared the results with EPA’s National Emission Inventory (NEI).     

Monitoring the multi-year carbon balance of a subarctic palsa mire with micrometeorological techniques

This article reports a dataset on 8 years of monitoring carbon fluxes in a subarctic palsa mire based on micrometeorological eddy covariance measurements. The mire is a complex with wet minerotrophic areas and elevated dry palsa as well as intermediate sub-ecosystems. The measurements document primarily the emission originating from the wet parts of the mire dominated by a rather homogenous cover of Eriophorum angustifolium. The CO(2)/CH(4) flux measurements performed during the years 2001-2008 showed that the areas represented in the measurements were a relatively stable sink of carbon with an average annual rate of uptake amounting to on average -46 g C m(-2) y(-1) including an equally stable loss through CH(4) emissions (18-22 g CH(4)-C m(-2) y(-1)). This consistent carbon sink combined with substantial CH(4) emissions is most likely what is to be expected as the permafrost under palsa mires degrades in response to climate warming.     

Structural analysis and flux associations of CO2, H2O,heat and ozone over cotton and grape canopies

Micrometeorological tower data, collected over grape and cotton canopies as part of the California ozone deposition experiment (CODE) during the summer of 1991, are used to examine the temporal association between fluxes, and the physical characteristics of the coherent structures which dominate transport for both stable nighttime and unstable daytime conditions. Flux was calculated using the eddy covariance technique and the dominant modes of flux transport determined by quadrant analysis. The mean flux densities for both the cotton and grape site showed the surface acting as a sink for CO₂ and ozone and a source of heat and H₂O during the day, as would be expected, while during the night it became a source for CO₂ and a sink for heat, but remained a sink for ozone and a source of H₂O. The flux association results indicated a single vegetated ozone sink for the grape site, but a vegetated as well as a non-vegetated sink for the cotton site. For both sites, structures simultaneously transporting significant flux contributions of CO₂, H₂O, heat and ozone dominate during unstable conditions, but differed during stable conditions, where unmixed single flux structures dominated over cotton but not over grape. Structure sizes were less than 10 m during nighttime conditions and ranged from 3 to 69 m during the day. The results of this study contribute empirical evidence about the relationship between ozone uptake and the physical and physiological state of vegetation, as well as the limitations placed on eddy scales in simulation models.     

Measurements and modelling of cloudwater deposition to moorland and forests

The objective of the study was to measure the size dependence of cloudwater deposition and associated average ionic fluxes to vegetated surfaces. Measurements were made over a forest canopy at Dunslair heights in south Scotland and a moorland site, Great Dun Fell, in northern England. Measurements were made using the gradient and eddy correlation techniques. Eddy correlation measurements were made using an ultrasonic anemometer, a Knollenberg forward scattering spectrometer probe (to measure liquid water fluxes and fluxes of droplets in 1 microm size intervals) and a GSI particulate volume monitor (to measure liquid-water fluxes). Measurements were made at Great Dun Fell of the size dependence of droplet deposition velocity, using the gradient technique with two Knollenberg probes. Simultaneous gradient and eddy correlation measurements were also made at Great Dun Fell of average cloud-water fluxes, together with chemical analysis of cloud water composition, using a continuous analysis system. At Dunslair Heights, eddy correlation measurements were made using both the Knollenberg and Gerber Scientific Instruments (GSI) probes, while simultaneous gradient measurements using two GSI probes were also attempted. Samples of cloud water were collected at Dunslair Heights, using passive string collectors for chemical analysis by ion chromatography. The major findings of the study were: 1. The droplet deposition velocities measured by the two techniques were similar. 2. The deposition velocities were a strong function of droplet size. Considerable resistance to deposition was evident for droplets of less than 5 microm radius. Deposition velocities for particles from about 6 to 8 microm exceeded those for momentum. 3. Except when the droplets were very small or the winds very light, bulk cloud-water deposition velocities were about 80% or more of the momentum deposition velocities to forests.     

Carbon dioxide fluxes over an urban park area

From September 2006 to October 2007 turbulent fluxes of carbon dioxide were measured at an urban tower station (26 m above ground level, z/z_h = 1.73) in Essen, Germany, using the eddy covariance technique. The site was located at the border between a public park area (70 ha) in the south–west of the station and suburban/urban residential as well as light commercial areas in the north and east of the tower. Depending on the land-use two different sectors (park and urban) were identified showing distinct differences in the temporal evolution of the surface-atmosphere exchange of CO₂. While urban fluxes appear to be governed by anthropogenic emissions from domestic heating and traffic (average flux 9.3 μmol m^?2 s^?1), the exchange of CO₂ was steered by biological processes when the park contributed to the flux footprint. The diurnal course during the vegetation period exhibited negative daytime fluxes up to ?10 μmol m^?2 s^?1 on average in summer. Nevertheless, with a mean of 0.8 μmol m^?2 s^?1 park sector fluxes were slightly positive, thus no net carbon uptake by the surface occurred throughout the year.In order to sum the transport of CO₂ a gap-filling procedure was performed by means of artificial neural network generalisation. Using additional meteorological inputs the daily exchange of CO₂ was reproduced using radial basis function networks (RBF). The resulting yearly sum of 6031 g m^?2 a^?1 indicates the entire study site to be a considerable source of CO₂.     

Adaptation of a speciation sampling cartridge for measuring ammonia flux from cattle feedlots using relaxed eddy accumulation

Improved measurements of ammonia losses from cattle feedlots are needed to quantify the national NH₃ emissions inventory and evaluate management techniques for reducing emissions. Speciation cartridges composed of glass honeycomb denuders and filter packs were adapted to measure gaseous NH₃ and aerosol NH₄⁺ fluxes using relaxed eddy accumulation (REA). Laboratory testing showed that a cartridge equipped with four honeycomb denuders had a total capture capacity of 1800 μg of NH₃. In the field, a pair of cartridges was deployed adjacent to a sonic anemometer and an open-path gas analyzer on a mobile tower. High-speed valves were attached to the inlets of the cartridges and controlled by a datalogger so that up- and down-moving eddies were independently sampled based on direction of the vertical wind speed and a user-defined deadband. Air flowed continuously through the cartridges even when not sampling by means of a recirculating air handling system. Eddy covariance measurement of CO₂ and H₂O, as measured by the sonic and open-path gas analyzer, were used to determine the relaxation factor needed to compute REA-based fluxes. The REA system was field tested at the Beef Research Unit at Kansas State University in the summer and fall of 2007. Daytime NH₃ emissions ranged between 68 and 127 μg m^?2 s^?1; fluxes tended to follow a diurnal pattern correlated with latent heat flux. Daily fluxes of NH₃ were between 2.5 and 4.7 g m^?2 d^?1 and on average represented 38% of fed nitrogen. Aerosol NH₄⁺ fluxes were negligible compared with NH₃ emissions. An REA system designed around the high-capacity speciation cartridges can be used to measure NH₃ fluxes from cattle feedlots and other strong sources. The system could be adapted to measure fluxes of other gases and aerosols.     

Isoprene and monoterpene fluxes measured above Amazonian rainforest and their dependence on light and temperature

Canopy scale emissions of isoprene and monoterpenes from Amazonian rainforest were measured by eddy covariance and eddy accumulation techniques. The peak mixing ratios at about 10 m above the canopy occurred in the afternoon and were typically about 90 ppt_v of α-pinene and 4–5 ppb_v of isoprene. α-pinene was the most abundant monoterpene in the air above the canopy comprising ≈50% of the total monoterpene mixing ratio. Measured isoprene fluxes were almost 10 times higher than α-pinene fluxes. Normalized conditions of 30°C and 1000 μmol m⁻² s⁻¹ were associated with an isoprene flux of 2.4 mg m⁻² h⁻¹ and a β-pinene flux of 0.26 mg m⁻² h⁻¹. Both fluxes were lower than values that have been specified for Amazon rainforests in global emission models. Isoprene flux correlated with a light- and temperature-dependent emission activity factor, and even better with measured sensible heat flux. The variation in the measured α-pinene fluxes, as well as the diurnal cycle of mixing ratio, suggest emissions that are dependent on both light and temperature. The light and temperature dependence can have a significant effect on the modeled diurnal cycle of monoterpene emission as well as on the total monoterpene emission.