Micrometeorological Measurements Research Articles

Estimativa do coeficiente Priestley-Taylor em floresta monodominante Cambarazal no Pantanal

O coeficiente Priestley-Taylor (α) foi calculado baseado na estimativa da evapotranspiração pelo método de Bowen para floresta monodominante Vochysia divergens no Pantanal, Brasil. A área em estudo estava localizada no noroeste do Pantanal a aproximadamente 160 km de Cuiabá, Mato Grossso, Brasil (16º39'50''S; 56º47'50''O). Medições micrometeorológicas contínuas, em uma torre a uma altura de 32 m de altura, foram feitas de janeiro a dezembro de 2007. A evapotranspiração variou de 2,50 mm dia-1 (estação seca) a 4,10 mm dia-1 (estação úmida). O coeficiente Pristley-Taylor (α) variou durante o ano com valores máximos e mínimos nas estações seca e úmida, respectivamente, com média anual de 0,65 ± 0,18 de acordo com o padrão climático da área em estudo em função do aumento do conteúdo de água no solo/lâmina d'água de inundação e/ou diminuição na demanda evaporativa. Com a determinação empírica das dimensões de α, as estimativas da evapotranspiração podem ser melhoradas para florestas de Vochysia divergens na planície de inundação do Pantanal.

Chambers, micrometeorological measurements, and the New Zealand Denitrification–Decomposition model for nitrous oxide emission estimates from an irrigated dairy-grazed pasture

Nitrous oxide (N2O) emissions from soils are notoriously variable in space and time. Measuring and understanding variance in these emissions is imperative for improving the accuracy of the greenhouse gas inventory and assessing the viability of mitigation options; but data for N2O emissions are rather limited. Farm-scale emissions data are also required for developing and verifying predictive model estimates. A measurement campaign was undertaken from 12 October to 1 November 2006 at a highly productive grass-clover irrigated dairy farm on a stony silt loam soil in North Canterbury, South Island, New Zealand. The ∼7 ha experimental field, grazed in two morning 6-h grazing sessions (21–22 October 2006) by 718 milking dairy cattle, received two irrigations during the measurements, one before the grazing event and the other during grazing period. We first compare the emission measurements using a chamber technique against those made using a micrometeorological technique with tuneable diode-laser technology. We then compare the measured emissions against emissions predicted by a process-based model (New Zealand Denitrification–Decomposition (NZ-DNDC)). Daily averaged micrometeorological measurements gave a pre-grazing emission of 35 g N2O N/ha/day that increased to >60 g N2O N/ha/day following grazing by the dairy herd. The average pre-grazing emission of 10 g N2O N/ha/day from the chambers increased to 25 g N2O N ha−1 day−1 following grazing. The emissions were simulated with NZ-DNDC model, which gave average daily emissions of 15 ± 9 g N2O N ha−1 day−1 for the pre-grazing period and 22 ± 6 g N2O N ha−1 day−1 for the post-grazing period. Here we describe these measurement approaches, compare their emission estimates and discuss the advantages of combining them for verification of emissions.

Lag time determination in DEC measurements with PTR-MS

Abstract. The disjunct eddy covariance (DEC) method has emerged as a popular technique for micrometeorological flux measurements of volatile organic compounds (VOCs). It has usually been combined with proton transfer reaction mass spectrometry (PTR-MS), an online technique for VOC concentration measurements. However, the determination of the lag time between wind and concentration measurements has remained an important challenge. To address this issue, we studied the effect of different lag time methods on DEC fluxes. The analysis was based on both actual DEC measurements with PTR-MS and simulated DEC data derived from high frequency H2O measurements with an infrared gas analyzer. Conventional eddy covariance fluxes of H2O served as a reference in the DEC simulation. The individual flux measurements with PTR-MS were rather sensitive to the lag time methods, but typically this effect averaged out when the median fluxes were considered. The DEC simulation revealed that the maximum covariance method was prone to overestimation of the absolute values of fluxes. The constant lag time methods, one based on a value calculated from the sampling flow and the sampling line dimensions and the other on a typical daytime value, had a tendency to underestimate. The visual assessment method and our new averaging approach utilizing running averaged covariance functions did not yield statistically significant errors and thus fared better than the habitual choice, the maximum covariance method. Given this feature and the potential for automatic flux calculation, we recommend using the averaging approach in DEC measurements with PTR-MS. It also seems well suited to conventional eddy covariance applications when measuring fluxes near the detection limit.

Energy Balance Closure Using Eddy Covariance Above Two Different Land Surfaces and Implications for CO2 Flux Measurements

Componentsofthesurfaceenergybalanceofamatureborealjackpineforestand a jack pine clearcut were analysed to determine the causes of the imbalance that is commonly observed in micrometeorological measurements. At the clearcut site (HJP02), a significant portion of the imbalance was caused by: (i) the overestimation of net radiation (Rn) due to the inclusion of the tower in the field of view of the downward facing radiometers, and (ii) the underestimation of the latent heat flux (λE) due to the damping of high frequency fluc- tuations in the water vapour mixing ratio by the sample tube of the closed-path infrared gas analyzer. Loss of low-frequency covariance induced by insufficient averaging time as well as systematicadvectionoffluxesawayfromtheeddy-covariance(EC)towerwerediscountedas significant issues. Spatial and temporal distributions of the total surface-layer heat flux (T), i.e. the sum of sensible heat flux (H )a ndλE, were well behaved and differences between the relative magnitudes of the turbulentfluxes for several investigated energy balance closure (C) classes were observed. Therefore, it can be assumed that micrometeorological processes that affected all turbulent fluxes similarly did not cause the variation in C. Turbulent fluxes measured at the clearcut site should not be forced to close the energy balance. However,

Linking urban aerosol fluxes in street canyons to larger scale emissions

Abstract. In this study we investigate ultrafine particle (UFP) fluxes using a first order eddy viscosity turbulence closure Computational Fluid Dynamics (CFD) model and determine the different factors that influence emissions of UFP into the urban boundary layer. Both vertical turbulent fluxes as well as the fluxes due to mean circulatory flow are shown to contribute to the overall ventilation characteristics of street canyons. We then derive a simple parameterised numerical prediction model for canyon top UFP venting which is then compared with tower based micrometeorological flux measurements obtained during the REPARTEE & CityFlux field experiments.

Using Temperature Fluctuation Measurements to Estimate Meteorological Inputs for Modelling Dispersion During Convective Conditions in Urban Areas

We examine the performance of several methods to estimate meteorological inputs for modelling dispersion in urban areas during convective conditions. Sensible heat flux, surface friction velocity and turbulent velocities are estimated from measurements of mean wind speed and the standard deviation of temperature fluctuations at a single level on a tower at two suburban sites and at one urban site in Riverside, California. These estimates are compared with observations made at these sites during a field study conducted in 2007. The sensible heat flux is overestimated in the urban area, while it is underestimated at a suburban site when temperature fluctuations are used in the free convection formulation to estimate heat flux. The bias in heat flux estimates can be reduced through a correction that depends on stability. It turns out that the bias in heat flux estimates has a minor effect on the prediction of surface friction velocity and turbulent velocities. Estimates of sensible heat flux, surface friction velocity and turbulent velocities are sensitive to estimates of aerodynamic roughness length, and we suggest estimating the aerodynamic roughness length through detailed micrometeorological measurements made during a limited field study. An examination of the impact of the uncertainty in estimating surface micrometeorology on concentrations indicates that, at small distances from a surface release, ground-level concentrations computed using estimates of heat flux and surface friction compare well with the those based on observed values: the bias is small and the 95% confidence interval of the ratio of the two concentrations is 1.7. However, at distances much larger than the Obukhov length, this confidence interval is close to 2.3 because errors in both friction velocity and heat flux affect plume spread. Finally, we show that using measurements of temperature fluctuations in estimating heat flux is an improvement on that based on the surface energy balance, even when net radiation measurements are available.

The annual ammonia budget of fertilised cut grassland – Part 2: Seasonal variations and compensation point modeling

Abstract. The net annual NH3 exchange budget of a fertilised, cut grassland in Central Switzerland is presented. The observation-based budget was computed from semi-continuous micrometeorological fluxes over a time period of 16 months and using a process-based gap-filling procedure. The data for emission peak events following the application of cattle slurry and for background exchange were analysed separately to distinguish short-term perturbations from longer-term ecosystem functioning. A canopy compensation point model of background exchange is parameterised on the basis of measured data and applied for the purposes of gap-filling. The data show that, outside fertilisation events, grassland behaves as a net sink for atmospheric NH3 with an annual dry deposition flux of −3.0 kg N ha−1 yr−1, although small NH3 emissions by the canopy were measured in dry daytime conditions. The median Γs ratio in the apoplast (=[NH4+]/[H+]) estimated from micrometeorological measurements was 620, equivalent to a stomatal compensation point of 1.3 μg NH3 m−3 at 15 °C. Non-stomatal resistance to deposition Rw was shown to increase with temperature and decrease with surface relative humidity, and Rw values were among the highest published for European grasslands, consistent with a relatively high ratio of NH3 to acid gases in the boundary layer at this site. Since the gross annual NH3 emission by slurry spreading was of the order of +20 kg N ha−1 yr−1, the fertilised grassland was a net NH3 source of +17 kg N ha−1 yr−1. A comparison with the few other measurement-based budget values from the literature reveals considerable variability, demonstrating both the influence of soil, climate, management and grassland type on the NH3 budget and the difficulty of scaling up to the national level.

Application of the Priestley–Taylor Approach in a Two-Source Surface Energy Balance Model

Abstract The Priestley–Taylor (PT) approximation for computing evapotranspiration was initially developed for conditions of a horizontally uniform saturated surface sufficiently extended to obviate any significant advection of energy. Nevertheless, the PT approach has been effectively implemented within the framework of a thermal-based two-source model (TSM) of the surface energy balance, yielding reasonable latent heat flux estimates over a range in vegetative cover and climate conditions. In the TSM, however, the PT approach is applied only to the canopy component of the latent heat flux, which may behave more conservatively than the bulk (soil + canopy) system. The objective of this research is to investigate the response of the canopy and bulk PT parameters to varying leaf area index (LAI) and vapor pressure deficit (VPD) in both natural and agricultural vegetated systems, to better understand the utility and limitations of this approximation within the context of the TSM. Micrometeorological flux measurements collected at multiple sites under a wide range of atmospheric conditions were used to implement an optimization scheme, assessing the value of the PT parameter for best performance of the TSM. Overall, the findings suggest that within the context of the TSM, the optimal canopy PT coefficient for agricultural crops appears to have a fairly conservative value of ∼1.2 except when under very high vapor pressure deficit (VPD) conditions, when its value increases. For natural vegetation (primarily grasslands), the optimal canopy PT coefficient assumed lower values on average (∼0.9) and dropped even further at high values of VPD. This analysis provides some insight as to why the PT approach, initially developed for regional estimates of potential evapotranspiration, can be used successfully in the TSM scheme to yield reliable heat flux estimates over a variety of land cover types.

Eddy Correlation Measurements of Ozone Fluxes over Coastal Waters West of Ireland

Measurements of ozone fluxes using the eddy‐correlation (EC) technique were carried out for the first time at the Mace Head atmospheric research station, on the west coast of Ireland between August‐October 2009. Vertical exchange of ozone was measured from a tower platform at 22 m above mean sea level to study fluxes over coastal waters excluding the tidal region. The results were averaged over 30 min and exhibited predominantly downward but also upward transport of ozone in the boundary layer. Data quality was found to be high based on inspection of cospectra and micrometeorological measurements. During the study period, a major physical influence on O3 fluxes was found to be wind speed. Measured fluxes were of the same magnitude as reported in previous open ocean studies ranging from approximately +0.2 to −0.5 μg m−2 s−1 (−0.017 μg m−2 s−1 on average, corresponding to a deposition velocity of 0.25 mm s−1 or a surface resistance of 4.13 s mm−1). These results are considered to represent ozone fluxes over shallow coastal waters west of Ireland for conditions during summer and fall not affected by phytoplankton blooms.

Dynamics of ammonia exchange with cut grassland: synthesis of results and conclusions of the GRAMINAE Integrated Experiment

Abstract. Improved data on biosphere-atmosphere exchange are fundamental to understanding the production and fate of ammonia (NH3) in the atmosphere. The GRAMINAE Integrated Experiment combined novel measurement and modelling approaches to provide the most comprehensive analysis of the interactions to date. Major inter-comparisons of micrometeorological parameters and NH3 flux measurements using the aerodynamic gradient method and relaxed eddy accumulation (REA) were conducted. These showed close agreement, though the REA systems proved insufficiently precise to investigate vertical flux divergence. Grassland management had a large effect on fluxes: emissions increased after grass cutting (−50 to 700 ng m−2 s−1 NH3) and after N-fertilization (0 to 3800 ng m−2 s−1) compared with before the cut (−60 to 40 ng m−2 s−1). Effects of advection and air chemistry were investigated using horizontal NH3 profiles, acid gas and particle flux measurements. Inverse modelling of NH3 emission from an experimental farm agreed closely with inventory estimates, while advection errors were used to correct measured grassland fluxes. Advection effects were caused both by the farm and by emissions from the field, with an inverse dispersion-deposition model providing a reliable new approach to estimate net NH3 fluxes. Effects of aerosol chemistry on net NH3 fluxes were small, while the measurements allowed NH3-induced particle growth rates to be calculated and aerosol fluxes to be corrected. Bioassays estimated the emission potential Γ = [NH4+]/[H+] for different plant pools, with the apoplast having the smallest values (30–1000). The main within-canopy sources of NH3 emission appeared to be leaf litter and the soil surface, with Γ up to 3 million and 300 000, respectively. Cuvette and within-canopy analyses confirmed the role of leaf litter NH3 emission, which, prior to cutting, was mostly recaptured within the canopy. Measured ammonia fluxes were compared with three models: an ecosystem model (PaSim), a soil vegetation atmosphere transfer model (SURFATM-NH3) and a dynamic leaf chemistry model (DCC model). The different models each reproduced the main temporal dynamics in the flux, highlighting the importance of canopy temperature dynamics (Surfatm-NH3), interactions with ecosystem nitrogen cycling (PaSim) and the role of leaf surface chemistry (DCC model). Overall, net above-canopy fluxes were mostly determined by stomatal and cuticular uptake (before the cut), leaf litter emissions (after the cut) and fertilizer and litter emissions (after fertilization). The dynamics of ammonia emission from leaf litter are identified as a priority for future research.

Biosphere-atmosphere exchange of CO<sub>2</sub> in relation to climate: a cross-biome analysis across multiple time scales

Abstract. The net ecosystem exchange of CO2 (NEE) varies at time scales from seconds to years and longer via the response of its components, gross ecosystem productivity (GEP) and ecosystem respiration (RE), to physical and biological drivers. Quantifying the relationship between flux and climate at multiple time scales is necessary for a comprehensive understanding of the role of climate in the terrestrial carbon cycle. Orthonormal wavelet transformation (OWT) can quantify the strength of the interactions between gappy eddy covariance flux and micrometeorological measurements at multiple frequencies while expressing time series variance in few energetic wavelet coefficients, offering a low-dimensional view of the response of terrestrial carbon flux to climatic variability. The variability of NEE, GEP and RE, and their co-variability with dominant climatic drivers, are explored with nearly one thousand site-years of data from the FLUXNET global dataset consisting of 253 eddy covariance research sites. The NEE and GEP wavelet spectra were similar among plant functional types (PFT) at weekly and shorter time scales, but significant divergence appeared among PFT at the biweekly and longer time scales, at which NEE and GEP were relatively less variable than climate. The RE spectra rarely differed among PFT across time scales as expected. On average, RE spectra had greater low frequency (monthly to interannual) variability than NEE, GEP and climate. CANOAK ecosystem model simulations demonstrate that "multi-annual" spectral peaks in flux may emerge at low (4+ years) time scales. Biological responses to climate and other internal system dynamics, rather than direct ecosystem response to climate, provide the likely explanation for observed multi-annual variability, but data records must be lengthened and measurements of ecosystem state must be made, and made available, to disentangle the mechanisms responsible for low frequency patterns in ecosystem CO2 exchange.

N 2O exchange over managed grassland: Application of a quantum cascade laser spectrometer for micrometeorological flux measurements

A novel quantum cascade laser absorption spectrometer (QCL-AS) was tested to monitor N 2O exchange fluxes over an intensively managed grassland using the eddy covariance approach. The instrument employs a continuous wave quantum cascade laser to scan over the absorption features of N 2O, CH 4 and water vapor at 7.8 μm. The precision of the N 2O flux measurements was determined to be 0.2 nmol m −2 s −1 but the accuracy can easily be affected by water vapor interferences twice as large. These water vapor interferences are not only due to the respective gas dilution effect but also due to an additional cross-sensitivity of the N 2O analyzer to water vapor (0.3 ppb N 2O/% H 2O). Both effects cause a negative bias of similar magnitude (0.3 nmol m −2 s −1 N 2O flux/mmol m −2 s −1 H 2O flux) in the flux measurements. While the dilution (or density) correction is a well known and routinely applied procedure, the magnitude of the analyzer cross-talk may depend on the specific instrumental setup and should be empirically determined. The comparison with static chamber measurements shows the necessity of the cross-talk correction; otherwise the QCL-AS based eddy covariance system would yield unrealistically large uptake of N 2O.

A new design of the LAPS land surface scheme for use over and through heterogeneous and non-heterogeneous surfaces: Numerical simulations and tests

Numerical simulations and tests with the recently redesigned land–air parameterization scheme (LAPS) are presented. In all experiments, supported either by one-point micrometeorological, 1D or 3D simulations, the attention has been directed to: (1) comparison of simulation outputs, expressing the energy transfer over and through heterogeneous and non-heterogeneous surfaces, versus observations and (2) analysis of uncertainties occurring in the solution of the energy balance equation at the land–air interface. To check the proposed method for aggregation of albedo, “propagating hole” sensitivity tests with LAPS over a sandstone rock grid cell have been performed with the forcing meteorological data for July 17, 1999 in Baxter site, Philadelphia (USA). Micrometeorological and biophysical measurements from the surface experiments conducted over crops and apple orchard in Serbia, Poland, Austria and France were used to test the operation of LAPS in calculating surface fluxes and canopy environment temperatures within and above plant covers of different densities. In addition, sensitivity tests with single canopy covers over the Central Europe region and comparison against the observations taken from SYNOP data using 3D simulations were made. Validation of LAPS performances over a solid surface has been done by comparison of 2 m air temperature observations against 5-day simulations over the Sahara Desert rocky ground using 3D model. To examine how realistically the LAPS simulates surface processes over a heterogeneous surface, we compared the air temperature measured at 2 m and that predicted by the 1D model with the LAPS as the surface scheme. Finally, the scheme behaviour over urban surface was tested by runs over different parts of a hypothetical urban area. The corresponding 1D simulations were carried out with an imposed meteorological dataset collected during HAPEX-MOBILHY experiment at Caumont (France). The quantities predicted by the LAPS compare well with the observations and the various subcomponents of the scheme appear to operate realistically.

Estimating nocturnal ecosystem respiration from the vertical turbulent flux and change in storage of CO 2

Micrometeorological measurements of nighttime ecosystem respiration can be systematically biased when stable atmospheric conditions lead to drainage flows associated with decoupling of air flow above and within plant canopies. The associated horizontal and vertical advective fluxes cannot be measured using instrumentation on the single towers typically used at micrometeorological sites. A common approach to minimize bias is to use a threshold in friction velocity, u *, to exclude periods when advection is assumed to be important, but this is problematic in situations when in-canopy flows are decoupled from the flow above. Using data from 25 flux stations in a wide variety of forest ecosystems globally, we examine the generality of a novel approach to estimating nocturnal respiration developed by van Gorsel et al. (van Gorsel, E., Leuning, R., Cleugh, H.A., Keith, H., Suni, T., 2007. Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u *-threshold filtering technique. Tellus 59B, 397–403, Tellus, 59B, 307-403). The approach is based on the assumption that advection is small relative to the vertical turbulent flux ( F C) and change in storage ( F S) of CO 2 in the few hours after sundown. The sum of F C and F S reach a maximum during this period which is used to derive a temperature response function for ecosystem respiration. Measured hourly soil temperatures are then used with this function to estimate respiration R Rmax. The new approach yielded excellent agreement with (1) independent measurements using respiration chambers, (2) with estimates using ecosystem light-response curves of F c + F s extrapolated to zero light, R LRC, and (3) with a detailed process-based forest ecosystem model, R cast. At most sites respiration rates estimated using the u *-filter, R ust, were smaller than R Rmax and R LRC. Agreement of our approach with independent measurements indicates that R Rmax provides an excellent estimate of nighttime ecosystem respiration.

Increases in air temperature can promote wind-driven dispersal and spread of plants

Long-distance dispersal (LDD) of seeds and pollen shapes the spatial dynamics of plant genotypes, populations and communities. Quantifying LDD is thus important for predicting the future dynamics of plants exposed to environmental changes. However, environmental changes can also alter the behaviour of LDD vectors: for instance, increasing air temperature may enhance atmospheric instability, thereby altering the turbulent airflow that transports seed and pollen. Here, we investigate temperature effects on wind dispersal in a boreal forest using a 10-year time series of micrometeorological measurements and a Lagrangian stochastic model for particle transport. For a wide range of dispersal and life history types, we found positive relations between air temperature and LDD. This translates into a largely consistent positive effect of +3 degrees C warming on predicted LDD frequencies and spread rates of plants. Relative increases in LDD frequency tend to be higher for heavy-seeded plants, whereas absolute increases in LDD and spread rates are higher for light-seeded plants for which wind is often an important dispersal vector. While these predicted increases are not sufficient to compensate forecasted range losses and environmental changes can alter plant spread in various ways, our results generally suggest that warming can promote wind-driven movements of plant genotypes and populations in boreal forests.

Micrometeorological measurements in Nigeria during the total solar eclipse of 29 March, 2006

The total solar eclipse of 29 March, 2006 which was visible at Ibadan (7.55°N, 4.56°E), south-western Nigeria was utilized to document atmospheric surface-layer effects of the eclipse for the first time in Nigeria. The meteorological parameters measured are global radiation, net radiation, wind speed (at different heights), atmospheric pressure and soil temperature (5, 10 and 30 cm), moisture and heat flux and rainfall. The results revealed remarkable dynamic atmospheric effects. The observations showed that the incoming solar radiation, net radiation and air temperature were significantly affected. There was an upsurge of wind speed just before the first contact of the eclipse followed by a very sharp decrease in wind speed due to the cooling and stabilization of the atmospheric boundary layer. The atmospheric pressure lags the eclipse maximum by 1 h 30 min, while the soil temperature at 5 and 10 cm remain constant during the maximum phase of the eclipse.

Temporal Common Trends of Topsoil Water Dynamics in a Humid Subtropical Forest Watershed

Research oriented toward understanding the hydrologic functioning of the relict “laurisilva” evergreen forests is scarce. This study focused on the analysis of temporal changes in soil water status under such humid subtropical stands and explored to what extent hydrologic fluxes may explain topsoil water dynamics. Hydrologic fluxes (potential evapotranspiration, canopy fog water dripping, and rainwater below the canopy) were computed for a 2‐yr period using in situ micrometeorological measurements in the Garajonay National Park cloud forest (Canary Islands). Time domain reflectometry (TDR) data were used to characterize soil water status at 0.15‐ and 0.30‐m depths in plots located at 1145, 1185, 1230, and 1270 m above sea level. The resulting eight daily TDR data sets were studied with dynamic factor analysis. The variability in the soil water status time series was simplified and successfully described (coefficient of efficiency = 0.717) with a single temporal trend dynamic factor model (DFM), representing unexplained variability common to all plots and monitoring depths. Comparison of DFMs with and without explanatory variables (i.e., hydrologic fluxes) indicates that unexplained variability in the observed data was partially reduced by the information provided by the hydrologic fluxes. The rainfall contribution to the soil surface, and to a lesser extent forest potential evapotranspiration, were necessary variables for describing temporal changes in topsoil water status; however, dripping fog water was found to be a negligible contributor. Dynamic factor analysis proved to be useful for studying the variability in multivariate hydrologic time series without the need of a priori detailed information about the underlying mechanisms governing soil water dynamics.

Dynamics of ammonia exchange with cut grassland: strategy and implementation of the GRAMINAE Integrated Experiment

Abstract. A major international experiment on ammonia (NH3) biosphere-atmosphere exchange was conducted over intensively managed grassland at Braunschweig, Germany. The experimental strategy was developed to allow an integrated analysis of different features of NH3 exchange including: a) quantification of nearby emissions and advection effects, b) estimation of net NH3 fluxes with the canopy by a range of micrometeorological measurements, c) analysis of the sources and sinks of NH3 within the plant canopy, including soils and bioassay measurements, d) comparison of the effects of grassland management options on NH3 fluxes and e) assessment of the interactions of NH3 fluxes with aerosol exchange processes. Additional technical objectives included the inter-comparison of different estimates of sensible and latent heat fluxes, as well as continuous-gradient and Relaxed Eddy Accumulation (REA) systems for NH3 fluxes. The prior analysis established the spatial and temporal design of the experiment, allowing significant synergy between these objectives. The measurements were made at 7 measurement locations, thereby quantifying horizontal and vertical profiles, and covered three phases: a) tall grass canopy prior to cutting (7 days), b) short grass after cutting (7 days) and c) re-growing sward following fertilization with ammonium nitrate (10 days). The sequential management treatments allowed comparison of sources-sinks, advection and aerosol interactions under a wide range of NH3 fluxes. This paper describes the experimental strategy and reports the grassland management history, soils, environmental conditions and air chemistry during the experiment, finally summarizing how the results are coordinated in the accompanying series of papers.

Fog reduces transpiration in tree species of the Canarian relict heath-laurel cloud forest (Garajonay National Park, Spain)

The ecophysiologic role of fog in the evergreen heath-laurel 'laurisilva' cloud forests of the Canary Islands has not been unequivocally demonstrated, although it is generally assumed that fog water is important for the survival and the distribution of this relict paleoecosystem of the North Atlantic Macaronesian archipelagos. To determine the role of fog in this ecosystem, we combined direct transpiration measurements of heath-laurel tree species, obtained with Granier's heat dissipation probes, with micrometeorological and artificial fog collection measurements carried out in a 43.7-ha watershed located in the Garajonay National Park (La Gomera, Canary Islands, Spain) over a 10-month period. Median ambient temperature spanned from 7 to 15 degrees C under foggy conditions whereas higher values, ranging from 9 to 21 degrees C, were registered during fog-free periods. Additionally, during the periods when fog water was collected, global solar radiation values were linearly related (r2=0.831) to those under fog-free conditions, such that there was a 75+/-1% reduction in median radiation in response to fog. Fog events greatly reduced median diurnal tree transpiration, with rates about 30 times lower than that during fog-free conditions and approximating the nighttime rates in both species studied (the needle-like leaf Erica arborea L. and the broadleaf Myrica faya Ait.). This large decrease in transpiration in response to fog was independent of the time of the day, tree size and species and micrometeorological status, both when expressed on a median basis and in cumulative terms for the entire 10-month measuring period. We conclude that, in contrast to the turbulent deposition of fog water droplets on the heath-laurel species, which may be regarded as a localized hydrological phenomenon that is important for high-altitude wind-exposed E. arborea trees, the cooler, wetter and shaded microenvironment provided by the cloud immersion belt represents a large-scale effect that is crucial for reducing the transpirational water loss of trees that have profligate water use, such as those of the 'laurisilva'.

Correlated change in normalized difference vegetation index and the seasonal trajectory of photosynthetic capacity in a conifer stand

Seasonal changes in canopy photosynthetic activity play an important role in carbon assimilation. However, few simulation models for estimating carbon balances have included them due to scarcity in quality data. This paper investigates some important aspects of the relationship between the seasonal trajectory of photosynthetic capacity and the time series of a common vegetation index (normalized difference vegetation index, NDVI), which was derived from on site micrometeorological measurements or smoothed and downscaled from satellite‐borne NDVI sensors. A parameter indicating the seasonality of canopy physiological activity, P E, was retrieved through fitting a half‐hour step process model, PROXELNEE, to gross primary production (GPP) estimates by inversion for carboxylation and light utilization efficiencies. The relative maximum rate of carboxylation (V rm), a parameter that indicates the seasonality of CO2 uptake potential under prevailing temperature, was then calculated from P E and daily average air temperature. Statistical analysis revealed that there were obvious exponential relationships between NDVI and the seasonal courses for both canopy physiological activities P E and V rm. Among them, the on‐site broadband NDVI provided a robust and consistent relationship with canopy physiological activities (R 2 = 0.84). The relationships between satellite‐borne NDVI time series with instantaneous canopy physiological activities at the time of satellite passing were also checked. The results indicate that daily step NDVI time series (data downscaled from composite temporal resolution NDVI) better represent the daily average activity of the canopy. These findings may enable us to retrieve the seasonal course of canopy physiological activity from widely available NDVI data series and, thus, to include it into carbon assimilation models. However, both smoothing methods for satellite‐borne NDVI time series may generate incorrect estimates and must be treated with care.