3,811 research outputs found

    Emissions by aerial routes from protected crop systems (greenhouses and crops grown under cover) : a position paper

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    This report describes the processes that may lead to emission of Plant Protection Products (PPP) from protected cultivation, through aerial routes. The introduction gives the background for this work and the limitations, outlining in particular why receptors other than air are not explicitly addressed here. Chapters 2 discusses the physical background of greenhouse air exchanges and the factors that affect it. Existing models for estimating ventilation of the different types of greenhouses are reviewed there. Chapter 3 gives a scientific argument about the processes and the factors that may affect aerial emissions of PPP from protected cultivations. The parameters that may have an high impact on the emission are identified there as well. A review of the knowledge needed and of the models that may be available for scoring each emission route is given in Chapter 4. In Chapter 5 a strategy is proposed to reduce/group the number of factors that are important (and to score their relevance) through some model calculations. An outline of the calculations that would be needed for ranking and eventually scoring the emissions and, possibly, highlight groupings of combinations that are similar with respect to emissions, is given

    Steering of fogging: control of humidity: temperature or transpiration

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    Fogging systems are increasingly used to cool greenhouses and prevent water stress. More recently, fogging systems are applied also in relatively low radiation environments (such as The Netherlands), for a better control of product quality than whitewashing and to reduce need for natural ventilation Âż thus allowing for higher CO2 concentrations to be maintained in the greenhouse. Most commonly the steering of such systems is done by setting an upper limit to the deficit of specific humidity that, whenever exceeded, triggers the fogging system. In both cases, however, one may wonder whether static and pre-fixed set points are the most effective choice. In the experiment presented in this paper, fogging and venting were controlled with the purpose of steering crop transpiration. The desired transpiration rate was the input of an algorithm that calculated on-line the required humidity and air temperature set points in view of the current weather factors. The set points were then the input of a standard P-controller that calculated vent opening and time of operation of the fogging system. In this paper, the resulting climate and actuator control operations are discussed and compared with a similar greenhouse controlled in a traditional fashion. The study concluded that a desired crop transpiration rate (an all-round indicator of crop well-being) could be used to select dynamic set points for the climate control in a greenhouse equipped with a fogging system

    Optical identifications of High Frequency Peakers

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    We present CCD observations of 13 objects from a complete sample of 55 bright High Frequency Peaker (HFP) radio sources, and provide optical identification for 12 of them. Images in R and V filters have been used to derive some additional information concerning the host of the radio source. Three hosts are likely to be galaxies, one resulted slightly extended, while the remaining 8 are likely distant quasars. Based on these identifications and those available in the literature, we find that the fraction of quasars in our HFP sample is significantly higher than in samples of Compact Steep-Spectrum and GHz-Peaked Spectrum radio sources.Comment: 7 pages, A&A accepte

    The Galactic structure and chemical evolution traced by the population of planetary nebulae

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    We use an extended and homogeneous data set of Galactic planetary nebulae (PNe) to study the metallicity gradients and the Galactic structure and evolution. The most up-to-date abundances, distances (calibrated with Magellanic Cloud PNe) have been employed, together with a novel homogeneous morphological classification, to characterize the different PN populations. We confirm that morphological classes have a strong correlation with PN Peimbert's Type, and also with their distribution on the Galactic landscape. We studied the alpha-element distribution within the Galactic disk, and found that the best selected disk population, together with the most reliable PN distance scale yields to a radial oxygen gradient of d[log(O/H)]/dR=-0.023 +- 0.006 dex/ kpc for the whole disk sample, and of d[log(O/H)]/dR= -0.035+-0.024, -0.023+-0.005, and -0.011+-0.013 dex/kpc respectively for Type I, II, and III PNe. Neon gradients for the same PN types confirm the trend. Accurate statistical analysis show moderately high uncertainties in the slopes, but also confirm the trend of steeper gradient for PNe with more massive progenitors, indicating a possible steepening with time of the Galactic disk metallicity gradient. The PN metallicity gradients presented here are consistent with the local metallicity distribution; furthermore, oxygen gradients determined with young and intermediate age PNe show good consistency with oxygen gradients derived respectively from other young (OB stars, HII regions) and intermediate (open cluster) Galactic populations. We also extend the Galactic metallicity gradient comparison by revisiting the open cluster [Fe/H] data from high resolution spectroscopy. The analysis suggests that they could be compliant with the same general picture of a steepening of gradient with time.Comment: ApJ, in pres
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