Absence Of Condensation Research Articles

Composite-boson formalism applied to strongly bound fermion pairs in a one-dimensional trap

We analyze a system of fermions in a one dimensional harmonic trap with attractive delta-interactions between different fermions species, as an approximate description of experiments involving atomic dimers. We solve the problem of two fermion pairs numerically using the so-called "coboson formalism" as an alternative to techniques which are based on the single-particle basis. This allows us to explore the strongly bound regime, approaching the limit of infinite attraction in which the composite particles behave as hard-core bosons. Our procedure is computationally inexpensive and illustrates how the coboson toolbox is useful for ultracold atom systems even in absence of condensation.

Environmental Modulation of Mechanical and Thermodynamic Forcing from Cold Pool Collisions

Abstract Cold pools can initiate new convection by increasing vertical velocity (mechanical forcing) and locally enhancing moisture content (thermodynamic forcing). This study investigates the impact of the environment on mechanical and thermodynamic forcing from cold pool collisions. An ensemble of high-resolution numerical simulations was conducted that tested the sensitivity of cold pool collisions to three parameters: 1) the initial temperature deficit of cold pools, 2) the initial distance between cold pools, and 3) the static stability and moisture content of the environment. These parameters are tested in the absence of condensation, surface fluxes, radiation, and wind shear. Colder initial cold pools increase mechanical and thermodynamic forcing owing to greater horizontal winds during collisions. For all environments tested, mechanical forcing peaked robustly at an optimal initial distance between the cold pools due to a balance between the creation and dissipation of kinetic energy, and the different phases of density current evolution. Thermodynamic forcing peaked for greater initial cold pool distances than those associated with mechanical forcing. Decreased low-level static stability and an increased vertical gradient in low-level moisture enhanced mechanical and thermodynamic forcing, respectively. It is also shown that the initial temperature deficit had the greatest impact on mechanical and thermodynamic forcing, followed by the environment, and finally the initial separation distance. Finally, cold pool collisions are classified as “mechanically strong” or “mechanically weak,” where mechanically strong collisions increased mechanical forcing beyond that driven by the initial outward spreading of the cold pools. An analogous classification of “thermodynamically strong/weak” is also presented.

Condensation phenomena in preferential attachment trees with neighbourhood influence

We introduce a model of evolving preferential attachment trees where vertices are assigned weights, and the evolution of a vertex depends not only on its own weight, but also on the weights of its neighbours. We study empirical measures associated with edges with endpoints having certain weights, and degrees of vertices having a given weight. We show that the former exhibits a condensation phenomenon under a certain critical condition, whereas the latter behaves like a mixture of a power law distribution, depending on the weight distribution. Moreover, in the absence of condensation, for any measurable set we prove almost-sure convergence of the measure of that set under the related measure. This generalises existing results on the Bianconi-Barabási tree as well as on an evolving tree model introduced by the second author. Finally, as an application of our results, we provide criteria under which the degree distribution of this model behaves like a power law, and prove a limiting statement about the growth of the neighbourhood of a fixed vertex.

Water condensate morphologies on a cantilevered microfiber

Water collection via fiber-based coalescers shows promise in mitigating increasing water scarcity, and most studies usually focus on the water collection by accumulating water through successive coalescences of fog in the absence of condensation. Here, we report on non-uniform condensate morphologies observed during air–vapor mixture condensation on a cantilevered microfiber. Due to the competition between thermal conduction resistance within the fiber and condensation heat transfer resistance on the fiber surface, the vapor diffusive flux along the fiber varies accordingly, engendering three representative condensate morphologies. We systematically examine the effects of fiber length, diameter, and material (constantan, 316L steel, and alumel) on these condensate morphologies. Scaling analyses are also provided to reveal the underlying physics. Our experimental investigations and theoretical analyses reported in this work shed more light on air–vapor mixture condensation mechanisms that could pave the way to future condensation-associated applications.

Thermal Regime of Automobile Exhaust System at Low Temperature

Currently operation of cars in large cities in winter is the most unfavorable from the point of view of the risk of blocking the exhaust system by condensate accumulated in it. Frequent starts during relatively short period of time, or the cycle of start-up – short run – and subsequent long-term parking at low temperatures are dangerous because the exhaust system does not have time to warm up and remove the accumulated condensate. Daily operation in such modes contributes to rapid accumulation of condensate, and subsequent long-term parking at ambient temperatures below 0˚C are equally dangerous because, depending on the design features of the exhaust system elements, condensation may occur and freeze in the exhaust system, icing can occur inside it or at its exit, causing inability to start the engine. Given that most of the territory of Russia is located in the areas of moderate and cold climate, the relevance of studies, aimed at identifying the patterns of condensate formation and accumulation in the exhaust system, at adjusting on this basis the frequency of condensate removal from the exhaust system, as well as at optimizing the design parameters of the exhaust systems, is quite evident. The objectives of this study were: to identify the features of changes in temperature of the elements of the exhaust system when the automobile engine warms up at low ambient temperature, the effect of various modes of the heater operation on the temperature of the elements of the exhaust system, as well as the features of the temperature change of individual elements of the exhaust system depending on time for various ambient temperatures. To achieve those objectives a series of experiments has been conducted to study the process of starting the «cold» engine, and of its warming up in idle mode. The found dependencies can be used to develop methodology to adjust the recommended periodicity of warming up of the exhaust system, as well as a model of a device that will ensure the absence of condensate in exhaust systems during the operation of cars in large cities during winter period.

Use of poly(ε-caprolactone)-based films for equilibrium-modified atmosphere packaging to extend the postharvest shelf life of garland chrysanthemum.

A uniaxial‐stretched poly(ε‐caprolactone)/poly(propylene carbonate; PCL/PPC) composite film was prepared using a twin‐screw extruder, and its utility as an equilibrium‐modified atmosphere packaging (EMAP) film extending the shelf life of garland chrysanthemums stored at 2~4°C was explored. The oxygen, carbon dioxide, and water vapor penetration properties, mechanical properties, and gas permselectivity of PCL/PPC film used to package garland chrysanthemums were determined and compared to those of controlled low‐density polyethylene (LDPE) and PCL films. Physicochemical properties such as package headspace gas composition, weight loss, leaf color, total chlorophyll content, ascorbic acid content, lipid peroxidation extent, and the sensory traits of garland chrysanthemums were investigated over a storage period of 14 days to compare the preservative effects of the various packages. PPC blending decreased the PCL gas and water vapor permeability and slightly increased the CO 2 permselectivity. These effects on gas and water vapor permeability, combined with the effects on gas permselectivity, enhanced preservation of packed garland chrysanthemums. Furthermore, an O2 inner atmosphere level of 2%~5%, and a CO 2 concentration not greater than 8%, was established by the PCL/PPC film in the absence of condensation. The results thus suggest that biodegradable film can be used as an EMAP film to better maintain the quality of freshly harvested garland chrysanthemums and to afford a longer shelf life during cold storage compared to LDPE film. Sensory evaluation indicated that the garland chrysanthemums were market‐acceptable after 14 days of storage; LDPE‐packed chrysanthemums were acceptable only up to 8 days of storage. The film thus improved storage life compared to that afforded by LDPE.

Sweeping gas membrane distillation: Numerical simulation of mass and heat transfer in a hollow fiber membrane module

A hollow fiber MD module was tested at various air and brine flow rates and temperatures. A model based on heat and mass transport was developed to predict permeate production rates. The dependence of permeate production rate on brine temperature, air flow rate and brine flow rate was successfully simulated. Numerical simulations support the selection of membrane characteristics and air and brine flow conditions for optimal performance in water desalination. Condensation was predicted to occur on the air side of the membrane due to saturation of the sweeping gas and is accounted for in the model. In the absence of condensation, temperature profiles in the module could not be predicted correctly. The ratio of length to diameter of the membrane module is of particular significance as it dictates the permeation rate for a specific pore size membrane. Small pores require higher aspect ratios than large pores to obtain the same permeate production rate. The membrane module used in this study has an effective pore size of 0.1μm, which renders membrane transport the dominant source of mass transfer resistance to through-pore water vapor transport. A module with a larger pore size and appropriate aspect ratio should produce permeate at a significantly higher rate.

The structure of nanoparticle nucleation in planar jets

The effects of large-scale mixing and vapor concentration on homogeneous nucleation rates are investigated via direct numerical simulation of dibutyl-phthalate (DBP) nucleation during cooling and mixing in three-dimensional planar jets. In the simulated cases, a heated jet doped with DBP issues into a co-flow of room-temperature air. As the two streams mix, the DBP vapor becomes highly supersaturated and particles are formed by nucleation. This particle formation takes place in the absence of condensation or coagulation. The simulation results provide a demonstration of how nucleation takes place in narrow regions where molecular mixing of the two streams occurs. When maximum nucleation rates occur in conditions where the nucleation rates are sensitive to ambient conditions, islands of nucleation form. There are two possible nucleation events: initial shear layer nucleation, and later nucleation in coherent structures or eddies generated by the velocity difference between the jet and the co-flow. A scatter plot diagram of observed dilution paths in temperature versus condensable vapor concentration space where nucleation rates are superimposed is shown to be a convenient tool for analyzing nucleation events. Convection by large-scale eddies gradually spreads the range of mixing paths in this space toward higher nucleation rates. The results also show that boundary conditions, including inlet concentration and velocity ratio, have both qualitative and quantitative effects on particle nucleation.

Long cycles in the infinite-range-hopping Bose–Hubbard model

In this paper we study the relation between long cycles and Bose–Einstein condensation in the infinite-range Bose–Hubbard model. We obtain an expression for the cycle density involving the partition function for a Bose–Hubbard Hamiltonian with a single-site correction. Inspired by the approximating Hamiltonian method we conjecture a simplified expression for the short cycle density as a ratio of single-site partition functions. In the absence of condensation we prove that this simplification is exact and use it to show that in this case the long cycle density vanishes. In the presence of condensation we can justify this simplification when a gauge-symmetry breaking term is introduced in the Hamiltonian. Assuming our conjecture is correct, we compare numerically the long cycle density with the condensate and find that although they coexist, in general, they are not equal.

Nonperturbative Yukawa theory at finite density and temperature

In-medium Yukawa theory is part of the thermodynamics of the standard model of particle physics and is one of the main building blocks of most effective field theories of fermionic systems. By computing its pressure we investigate the nonperturbative thermodynamics at finite temperature and density using the optimized perturbation theory framework. Our calculations are valid for arbitrary fermion and scalar masses, temperature, chemical potential, and not restricted to weak coupling. The model is considered in the presence as well as in the absence of condensates. Comparison with nonperturbative results shows that second-order perturbation theory fails in the first case but performs rather well when condensates are absent, even at high-temperature regimes.

Infinite Cycles in Boson Lattice Models

We study the relationship between long cycles and Bose-Einstein condensation (BEC) in the case of several models. A convenient expression for the density of particles on cycles of length $q$ is obtained, in terms of $q$ unsymmetrised particles coupled with a boson field. Using this formulation we reproduce known results on the Ideal Bose Gas, Mean-Field and Perturbed Mean-Field Models, where the condensate density exactly equals the long cycle density. Then we consider the Infinite-Range-Hopping Bose-Hubbard model, with and without hard-cores. For the hard-core case, we find we can disregard the hopping contribution of the q unsymmetrised particles to obtain an exact expression for the density of particles on long cycles. It is shown that only the cycle of length one contributes to the cycle density. We conclude that while the existence of a non-zero long cycle density coincides with the occurrence of Bose-Einstein condensation, the respective densities are not necessarily equal. For the case of a finite on-site repulsion, we obtain an expression for the cycle density by neglecting the $q$ unsymmetrised hop. Inspired by the Approximating Hamiltonian method we conjecture a simplified expression for the short cycle density as a ratio of single-site partition functions. In the absence of condensation we prove that this simplification is exact and find that in this case the long-cycle density vanishes. In the presence of condensation we justify this simplification when a gauge-symmetry breaking term is introduced in the Hamiltonian. Assuming our conjecture is correct, we compare numerically the long-cycle density with the condensate and again find that though they coexist, in general they are not equal.

Turbulence and heat exchange in condensing vapor-liquid flow

Turbulence and heat exchange during condensation of a vapor stream countercurrently flowing to a subcooled liquid stream in a slightly inclined channel has been investigated by direct numerical simulation (DNS). Condensation rates and imposed pressure gradients have been varied, and capillary-gravity waves have been allowed to develop at the (deformable) vapor-liquid interface. These simulations extend our previous DNS of turbulence and scalar exchange in stratified gas-liquid flows without condensation. The previous studies indicated that for conditions in which the gas-liquid interface remained continuous, i.e., did not “break,” scalar exchange rates on both the gas and liquid sides were largely determined by sweeps which brought high momentum fluid from the bulk flow to the interface. As sweep frequencies were found to scale with interfacial friction velocities, scalar exchange coefficients could be parametrized with a surface renewal theory. The issue addressed in the current work is how these findings are altered by condensation which acts somewhat like suction through a wall on the vapor side and injection through a wall on the liquid side. Both suction and injection have been found to affect shear stresses, turbulence characteristics, and scalar exchange rates, and hence similar effects might be expected during condensation. The present simulations indicate that the turbulence characteristics in both phases are affected, with turbulence intensities and Reynolds stresses being enhanced on the vapor side and attenuated on the liquid side. For a given imposed pressure gradient, the interfacial shear stress decreases as a result of the interfacial momentum exchange due to condensation. Interfacial waves are also found to be damped by condensation and the streamwise vortical structures on the liquid side are attenuated. The frequencies of sweeps and ejections, however, do scale with the interfacial friction velocity, reduced due to condensation, as does the liquid-side heat transfer coefficient. The simulations indicate that the scaling relationship between the interfacial friction velocity and the liquid-side heat transfer coefficient is similar to that in the absence of condensation, although the interfacial friction velocity itself is different, being dependent on condensation rates. As condensation rates depend in turn on the liquid-side heat transfer, their prediction becomes a coupled problem. A procedure for determining condensation rates as a function of imposed pressure gradient and liquid subcooling is derived from the simulations.

The effect of sizing and paper structure on paperboard for retortable packaging

This thesis reports on an investigation of the mechanisms of edge wicking in retortable paperboard. Retortable board is used for packaging preserved food, a process which requires that the package and its contents be sterilised by exposure to high temperature steam for up to three hours. The board used must thus have higher water repellence than traditional liquid packaging. Water vapour that condenses on the cut edges on the outside of the board causes particular concern. The paperboards studied were made from refined and unrefined bleached softwood kraft pulp and from refined unbleached softwood kraft pulp in one or two plies with different sizing levels and structures. Samples of each of the boards were immersed in a water bath at 95 °C. Other samples from the same boards were exposed to a combination of saturated steam at 130 °C and condensed steam in a special apparatus developed for this purpose. The board samples were placed on a cooling plate to simulate the temperature gradient when a container of cold food is sterilised with heated steam. Still other samples were exposed to heated steam in the absence of condensation by hanging them freely in the apparatus. The extent of edge wicking in the boards was determined by gravimetric measurements and also by near-infrared (NIR) spectroscopy, a technique that yields more information about the moisture content profiles and the mechanisms of liquid sorption. The moisture content profiles of boards exposed to saturated steam in the absence of condensation show more uniform water uptake. This phenomenon can be explained by rapid vapour phase transport throughout the pore structure, followed by slower water uptake in the fibres. By contrast, in samples exposed to both heated steam and condensed steam, there was both liquid sorption in the fibre network and rapid vapour phase transport of the steam. The moisture content in those samples was much higher close to the edge and lower behind the liquid frontier. The edge wicking of high temperature water was greatest in board that was not fully sized, in low density board, and in board made from unrefined pulp. The greater edge wicking in board made from unbleached pulp can be accounted for in terms of its greater swelling potential. In the low density board and the board made from unrefined pulp, the lumens at the unsized edge and the weaker bonding strength are suggested to affect the results. Capillary sorption takes place in lumens and delaminated bonds at the edge and then any further propagation takes place entirely by fibre diffusion. Capillary sorption may also be important in damaged areas where broken fibre-fibre bonds are exposed to liquid. The weaker bonded area in low density board therefore also contributes to water sorption. Edge wicking in the boards placed on the cooling plate and exposed to a combination of saturated and condensed steam was of a different nature. For these samples, density was not an important factor. The difference is due to the different mechanism at work in this case, where the relative area of the edge exposed to condensed steam was larger in the high density boards than in the low density boards. In the high density boards, condensed steam was sorbed in the structure by diffusion in the fibre structure, whereas in low density boards, the dominant effect was vapour phase transport in the void spaces followed by water uptake in the fibres.

Determination of the in-plane wetting of a board structure using NIR spectroscopy

This thesis reports on an investigation of the mechanisms of edge wicking in retortable paperboard. Retortable board is used for packaging preserved food, a process which requires that the package and its contents be sterilised by exposure to high temperature steam for up to three hours. The board used must thus have higher water repellence than traditional liquid packaging. Water vapour that condenses on the cut edges on the outside of the board causes particular concern. The paperboards studied were made from refined and unrefined bleached softwood kraft pulp and from refined unbleached softwood kraft pulp in one or two plies with different sizing levels and structures. Samples of each of the boards were immersed in a water bath at 95 °C. Other samples from the same boards were exposed to a combination of saturated steam at 130 °C and condensed steam in a special apparatus developed for this purpose. The board samples were placed on a cooling plate to simulate the temperature gradient when a container of cold food is sterilised with heated steam. Still other samples were exposed to heated steam in the absence of condensation by hanging them freely in the apparatus. The extent of edge wicking in the boards was determined by gravimetric measurements and also by near-infrared (NIR) spectroscopy, a technique that yields more information about the moisture content profiles and the mechanisms of liquid sorption. The moisture content profiles of boards exposed to saturated steam in the absence of condensation show more uniform water uptake. This phenomenon can be explained by rapid vapour phase transport throughout the pore structure, followed by slower water uptake in the fibres. By contrast, in samples exposed to both heated steam and condensed steam, there was both liquid sorption in the fibre network and rapid vapour phase transport of the steam. The moisture content in those samples was much higher close to the edge and lower behind the liquid frontier. The edge wicking of high temperature water was greatest in board that was not fully sized, in low density board, and in board made from unrefined pulp. The greater edge wicking in board made from unbleached pulp can be accounted for in terms of its greater swelling potential. In the low density board and the board made from unrefined pulp, the lumens at the unsized edge and the weaker bonding strength are suggested to affect the results. Capillary sorption takes place in lumens and delaminated bonds at the edge and then any further propagation takes place entirely by fibre diffusion. Capillary sorption may also be important in damaged areas where broken fibre-fibre bonds are exposed to liquid. The weaker bonded area in low density board therefore also contributes to water sorption. Edge wicking in the boards placed on the cooling plate and exposed to a combination of saturated and condensed steam was of a different nature. For these samples, density was not an important factor. The difference is due to the different mechanism at work in this case, where the relative area of the edge exposed to condensed steam was larger in the high density boards than in the low density boards. In the high density boards, condensed steam was sorbed in the structure by diffusion in the fibre structure, whereas in low density boards, the dominant effect was vapour phase transport in the void spaces followed by water uptake in the fibres.

Degradation risk assessment of external envelopes: A practical engineering approach

A method is proposed, which allows evaluation of moisture in building envelopes taking into account sorption properties of timber or other materials with pronounced sorption. This is essential in many situations when apparently no risk of degradation of the envelope occurs in the absence of condensation. The method implemented was the 3D numerical code, which allows to take into account the risks of thermal bridges. The results of computation can be compared to standard requirements on moisture content in the envelope.

Is KCl(g) Corrosive at Temperatures Above its Dew Point? Influence of KCl(g) on Initial Stages of the High Temperature Corrosion of 11% Cr Steel at 600°C

The influence of gaseous KCl on the high temperature oxidation of CrMoV11 1 (X20) steel at 600degreesC is reported. The sample temperature was above the dew point of KCl, the partial pressure of KCl being about 5ppm. The samples were investigated by a number of surface analytical techniques including grazing angle XRD, SEM/EDX, and SAM. CrMoV11 1 steel shows protective behaviour in clean dry O-2 and O-2/H2O environment because of the formation of a chromium-rich oxide (alpha-(Fe,Cr)(2)O-3). It is Often considered that alkali salts accelerate the corrosion of steel only when present on the surface in solid or liquid form. In contrast, the present result shows that gaseous KCl is very corrosive, also in the absence of condensation. KCl(g) reacts with chromium in the scale, forming K2CrO4(s). This depletes the protective oxide in chromium and leads to the formation of non-protective hematite, Fe2O3.

Modelling and analyses of heat exchangers in a biomass boiler plant

A boiler plant is presented, in which the fuel is dried before combustion in a silo with air. The drying air is heated in a recuperative heat exchanger by the heat of flue gases. Hot air is then blown through the bed of fuel in the drying silo, while the fuel dries and the air cools down and becomes humidified. Heat of the moist exhaust air of the silo is recovered for the drying air and combustion air by a recuperative heat exchanger. Modelling of the thermal behaviour of the plant helps in understanding complex interdependencies of the two heat exchangers, the boiler and the dryer. The models of the heat exchangers and applications in analysing the boiler system are described in this paper. Calculating the combinations of extreme operational conditions gives the input data needed in comparing different types of heat exchangers, dimensioning the heat transfer area, choosing the control strategy and selecting the operating parameters and set-values of the control system. Results of verification measurements and practical operation at a 40 kWth pilot plant and a 500 kWth demonstration plant are also discussed. Using engineering correlation formulas for heat and mass transfer, an adequate accuracy between the model and the measurements was achieved. Fouling was detected to be a major problem with the flue gas heat exchanger. However, in absence of condensation, the increase of a fouling layer with respect to time was observed to be low. Fouling was also a problem with the drying exhaust gas heat exchanger, but after the installation of a simple dust collector, a reasonable cleaning period was achieved. A mixed-flow configuration was found to be the most appropriate for the flue gas heat exchanger. In order to avoid condensation of the flue gas the drying exhaust gas heat exchanger is indispensable in Finnish climate in the considered system. In addition to this, it decreases the need of fuel. A parallel-flow type was found the most appropriate as the drying exhaust gas heat exchanger. Copyright © 2004 John Wiley & Sons, Ltd.

Use of Tube Condensation as an Indicator of Endotracheal Tube Placement

Study objective: To determine whether condensation on the inner surface of the endotracheal tube (vapor trail) is a reliable indicator of intratracheal placement. Methods: Twenty-seven separate experiments were conducted on 10 conditioned, mongrel dogs weighing 15 to 20 kg each. After induction of anesthesia, an endotracheal tube was placed in the trachea under direct visualization. A second, identical endotracheal tube was then placed in the esophagus. An attending emergency physician, blinded to tube placement, then used a bag-valve apparatus to manually ventilate each endotracheal tube in turn. Five ventilations were performed on each tube, and the presence or absence of condensation on the inner surface of the tube was recorded. A second physician, blinded both to tube placement and to the actions of the first assessor, then repeated the ventilation and assessment of both tubes. Results: Vapor trail was observed in 27 (100%) of 27 endotracheal tubes correctly placed in the trachea (95% confidence interval [CI], 90% to 100%) and in 23 (83%) of 27 tubes placed in the esophagus (95% CI, 66% to 96%). Physicians concurred in all cases. Absence of vapor trail was 15% sensitive (95% CI, 4% to 34%) for indicating incorrect (esophageal) tube placement. Conclusion: In this model, condensation on the inner surface of the endotracheal tube was common after placement within the esophagus. If these results are confirmed in human studies, the presence of a vapor trail should not be used as a clinical indicator of correct endotracheal tube placement. [Kelly JJ, Eynon CA, Kaplan JL, de Garavilla L, Dalsey WC: Use of tube condensation as an indicator of endotracheal tube placement. Ann Emerg Med May 1998;31:575-578.]

Identification du coefficient de transfert thermique sur la surface externe d'un échangeur de chaleur à tubes et ailettes planes en fonction de l'humidité de l'air, en l'absence de condensation

The following study, which is rather oriented towards experimentation, shows the influence of the humidity content of air on heat transfer. This first article concerns heat transfer between the external fluid (moist air) and the internal fluid (water containing glycol, whose thermal behavior inside circular tubes is well-known) in a heat exchanger of the same type as those used in automotive air conditioning (horizontal copper tubes and plane aluminium fins), in the absence of condensation. The most difficult part of this experimental work is the measurement and control of the air humidity, since one has to make sure that the measurement incertainties are not significant compared to the precision of the calculation of the heat transfer coefficient. The conclusion is that, for this type of exchanger, the heat transfer coefficient decreases with air humidity in the absence of condensation (dry wall). Some correlations have been developed with respect to the relative air humidity. An analog experimental investigation, but this time carried out in the presence of condensation (partially or completely wetted wall), is about to be completed; the obtained results will be communicated later on.

Epiphytische Flechten im Bereich einer chilenischen „Nebeloase“ (Fray Jorge) II. Ökophysiologische Charakterisierung von CO2-Gaswechsel und Wasserhaushalt

In a previous communication (Part I) the epiphytic lichen vegetation in the region of the fog oasis Fray Jorge was reported. For 2 lichen communities, the habitat conditions have been characterized through microclimatological measurements. The present paper considers CO 2 gas exchange and water relations of these lichens. Under controlled conditions, photosynthesis and respiration are reported as influenced by water content of the thalli, light intensity, and temperature, for selected species ( Everniopsis trulla and Usnea lacerata from the Oropogonetum loxensis and Heterodermia spinulosa and Ramalina cactacearum from the Ramalinetum cactacearum). The maximal rates of net photosynthesis and dark respiration of the 4 species were very similar. They exhibited medium range of CO 2 exchange capacity when compared with maximum rates of species from other climatic regions. Moistened with liquid water, photosynthetic activity began at a thallus water content of 26 to 29 % (on a dry weight basis). Subsequently, net photosynthesis increased almost linearly with increasing water content. Very high water content resulted in a substantial depression of CO 2 uptake for the species of the Ramalinetum. This apparently is due to increased diffusion resistances within the fully-moistened thalli. Characteristically, such a depression is much smaller or nonexistent for the species of the Oropogonetum. Water vapor uptake in the absence of condensation by the dry lichen thalli can also enable photosynthetic activity. For example, Ramalina cactacearum already exhibited CO 2 uptake in equilibrium with a relative air humidity of 82 %, which corresponds to a water potential of —251 bar (at 10°C). In equilibrium with nearly saturated air, the lichens reached rates of photosynthesis which are similar to those which are attained at optimal water contents after moistening with liquid water. However with water vapor uptake, the same rates of net photosynthesis are possible at substantially lower thalli water contents than would be the case if the thalli were moistened with liquid water. Differences in activation of mitochondrial respiration and in the diffusive conductance of the thalli for CO 2 may be the reasons for thie phenomenon. This has important implications for production ecology. Based upon the CO 2 exchange measurements in the laboratory, the dependency of net photosynthesis of the experimental lichens on water status, light and temperature is presented in interpolation diagrams. These allow estimation of rates of CO 2 uptake for a given combination of environmental factors. With the weather data obtained from the field measurements it is thus possible at least approximately to simulate gas exchange performance of the lichens under their natural conditions. Fog condensation results in water contents of the lichen thalli which — according to the simulations — allow maximal rates of net photosynthesis. With typical fog weather conditions, a positive net CO 2 uptake is possible for the species of the Oropogonetum from briefly after sunrise until about 13 h when the moisture compensation point is finally reached. The daily photosynthetic CO 2 gain under such circumstances amounts to about 5.47 mg CO 2 per g dry weight. Conditions for production are much less favourable for the species of the Ramalinetum. However, the simulations show that in spite of the lack of moistening by fog in these habitats, water vapor uptake from the ambient air alone enables carbon gain in the morning. The duration and rate of net photosynthesis is very dependent on the humidity conditions during the night and on the rate of drying by solar radiation in the morning hours. Under conditions of high humidity at night, Heterodermia spinulosa attains a daily gain of 2.39 mg CO 2 per g. The capability of the thalli to use air humidity as their main water source and their ability to conduct positive photosynthesis at extremely low thallus water potentials, are necessary characteristics for the existence of these species in the Ramalinetum.