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Combustion characteristics of water-insoluble elemental and organic carbon in size selected ambient aerosol particles

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Atmos. Chem. Phys., 5, 1905–1913, 2005 www.atmos-chem-phys.org/acp/5/1905/ SRef-ID: 1680-7324/acp/2005-5-1905 European Geosciences Union Atmospheric Chemistry and Physics Combustion characteristics of water-insoluble elemental and organic carbon in size selected ambient aerosol particles K. Wittmaack GSF – National Research Centre for Environment and Health, Institute of Radiation Protection, 85758 Neuherberg, Germany Received: 1 March 2005 – Published in Atmos. Chem. Phys. Discuss.: 13 April 2005 Revised: 28 June 2005 – Accepted: 28 June 2005 – Published: 27 July 2005 Abstract. Combustion of elemental carbon (EC) and organic carbon (OC) contained in ambient aerosol matter was explored using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). To ease identification of the particles of interest and to avoid or at least reduce interaction with simultaneously sampled inorganic oxides and salts, the approach used in this work differed in two ways from commonly applied procedures. First, rather than using a mixture of particles of vastly different sizes, as in PM10 or PM2.5, aerosol matter was collected in a 5-stage impactor. Second, the water soluble fraction of the collected matter was removed prior to analysis. Diesel soot particles, which appeared in the well-known form of chaintype aggregates, constituted the major fraction of EC. In contrast, OC containing particles were observed in a variety of shapes, including a sizable amount of bioaerosol matter appearing mostly in the size range above about 1 µm. During heating in ambient air for 1 h, diesel soot particles were found to be stable up to 470 C, but complete combustion occurred in a narrow temperature interval between about 480 and 510 C. After diesel soot combustion, minute quantities of “ash” were observed in the form of aggregated tiny particles with sizes less than 10 nm. These particles could be due to elemental or oxidic contaminants of diesel soot. Combustion of OC was observed over a wide range of temperatures, from well below 200 C to at least 500 C. Incompletely burnt bioaerosol matter was still found after heating to 600 C. The results imply that the EC fraction in aerosol matter can be overestimated significantly if the contribution of OC to a thermogram is not well separated. 1 Introduction Carbonaceous particles constitute a significant if not a large fraction of atmospheric aerosol matter (Molnár et al., 1999; Putaud et al., 2004; ten Brink et al., 2004). It is common practice to distinguish two categories, organic carbon (OC) and elemental carbon (EC). Quantitative measurements of OC and EC are hampered by sometimes severe sampling artefacts (Kirchstetter et al., 2001; Mader at al., 2003; ten Brink et al., 2004) as well as by the fact that analytical methods based on the use of fundamental properties are not available. The mass of OC and EC in a given sample is usually determined by combustion, applying certain protocols which define the temperature and duration of heating, the composition of the gas atmosphere, the method of analysing gaseous carbon evolving from the sample, the control of sample charring etc. Depending on the details of the protocols, the results for EC were found to differ significantly (Hitzenberger et al., 1999; Schmid et al., 2001; ten Brink et al., 2004), in extreme cases by as much as a factor of 12 (Schmid et al., 2001). Part of the problem relates to the fact that OC comprises a diverse mixture of materials ranging from small organic molecules to bioaerosols (Cachier, 1998). These various components of OC exhibit different characteristic temperatures of desorption, pyrolysis and combustion. EC containing particles, on the other hand, may be expected to feature a comparatively well defined combustion behaviour. Preliminary evidence in this respect was recently obtained using scanning electron microscopy (SEM) to trace the incineration of diesel soot carbon nanoparticles after different steps of 1 h heating in ambient air (Wittmaack, 2004). The diesel particles, which are known to be composed of EC in the form of graphitic crystallites (Ishiguro et al., 1997; Wentzel et al., 2003), remained essentially unchanged after heating to 400 C, but had disappeared completely after heating to 500 C. Correspondence to: K. Wittmaack (wittmaack@gsf.de) © 2005 Author(s). This work is licensed under a Creative Commons License. 1906 K. Wittmaack: Combustion characteristics of water-insoluble elemental and organic carbon The purpose of this study was to extend the previous work with the aim of arriving at a detailed picture of the combustion characteristics of a variety of OC containing particles as well as of diesel soot, the latter often comprising the largest fraction of EC particulate matter. The results of such an investigation might help to interpret the different peaks routinely observed in thermograms of ambient aerosol matter at temperatures between 100 and 850 C (Chow et al., 2001). A detailed peak assignment has not been possible in previous work. The approach explored here differs in several ways from commonly used procedures for determining EC and OC in ambient aerosol matter. The standard approach is collect particles with aerodynamic diameters less than 10 or 2.5 µm (PM10 or PM2.5, respectively) on quartz fibre filters (Hitzenberger et al., 1999; Schmid et al., 2001; Mader et al., 2003; ten Brink et al., 2004) and to determine the combustion characteristics of the as-sampled aerosol matter, with or without determining the changes in optical transmission or reflectance during the heating procedure (Cachier, 1998; Chow et al., 2001). EC and OC measurements on size fractionated aerosols collected in impactors are rare (Maenhaut et al., 2002). To the author’s knowledge, a detailed comparison of the thermograms observed in different size ranges has only been reported once (Novakov et al., 2000). The other difference to previous studies is that in this work the water soluble aerosol matter was extracted prior to analysis and heat treatment. The procedure, tested (Wittmaack et al., 2002) and applied before (Wittmaack, 2004), has several advantages. First, many OC and EC containing particles, which may be covered by inorganic matter after prolonged sampling, become visible in SEM analysis. Second, combustion artefacts due to interaction with inorganic matter (Chow et al., 2001) are removed or at least reduced. Related analyses on PM2.5 samples performed in the author’s laboratory by proton induced X-ray emission (PIXE), before and after soluble-matter extraction with water, revealed large to high solubility, not only for critical alkali elements (Novakov and Corrigan, 1995) like Na (∼90%) and K (∼60%) but also for transition elements like Zn (∼80%) and Fe (∼40%). Third, water soluble organic carbon (WSOC) is removed from the sample and may be analysed separately. The WSOC contains a significant fraction of medium-mass organic matter, often referred to as humic-like substances, HULIS (Krivácsy et al., 2000). The term, originally coined for organic matter found in alkaline extracts of dust samples (Havers et al., 1998), is rather misleading. The mere fact that electrospray mass spectra of the extracted organic matter reveal a broad band of lines centred at about m/z 160 (Krivácsy et al., 2000) does not provide a justification for a wording that implies reasonably detailed knowledge of the composition of WSOC. Thorough electrospray mass spectrometric analyses of a variety of fulvic acids, the water soluble components of humic acids, revealed broad spectra extending from about m/z 100 to m/z 500 and beyond (Plancque et al., 2001). Similarly, Atmos. Chem. Phys., 5, 1905–1913, 2005 time-of-flight secondary ion mass spectra of solidified fulvic acids were found to exhibit tails up to at least m/z 3000 (Szymczak et al., 2000). The proposed structure of fulvic acid molecules features a benezene ring substituted with carboxyl groups and alkyl chains (Plancque et al., 2001). In a study related to this work, Novakov et al. (2000) examined the changes in the thermograms resulting from the removal of soluble matter in either acetone or water. A rather large (40 to 70%) loss of BC was observed with the water treated samples. A similar loss was not observed after extraction with acetone. The dislodgment of BC particles in water was tentatively attributed to the rapid dissolution of sulfate. It is worth noting that aerosol matter was collected on filters residing on perforated disks with holes having a diameter of 1 to 2 mm. Hence the sampled aerosol matter accumulated to form a large set of hillock. This special feature might have been responsible for the particularly pronounced dislodgment of BC particles. In fact, a similar BC removal was never observed with normal uniform deposits on 37 or 47 mm quartz filters (Novakov, 2005)1 . 2 Methods In this study aerosol matter was sampled using a 5-stage Berner-type impactor (Hauke GmbH) operated at a flow rate of 80 l/min. The nozzles and the corresponding aerosol deposits are arranged along a circle with a diameter of 50 mm. Annular aluminium foils, 12 µm thick, with inner and outer diameters of 32 and 70 mm, respectively, served as impaction substrates. To avoid contamination problems during the subsequent analysis the foils were not greased. Sampling was carried out on the campus of GSF, located on the northern outskirts of Munich, during a 22.5 h period in May 2000. The sampled aerosol deposits were treated as follows. The total mass of the aerosol deposits on each foil was first determined gravimetrically. Water-soluble matter was then removed by depositing the impaction foils face down in 10 ml of deionised water for 1 h. The concentration of inorganic ions was determined by ion chromatography (Wittmaack and Keck, 2004). After drying of the impaction foils, the mass of the residual water-insoluble deposits was also determined gravimetrically. The water extracts showed minor quantities of diesel soot particles dispersed in the solution. The estimated loss was well below 10%. Small sections containing between eight spots in stage 1 and one spot in stage 4 were cut from the impaction foils for separate heat treatment and analysis (for stage assignment, number of nozzles, nozzle diameter, and aerodynamic cut diameter see Fig. 1a). Each section was heated only once in ambient air, to temperatures between 200 and 600 C, for 1 h. This procedure is somewhat reminiscent of the two-step protocol used by Cachier et al. (1989) and the constant heat-rate protocol of Novakov 1 Novakov, T.: personal communication, 2005. www.atmos-chem-phys.org/acp/5/1905/ COVERAGE (ML) et al. (2000), both groups applying an oxidative environment from the beginning of sample heating. According to Cachier (1989) heating in an oxidative gas prevents charring due to pyrolysis of organic matter, a problem routinely encountered during intial heat treatment in pure helium. If charring had occurred in the experiments of this study, the produced black carbon (BC) should become observable as a distinct change of the carbon signal, at the same or almost the same temperature as EC. Prior to SEM analysis the samples were covered with a conducting layer of sputter deposited platinum (nominal thickness 3 nm). SEM images were recorded using a model JSM-6300F scanning electron microscope (Jeol), equipped with a field emission electron gun operated at 5 kV. A model 6524 Si(Li) detector (Link Analytical) served for acquiring energy dispersive X-ray (EDX) spectra. To achieve optimum sensitivity for light elements, the detector was operated with an open window. EDX analysis was usually carried out while scanning the electron beam over an area of 60×42 µm. Occasionally small-area analysis (4×3 µm) was performed to determine the composition of individual mineral particles. For that purpose the beam energy was increased to 15 keV (window closed). MASS per STAGE (µg) K. Wittmaack: Combustion characteristics of water-insoluble elemental and organic carbon 1907 4 10 3 10 stage no.: 1 nozzles: 220 noz. dia (mm): 0.3 cut dia (µm): 0.04 2 76 0.4 0.14 3 18 1.0 0.49 4 21 1.9 1.72 5 7 6.1 6.0 2 10 10 water insoluble a as sampled 3 10 ρ = 2 g/cm 1 water insoluble fraction b -1 10 0.01 0.1 1 MEAN DIAMETER (µm) 10 Fig. 1. Gross characterisation of the analysed aerosol samples in terms of (a) the mass per stage and (b) the mean number of layers in units of the mean particle diameter. The impactor specifications are listed in the top part of panel (a). For conversion of mass to mass concentration (µg/cm3 ), the data in panel (a) must be divided by the volume of sampled air, i.e., 102 m3 . 3 Results and discussion 3.1 Gross description of samples The size dependent differences between the aerosol deposits collected in the different impactor stages are illustrated in Fig. 1. The total mass after sampling ranged from 50 to 350 µg per stage, the water insoluble mass from about 10 to 150 µg (Fig. 1a). Different from stages 4 and 5, which contained a large amount of water insoluble mineral dust, the soluble fraction was typically 60% in stages 1–3 (mostly ammonium sulphate and nitrate). This result is in accordance with recent studies on PM2.5 matter (Wittmaack and Keck, 2004). Assuming that the residual insoluble aerosol matter had a mean mass density of 2 g/cm3 (Berner and Lrzer, 1980; Wittmaack, 2002a) and was uniformly distributed over an area equivalent of the respective nozzle area, the mean coverage per stage can be calculated in units of monolayers (ML), as shown in Fig. 1b (the thickness of one monolayer was set equal to the geometric mean aerodynamic diameter in the respective impactor stage). In stages 1 and 3 the deposits were found to be about five ML thick, even thicker in stage 2 (about 18 ML), but only 0.3 ML in stage 4. Accordingly, the nominal areas of deposition should to be densely covered in stages 1–3, but only partially in stage 4. However, due to enhanced deposition near the jet axis, described in detail recently (Wittmaack, 2002b), the local thickness in the centre of the deposit was larger, possibly by up to a factor of two. The SEM and EDX data presented below are in accordance with this estimate. The fractional coverage in stage 5 www.atmos-chem-phys.org/acp/5/1905/ was found to be even smaller than in stage 4. Hence stage 5 was not included in the detailed analysis described below. 3.2 SEM analysis Evidence for a significant concentration of diesel soot particles in the ambient air at the GSF sampling site has already been presented before (Wittmaack, 2004; Wittmaack et al., 2005). Figure 2 shows examples of these densely packed carbon nanoparticles near the centre of deposits in impactor stage 4 (panel a, unheated; referred to as room temperature, RT) and stage 3 (panel b, after heating to 300 C; in what follows the term “after heating to” will be skipped for brevity, only the temperature of the heating cycle will be quoted). There is no detectable difference in the morphology of the nanoparticles for the RT and 300 C samples. In the stage 3/510 C sample (Fig. 2c), on the other hand, diesel soot particles are no longer detectable, i.e. they have been lost by combustion. As a results the much larger mineral particles become clearly observable. EDX analysis showed that these particles are composed mostly of Si, Fe, and O, with additional contributions due to K, Ti and Ca. A closer inspection of Fig. 2c shows some aggregated very small particles on the much larger mineral particles. The tiny particles, which could be misinterpreted as residual carbon nanoparticles, are actually some kind of ash presumably left behind as a result of the combustion of the diesel soot particles at 510 C. Figure 3 illustrates the differences Atmos. Chem. Phys., 5, 1905–1913, 2005 1908 K. Wittmaack: Combustion characteristics of water-insoluble elemental and organic carbon a a b b c c Fig. 3 Fig. 2. SEM images showing diesel soot particles (a) in an asprepared sample (stage 4), (b) after heating to 300 C (stage 3) and (c) to 510 C (stage 3). The magnification is the same in all three panels. Fig. 3. High-resolution SEM images of diesel soot particles in asprepared samples, (a) stage 1 and (b) stage 2. (c) Residues left behind in stage 2 after sample heating to 600 C. The magnification is the same in all three panels. in morphology of the two types of nanoparticles. Two highresolution images of diesel soot particles observed at RT outside the central area of aerosol deposition in stages 1 and 2 are presented in Figs. 3a and b, respectively. Similar chain aggregates have been described repeatedly (Bérubé et al., 1999; Wentzel et al., 2003; Van Gulijk et al., 2004; Wittmaack, 2004). The important aspect to note with reference to Figs. 3a and b is that the individual aggregated particles feature distinctly different sizes, about 20 nm in panel a, but between 30 and 40 nm in panel b. Almost the same differences in the size of diesel soot particles have been reported recently (Wentzel et al., 2003), but a reason for the difference could not be provided. More work appears to be necessary to fully understand the growth mechanism of diesel soot particles. An emerging technique for advanced chemical characterisation of diesel soot is Raman microspectroscopy (Sadetzky et al., 2005). For comparison, the residues of combustion in stage 2/600 C are shown in Fig. 3c. The individual particles in the aggregates have sizes less than 10 nm. Owing to an estimated resolution in the SEM analysis of 2–5 nm, the actual size could well be less than 5 nm. Because of the very small amount of material contained in these particles, compositional analysis is extremely difficult, if not impossible. EDX analysis at 5 keV showed very faint signals due to silicon, barely above the bremsstrahlung background. The equilibrium value of MeCpG steps (,+14 deg.) [31,44]. In comparison, methylation has a significantly lower stability cost when happening at major groove positions, such as 211 and 21 base pair from dyad (mutations 9 and 12), where the roll of the nucleosome bound conformation (+10 deg.) is more compatible with the equilibrium geometry of MeCpG steps. The nucleosome destabilizing effect of cytosine methylation increases with the number of methylated cytosines, following the same position dependence as the single methylations. The multiple-methylation case reveals that each major groove meth- PLOS Computational Biology | www.ploscompbiol.org 3 November 2013 | Volume 9 | Issue 11 | e1003354 DNA Methylation and Nucleosome Positioning ylation destabilizes the nucleosome by around 1 kJ/mol (close to the average estimate of 2 kJ/mol obtained for from individual methylation studies), while each minor groove methylation destabilizes it by up to 5 kJ/mol (average free energy as single mutation is around 6 kJ/mol). This energetic position-dependence is the reverse of what was observed in a recent FRET/SAXS study [30]. The differences can be attributed to the use of different ionic conditions and different sequences: a modified Widom-601 sequence of 157 bp, which already contains multiple CpG steps in mixed orientations, and which could assume different positioning due to the introduction of new CpG steps and by effect of the methylation. The analysis of our trajectories reveals a larger root mean square deviation (RMSD) and fluctuation (RMSF; see Figures S2– S3 in Text S1) for the methylated nucleosomes, but failed to detect any systematic change in DNA geometry or in intermolecular DNA-histone energy related to methylation (Fig. S1B, S1C, S4–S6 in Text S1). The hydrophobic effect should favor orientation of the methyl group out from the solvent but this effect alone is not likely to justify the positional dependent stability changes in Figure 2, as the differential solvation of the methyl groups in the bound and unbound states is only in the order of a fraction of a water molecule (Figure S5 in Text S1). We find however, a reasonable correlation between methylation-induced changes in hydrogen bond and stacking interactions of the bases and the change in nucleosome stability (see Figure S6 in Text S1). This finding suggests that methylation-induced nucleosome destabilization is related to the poorer ability of methylated DNA to fit into the required conformation for DNA in a nucleosome. Changes in the elastic deformation energy between methylated and un-methylated DNA correlate with nucleosomal differential binding free energies To further analyze the idea that methylation-induced nucleosome destabilization is connected to a worse fit of methylated DNA into the required nucleosome-bound conformation, we computed the elastic energy of the nucleosomal DNA using a harmonic deformation method [36,37,44]. This method provides a rough estimate of the energy required to deform a DNA fiber to adopt the super helical conformation in the nucleosome (full details in Suppl. Information Text S1). As shown in Figure 2, there is an evident correlation between the increase that methylation produces in the elastic deformation energy (DDE def.) and the free energy variation (DDG bind.) computed from MD/TI calculations. Clearly, methylation increases the stiffness of the CpG step [31], raising the energy cost required to wrap DNA around the histone octamers. This extra energy cost will be smaller in regions of high positive roll (naked DNA MeCpG steps have a higher roll than CpG steps [31]) than in regions of high negative roll. Thus, simple elastic considerations explain why methylation is better tolerated when the DNA faces the histones through the major groove (where positive roll is required) that when it faces histones through the minor groove (where negative roll is required). Nucleosome methylation can give rise to nucleosome repositioning We have established that methylation affects the wrapping of DNA in nucleosomes, but how does this translate into chromatin structure? As noted above, accumulation of minor groove methylations strongly destabilizes the nucleosome, and could trigger nucleosome unfolding, or notable changes in positioning or phasing of DNA around the histone core. While accumulation of methylations might be well tolerated if placed in favorable positions, accumulation in unfavorable positions would destabilize the nucleosome, which might trigger changes in chromatin structure. Chromatin could in fact react in two different ways in response to significant levels of methylation in unfavorable positions: i) the DNA could either detach from the histone core, leading to nucleosome eviction or nucleosome repositioning, or ii) the DNA could rotate around the histone core, changing its phase to place MeCpG steps in favorable positions. Both effects are anticipated to alter DNA accessibility and impact gene expression regulation. The sub-microsecond time scale of our MD trajectories of methylated DNAs bound to nucleosomes is not large enough to capture these effects, but clear trends are visible in cases of multiple mutations occurring in unfavorable positions, where unmethylated and methylated DNA sequences are out of phase by around 28 degrees (Figure S7 in Text S1). Due to this repositioning, large or small, DNA could move and the nucleosome structure could assume a more compact and distorted conformation, as detected by Lee and Lee [29], or a slightly open conformation as found in Jimenez-Useche et al. [30]. Using the harmonic deformation method, we additionally predicted the change in stability induced by cytosine methylation for millions of different nucleosomal DNA sequences. Consistently with our calculations, we used two extreme scenarios to prepare our DNA sequences (see Fig. 3): i) all positions where the minor grooves contact the histone core are occupied by CpG steps, and ii) all positions where the major grooves contact the histone core are occupied by CpG steps. We then computed the elastic energy required to wrap the DNA around the histone proteins in unmethylated and methylated states, and, as expected, observed that methylation disfavors DNA wrapping (Figure 3A). We have rescaled the elastic energy differences with a factor of 0.23 to match the DDG prediction in figure 2B. In agreement with the rest of our results, our analysis confirms that the effect of methylation is position-dependent. In fact, the overall difference between the two extreme methylation scenarios (all-in-minor vs all-in-major) is larger than 60 kJ/mol, the average difference being around 15 kJ/ mol. We have also computed the elastic energy differences for a million sequences with CpG/MeCpG steps positioned at all possible intermediate locations with respect to the position (figure 3B). The large differences between the extreme cases can induce rotations of DNA around the histone core, shifting its phase to allow the placement of the methylated CpG steps facing the histones through the major groove. It is illustrative to compare the magnitude of CpG methylation penalty with sequence dependent differences. Since there are roughly 1.5e88 possible 147 base pairs long sequence combinations (i.e., (4n+4(n/2))/2, n = 147), it is unfeasible to calculate all the possible sequence effects. However, using our elastic model we can provide a range of values based on a reasonably large number of samples. If we consider all possible nucleosomal sequences in the yeast genome (around 12 Mbp), the energy difference between the best and the worst sequence that could form a nucleosome is 0.7 kj/mol per base (a minimum of 1 kJ/mol and maximum of around 1.7 kJ/mol per base, the first best and the last worst sequences are displayed in Table S3 in Text S1). We repeated the same calculation for one million random sequences and we obtained equivalent results. Placing one CpG step every helical turn gives an average energetic difference between minor groove and major groove methylation of 15 kJ/ mol, which translates into ,0.5 kJ/mol per methyl group, 2 kJ/ mol per base for the largest effects. Considering that not all nucleosome base pair steps are likely to be CpG steps, we can conclude that the balance between the destabilization due to CpG methylation and sequence repositioning will depend on the PLOS Computational Biology | www.ploscompbiol.org 4 November 2013 | Volume 9 | Issue 11 | e1003354 DNA Methylation and Nucleosome Positioning Figure 3. Methylated and non-methylated DNA elastic deformation energies. (A) Distribution of deformation energies for 147 bplong random DNA sequences with CpG steps positioned every 10 base steps (one helical turn) in minor (red and dark red) and major (light and dark blue) grooves respectively. The energy values were rescaled by the slope of a best-fit straight line of figure 2, which is 0.23, to por la lectura a través de la lectura de la prensa. La educación en los medios las fuerzas dispersas en función de los soportes mediáticos y orientarse más hacia la educación en medios que al dominio adquiere pleno derecho y entidad en la sección sexta titulada «competencias sociales y cívi- técnico de los aparatos. cas» que indica que «los alum- nos deberán ser capaces de juz- gar y tendrán espíritu crítico, lo que supone ser educados en los las programaciones oficiales, ya que, a lo largo de un medios y tener conciencia de su lugar y de su influencia estudio de los textos, los documentalistas del CLEMI en la sociedad». han podido señalar más de una centena de referencias a la educación de los medios en el seno de disciplinas 4. Un entorno positivo como el francés, la historia, la geografía, las lenguas, Si nos atenemos a las cifras, el panorama de la las artes plásticas : trabajos sobre las portadas de educación en medios es muy positivo. Una gran ope- prensa, reflexiones sobre temas mediáticos, análisis de ración de visibilidad como la «Semana de la prensa y publicidad, análisis de imágenes desde todos los ángu- de los medios en la escuela», coordinada por el CLE- los, reflexión sobre las noticias en los países europeos, MI, confirma año tras año, después de 17 convocato- información y opinión rias, el atractivo que ejerce sobre los profesores y los Esta presencia se constata desde la escuela mater- alumnos. Concebida como una gran operación de nal (2 a 6 años) donde, por ejemplo, se le pregunta a complementariedad entre la escuela y los profesiona- los niños más pequeños si saben diferenciar entre un les de los medios, alrededor del aprendizaje ciudada- periódico, un libro, un catálogo, a través de activida- no de la comunicación mediática, este evento moviliza des sensoriales, si saben para qué sirve un cartel, un durante toda una semana un porcentaje elevado de periódico, un cuaderno, un ordenador si son capa- centros escolares que representan un potencial de 4,3 ces de reconocer y distinguir imágenes de origen y de millones de alumnos (cifras de 2006). Basada en el naturaleza distintas. Podríamos continuar con más voluntariado, la semana permite desarrollar activida- ejemplos en todos los niveles de enseñanza y práctica- des más o menos ambiciosas centradas en la introduc- Páginas 43-48 ción de los medios en la vida de la escuela a través de la instalación de kioscos, organización de debates con profesionales y la confección por parte de los alumnos de documentos difundidos en los medios profesionales. Es la ocasión de dar un empujón a la educación en medios y de disfrutarlos. Los medios –un millar en 2006– se asocian de maneras diversas ofreciendo ejemplares de periódicos, acceso a noticias o a imágenes, proponiendo encuentros, permitiendo intervenir a los jóvenes en sus ondas o en sus columnas Esta operación da luz al trabajo de la educación en medios y moviliza a los diferentes participantes en el proyecto. 5. La formación de los docentes La formación es uno de los pilares principales de la educación en los medios. Su función es indispensable ya que no se trata de una disciplina, sino de una enseñanza que se hace sobre la base del voluntariado y del compromiso personal. Se trata de convencer, de mostrar, de interactuar. En primer lugar es necesario incluirla en la formación continua de los docentes, cuyo volumen se ha incrementado desde 1981 con la aparición de una verdadera política de formación continua de personal. Es difícil dar una imagen completa del volumen y del público, pero si nos atenemos a las cifras del CLEMI, hay más de 24.000 profesores que han asistido y se han involucrado durante 2004-05. 5.1. La formación continua En la mayoría de los casos, los profesores reciben su formación en contextos cercanos a su centro de trabajo, o incluso en este mismo. Después de una política centrada en la oferta que hacían los formadores, se valora más positivamente la demanda por parte del profesorado, ya que sólo así será verdaderamente fructífera. Los cursos de formación se repartieron en varias categorías: desde los formatos más tradicionales (cursos, debates, animaciones), hasta actividades de asesoramiento y de acompañamiento, y por supuesto los coloquios que permiten un trabajo en profundidad ya que van acompañados de expertos investigadores y profesionales. Citemos, por ejemplo en 2005, los coloquios del CLEMI-Toulouse sobre el cine documental o el del CLEMI-Dijon sobre «Políticos y medios: ¿connivencia?». Estos coloquios, que forman parte de un trabajo pedagógico regular, reagrupan a los diferentes participantes regionales y nacionales alrededor de grandes temas de la educación en medios y permiten generar nuevos conocimientos de aproximación y una profundización. Páginas 43-48 Hay otro tipo de formación original que se viene desarrollando desde hace menos tiempo, a través de cursos profesionales, como por ejemplo, en el Festival Internacional de Foto-periodismo «Visa para la imagen», en Perpignan. La formación se consolida en el curso, da acceso a las exposiciones, a las conferencias de profesionales y a los grandes debates, pero añade además propuestas pedagógicas y reflexiones didácticas destinadas a los docentes. Estas nuevas modalidades de formación son también consecuencia del agotamiento de la formación tradicional en las regiones. Los contenidos más frecuentes en formación continua conciernen tanto a los temas más clásicos como a los cambios que se están llevando a cabo en las prácticas mediáticas. Así encontramos distintas tendencias para 2004-05: La imagen desde el ángulo de la producción de imágenes animadas, el análisis de la imagen de la información o las imágenes del J.T. La prensa escrita y el periódico escolar. Internet y la información en línea. Medios y educación de los medios. 5.2 La formación inicial La formación inicial está aun en un grado muy ini- cial. El hecho de que la educación en medios no sea una disciplina impide su presencia en los IUFM (Institutos Universitarios de Formación de Maestros) que dan una prioridad absoluta a la didáctica de las disciplinas. En 2003, alrededor de 1.400 cursillistas sobre un total de 30.000 participaron en un momento u otro de un módulo de educación en medios. Estos módulos se ofrecen en función del interés que ese formador encuentra puntualmente y forman parte a menudo de varias disciplinas: documentación, letras, historia-geografía Estamos aún lejos de una política concertada en este dominio. La optativa «Cine-audiovisual» ha entrado desde hace muy poco tiempo en algunos IUFM destinada a obtener un certificado de enseñanza de la opción audiovisual y cine. Internet tiene cabida también en los cursos de formación inicial, recientemente con la aparición de un certificado informático y de Internet para los docentes, dirigido más a constatar competencias personales que a valorar una aptitud para enseñarlos. 6. ¿Y el futuro? El problema del futuro se plantea una vez más por la irrupción de nuevas técnicas y nuevos soportes. La difusión acelerada de lo digital replantea hoy muchas cuestiones relativas a prácticas mediáticas. Muchos Comunicar, 28, 2007 47 Comunicar, 28, 2007 Enrique Martínez-Salanova '2007 para Comunicar 48 trabajos que llevan el rótulo de la educación en medios solicitan una revisión ya que los conceptos cambian. La metodología elaborada en el marco de la educación en medios parece incluso permitir la inclinación de la sociedad de la información hacia una sociedad del conocimiento, como defiende la UNESCO. En Francia, se necesitaría unir las fuerzas dispersas en función de los soportes mediáticos y orientarse más hacia la educación en medios que al dominio técnico de los aparatos. Los avances recientes en el reconocimiento de estos contenidos y las competencias que supondrían podrían permitirlo. Referencias CLEMI/ACADEMIE DE BORDEAUX (Ed.) (2003): Parcours médias au collège: approches disciplinaires et transdisciplinaires. Aquitaine, Sceren-CRDP. GONNET, J. (2001): Education aux médias. Les controverses fécondes. Paris, Hachette Education/CNDP. SAVINO, J.; MARMIESSE, C. et BENSA, F. (2005): L’éducation aux médias de la maternelle au lycée. Direction de l’Enseignement Scolaire. Paris, Ministère de l’Education Nationale, Sceren/CNDP, Témoigner. BEVORT, E. et FREMONT, P. (2001): Médias, violence et education. Paris, CNDP, Actes et rapports pour l’éducation. – www.clemi.org: fiches pédagogiques, rapports et liens avec les pages régionales/académiques. – www.ac-nancy-metz.fr/cinemav/quai.html: Le site «Quai des images» est dédié à l’enseignement du cinéma et de l’audiovisuel. – www.france5.fr/education: la rubrique «Côté profs» a une entrée «education aux médias». – www.educaunet.org: Programme européen d’éducation aux risques liés à Internet. dResedfeleexliobnuetsacón Páginas 43-48
Combustion characteristics of water-insoluble elemental and organic carbon in size selected ambient aerosol particles
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Combustion characteristics of water-insoluble elemental and organic carbon in size selected ambient aerosol particles

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