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International audience; Estimating accurately dust emission flux during aeolian erosion events is crucial for quantifying the amount of dust in the atmosphere. The rare existing field experiments quantifying such flux were mainly performed using the flux-gradient (FG) method. Here, we present the first intercomparison of the size-resolved dust fluxes estimated by both the FG and the eddy-covariance (EC) methods during several erosion events. Both methods were applied simultaneously during the WIND-O-V (WIND erOsion in presence of sparse Vegetation)'s 2017 field experiment over an isolated erodible bare plot in South Tunisia. Overall, both methods predict similar dust fluxes for particle smaller than about 4 μm. For coarser particles, the EC method predicts a smaller dust flux than the FG method. Factors explaining this difference are discussed such as the different sampling heads used by the dust particle counters of both methods, or the possible weakening of the constant dust flux layer at the location of the upper dust particle counter of the FG method. This intercomparison highlights the difficulties and advantages of each method as well as their complementarity.

Turbulent transport of mineral dust away from erodible plots is usually assumed similar to the momentum transport. However, observations from the WIND-O-V (WIND erOsion in presence of sparse Vegetation's) 2017 field experiment over an isolated flat surface in Tunisia showed a dissimilar turbulent transport between dust and momentum. Here, the origin of this dissimilarity is explored from a numerical experiment using a detailed physically based erosion model based on a large-eddy simulation airflow model. Simulations support the findings of the WIND-O-V campaign, confirming the key role played by the dust emission intermittency to the transport dissimilarity with the momentum, this later one being more continuously absorbed at the surface. Simulations reveal that this dissimilarity diminishes with height as the intermittency of dust emission is progressively lost during the turbulent transport-mixing process. With wind intensity, the dissimilarity diminishes as well, with dust emissions becoming more spatially homogeneous, and thus less intermittent. Our simulations further highlight the additional role played by the fetch length limitation of the erodible plot to the turbulent transport dissimilarity. In presence of a short fetch, the dissimilarity between dust and momentum turbulent transports increases with height as the dust flux footprint integrates dust emission conditions from different surrounding surfaces. This latter process depends on the characteristics of the surrounded surfaces and is expected to be significant in semiarid regions.

Predicting the particle size distribution (PSD) of near-surface turbulent dust flux ( F wc ) is a key issue for estimating the size of atmospheric mineral dust. Existing dust emission schemes differ in their parametrization of the emitted dust ( F emi ) PSD, defining differently the surface inter-particle cohesive force and the influence of wind intensity. Moreover, these schemes have often been validated-fitted against field measurements, assuming PSD similarity between F wc and F emi . Here, we investigate numerically the main factors influencing F wc -PSD during erosion events. To this effect, we developed a 1D dust-dispersal model. After evaluating the model against published results, it is shown that F wc -PSD is influenced by both deposition and F emi -PSD. This latter one is shaped by the inter-particle cohesive bond exponent and the surface dust PSD. A time-to-space conversion of the dust flux variations reveals an increasing enrichment of F wc in small particles compared to F emi . This enrichment remains lower than a few percent of the total dust flux (in number) for fetch lower than 100 m, but it can rise to more than 10% for fetch longer than 1 km. This fetch dependence of F wc -PSD is explained by the slow deposition of particles having the lowest deposition velocities. Importantly, this difference between F wc and F emi PSDs is accentuated with wind intensity, with F emi -PSD dominated by particles with large deposition velocities, and in presence of a large-scale background dust concentration. The role played by the deposition process in shaping the F wc -PSD should be considered when evaluating dust emission schemes against near-surface field measurements.

International audience; Atmospheric particles play a major role in both air quality and climate change. Formation of secondary particles in the atmosphere has been observed over many different environments and is believed to provide up to half of the atmospheric cloud condensation nuclei (CCN) at a global scale. However, high uncertainties are still remaining in the description of mechanisms involved in new particle formation (NPF). Especially, more evidences of the implication of biogenic volatile organic compounds (BVOCs) in NPF from field studies are still needed. To investigate this question, two field campaigns have been set up during July 2014 and July 2015, in the French Landes forest (south west of France). Summer 2015 was characterized by a strong hydric stress, whereas summer 2014 was rainy. In 2015, frequent nocturnal NPF was observed, reaching a frequency of occurrence of similar to 55% of the nights, while only one event was observed in 2014. In July 2015, monoterpene mixing ratios (dominated by alpha- and beta-pinene) were higher, mostly due to high ambient temperatures and drought. A focus was made on the 2015 field campaign, where NPF was mostly observed. The mean diurnal variation of the ratio between alpha- and beta-pinene mixing ratios highlighted in-canopy reactivity of monoterpenes with ozone in the early night. This hypothesis was reinforced by the increasing gas phase levels of pinonaldehyde and nopinone, the main first-generation products arising from alpha- and beta-pinene ozonolysis, at night, before NPF started. It strongly suggests that monoterpene oxidation further generated very-low volatility gases involved in NPF. This finding is also supported by the high concentrations of the SOA traditional biogenic tracers, e.g. pinic and pinonic acids, quantified in the particulate phase. The role of BVOCs in NPF is thus highlighted, as well as the importance of nighttime NPF.

Soil deposition is an essential pathway for tropospheric ozone (O3) removal, but its controlling factors remain unclear. Here, we explored the variability of soil O3 resistance in response to soil texture. To this aim, data of O3 deposition over bare soil obtained from micrometeorological measurements under contrasted meteorological conditions for five sites were used. The results obtained are twofold: (i) soil resistance (Rsoil) increased with soil surface relative humidity (RHsurf), but (ii) this relationship exhibited large site-by-site variability. Further analysis showed that the minimum soil resistance (corresponding to completely dry soil surface or RHsurf = 0%) and the increase of Rsoil with RHsurf are both linked to soil clay content. These results can be explained by (i) the soil surface available for O3 deposition at a microscopic scale which is a function of the soil specific surface area, and (ii) the capacity of a soil to adsorb water according to its clay content and therefore to reduce the surface active for O3 deposition. From these results, a new parameterization has been established to estimate Rsoil as a function of RHsurf and soil clay fraction.

International audience; During two growing seasons of a maritime pine stand, in 2014 and 2015, ozone (O-3) fluxes have been determined using the eddy covariance (EC) method and compared to the outputs of a big-leaf O-3 deposition model including stomatal, cuticular and soil pathways. The model developed in this study generally allowed to properly reproduce the measured ozone deposition. Ozone fluxes showed a strong reduction during two water stressed periods in September 2014 and July 2015. The model partly explain this fall due to the reduction of stomatal deposition. Despite this stomatal closure, measured O-3 fluxes presented systematically lower negative values than the model outputs, and sometimes even positive values around midday during periods marked by strong water stress. In other words, the difference between observed and modelled O-3 fluxes (hereinafter referred to as the residual O-3 flux) is systematically positive on daytime during these water-stressed periods. This positive residual flux traduced the existence of an O-3 source below the flux measurement level, responsible for positive fluxes that counterbalance deposition fluxes. We developed an O-3 production module based on a terpene emission algorithm and an OH concentration proxy, to try to explain the observed ozone production. As this parametrisation allowed us to reproduce well the daily and inter-daily dynamics of the residual O-3 flux, it confirms that the latter actually resulted from O-3 production processes. This ozone production is here highlighted for the first time using O-3 fluxes measurements by the EC method. The chemical reactions possibly involved in O-3 production processes in this maritime pine forest have been discussed and different mechanisms are proposed, based on peroxy radicals chemistry or stress-induced BVOCs.

Ozone is a potentially phyto-toxic air pollutant, which can cause leaf damage and drastically alter crop yields, causing serious economic losses around the world. The VULNOZ (VULNerability to OZone in Anthropised Ecosystems) project is a biology and modeling project that aims to understand how plants respond to the stress of high ozone concentrations, then use a set of models to (i) predict the impact of ozone on plant growth, (ii) represent ozone deposition fluxes to vegetation, and finally (iii) estimate the economic consequences of an increasing ozone background the future. In this work, as part of the VULNOZ project, an innovative representation of ozone deposition to vegetation was developed and implemented in the CHIMERE regional chemistry-transport model. This type of model calculates the average amount of ozone deposited on a parcel each hour, as well as the integrated amount of ozone deposited to the surface at the regional or country level. Our new approach was based on a refinement of the representation of crop types in the model and the use of empirical parameters specific to each crop category. The results obtained were compared with a conventional ozone deposition modeling approach, and evaluated against observations from several agricultural areas in France. They showed that a better representation of the distribution between stomatal and non-stomatal ozone fluxes was obtained in the empirical approach, and they allowed us to produce a new estimate of the total amount of ozone deposited on the subtypes of vegetation at the national level.

International audience; Improving the understanding of processes related to atmospheric particle sources is essential to better assess future climate. Especially, how biogenic volatile organic compounds (BVOCs) are involved in new particle formation (NPF) is still unclear, highlighting the need for field studies in sites that have not yet been explored. Weakly anthropised, mostly composed of maritime pines (known as strong monoterpene emitters), vast and under the influence of sea spray inputs, the Landes forest (located in the southwestern part of France) is a suitable ecosystem to explore these questions. The aim of the present work was to investigate for the first time NPF in the Landes forest, and to identify the conditions for NPF. During a field campaign conducted in July 2015, clear NPF was observed during nighttime, at a high frequency rate (37.5%), whereas only two daytime episodes were observed. Growth rates during NPF events were in the range 9.0-15.7 nm h(-1), and nucleation rates (J(10)) in the range 0.8-8 particles cm(3) s(-1), typically in the range of reported values from rural sites. Nocturnal NPF started at sunset, lagging the reductions of temperature and ozone concentration as well as the increase of relative humidity, atmospheric stability and monoterpene concentration. We established that NPF occurred during more stratified atmosphere episodes, reflecting that NPF is more influenced by local processes at the Landes forest site (Bilos). Concentration of the sum of monoterpenes, here mainly alpha- and beta-pinene, was observed to be maximal during NPF episodes. On the contrary, ozone concentration was lower, which may indicate a larger consumption during nights where NPF episodes occur. Results strongly suggest the contribution of BVOC oxidation to nocturnal NPF, in both nucleation and the growth stages.

[1] This study evaluates ozone (O3) deposition to an agricultural field over a period of 2 years. A two-layer soil-vegetation-atmosphere-transfer (Surfatm-O3) model is used to partition the O3 flux between the soil, the cuticular, and the stomatal pathways. The comparison between measured and modeled O3 fluxes exhibited a good agreement, independently of the canopy structure and coverage and the climatic conditions, which implicitly validates the O3 flux partitioning. The total, soil, cuticular, and stomatal O3 budgets are then established from the modeling. Total ecosystem O3 deposition over the 2 year period was 87.5 kg ha−1. Clearly, nonstomatal deposition dominates the deposition budget, especially the soil component which represented up to 50% of the total deposition. Nevertheless, the physiological and phenological differences of maize and winter wheat induced large difference in the stomatal deposition budgets of these two crops. Then, the effect of simplified parameterizations for soil and cuticular resistances currently used in other models on the O3 budget is tested. Independently, these simplified parameterizations cause an underestimation of the O3 deposition ranging between 0% and 11.2%. However, the combination of all simplifications resulted in an underestimation of the total O3 deposition by about 20%. Finally, crop yield loss was estimated to be 1.5–4.2% for the winter wheat, whereas maize was not affected by O3.

Tropospheric ozone (O3) is a known greenhouse gas responsible for impacts on human and animal health and ecosystem functioning. In addition, O3 plays an important role in tropospheric chemistry, together with nitrogen oxides. The determination of surface-atmosphere exchange fluxes of these trace gases is a prerequisite to establish their atmospheric budget and evaluate their impact onto the biosphere. In this study, O3, nitric oxide (NO) and nitrogen dioxide (NO2) fluxes were measured using the aerodynamic gradient method over a bare soil in an agricultural field. Ozone and NO fluxes were also measured using eddy-covariance and automatic chambers, respectively. The aerodynamic gradient measurement system, composed of fast response sensors, was capable to measure significant differences in NO and O3 mixing ratios between heights. However, due to local advection, NO2 mixing ratios were highly non-stationary and NO2 fluxes were, therefore, not significantly different from zero. The chemical reactions between O3, NO and NO2 led to little ozone flux divergence between the surface and the measurement height (less than 1% of the flux on average), whereas the NO flux divergence was about 10% on average. The use of fast response sensors allowed reducing the flux uncertainty. The aerodynamic gradient and the eddy-covariance methods gave comparable O3 fluxes. The chamber NO fluxes were down to 70% lower than the aerodynamic gradient fluxes, probably because of either the spatial heterogeneity of the soil NO emissions or the perturbation due to the chamber itself.

International audience; Forested landscapes often exhibit large spatial variability in vertical and horizontal foliage distributions. This variability may impact canopy-atmosphere ex- changes through its action on the development of turbulent structures. Here we inves- tigate in neutral stratification the turbulent structures encountered in a maritime pine forest characterised by a high, dense foliated layer associated with a deep and sparse trunk space. Both stand and edge regions are considered. In situ measurements and the outputs of large-eddy simulations are used and analysed together. In stand conditions, far from the edge, canopy-top structures appear stronlgy damped by the dense crown layer. Turbulent wind fluctuations within the trunk space, where the momentum flux vanishes, are closely related to these canopy-top structures through pressure diffusion. Consequently, auto-correlation and spectral analyses are not quite appropriate to characterise the vertical scale of coherent structures in this type of canopy, as pressure diffusion enhances the actual scale of structures. At frequencies higher than those associated with canopy-top structures, wind fluctuations related to wake structures developing behind tree stems are observed within the trunk space. They manifest themselves in wind velocity spectra as secondary peaks in the inertial subrange re- gion, confirming the hypothesis of spectral short-cuts in vegetation canopies. In the edge region specific turbulent structures develop just below the crown layer, in addi- tion to canopy-top structures. They are generated by the wind shear induced by the sub-canopy wind jet that forms at the edge. These structures provide a momentum exchange mechanism similar to that observed at the canopy top but in the opposite di- rection and with a lower magnitude. They may develop as in plane mixing-layer flows, with some perturbations induced by canopy-top structures. Wake structures are also observed within the trunk space in the edge region.

Measurements of carbon dioxide fluxes were performed over a temperate intertidal mudflat in southwestern France using the micrometeorological Eddy Covariance (EC) technique. EC measurements were carried out in two contrasting sites of the Arcachon flat during four periods and in three different seasons (autumn 2007, summer 2008, autumn 2008 and spring 2009). In addition, satellite images of the tidal flat at low tide were used to link the net ecosystem CO2 exchange (NEE) with the occupation of the mudflat by primary producers, particularly by Zostera noltii meadows. CO2 fluxes during the four deployments showed important spatial and temporal variations, with the flat rapidly shifting from sink to source with the tide. Absolute CO2 fluxes showed generally small negative (influx) and positive (efflux) values, with larger values up to −13 μmol m−2 s−1 for influxes and 19 μmol m−2 s−1 for effluxes. Low tide during the day was mostly associated with a net uptake of atmospheric CO2. In contrast, during immersion and during low tide at night, CO2 fluxes where positive, negative or close to zero, depending on the season and the site. During the autumn of 2007, at the innermost station with a patchy Zostera noltii bed (cover of 22 ± 14% in the wind direction of measurements), CO2 influx was −1.7 ± 1.7 μmol m−2 s−1 at low tide during the day, and the efflux was 2.7 ± 3.7 μmol m−2 s−1 at low tide during the night. A gross primary production (GPP) of 4.4 ± 4.1 μmol m−2 s−1 during emersion could be attributed to microphytobenthic communities. During the summer and autumn of 2008, at the central station with a dense eelgrass bed (92 ± 10%), CO2 uptakes at low tide during the day were −1.5 ± 1.2 and −0.9 ± 1.7 μmol m−2 s−1, respectively. Night time effluxes of CO2 were 1.0 ± 0.9 and 0.2 ± 1.1 μmol m−2 s−1 in summer and autumn, respectively, resulting in a GPP during emersion of 2.5 ± 1.5 and 1.1 ± 2.0 μmol m−2 s−1, respectively, attributed primarily to the seagrass community. At the same station in April 2009, before Zostera noltii started to grow, the CO2 uptake at low tide during the day was the highest (−2.7 ± 2.0 μmol m−2 s−1). Influxes of CO2 were also observed during immersion at the central station in spring and early autumn and were apparently related to phytoplankton blooms occurring at the mouth of the flat, followed by the advection of CO2-depleted water with the flooding tide. Although winter data as well as water carbon measurements would be necessary to determine a precise CO2 budget for the flat, our results suggest that tidal flat ecosystems are a modest contributor to the CO2 budget of the coastal ocean.

The effects of management practices on energy, water and carbon exchanges were investigated in a young pine plantation in south-west France. In 2009-10, carbon dioxide (CO(2)), H(2)O and heat fluxes were monitored using the eddy covariance and sap flow techniques in a control plot (C) with a developed gorse layer, and an adjacent plot that was mechanically weeded and thinned (W). Despite large differences in the total leaf area index and canopy structure, the annual net radiation absorbed was only 4% lower in plot W. We showed that higher albedo in this plot was offset by lower emitted long-wave radiation. Annual evapotranspiration (ET) from plot W was 15% lower, due to lower rainfall interception and transpiration by the tree canopy, partly counterbalanced by the larger evaporation from both soil and regrowing weedy vegetation. The drainage belowground from plot W was larger by 113 mm annually. The seasonal variability of ET was driven by the dynamics of the soil and weed layers, which was more severely affected by drought in plot C. Conversely, the temporal changes in pine transpiration and stem diameter growth were synchronous between sites despite higher soil water content in the weeded plot. At the annual scale, both plots were carbon sinks, but thinning and weeding reduced the carbon uptake by 73%: annual carbon uptake was 243 and 65 g C m(-2) on plots C and W, respectively. Summer drought dramatically impacted the net ecosystem exchange: plot C became a carbon source as the gross primary production (GPP) severely decreased. However, plot W remained a carbon sink during drought, as a result of decreases in both GPP and ecosystem respiration (R(E)). In winter, both plots were carbon sources, plots C and W emitting 67.5 and 32.4 g C m(-2), respectively. Overall, this study highlighted the significant contribution of the gorse layer to mass and energy exchange in young pine plantations.

During the last few decades, many studies have been performed to determine water and carbon budgets of broadleaf and deciduous forests, crops and grasslands. However, since most measurements have been made in different regions and at different periods, it is difficult to compare the results directly. In order to evaluate accurately the respective contribution of various agroecosystems to global water and carbon exchanges, it is necessary to compare data obtained in similar climatic and weather conditions. To address this question, we present the results from simultaneous measurements carried out during one year over three typical agroecosystems of the Les Landes region in South-West France:~an agricultural field with maize from 29 May to 18 October, a young (5 year-old) pine forest and a mature (37 year-old) pine forest. All measurements were collected as part of the Regional Experiment component of the CarboEurope-IP project. During most of the year, the agricultural field without vegetation is a source of CO2, but from late June to early September the maize crop becomes a stronger carbon sink than the forests. Over the whole measurement period the three agroecosystems behave as CO2 sinks with carbon storage of about 335, 210 and 160 g C m−2 for the young forest, the mature forest and the agricultural field, respectively. We investigated the influence of climatic conditions on Gross Primary Production (GPP) of the three ecosystems and observed a predominant role of vapour pressure deficit (VPD) for forests and of photosynthetic photon flux density (FPP) for maize. Daily Water Use Efficiencies (WUE) of the three ecosystems were evaluated and expressed as functions of the mean daily vapour pressure deficit (VPD). Similar trends were observed for the two forests, which suggests that for a given species WUE is independent of stand age. The WUE of the maize crop at maturity was also found to depend upon VPD, but it is about twice as large as for the forests, owing to the physiological advantages of C4 species.

Understanding how ozone is deposited on vegetation canopies is needed to perform tropospheric greenhouse gas budgets and evaluate the associated damage on vegetation. In this study, we propose a new multilayer scheme of ozone deposition on vegetation canopies that predicts stomatal, cuticular and soil deposition pathways separately. This mechanistic ozone deposition scheme is based on the multi-layer, multi-leaf mass and energy transfer model MuSICA. This model was chosen because it explicitly simulates the processes of rain interception, through fall and evaporation at different depths within the vegetation canopy, so that ozone deposition on wet leaf cuticles can be explicitly modelled with ozone dissolution, diffusion and chemical reaction inside the water films. The model was evaluated against a 3-year dataset of ozone, CO2 and evapotranspiration flux measurements over a winter wheat field near Paris, France (ICOS Fr-GRI). Only periods with fully developed canopies (including senescence) were considered to minimise the contribution of soil deposition to the total ozone flux. Before senescence, the model could reproduce the measured ozone deposition rates as well as the CO2 and water vapour fluxes. During senescence, large ozone deposition rates were observed under wet canopy conditions that could only be explained by first-order reaction rates in the water film of around 105 s−1. Such reaction rates are not compatible with the chemical composition of rainwater. We therefore hypothesise that, during senescence, the cell content leaks out of the leaves when they become wet, exposing anti-oxidants to ozone. These results provide for the first time a mechanistic explanation of the commonly observed increase in ozone deposition rates during rain or dew formation.

International programmes such as EUROFLUX focus on the analysis of long-term fluxes and energy budgets in the biosphere. Reliable estimates of hourly energy budgets require an accurate estimation of soil heat flux, that is often non-negligible even in a forest, and can be predominant during the night. Over long periods of time such as one to several months, its contribution can also be significant. The present work has been carried out to get good estimates of the soil heat flux in a maritime pine stand in the southwest of France, one of the 15 EUROFLUX sites. Using a whole year’s worth of data, soil heat flux was estimated by a two-step version of the null-alignment method using soil temperature, water content and bulk density measurements between the soil surface and a depth of 1 m. A data subset was firstly used to estimate and model the soil thermal conductivity at various depths. The full data set was then used with the modelled conductivity to estimate heat storage between the surface and a reference depth, and calculate the heat flux at the soil surface. Throughout the investigated year and at a 30 min time scale, the soil heat flux represents 5‐10% of the incident net radiation, i.e. 30‐50% of the net radiation over the understorey. Cumulative values from September 1997 to March 1998 reach a maximum of 70 MJ m 2 , which represents nearly 50% of the cumulative values of transmitted net radiation (140 MJ m 2 ) over the same period. These estimates of soil heat flux allowed the energy budgets of the whole stand and the understorey to be closed, and showed that the storage terms are significant not only at a 30 min time scale but also at longer time scales (a few weeks). An attempt was finally made to model soil heat flux from meteorological data, which has rarely been done for a forest soil and over a long-term data set. In most of the existing models, soil heat flux is taken as a fraction of net radiation or sensible heat flux. Here, the litter acts as a mulch at the soil surface so that the only significant terms of the energy balance at this level are soil heat flux, transmitted net radiation and turbulent sensible heat flux. Soil heat flux is shown to be a linear combination of (1) net radiation above the understorey with a clear dependence of the coefficient on the soil cover fraction, and (2) the difference between the air and litter temperatures, with little influence of soil water content or wind speed on the coefficient. © 2001 Elsevier Science B.V. All rights reserved.

The eddy covariance (EC) method is widely considered as the reference method for heat and trace gas exchange flux measurements. However, for some species of interest, the aerodynamic gradient method (AG) is still a valuable method. Furthermore, some useful datasets are based on the AG method. In this study we compare the EC and the AG methods for latent (LE) and sensible (H) heat, carbon dioxide (Fc) fluxes and ozone deposition velocity ( V d O 3 ) over a maize field near Paris. The AG method gave roughly 15% smaller Fc and LE, similar H, and 40% larger V d O 3 than the EC method. The differences between the two methods are discussed. In particular, the effects of the displacement height and heights of measurements on the AG fluxes are explored and the similarity among heat, CO2, H2O and O3 is tested. Furthermore, the vertical divergence of the flux above the canopy is estimated with the AG method.

Terrestrial ecosystems represent a major sink for ozone (O3) and also a critical control of tropospheric O3 budget. However, due to its deleterious effects, plant functioning is affected by the ozone absorbed. It is thus necessary to both predict total ozone deposition to ecosystems and partition the fluxes in stomatal and non-stomatal pathways. The Surfatm-O3 model was developed to predict ozone deposition to agroecosystems from sowing to harvest, taking into account each deposition pathways during bare soil, growth, maturity, and senescence periods. An additional sink was added during senescence: stomatal deposition for yellow leaves, not able to photosynthesise but transpiring. The model was confronted to measurements performed over three maize crops in different regions of France. Modelled and measured fluxes agreed well for one dataset for any phenological stage, with only 3 % difference over the whole cropping season. A larger discrepancy was found for the two other sites, 16 % and 19 % over the entire study period, especially during bare soil, early growth and senescence. This was attributed to site-specific soil resistance to ozone and possible chemical reactions between ozone and volatile organic compounds emitted during late senescence. Considering both night-time and daytime conditions, non-stomatal deposition was the major ozone sink, from 100 % during bare soil period to 70–80 % on average during maturity. However, considering only daytime conditions, especially under optimal climatic conditions for plant functioning, stomatal flux could represent 75 % of total ozone flux. This model could improve estimates of crop yield losses and projections of tropospheric ozone budget.

As forest edges are a major source of heterogeneity in fragmented landscapes, the atmospheric flow over forested areas is often under their influence. Understanding how far the upstream edge has an impact on the turbulent wind flow in a forest canopy is important, in particular for scalar flux measurement. In this study, edge and stand flows over a maritime pine forest characterized by a dense crown layer located above a deep and sparse trunk space are analysed in detail from in situ measurements and large-eddy simulations (LES). The LES model used here appears to simulate remarkably well most characteristics of the turbulent wind flow for this particular canopy structure. It is shown that the main characteristics of the edge flow in this case differ from those usually observed in forests with a more uniform vertical foliage distribution. The main differences are (i) the development of turbulence above the canopy occurring closer to the edge, (ii) the absence of a well-defined enhanced gust zone around the top of the canopy, (iii) the presence of a large secondary wind maximum within the trunk space, and (iv) the development of a positive momentum flux layer below the crown layer. Most of these differences are related to the presence of a substantial sub-canopy wind jet induced by the wind flow through the trunk space at the edge. The secondary velocity maximum induced by this wind jet differs from that observed in homogeneous stand conditions, where it seems to be related to the mesoscale pressure gradient. The wind jet appears to decrease very slowly with distance from the edge, so that edge effects are still significant at 9 h from the edge (where h is the mean canopy height). The length of the adjustment region is shown to be greater than 10–15 h , and to depend on the depth of the trunk space. In very fragmented forested areas with deep and sparse trunk space, within-canopy flow may always be under the influence of edges.

The variables controlling ozone deposition onto bare soil are still unknown and it is necessary to understand this pathway well, as it represents a significant sink for ozone. Eddy-covariance measurements of ozone (O3) fluxes were performed over bare soils in agricultural land. Three datasets with contrasted meteorological conditions and soil nitric oxide (NO) emissions were used to study the factors controlling soil deposition. It is considered that ozone deposition can be represented with an aerodynamic resistance (Ra), a quasi-laminar boundary layer resistance (Rb O3), and an additional resistance, named soil resistance (Rsoil). Although it is assumed in previous studies that soil resistance is a function of soil water content (SWC) and could be considered constant as variation of SWC at monthly scale are generally weak, the results of this study indicate that SWC is not the main factor controlling Rsoil which shows daily and hourly variations. The main factor controlling soil resistance is the surface relative humidity which is positively correlated with Rsoil, contrary to non stomatal resistance onto canopies which show a negative correlation with relative humidity. The relationship between Rsoil and the surface relative humidity is probably due to a decrease in the surface available for ozone deposition, due to an increasing adsorption of water molecules onto the ground with relative humidity. A new parameterisation of Rsoil was established, where Rsoil is a function of the surface relative humidity only (Rsoil = Rsoil min × e(k×RHsurf), and Rsoil min = 21 ± 1.01 s m−1 and k = 0.024 ± 0.001, mean ± SD). The measured and parameterised ozone deposition velocities agree well when soil NO emissions are negligible. However, when there are large soil NO emissions, the parameterised ozone deposition strongly underestimates the measured deposition velocity even if the chemical destruction of ozone by reaction with NO in the air column was evaluated to be negligible. This suggests that soil NO emissions enhance soil ozone deposition by chemical reaction at or near the soil surface. The new parameterisation allows a better estimation of soil deposition, especially during daytime when Rsoil is overestimated using previously published parameterisations. It is an important step towards a better parameterisation of the non-stomatal uptake of ozone.

Ozone (O-3) flux measurements, obtained by eddy-covariance technique, over a developed maize crop were used to partition the overall O-3 deposition between stomatal and non-stomatal uptakes. Data were analysed using a big-leaf model, which was developed from current knowledge on O-3 deposition. The classical parameters used in dry deposition models (i.e. the in-canopy aerodynamic resistance R-ac, the intrinsic ground resistance R-ig and the cuticular resistance R-cut) were determined for the maize crop from the relationship between the experimental non-stomatal conductance (g(ns)) and the friction velocity (u(*)) in dry conditions (relative humidity (RH) < 60%). g. was determined as the difference between the O-3 canopy conductance (g(c)) and the O-3 stomatal conductance (g(s)), where g(s) was estimated by a method which combines the Penman-Monteith approach and the use of the CO2 assimilation flux. Data analysis revealed that chemical reactions between O-3 and nitrogen monoxide (NO) between the canopy top and the O-3 flux measurement level (z(m)) could induce high values of the observed O-3 conductance, not representative of ozone deposition to the canopy. The actual O-3 canopy conductance was derived from the observed O-3 conductance by including a correction term function of z(m) and the NO concentration at this height, based on the previous studies on O-3 destruction above canopies. The estimations of R-ac, R-ig and R-cut given by the non-linear regression of g(ns) vs u(*) are in agreement with previously published results. Our analysis also confirms previous studies which have shown that the cuticular conductance (g(cut)) increases exponentially with RH, and we propose a new parameterization of g(cut) as a function of RH, based on experimental evidence. Using our model to partition the total O-3 deposition to the canopy, we showed that the relative contributions of stomatal and non-stomatal uptakes varied strongly with the physiological activity of the maize and the meteorological conditions. This point is of major importance for studies dedicated to the impact of ozone on plant physiology, since it emphasizes the necessity to determine accurately the amount of O-3 actually absorbed by the plants via their stomatal activity. (C) 2009 Elsevier B.V. All rights reserved.

International audience; Carbon dioxide, water vapour and energy fluxes were measured above and within a maritime pine forest during an atypical year with long-lasting reduced soil water availibility. Energy balance closure was adequately good at both levels. As compared with what is usually observed at this site the ecosystem dissipated less energy via latent heat flux and more via sensible heat flux. The understorey canopy was responsible for a variable, significant component of the whole canopy fluxes of water vapour and carbon dioxide. The annual contribution of the understorey was 38% (154 mm) of the overall evaporation (399 mm) and 32% (89 mm) of the overall sensible heat flux (274 mm). The participation of the understorey reached 45% of the overall evaporation and 30% of the daytime overall assimilation during significant soil water deficit periods in summertime. Even during winter, understorey photosynthesis was consistent as it compensated soil and understorey respiration. The ecosystem behaved as a sink of carbon, with a negative annual carbon budget (-57 gC m-2). However, due to high soil water deficit, the annual ecosystem GPP was 40% less than usually observed at this site. This budget resulted from a sink of -131 gC m-2 for the overstorey and a source of +74 gC m-2 for the understorey. Moreover, on an annual basis the overstorey layer contributed to almost two thirds of the ecosystem respiration. Finally, the effect of long-lasting soil water deficit on the maritime pine forest was found more important than the effect of the heat wave and drought of summer 2003.

Over the past 15 years atmospheric surface-layer experiments over heterogeneous canopies have shown that the vertical transfer of sensible heat and water vapour exhibit a strong dissimilarity. In particular, the sensible-heat-to-water-vapour transport efficiencies generally exceed unity. One of the main consequences is that evaporation (latent heat flux) computed by the flux-variance method is overestimated, as persistently demonstrated by comparisons with evaporation obtained with the eddy-correlation method. Various authors proposed to take into account the temperature–humidity dissimilarity to extend the applicability of the flux-variance method in order to compute evaporation from non-uniform surfaces. They attempted to connect the sensible-heat-to-water-vapour transport efficiency (λ) to the correlation coefficient between temperature and humidity turbulent fluctuations (R Tq ). This approach was found to be successful over ‘wet’ surfaces for which λ can be approximated by R Tq and ‘dry’ surfaces for which λ can be approximated by 1/R Tq . However, no solution has been proposed until now for intermediate hydrological conditions. We investigated this question using eddy-correlation measurements above and inside a pine forest canopy. For both levels, our data present a strong likeness with previously published results over heterogeneous surfaces. In particular, they confirm that λ is R Tq in wet conditions and 1/R Tq in dry conditions. Moreover, we defined the range of the Bowen ratio (Bo) values for which those two approximations are valid (below 0.1 and greater than 1, respectively) and established a relationship between λ, R Tq and Bo for the intermediate range of Bo. We are confident that this new parameterization will enlarge the applicability of the flux-variance method to all kinds of heterogeneous surfaces in various hydrological conditions

Quantification of sources and sinks of carbon at global and regional scales requires not only a good description of the land sources and sinks of carbon, but also of the synoptic and mesoscale meteorology. An experiment was performed in Les Landes, southwest France, during May¿June 2005, to determine the variability in concentration gradients and fluxes of CO2. The CarboEurope Regional Experiment Strategy (CERES; see also http://carboregional.mediasfrance.org/index) aimed to produce aggregated estimates of the carbon balance of a region that can be meaningfully compared to those obtained from the smallest downscaled information of atmospheric measurements and continental-scale inversions. We deployed several aircraft to concentration sample the CO2 and fluxes over the whole area, while fixed stations observed the fluxes and concentrations at high accuracy. Several (mesoscale) meteorological modeling tools were used to plan the experiment and flight patterns. Results show that at regional scale the relation between profiles and fluxes is not obvious, and is strongly influenced by airmass history and mesoscale flow patterns. In particular, we show from an analysis of data for a single day that taking either the concentration at several locations as representative of local fluxes or taking the flux measurements at those sites as representative of larger regions would lead to incorrect conclusions about the distribution of sources and sinks of carbon. Joint consideration of the synoptic and regional flow, fluxes, and land surface is required for a correct interpretation. This calls for an experimental and modeling strategy that takes into account the large spatial gradients in concentrations and the variability in sources and sinks that arise from different land use types. We briefly describe how such an analysis can be performed and evaluate the usefulness of the data for planning of future networks or longer campaigns with reduced experimental efforts.

International audience; The physiological and physical processes controlling ozone dry deposition to vegetated surfaces are still not fully understood. In particular, the role of the understorey and the possible action of dew on ozone deposition have not received much attention so far. This paper presents the results of an experiment aimed at quantifying ozone dry deposition to a maritime pine forest in the “Les Landes” area in France. Ozone deposition fluxes were measured using the eddy-covariance technique above and within the canopy. We investigate the factors acting on ozone deposition in both dew-wetted and dry conditions. The values obtained for the ozone deposition velocity are well in the range of previously published measurements over coniferous forests. For the present forest, ozone uptake by the understorey is a significant portion of ozone deposition to the whole pine stand. The understorey contributes more to the overall ozone flux than to the other measured scalar fluxes (sensible heat and water vapour). During dry nights the surface conductance for ozone and the friction velocity are strongly correlated, showing that ozone deposition is largely controlled by dynamical processes. During the day, in dry conditions, the canopy stomatal conductance is the major parameter controlling ozone deposition. However, in winter, when the stomatal conductance is low, the influence of dynamical processes persists during day-time. It is also found that surface wetness associated with dew significantly enhances ozone deposition, during the night as well as in the morning.

Measurements of turbulent exchange in the understorey of a forest canopy are necessary to understand and model both the functioning of the lower stratum and its contribution to turbulent exchange at the whole canopy scale. Eddy covariance measurements of eddy fluxes just above the floor of a forest canopy must be thoroughly validated, given the particular conditions prevailing there. Our objective is two-fold: (i) check the overall quality of such eddy flux measurements through the analysis of the understorey energy balance closure and (ii) define quality criteria for each half-hourly sample, based on the residual term of the energy balance. A subset of the EUROFLUX data base, collected within a pine forest canopy in south-west France, was used for this purpose. During this experiment, all heat storage terms were carefully measured, which allowed the closure of the understorey energy balance to be rigorously tested. As in most experiments storage term measurements are not available, we also developed a method to estimate them, in order to apply the above-mentioned data selection method. The energy balance closure was found to be quite satisfactory (the slope of the sum of eddy fluxes and storage terms versus transmitted net radiation is 0.99, the intercept is less than 1 W m 2 , r 2 is 0.94, there is no deviation from a linear trend). The data selection procedure allows a fair description of the daily and day-to-day variation of turbulent fluxes while rejecting the most dubious data, whether experimental or estimated storage terms are used. This analysis proves the validity of eddy flux measurements in the lower part of the forest and offers tools for flux data selection, depending on the type of studies such data are intended for. © 2001 Elsevier Science B.V. All rights reserved.

International audience; Two experiments were performed in a confierous forest (maritime pine) in the southwest of France, one in 1994 and the other in 1995. Two sites were chosen, differing by age, height and structure of the trees, as well as the nature of the understorey. In both cases measurements of turbulent fluxes were made at two levels above and within the forest canopy, using sonic anemometers and open-path infrared CO2-H2O analysers. The flux differences derived from the two measurement levels allowed the Radiation and Water Use Efficiencies (RUE and WUE, respectively) to be evaluated for both canopy crowns. The results are based on the analysis of about ten days from each experiment. For both campaigns RUE is significantly larger during cloudy conditions when the fraction of diffuse radiation () increases. An empirical linear relation between RUE and is established for each site, with a smaller intercept and a larger slope for the older forest. In clear conditions (), RUE is about 30 % lower for this forest. Tree photosynthesis, estimated as the net CO2 flux of the foliated layer Fc, appears poorly correlated (r2 < 0.4) with transpiration (net water vapour flux E). This is shown to result from strong variations in the atmospheric saturation deficit D during both campaigns. At both sites WUE turns out to be a hyperbolic function of D (). The coefficient k is 50 % larger for the younger forest. This is in agreement with the values obtained for RUE, and indicates that photosynthetic rates decrease with the age of the trees.

International audience; An experiment was conducted in a pine forest in southwestern France during late spring 1992. The aim was fourfold: testing various flux measurement methodologies for chemically reactive species; quantifying the exchanges between the forest and the atmosphere; analyzing the involved mechanisms; and studying their influence on the chemistry of the surface boundary layer. This paper presents preliminary results obtained on the dry deposition of ozone and submicronic aerosol particles, measured using eddy correlation. Once properly normalized, the spectra and cospectra of all scalar species exhibit universal shapes over the whole frequency range. However, evidence is provided that under some meteorological conditions the time series of turbulent variables can be affected by nonstationary trends, or low-frequency fluctuations that do not contribute to vertical transfer but whose presence can induce large errors in the calculated fluxes. The time variations of the deposition velocities for ozone and aerosol particles are then presented over 2 days with different meteorological conditions. The deposition velocities are shown to be consistent with other reported studies. Dry deposition of ozone appears to be mainly governed by the stomatal resistance, whereas friction velocity and atmospheric instability in the boundary layer seem to govern the deposition of aerosol particles.