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The concept of canopy-scale resistances was developed to investigate and evaluate the transfer of momentum, heat and mass from the leaf surface to the canopy air space and to the atmosphere. Therefore, reliable estimates of resistances are of fundamental importance for studying the ecosystem scale fluxes and land-atmosphere interaction. The canopy-scale resistance has two components: the leaf boundary layer resistance and canopy-air-to-atmosphere resistance. In big-leaf conceptualizations, canopy-scale resistances are represented in a single term called aerodynamic resistance, which refers to the resistance between an idealized ‘big-leaf’ and the atmosphere for the transfer of momentum, heat and mass. A decent amount of literature exists on the estimation of aerodynamic resistances for various ecosystems based on the roughness length parametrizations and atmospheric stability correction. Most of these parametrizations do not include the leaf boundary layer explicitly and therefore rely on a conceptual 'aerodynamic temperature' at some distance above the leaf surface. This gap hampers reliable modelling of canopy gas exchange (transpiration and CO2 assimilation) as these processes happen directly at the leaf surface and strongly rely on accurately capturing the leaf surface temperature. To bridge this gap, an additional resistance based on a ‘kB-1' parametrization is commonly added to the classical aerodynamic resistance. The objective of the present study is to estimate the total resistance to heat transfer from the heat exchanging surfaces to the measurement height and to find the most appropriate mathematical formulation for this resistance. We used radiometric and eddy covariance (EC) measurements from a wide range of land cover types and estimated the total resistance to heat transport using measured fluxes and radiometric surface temperatures by inverting the flux-profile equation. We also performed a comprehensive comparison of total resistance estimates with commonly used stability and roughness-based resistance formulations, including ‘KB-1' parametrizations and the momentum flux resistance inverted from EC measurements. We found that total resistances were consistently greater than the roughness length-based resistance parametrizations at most of the study sites. We further found that the difference between the total and aerodynamic resistance can be largely explained by dominant leaf sizes at the individual sites. Based on these results, we propose a consistent canopy resistance formulation by explicitly considering leaf sizes and leaf boundary layer resistances in combination with an adequate representation of aerodynamic canopy-atmosphere resistance. This approach will enable a consistent coupling of the aerodynamic process with physiological leaf-scale processes such as photosynthesis and stomatal control, which depend on and interact with leaf temperature, and aerodynamic stability.

International audience; Global evaporation monitoring from Earth observation thermal infrared satellite missions is historically challenged due to the unavailability of any direct measurements of aerodynamic temperature. State-of-the-art one-source evaporation models use remotely sensed radiometric surface temperature as a substitute for the aerodynamic temperature and apply empirical corrections to accommodate for their inequality. This introduces substantial uncertainty in operational drought mapping over complex landscapes. By employing a non-parametric model, we show that evaporation can be directly retrieved from thermal satellite data without the need of any empirical correction. Independent evaluation of evaporation in a broad spectrum of biome and aridity yielded statistically significant results when compared with eddy covariance observations. While our simplified model provides a new perspective to advance spatio-temporal evaporation mapping from any thermal remote sensing mission, the direct retrieval of aerodynamic temperature also generates the highly required insight on the critical role of biophysical interactions in global evaporation research.

Land surface temperature (LST) is a preeminent state variable that controls the energy and water exchange between the Earth’s surface and the atmosphere. At the landscape-scale, LST is derived from thermal infrared radiance measured using space-borne radiometers. At the plot-scale, the flux tower recorded longwave radiation components are inverted to retrieve LST. Since the down-welling longwave component was not measured routinely until recently, usually only the up-welling longwave component is used for the plot-scale LST retrieval. However, we found that ignoring reflected down-welling longwave radiation for plot-scale LST estimations can lead to substantial error. This also has important implications for estimating the correct surface emissivity using flux tower measurements, which is needed for plot-scale LST retrievals. The present study proposes a new method for plot-scale emissivity and LST estimation and addresses in detail the consequences of omitting down-welling longwave radiation as frequently done in the literature. Our analysis uses ten eddy covariance sites with different land cover types and found that the LST values obtained using both up-welling and down-welling longwave radiation components are 0.5 to 1.5 K lower than estimates using only up-welling longwave radiation. Furthermore, the proposed method helps identify inconsistencies between plot-scale radiometric and aerodynamic measurements, likely due to footprint mismatch between measurement approaches. We also found that such inconsistencies can be removed by slight corrections to the up-welling longwave component and subsequent energy balance closure, resulting in realistic estimates of surface emissivity and consistent relationships between energy fluxes and surface-air temperature differences. Landscape-scale daytime LST obtained from satellite data (MODIS TERRA) was strongly correlated with our plot-scale estimates for most of the sites, but higher by several Kelvin at two sites. We also quantified the uncertainty in estimated LST and surface emissivity using the different methods and found that the proposed method does not result in increased uncertainty. The results of this work have significant implications for the combined use of aerodynamic and radiometric measurements to understand the interactions and feedbacks between LST and surface-atmosphere exchange processes.

A detailed representation of plant hydraulic traits and stomatal closure in land surface models (LSMs) is a prerequisite for improved predictions of ecosystem drought response. This work presents the integration of a macroscopic root water uptake (RWU) model based on the hydraulic architecture approach in the LSM of the Terrestrial Systems Modeling Platform. The novel RWU approach is based on three parameters derived from first principles that describe the root system equivalent conductance, the compensatory RWU conductance, and the leaf water potential at stomatal closure, which defines the water stress condition for the plants. The developed RWU model intrinsically accounts for changes in the root density as well as for the simulation of the hydraulic lift process. The standard and the new RWU approach are compared by performing point-scale simulations for cropland over a sheltered minirhizotron facility in Selhausen, Germany, and validated against transpiration fluxes estimated from sap flow and soil water content measurements at different depths. Numerical sensitivity experiments are carried out using different soil textures and root distributions in order to evaluate the interplay between soil hydrodynamics and plant characteristics, and the impact of assuming time-constant plant physiological properties. Results show a good agreement between simulated and observed transpiration fluxes for both RWU models, with a more distinct response under water stress conditions and with uncertainty in the soil parameterization prevailing to the differences due to changes in the model formulation. The hydraulic RWU model exhibits also a lower sensitivity to the root distributions when simulating the onset of the water stress period. Finally, an analysis of variability across the soil and root scenarios indicates that differences in soil water content are mainly influenced by the root distribution, while the transpiration flux in both RWU models is additionally determined by the soil characteristics. ispartof: AGRICULTURAL AND FOREST METEOROLOGY vol:269 pages:28-45 status: published

‘Aerodynamic resistance’ (hereafter ra) is a preeminent variable in the modelling of evapotranspiration (ET), and its accurate quantification plays a critical role in determining the performance and consistency of thermal remote sensing-based surface energy balance (SEB) models for estimating ET at local to regional scales. Atmospheric stability links ra with land surface temperature (LST) and the representation of their interactions in the SEB models determines the accuracy of ET estimates.The present study investigates the influence of ra and its relation to LST uncertainties on the performance of three structurally different SEB models by combining nine OzFlux eddy covariance datasets from 2011 to 2019 from sites of different aridity in Australia with MODIS Terra and Aqua LST and leaf area index (LAI) products. Simulations of the latent heat flux (LE, energy equivalent of ET in W/m2) from the SPARSE (Soil Plant Atmosphere and Remote Sensing Evapotranspiration), SEBS (Surface Energy Balance System) and STIC (Surface Temperature Initiated Closure) models forced with MODIS LST, LAI, and in-situ meteorological datasets were evaluated using observed flux data across water-limited (semi-arid and arid) and radiation-limited (mesic) ecosystems.Our results revealed that the three models tend to overestimate instantaneous LE in the water-limited shrubland, woodland and grassland ecosystems by up to 60% on average, which was caused by an underestimation of the sensible heat flux (H). LE overestimation was associated with discrepancies in ra retrievals under conditions of high atmospheric instability, during which errors in LST (expressed as the difference between MODIS LST and in-situ LST) apparently played a minor role. On the other hand, a positive bias in LST coincides with low ra and causes slight underestimation of LE at the water-limited sites. The impact of ra on the LE residual error was found to be of the same magnitude as the influence of errors in LST in the semi-arid ecosystems as indicated by variable importance in projection (VIP) coefficients from partial least squares regression above unity. In contrast, our results for mesic forest ecosystems indicated minor dependency on ra for modelling LE (VIPa.

The surface energy balance (SEB) is defined as the balance between incoming energy from the sun and outgoing energy from the Earth’s surface. All components of the SEB depend on land surface temperature (LST). Therefore, LST is an important state variable that controls the energy and water exchange between the Earth’s surface and the atmosphere. LST can be estimated radiometrically, based on the infrared radiance emanating from the surface. At the landscape scale, LST is derived from thermal radiation measured using satellites. At the plot scale, eddy covariance flux towers commonly record downwelling and upwelling longwave radiation, which can be inverted to retrieve LST using the grey body equation : Rlup = εσ Ts4 + (1 − ε) R ldw (1)where Rlup is the upwelling longwave radiation, Rldw is the downwelling longwave radiation, ε is the surface emissivity, Ts is the surface temperature and σ is the Stefan-Boltzmann constant. The first term is the temperature-dependent part, while the second represents reflected longwave radiation. Since in the past downwelling longwave radiation was not measured routinely using flux towers, it is an established practice to only use upwelling longwave radiation for the retrieval of plot-scale LST, essentially neglecting the reflected part and shortening Eq. 1 to: Rlup = εσ Ts4 (2)Despite widespread availability of downwelling longwave radiation measurements, it is still common to use the short equation (Eq. 2) for in-situ LST retrieval. This prompts the question if ignoring the downwelling longwave radiation introduces a bias in LST estimations from tower measurements. Another associated question is how to obtain the correct ε needed for in-situ LST retrievals using tower-based measurements.The current work addresses these two important science questions using observed fluxes at eddy covariance towers for different land cover types. Additionally, uncertainty in retrieved LST and emissivity due to uncertainty in input fluxes was quantified using SOBOL-based uncertainty analysis (SALib). Using landscape-scale emissivity obtained from satellite data (MODIS), we found that the LST obtained using the complete equation (Eq. 1) is 0.5 to 1.5 K lower than the short equation (Eq. 2). Also, plot-scale emissivity was estimated using observed sensible heat flux and surface-air temperature differences. Plot-scale emissivity obtained using the complete equation was generally between 0.8 to 0.98 while the short equation gave values between 0.9 to 0.98, for all land cover types. Despite additional input data for the complete equation, the uncertainty in plot-scale LST was not greater than if the short equation was used. Landscape-scale daytime LST obtained from satellite data (MODIS TERRA) were strongly correlated with our plot-scale estimates, but on average higher by 0.5 to 9 K, regardless of the equation used. However, for most sites, the correspondence between MODIS TERRA LST and retrieved plot-scale LST estimates increased significantly if plot-scale emissivity was used instead of the landscape-scale emissivity obtained from satellite data.

The Earth’s surface is both an important source and sink for atmospheric trace compounds (gases and aerosols). This chapter provides a detailed overview of methods to determine immission, dry deposition fluxes, and deposition velocities of trace substances to the biosphere. In particular, relevant measurement techniques are described, and their requirements and limitations for determining accurate fluxes are analyzed. Furthermore, several quality control procedures are outlined. Typical applications are explained in detail, taking into account current and future challenges and developments.

Evapotranspiration (EET) observed by eddy covariance (EC) towers is composed of physical evaporation (EE) from wet surfaces and biological transpiration (ET), that involves soil moisture uptake by roots and water vapor transfer regulated through the canopy-stomatal conductance (gC) during photosynthesis. ET plays a dominant role in the global water cycle and represents 80% of the total terrestrial EET. Understanding the magnitude and variability of ET are critical to assess the ecophysiological responses of vegetation to drought. While separating ET signals from lumped EET observations and/or simulating ET by terrestrial systems models is insufficiently constrained owing to the large uncertainties in disentangling gC from the aggregated canopy-substrate conductance (gcS), evaluating ecosystem ET derived through partitioning EET observations (or model simulation) is also challenging due to the absence of any ecosystem-scale measurements of this biotic flux and gC. To date, the main methods for partitioning EC-EET observations are largely based on regressing EET with gross photosynthesis (Pg) and atmospheric vapor pressure deficit (DA) observations. However, such methods ignore the essential feedback of the surface energy balance (SEB) and canopy temperature (TC) on gC and ET.This study demonstrates partitioning EET observations into ET and EE [soil evaporation (EEs) and interception evaporation (EEi)] through an ‘analytical solution’ of gC, TC and canopy vapor pressures by employing a Shuttleworth-Gurney vegetation-substrate energy balance model with minimal complexity. The model is called TRANSPIRE (Top-down partitioning evapotRANSPIRation modEl), which ingests remote sensing land surface temperature (LST) and leaf area index (Lai) information in conjunction with meteorological, sensible heat flux (H) and EET observations from EC tower into the SEB equations for retrieving canopy and soil temperatures (TS, TC), gC, and ET.ET estimates from TRANSPIRE were tested and evaluated with a remote sensing based ET estimate from an analytical model (STIC1.2), where lumped EET was partitioned by employing a moisture availability constraints across an aridity gradient in the North Australian Tropical Transect (NATT) by using time-series of 8-day MODIS Terra LST and LAI products in conjunction with EC measurements from 2011 to 2018. Both methods captured the seasonal pattern of ET/EET ratio in a very similar way. While ET accounted for 60±10% of the annual EET in the tropical savanna, ET in the arid mulga contributed 75±12% of the annual EET. Seasonal variation of ET was higher in the arid, semi-arid ecosystems (50 - 90%), as compared to the humid tropical ecosystem (10 - 50%). The TRANSPIRE model reasonably captured ET variations along with soil moisture and precipitation dynamics in both sparse and homogeneous vegetation and showed the potential of partitioning EET observations for cross-site comparison with a variety of models.

Land surface temperature (LST) is an important variable that controls the energy and water exchange between the Earth’s surface and the atmosphere. It is widely used to estimate evapo-transpiration and vegetation water stress through surface energy balance models. On a large scale, LST is obtained from space-borne instruments (remote sensing) consisting of radiometers measuring the thermal radiance from the underlying surface. On the other hand, at plot scale, flux towers record longwave radiation, which can be used to estimate LST locally. The up-welling and down-welling longwave radiation measured by radiometers mounted on the eddy covariance towers can be inverted to deduce LST using the Stefan-Boltzmann law and longwave balance (Eq.1): L↑ = εσ Ts4+ (1− ε)L↓ (1)where L↑ = upwelling longwave radiation (Wm−2), L↓ = downwelling longwave radiation (Wm−2), ε = surface emissivity, Ts = surface temperature (K) and σ = Stefan Boltzmann constant (Wm-2K-4).Since down-welling longwave radiation was not measured routinely for a long time, the second term in Eq. 1 is commonly omitted, arguing that emissivity is close to unity and therefore Eq. 1 can be approximated by Eq. 2: L↑= εσTs4 (2)Even with the availability of down-welling longwave measurements it is very common to use Eq. 2. This gives rise to the query if the simplified equation is adequate to estimate LST from flux tower measurements. Another question associated with this method is how to obtain the correct surface emissivity (SE) values needed for LST retrievals.The present work addresses these two important issues by using FLUXNET data for different land cover types (mulga, tropical savanna, and eucalyptus forest). SE was estimated by comparing measured sensible heat flux (H) with estimated radiometric surface-air temperature difference (ΔT) and assuming that ΔT=0 if H=0 (Holmes et al., 2009). Our FLUXNET-based estimate of LST and SE was compared with space-borne measurements (MODIS). We found that LST values obtained using Eq. 1 were more strongly correlated with MODIS (MOD11) estimates, compared to Eq. 2. However, the SE derived using Eq. 1 was much lower than the MODIS emissivities, whereas surface emissivity based on Eq. 2 was very close to the MODIS values. Generally, we found that, even at the high emissivity values taken from MODIS, the estimated LST values differed significantly (2 K or more) between the two equations for all ecosystems.Considering that Eq. 1 physically correct equation, whereas Eq. 2 is an approximation, our analysis suggests that results based on Eq. 2 are likely biased and should be considered with caution. It further questions the implication of large scale SE at plot scale. References: Holmes, T. R. H., Jeu, R. A. M. D., Owe, M., & Dolman, A. J. (2009). Land surface temperature from Ka band (37 GHz) passive microwave observations. Journal of Geophysical Research: Atmospheres, 114(D4). https://doi.org/10.1029/2008JD010257

Thermal infrared sensing of evapotranspiration (E) through surface energy balance (SEB) models is challenging due to uncertainties in determining the aerodynamic conductance (g(A)) and due to inequalities between radiometric (T-R) and aerodynamic temperatures (T-0). We evaluated a novel analytical model, the Surface Temperature Initiated Closure (STIC1.2), that physically integrates T-R observations into a combined Penman-Monteith Shuttleworth-Wallace (PM-SW) framework for directly estimating E, and overcoming the uncertainties associated with T0 and gA determination. An evaluation of STIC1.2 against high temporal frequency SEB flux measurements across an aridity gradient in Australia revealed a systematic error of 10-52% in E from mesic to arid ecosystem, and low systematic error in sensible heat fluxes (H) (12-25%) in all ecosystems. Uncertainty in TR versus moisture availability relationship, stationarity assumption in surface emissivity, and SEB closure corrections in E were predominantly responsible for systematic E errors in arid and semi-arid ecosystems. A discrete correlation (r) of the model errors with observed soil moisture variance (r = 0.33-0.43), evaporative index (r = 0.77-0.90), and climatological dryness (r = 0.60-0.77) explained a strong association between ecohydrological extremes and T-R in determining the error structure of STIC1.2 predicted fluxes. Being independent of any leaf-scale biophysical parameterization, the model might be an important value addition in working group (WG2) of the Australian Energy and Water Exchange (OzEWEX) research initiative which focuses on observations to evaluate and compare biophysical models of energy and water cycle components. Plain Language Summary Evapotranspiration modeling and mapping in arid and semi-arid ecosystems are uncertain due to empirical approximation of surface and atmospheric conductances. Here we demonstrate the performance of a fully analytical model which is independent of any leaf-scale empirical parameterization of the conductances and can be potentially used for continental scale mapping of ecosystem water use as well as water stress using thermal remote sensing satellite data.d

The failure of the Monin–Obukhov Similarity Theory (MOST) in the roughness sublayer is a major problem for the estimation of fluxes over tall forests, whenever indirect methods that rely on MOST, such as flux-gradient or the variance method, are involved. While much research focuses on micrometeorological measurements over temperate-climate forests, very few studies deal with such measurements over tropical forests. In this paper, we show evidence that some similarity functions over the Amazon forest are somewhat different from temperate forests. Comparison of the nondimensional scalar gradients canonical values for the inertial sublayer with our measurements in the roughness sublayer showed smaller deviations than what is usually reported for temperate forests. Although the fluxes of water vapor and CO2 derived from mean profiles show considerable scatter when compared with the eddy covariance measurements, using calibrated dimensionless gradients it is possible to estimate their mean daily cycle during the period of measurement (36 days in May and June, transition between rainy and dry season). Moreover, since mean ozone profiles were available, although without the corresponding eddy covariance measurements, mean daily ozone fluxes were calculated with the flux-gradient method, yielding a nighttime value of −0.05 and a daily peak of −0.45 μg m−2 s−1 (−1.04 and −9.37 nmol m−2 s−1, respectively). These values are comparable to previously measured fluxes in the literature for the Amazon forest.

Vegetation is responsible for more than 50% of the land-atmosphere water vapour fluxes.It affects both the atmospheric moisture content and heat by its control on flux partitioning into latent and sensible heat. The convective exchange of latent and sensible heat is generally assumed to be driven by wind (forced convection). To test this assumption, we analyzed the dependence of the heat transfer coefficient (HTC) on wind speed using eddy covariance datafrom sites along the North Australian Tropical Transect. Here we deduced the HTC from observed sensible heat flux, air temperature and radiometric surface temperature. In most cases, we did not find a significant correlation between HTC and wind speed. In contrast, many cases seemed to suggest a significant dependence of HTC on the surface-air tem-perature gradient (DT) itself. This would implicate buoyancy-driven vegetation atmosphere exchange rather than wind-driven. However, we found that the correlation between HTC and DT is very sensitive to the formulation used for radiometric surface temperature estimation and the assumed surface emissivity. In fact, in many cases, surface temperatures deduced from radiometric data were unrealistic when compared with measured air temperatures and sensible heat flux, while realistic values could only be achieved with very low surface emissivity values that are well outside the commonly used range. This analysis raises questions about our fundamental understanding of radiative surface energy exchange and/or the measurements of sensible and radiative heat fluxes. &nbsp

While modeling approaches of evapotranspiration (λE) perform reasonably well when evaluated at daily or monthly timescales, they can show systematic deviations at the sub-daily timescale, which results in potential biases in modeled λE to global climate change. Here we decompose the diurnal variation of heat fluxes and meteorological variables into their direct response to incoming solar radiation (Rsd) and a phase shift to Rsd. We analyze data from an eddy-covariance (EC) station at a temperate grassland site, which experienced a pronounced summer drought. We employ three structurally different modeling approaches of λE, which are used in remote sensing retrievals, and quantify how well these models represent the observed diurnal cycle under clear-sky conditions. We find that energy balance residual approaches, which use the surface-to-air temperature gradient as input, are able to reproduce the reduction of the phase lag from wet to dry conditions. However, approaches which use the vapor pressure deficit (Da) as the driving gradient (Penman–Monteith) show significant deviations from the observed phase lags, which is found to depend on the parameterization of surface conductance to water vapor. This is due to the typically strong phase lag of 2–3 h of Da, while the observed phase lag of λE is only on the order of 15 min. In contrast, the temperature gradient shows phase differences in agreement with the sensible heat flux and represents the wet–dry difference rather well. We conclude that phase lags contain important information on the different mechanisms of diurnal heat storage and exchange and, thus, allow a process-based insight to improve the representation of land–atmosphere (L–A) interactions in models.

An important current problem in micrometeorology is the characterization of turbulence in the roughness sublayer (RSL), where most of the measurements above tall forests are made. There, scalar turbulent fluctuations display significant departures from the predictions of Monin?Obukhov similarity theory (MOST). In this work, we analyze turbulence data of virtual temperature, carbon dioxide, and water vapor in the RSL above an Amazonian forest (with a canopy height of 40 m), measured at 39.4 and 81.6 m above the ground under unstable conditions. We found that dimensionless statistics related to the rate of dissipation of turbulence kinetic energy (TKE) and the scalar variance display significant departures from MOST as expected, whereas the vertical velocity variance follows MOST much more closely. Much better agreement between the dimensionless statistics with the Obukhov similarity variable, however, was found for the subset of measurements made at a low zenith angle Z, in the range 0°

Canopy and aerodynamic conductances (gC and gA) are two of the key land surface biophysical variables that control the land surface response of land surface schemes in climate models. Their representation is crucial for predicting transpiration (λET) and evaporation (λEE) flux components of the terrestrial latent heat flux (λE), which has important implications for global climate change and water resource management. By physical integration of radiometric surface temperature (TR) into an integrated framework of the Penman–Monteith and Shuttleworth–Wallace models, we present a novel approach to directly quantify the canopy-scale biophysical controls on λET and λEE over multiple plant functional types (PFTs) in the Amazon Basin. Combining data from six LBA (Large-scale Biosphere-Atmosphere Experiment in Amazonia) eddy covariance tower sites and a TR-driven physically based modeling approach, we identified the canopy-scale feedback-response mechanism between gC, λET, and atmospheric vapor pressure deficit (DA), without using any leaf-scale empirical parameterizations for the modeling. The TR-based model shows minor biophysical control on λET during the wet (rainy) seasons where λET becomes predominantly radiation driven and net radiation (RN) determines 75 to 80 % of the variances of λET. However, biophysical control on λET is dramatically increased during the dry seasons, and particularly the 2005 drought year, explaining 50 to 65 % of the variances of λET, and indicates λET to be substantially soil moisture driven during the rainfall deficit phase. Despite substantial differences in gA between forests and pastures, very similar canopy–atmosphere "coupling" was found in these two biomes due to soil moisture-induced decrease in gC in the pasture. This revealed the pragmatic aspect of the TR-driven model behavior that exhibits a high sensitivity of gC to per unit change in wetness as opposed to gA that is marginally sensitive to surface wetness variability. Our results reveal the occurrence of a significant hysteresis between λET and gC during the dry season for the pasture sites, which is attributed to relatively low soil water availability as compared to the rainforests, likely due to differences in rooting depth between the two systems. Evaporation was significantly influenced by gA for all the PFTs and across all wetness conditions. Our analytical framework logically captures the responses of gC and gA to changes in atmospheric radiation, DA, and surface radiometric temperature, and thus appears to be promising for the improvement of existing land–surface–atmosphere exchange parameterizations across a range of spatial scales.

Missing data in river flow records represent a loss of information and a serious drawback in water management. In this work, we introduce gapIt, a user-driven case-based reasoning tool for infilling gaps in daily mean river flow records. Given a set of flow time series, gapIt builds a database of artificial gaps for which it computes several flow estimates, to find the best combinations of infilling algorithm and automatically selected donor station(s), according to state-of-the-art performance indicators. We obtained satisfactory results with Nash-Sutcliffe >0.7 for more than half of the ~5000 synthetic gaps of various lengths and positions, randomly created along the available records. gapIt was evaluated on 24 daily river discharge time series recorded in Luxembourg over seven years from 01/01/2007 to 31/12/2013. We also discuss the benefits of coupling this approach with user-expertise for an improved infilling of real data gaps. We propose a user-driven case-based reasoning tool to infill gaps in flow records.With data analytics the tool finds the best combination: algorithm - donor stations.Satisfactory results were obtained on synthetic gaps of different characteristics.The tool bridges a gap between data-driven and user-expertise approaches.

Nitrous acid (HONO) is a precursor of the hydroxyl radical (OH), a key oxidant in the degradation of most air pollutants. Field measurements indicate a large unknown source of HONO during the day time. Release of nitrous acid (HONO) from soil has been suggested as a major source of atmospheric HONO. We hypothesize that nitrite produced by biological nitrate reduction in oxygen-limited microzones in wet soils is a source of such HONO. Indeed, we found that various contrasting soil samples emitted HONO at high water-holding capacity (75-140%), demonstrating this to be a widespread phenomenon. Supplemental nitrate stimulated HONO emissions, whereas ethanol (70% v/v) treatment to minimize microbial activities reduced HONO emissions by 80%, suggesting that nitrate-dependent biotic processes are the sources of HONO. High-throughput Illumina sequencing of 16S rRNA as well as functional gene transcripts associated with nitrate and nitrite reduction indicated that HONO emissions from soil samples were associated with nitrate reduction activities of diverse Proteobacteria. Incubation of pure cultures of bacterial nitrate reducers and gene-expression analyses, as well as the analyses of mutant strains deficient in nitrite reductases, showed positive correlations of HONO emissions with the capability of microbes to reduce nitrate to nitrite. Thus, we suggest biological nitrate reduction in oxygen-limited microzones as a hitherto unknown source of atmospheric HONO, affecting biogeochemical nitrogen cycling, atmospheric chemistry, and global modeling.

The diurnal forcing of solar radiation is the largest signal within the Earth system and dominates the diurnal cycle of the turbulent heat fluxes and evapotranspiration (λE) over land. Incoming solar radiation (Rsd) also shapes temperature, vapor pressure deficit and wind speed known as important controls on λE. Current process-based λE schemes used in remote sensing and land–surface modeling differ in how these controls on λE are represented and which input variables are required. Here, we analyze how well different surface energy balance schemes are able to reproduce the diurnal cycle and how the diurnal signals of observed input variables actually influence the resulting diurnal pattern of λE. As additional constraint for model evaluation we estimate a linear and a non-linear phase shift component of a surface variable (e.g. λE) to incoming solar radiation. We illustrate our analysis with observations from an eddy covariance station at a temperate grassland site in Luxembourg with a focus on clear sky conditions. During the field campaign in 2015 a summer drought led to a dry-down of soil moisture which allows for studying the effect of wet and dry conditions on the diurnal cycle. We found a remarkable, almost linear relationship of λE with Rsd, which exhibits a significant positive phase lag during wet periods. This phase lag in λE was compensated by a preceding phase lag of the sensible heat flux. Vapor pressure deficit (Da, often used as input for Penman–Monteith based approaches) exhibits a strong phase lag, which is driven by air temperature reflecting large diurnal heat storage changes in the lower atmosphere. This large phase lag in Da, which is not seen in λE, explains why actual and potential evapotranspiration approaches can show systematic deviations from observations at the sub-daily time scale and highlight the need for a time-dependent non-linear compensation through the conductance parameterization. The surface to air temperature gradient used as input in energy balance residual approaches corresponds rather well with its linear response and phase lag to the observed sensible heat flux under both, wet and dry conditions. This simplifies the conductance parameterization and explains the better correlation of these models at the sub-daily time scale. We conclude that the analysis of phase lags at the sub-daily time-scale provides valuable information on the drivers of the surface–atmosphere exchange, which can be used to evaluate and improve the representation of land–atmosphere coupling in land–surface schemes.

Here we demonstrate a novel method to physically integrate radiometric surface temperature (TR) into the Penman-Monteith (PM) formulation for estimating the terrestrial sensible and latent heat fluxes (H and λE) in the framework of a modified Surface Temperature Initiated Closure (STIC). It combines TR data with standard energy balance closure models for deriving a hybrid scheme that does not require parameterization of the surface (or stomatal) and aerodynamic conductances (gS and gB). STIC is formed by the simultaneous solution of four state equations and it uses TR as an additional data source for retrieving the “near surface” moisture availability (M) and the Priestley-Taylor coefficient (α). The performance of STIC is tested using high-temporal resolution TR observations collected from different international surface energy flux experiments in conjunction with corresponding net radiation (RN), ground heat flux (G), air temperature (TA), and relative humidity (RH) measurements. A comparison of the STIC outputs with the eddy covariance measurements of λE and H revealed RMSDs of 7–16% and 40–74% in half-hourly λE and H estimates. These statistics were 5–13% and 10–44% in daily λE and H. The errors and uncertainties in both surface fluxes are comparable to the models that typically use land surface parameterizations for determining the unobserved components (gS and gB) of the surface energy balance models. However, the scheme is simpler, has the capabilities for generating spatially explicit surface energy fluxes and independent of submodels for boundary layer developments.

This study reports NO, NO2 and O3 mixing ratios and flux measurements using the eddy covariance method during a 7-month period over an oilseed-rape field, spanning an organic and a mineral fertilisation event. Cumulated NO emissions during the whole period were in agreement with previous studies and showed quite low emissions of 0.26 kg N ha−1 with an emission factor of 0.27 %, estimated as the ratio between total N emitted in the form of NO and total N input. The NO emissions were higher following organic fertilisation in August due to conditions favouring nitrification (soil water content around 20 % and high temperatures), while mineral fertilisation in February did not result in high emissions. The ozone deposition velocity increased significantly after organic fertilisation. The analysis of the chemical and turbulent transport times showed that reactions between NO, NO2 and O3 below the measurement height occurred constantly throughout the 7-month period. Following organic fertilisation, the NO ground fluxes were 30 % larger than the NO fluxes at the measurement height (3.2 m), while the NO2 fluxes switched from deposition to emission during certain periods, being negative at the surface and positive at the measurement height. This phenomenon of apparent NO2 emissions appears to be significant during strong NO emissions and high O3 ambient mixing ratios, even on a bare soil during August.

Gaseous nitrous acid (HONO), the protonated form of nitrite, contributes up to similar to 60% to the primary formation of hydroxyl radical (OH), which is a key oxidant in the degradation of most air pollutants. Field measurements and modeling studies indicate a large unknown source of HONO during daytime. Here, we developed a new tracer method based on gas-phase stripping-derivatization coupled to liquid chromatography-mass spectrometry (LC-MS) to measure the N-15 relative exceedance, psi(N-15), of HONO in the gas-phase. Gaseous HONO is quantitatively collected and transferred to an azo dye, purified by solid phase extraction (SPE), and analyzed using high performance liquid chromatography coupled to mass spectrometry (HPLC-MS). In the optimal working range of psi(N-15) = 0.2-0.5, the relative standard deviation of psi(N-15) is

[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.

Soils can be a source and sink for atmospheric nitrous oxide (N2O). Consumption of N2O has been reported for anoxic soils and sediments rich in organic matter and depleted in nitrates ( NO 3 − ), and also for some dry, oxic soils. However, the mechanisms and controls of N2O consumption in dry soil are not clear. Here, we report on a field study in China (Taihang mountain region, Shijiazhuang), in which N2O uptake by a sandy loam soil was measured for the greater part of the season (from April to October in 2011), and on four incubation experiments, in which we tried to reveal the roles of water content and oxygen (O2) concentrations on N2O consumption. Flux measurements in the field were made bi-weekly on unfertilized cropped land with static flux chambers (5 replicates) for 6 months. The results show that N2O–N fluxes ranged from −26.0 to −726.6 μg m−2 h−1. Consumption of N2O was largest when the soil was dry (5–20% soil water filled pore space). In the incubation experiments, N2O consumption and N2 production were measured in (an)aerobic soil with soil moisture content ranging from 1% to 50% (wt/wt) and with N2O addition, using a thermostatic, robotized incubation system. Under anaerobic conditions, N2O was rapidly consumed at water content of >10% (wt/wt). However, a significant consumption also occurred at 1% soil moisture. Under aerobic conditions, N2O consumption increased with increasing soil moisture content, but significant consumption was still measured at 2% moisture. Sterilization of oxic soil completely blocked N2O consumption, suggesting that the consumption had a biological nature. In conclusion, the steady N2O consumption measured in the field was confirmed by the laboratory experiments, but the relationship with soil moisture content was reversed. Further studies are required to understand this apparent anomaly.

This study reports NO, NO2 and O3 mixing ratios and flux measurements using the eddy-covariance method during a 7-months period over an oilseed rape field, spanning an organic and a mineral fertilisation event. Mean NO emissions during the whole period were in agreement with previous studies and showed quite small emissions of 0.26 kg N ha−1 with an emission factor of 0.27 %, estimated as the ratio between total N emitted in form of NO and total N input. The NO emissions were higher following organic fertilisation in August due to conditions favouring nitrification (soil water content around 20 % and high temperatures), while mineral fertilisation in February did not result in large emissions. The ozone (O3) deposition velocity was significantly larger following organic fertilisation. We argue that reaction of O3 with emitted NO or reaction of O3 at the surface did not explain this finding, but we propose that reactions of O3 with VOCs emitted by the slurry were the main reason. The analysis of the chemical and turbulent transport times showed that reactions between NO, NO2 and O3 below the measurement height occurred at all time during the 7-months period. Following organic fertilisation, the NO ground fluxes were 30 % larger than the NO fluxes at the measurement height (3.2 m) , while the NO2 fluxes changed sign during some periods, being negative at the surface and positive at the measurement height. This phenomenon of "apparent NO2 emissions" reveals to be important during strong NO emissions and high O3 ambient mixing ratios, even on a bare soil during August.

We studied the Nocturnal Boundary Layer - CLN on area of primary tropical forest northeast of Amazonas State in Amazon Tall Tower Observatory - ATTO. We applied the method of Sun et al. (2012) to characterization three regimes dinamic stability in CLN: 1) a weak turbulence, with low average wind speed; 2) strong turbulence, with high avarage wind speed and 3) turbulent intermittency and nonstationarity conditions. There are seasonal variations in the values of "c" and from one regime to another. We investigated some of the main statistical characteristics of CO 2 concentrations - "c" for each one the turbulent regimes. "c" slightly increases with the increase of the value of a characteristic turbulence velocity scale, V TKE until a threshold value, V L , from which a sudden increase in the values of "c" is observed.This find is important consequences for parametrization of meteorological variables in the NBL and modelling of the atmospheric flod in the interface forest-atmosferic.

We have combined chemical and micrometeorological measurements to investigate the formation and distribution of HONO throughout a forest canopy. HONO was measured simultaneously at two heights, close to the forest floor and just above canopy. The turbulent exchange between the forest and the atmosphere above was studied using vertical profiles of eddy covariance measurements of wind velocity, sonic temperature, water vapour and CO2. HONO mixing ratios at both heights showed typical diel cycles with low daytime values (~80 ppt) and high nighttime values (up to 500 ppt), but were influenced by various sources and sinks leading to mixing ratio differences (above canopy minus below) of up to +240 ppt at nighttime. In the late afternoon and early night mixing ratios increased at higher rates near the forest floor, indicating a possible ground source. Due to the simultaneous decoupling of the forest from the air layer above the canopy, mixing ratio differences reached about −170 ppt. From the late night until the early morning mixing ratios above the forest were typically higher than close to the forest floor. For some cases, this could be attributed to advection above the forest, which only partly penetrated the canopy. Measured photolysis frequencies above and below the forest canopy differed by a factor of 10–25 resulting in HONO lifetimes of about 10 min above and 100–250 min below the canopy at noontime. However, these differences of the main daytime HONO sink were not evident in the mixing ratio differences, which were close to zero during the morning hours. Effective turbulent exchange due to a complete coupling of the forest to the air layer above the canopy in the morning has offset the differences caused by the daytime photolytic sink and added to the interplay between different HONO production and loss processes.

The concentrations of the water-soluble inorganic aerosol species, ammonium (NH4 +), nitrate (NO3 −), chloride (Cl−), and sulfate (SO4 2−), were measured from September to November 2002 at a pasture site in the Amazon Basin (Rondonia, Brazil) (LBA-SMOCC). Measurements were conducted using a semi-continuous technique (Wet-annular denuder/Steam-Jet Aerosol Collector: WAD/SJAC) and three integrating filter-based methods, namely (1) a denuder-filter pack (DFP: Teflon and impregnated Whatman filters), (2) a stacked-filter unit (SFU: polycarbonate filters), and (3) a High Volume dichotomous sampler (HiVol: quartz fiber filters). Measurements covered the late dry season (biomass burning), a transition period, and the onset of the wet season (clean conditions). Analyses of the particles collected on filters were performed using ion chromatography (IC) and Particle-Induced X-ray Emission spectrometry (PIXE). Season-dependent discrepancies were observed between the WAD/SJAC system and the filter-based samplers. Dur...

The Amazon Basin plays key roles in the carbon and water cycles, climate change, atmospheric chemistry, and biodiversity. It already has been changed significantly by human activities, and more pervasive change is expected to occur in the next decades. It is therefore essential to establish long-term measurement sites that provide a baseline record of present-day climatic, biogeochemical, and atmospheric conditions and that will be operated over coming decades to monitor change in the Amazon region as human perturbations increase in the future. The Amazon Tall Tower Observatory (ATTO) has been set up in a pristine rain forest region in the central Amazon Basin, about 150 km northeast of the city of Manaus. An ecological survey including a biodiversity assessment has been conducted in the forest region surrounding the site. Two 80 m towers have been operated at the site since 2012, and a 325 m tower is nearing completion in mid-2015. Measurements of micrometeorological and atmospheric chemical variables were initiated in 2012, and their range has continued to broaden over the last few years. The meteorological and micrometeorological measurements include temperature and wind profiles, precipitation, water and energy fluxes, turbulence components, soil temperature profiles and soil heat fluxes, radiation fluxes, and visibility. A tree has been instrumented to measure stem profiles of temperature, light intensity, and water content in cryptogamic covers. The trace gas measurements comprise continuous monitoring of carbon dioxide, carbon monoxide, methane, and ozone at 5 to 8 different heights, complemented by a variety of additional species measured during intensive campaigns (e.g., VOC, NO, NO2, and OH reactivity). Aerosol optical, microphysical, and chemical measurements are made above the canopy as well as in the canopy space. They include light scattering and absorption, aerosol fluorescence, number and volume size distributions, chemical composition, cloud condensation nuclei (CCN) concentrations, and hygroscopicity. Initial results from ecological, meteorological, and chemical studies at the ATTO site are presented in this paper.

Nowadays, eddy covariance is the state-of-the-art method to quantify turbulent exchange fluxes in the surface boundary layer. In the absence of instruments suitable for high-frequency measurements, fluxes can also be determined using e.g., chamber techniques. However, up to date fluxes of depositing compounds were rarely determined using chamber techniques, mainly due to a modification of the aerodynamic conditions for the trace gas transport within the chamber. In this study, we present ozone (O3) deposition fluxes measured by the dynamic chamber technique and validate them against the eddy covariance (EC) method for a natural grassland site in Germany. The chamber system presented in Pape et al. (2009) was used and optimized to (i) reduce the likelihood of non-stationarities, (ii) yield 30 min averages of flux measurements, and (iii) supply simultaneous profile measurements. The raw O3 fluxes of the dynamic chamber were corrected for gas-phase chemistry in the chamber volume and for the modification of the aerodynamic and boundary layer resistances. Simultaneously measured carbon dioxide and water vapor fluxes by both methods compared well during daytime documenting an equal vegetation activity inside and outside the chambers. The final corrected O3 deposition fluxes of both methods deviated on average by only 11% during daytime. The findings demonstrate the capability of the dynamic chamber method to capture representative O3 deposition fluxes for grassland ecosystems, even when the canopy height is similar to the chamber height. The canopy resistance to O3 was assessed by both methods and showed a characteristic diurnal cycle with minimum hourly median values of 180 s m−1 (chambers) and 150 s m−1 (EC) before noon. During nighttime the fluxes and resistances showed a higher uncertainty for both methods due to frequent low wind associated with non-stationary conditions at the experimental site. Canopy resistances for nitrogen dioxide (NO2) deposition were determined analogously with the chambers and were on average 86% higher than for O3.

Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where experimental data are available. In tropical developing countries, however, experimental studies are lacking. This paper summarizes available data and assesses the impacts of human activities on N fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape transformations over the last century associated with rapid socioeconomic changes. N yield calculations conducted in several watersheds of different anthropogenic influences revealed that disturbed watersheds export more N per unit area than undisturbed forested watersheds. Export of N from urban watersheds ranged from 4.8 kg ha−1 year−1 in the Ri o Bayamon watershed to 32.9 kg ha−1 year−1 in the highly urbanized Rio Piedras watershed and 33.3 kg ha−1 year−1 in the rural-agricultural Rio Grande de Anasco watershed. Along with land use, mean annual runoff explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan Area receives primary treatment before it is discharged into the Atlantic Ocean. These discharges are N-rich and export large amounts of N to the ocean at a rate of about 140 kg ha−1 year−1. Data on wet deposition of inorganic N (NH 4 + + NO 3 − ) suggest that rates of atmospheric N deposition are increasing in the pristine forests of Puerto Rico. Stationary and mobile sources of NOx (NO+NO2) and N2O generated in the large urban centers may be responsible for this trend. Comprehensive measurements are required in Puerto Rico to quantitatively characterize the local N cycle. More research is required to assess rates of atmospheric N deposition, N fixation in natural and human-dominated landscapes, N-balance associated with food and feed trade, and denitrification.

The input of nitrogen (N) to ecosystems has increased dramatically over the past decades. While total (wet+dry) N deposition has been extensively determined in temperate regions, only very few data sets of N wet deposition exist for tropical ecosystems, and moreover, reliable experimental information about N dry deposition in tropical environments is lacking. In this study we estimate dry and wet deposition of inorganic N for a remote pasture site in the Amazon Basin based on in-situ measurements. The measurements covered the late dry (biomass burning) season, a transition period and the onset of the wet season (clean conditions) (12 September to 14 November 2002) and were a part of the LBA-SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia – Smoke, Aerosols, Clouds, Rainfall, and Climate) 2002 campaign. Ammonia (NH3), nitric acid (HNO3), nitrous acid (HONO), nitrogen dioxide (NO2), nitric oxide (NO), ozone (O3), aerosol ammonium (NH4+) and aerosol nitrate (NO3-) were measured in real-time, accompanied by simultaneous meteorological measurements. Dry deposition fluxes of NO2 and HNO3 are inferred using the ''big leaf multiple resistance approach'' and particle deposition fluxes are derived using an established empirical parameterization. Bi-directional surface-atmosphere exchange fluxes of NH3 and HONO are estimated by applying a ''canopy compensation point model''. N dry and wet deposition is dominated by NH3 and NH4+, which is largely the consequence of biomass burning during the dry season. The grass surface appeared to have a strong potential for daytime NH3 emission, owing to high canopy compensation points, which are related to high surface temperatures and to direct NH3 emissions from cattle excreta. NO2 also significantly accounted for N dry deposition, whereas HNO3, HONO and N-containing aerosol species were only minor contributors. Ignoring NH3 emission from the vegetation surface, the annual net N deposition rate is estimated to be about −11 kgN ha-1 yr-1. If on the other hand, surface-atmosphere exchange of NH3 is considered to be bi-directional, the annual net N budget at the pasture site is estimated to range from −2.15 to −4.25 kgN ha-1 yr-1.

[1] Relaxed eddy accumulation (REA) systems that employ one single long inlet tube are prone to measurement uncertainties caused by (a) an imprecisely determined lag time between the change of sign in the vertical wind velocity and the switching of the splitter valves and (b) attenuation of high-frequency concentration fluctuations in the tube flow. However, there is currently no commonly applied procedure to address these uncertainties. In this study, we first evaluated the lag time error of the volume flow, mass flow, and cross-correlation method (online and offline) and experimentally determined the magnitude of high-frequency attenuation for a 21.5 m long inlet tube of an operating REA system. In a second step, we simulated the impact for different artificial lag time errors and low-pass filter strengths on the REA concentration differences and, thus, on the REA flux, using high-frequency time series of temperature, O3, CO2, and H2O. The reduction of scalar fluxes was mainly correlated with increasing switching frequencies and ranged for typical lag time errors of the investigated REA system between

This study investigates the influence of leaf surface water films on the deposition of ozone (O3) and peroxyacetyl nitrate (PAN) under controlled laboratory conditions. A twin cuvette system was used to simulate environmental variables. We observed a clear correlation between the O3 deposition on plants (Quercus ilex) and the relative humidity (RH) under both, light and dark conditions. During the light period the observed increase of the O3 deposition was mainly attributed to the opening of leaf stomata, while during the absence of light the liquid surface films were the reason for O3 deposition. This finding was supported by experimentally induced stomatal closure by the infiltration of abscisic acid. In the case of PAN, no relationship with RH was found during the dark period, which indicates that the non-stomatal deposition of PAN is not affected by the liquid surface films. Consequently, the ratio of the O3 and PAN deposition velocities is not constant when relative humidity changes, which is in contrast to assumptions made in many models. The flux partitioning ratio between non-stomatal and stomatal deposition as well as between non-stomatal and total deposition was found to be Rnsto/sto = 0.21 – 0.40, Rnsto/tot = 0.18 – 0.30 for O3 and Rnsto/sto = 0.26 – 0.29, Rnsto/tot = 0.21 – 0.23 for PAN. Furthermore, we demonstrate that the formation of the liquid surface film on leaves and the non-stomatal O3 deposition are depending on the chemical composition of the particles deposited on the leaf cuticles as proposed previously.

Hydroxyl radicals (OH) are a key species in atmospheric photochemistry. In the lower atmosphere, up to ~30% of the primary OH radical production is attributed to the photolysis of nitrous acid (HONO), and field observations suggest a large missing source of HONO. We show that soil nitrite can release HONO and explain the reported strength and diurnal variation of the missing source. Fertilized soils with low pH appear to be particularly strong sources of HONO and OH. Thus, agricultural activities and land-use changes may strongly influence the oxidizing capacity of the atmosphere. Because of the widespread occurrence of nitrite-producing microbes, the release of HONO from soil may also be important in natural environments, including forests and boreal regions.

Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, normally exceed by far the values predicted by the assumption of a photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. Here, we present budget calculations of HONO based on simultaneous measurements of all relevant species including HONO and OH at two different measurement heights, i.e. 1 m above ground and about 2 to 3 m above canopy (24 m above ground), conducted in boreal forest environment. We observed mean HONO concentrations during daytime of about 6.5 × 108 molecules cm−3 (26 ppt), more than twenty times higher than expected from the PSS, 0.2 × 108 molecules cm−3 (1 ppt). To close the budgets in both heights a strong additional source term during daytime is required. This unidentified source is maximal at noon (up to 1.1 × 106 molecules cm−3 s−1, 160 ppt h−1) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and also other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.

A detailed description of the WAD/SJAC and the analytical procedures is given elsewhere (Slanina et al., 2001; Wyers et al., 1993). A simplified sketch of the sampling system is shown in Fig. 2. The air flow through the instrument (∼17 l min−1, STP) was generated by a scroll pump outside the wooden house and could be adjusted with a needle valve. The flow was measured continuously (1 min time resolution) with a mass flow meter (Bronkhorst, F-112ACHA-55-V). After the sample air passed the steel elbow and/or pre-impactor and the PFA Teflon tubing, it entered a horizontally aligned WAD that scavenges soluble gaseous species. Trace gases (such as NH3, HNO3, HNO2, HCl and SO2) were collected in a 10−4 M NaHCO3 absorption solution. The liquid input was controlled automatically by an infrared sensor and a switching valve and the liquid was continuously pumped out of the denuder at a flow rate of 0.5– 0.6 ml min−1 by a peristaltic pump. The liquid effluent was collected in a sample reservoir (“gas sample”, see Fig. 2). Artifacts due to evaporation of aerosol phase species in the WAD can be excluded because the characteristic time for formation/evaporation of NH4NO3 is >10 s (Dlugi, 1993), while the mean residence time of the sample air in the WAD is ∼0.002 s (annulus volume: 0.0018 l, flow rate: ∼17 l min−1). After the WAD, the air entered a reservoir where it was mixed with steam of highly purified water. The supersaturation causes aerosol particles to grow rapidly (within 0.1 s) into droplets of at least 2μm diameter. These droplets, containing the dissolved aerosol species were then collected in a cyclone (Khlystov et al., 1995). The cyclone effluent (“aerosol sample”) was transferred into the sample reservoir by a peristaltic pump at a flow rate of 0.5–0.6 ml min−1.

In 2011 the currently highest atmospheric research tower of Amazonia was erected at the ATTO site (02°08’38,8’’S, 58°59’59,5’’W) (whereas ATTO stands for Amazon Tall Tower Observatory), which is monitoring concentration gradients regarding 5 trace gases (H2O, CO2, O3, NO, NO2) from 8 different heights between 0,05 m and 79,3 m, which enables the possibility to get new results regarding transport processes in and above the canopy. Never before there have been made profile measurements up to that height in the Amazonian rainforest.

Neste trabalho, foram detectadas as escalas temporais das estruturas coerentes (ECs) em dois niveis acima da reserva florestal do Uatuma, Amazonas. Os dados observacionais utilizados foram coletados na torre micrometeorologica de 81 m instalada na area de estudo do projeto ATTO. As ECs detectadas a uma altura de 41 m apresentaram escala temporal menor que as observadas em 81 m.

Fast ozone (O(3)) measurements (1-50 Hz) in the atmosphere are required for airborne studies and for the measurement of ground-based O(3) fluxes by the eddy covariance technique. Fast response analyzers, based on heterogeneous chemiluminescence, need dye coated sensor discs on which the chemiluminescence is generated. In this study, we present three new preparation methods for those sensor discs. Currently available sensor discs exhibit a fast temporal decay of sensitivity, resulting in short duty times which is troublesome for many field applications. To produce sensor discs that provide more stable signals over time, three dyes and nine energy transfer reagents were tested (as well as different stoichiometric mixtures). The resulting optimal method saves 80% of the solid chemicals and shows a duty ozone dose that is prolonged by a factor of 3.5, revealing the same average sensitivity as currently available discs. In addition, we observed a strong effect of the adsorption matrix on the O(3) sensitivity, although silica discs from the same manufacturer were used. Application of the new sensor discs during field measurements showed that the results are consistent with the laboratory data.

From Soil to Sky Trace gases emitted either through the activity of microbial communities or from abiotic reactions in the soil influence atmospheric chemistry. In laboratory column experiments using several soil types, Oswald et al. (p. 1233 ) showed that soils from arid regions and farmlands can produce substantial quantities of nitric oxide (NO) and nitrous acid (HONO). Ammonia-oxidizing bacteria are the primary source of HONO at comparable levels to NO, thus serving as an important source of reactive nitrogen to the atmosphere.