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R. M. Pick - One of the best experts on this subject based on the ideXlab platform.
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Third order susceptibilities in supercooled liquids and the "Box Model" theory versus experiments
Journal of Chemical Physics, 2014Co-Authors: R. M. PickAbstract:We give here the exact expression of the time dependent third order polarization predicted by the "Box Model" when a supercooled liquid is permanently subjected since time t = 0 to a strong electric field E(t) = E0 sin(ωt) , Dielectric Hole Burning experiment. We derive different forms of the time dependent susceptibilities, both at frequencies ω and 3ω, and examine particularly one of them. We also show that its susceptibility at ω should exhibit, at short times, presently undetected oscillations at frequency 2ω that are a signature of the "Box Model." We finally compare, for a large frequency range, the ω and 3ω time independent susceptibilities with the corresponding measurements on glycerol at 204.7 K. The agreement is good at frequency ω but, as already shown in Ladieu et al. [J. Chem. Phys. 134, 194507 (2011)], it is not the case at 3ω.
Thomas A M Pugh - One of the best experts on this subject based on the ideXlab platform.
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Modelling chemistry in the nocturnal boundary layer above tropical rainforest and a generalised effective nocturnal ozone deposition velocity for sub-ppbv NO_x conditions
Journal of Atmospheric Chemistry, 2010Co-Authors: Thomas A M Pugh, James Ryder, Sarah J. Moller, Carole Helfter, Douglas Lowe, James D. Lee, A. Robert Mackenzie, Eiko Nemitz, C. Nicholas HewittAbstract:Measurements of atmospheric composition have been made over a remote rainforest landscape. A Box Model has previously been demonstrated to Model the observed daytime chemistry well. However the Box Model is unable to explain the nocturnal measurements of relatively high [NO] and [O_3], but relatively low observed [NO_2]. It is shown that a one-dimensional (1-D) column Model with simple O_3-NO_x chemistry and a simple representation of vertical transport is able to explain the observed nocturnal concentrations and predict the likely vertical profiles of these species in the nocturnal boundary layer (NBL). Concentrations of tracers carried over from the end of the night can affect the atmospheric chemistry of the following day. To ascertain the anomaly introduced by using the Box Model to represent the NBL, vertically-averaged NBL concentrations at the end of the night are compared between the 1-D Model and the Box Model. It is found that, under low to medium [NO_x] conditions (NO_x 1 ppbv), the effect on the chemistry due to the vertical distribution of the species means no Box Model can adequately represent chemistry in the NBL without modifying reaction rate coefficients.
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simulating atmospheric composition over a south east asian tropical rainforest performance of a chemistry Box Model
Atmospheric Chemistry and Physics, 2009Co-Authors: Thomas A M Pugh, A R Mackenzie, C N Hewitt, Ben Langford, Peter Edwards, K L Furneaux, Dwayne E Heard, J R Hopkins, C E Jones, A KarunaharanAbstract:Abstract. Atmospheric composition and chemistry above tropical rainforests is currently not well established, particularly for south-east Asia. In order to examine our understanding of chemical processes in this region, the performance of a Box Model of atmospheric boundary layer chemistry is tested against measurements made at the top of the rainforest canopy near Danum Valley, Malaysian Borneo. Multi-variate optimisation against ambient concentration measurements was used to estimate average canopy-scale emissions for isoprene, total monoterpenes and nitric oxide. The excellent agreement between estimated values and measured fluxes of isoprene and total monoterpenes provides confidence in the overall Modelling strategy, and suggests that this method may be applied where measured fluxes are not available, assuming that the local chemistry and mixing are adequately understood. The largest contributors to the optimisation cost function at the point of best-fit are OH (29%), NO (22%) and total peroxy radicals (27%). Several factors affect the Modelled VOC chemistry. In particular concentrations of methacrolein (MACR) and methyl-vinyl ketone (MVK) are substantially overestimated, and the hydroxyl radical (OH) concentration is substantially underestimated; as has been seen before in tropical rainforest studies. It is shown that inclusion of dry deposition of MACR and MVK and wet deposition of species with high Henry's Law values substantially improves the fit of these oxidised species, whilst also substantially decreasing the OH sink. Increasing OH production arbitrarily, through a simple OH recycling mechanism , adversely affects the Model fit for volatile organic compounds (VOCs). Given the constraints on isoprene flux provided by measurements, a substantial decrease in the rate of reaction of VOCs with OH is the only remaining option to explain the measurement/Model discrepancy for OH. A reduction in the isoprene+OH rate constant of 50%, in conjunction with increased deposition of intermediates and some modest OH recycling, is able to produce both isoprene and OH concentrations within error of those measured. Whilst we cannot rule out an important role for missing chemistry, particularly in areas of higher isoprene flux, this study demonstrates that the inadequacies apparent in Box and global Model studies of tropical VOC chemistry may be more strongly influenced by representation of detailed physical and micrometeorological effects than errors in the chemical scheme.
D. L'hote - One of the best experts on this subject based on the ideXlab platform.
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Third harmonics nonlinear susceptibility in supercooled liquids: A comparison to the Box Model
Journal of Chemical Physics, 2011Co-Authors: C. Brun, C. Crauste-thibierge, F. Ladieu, D. L'hoteAbstract:The Box Model, originally introduced to account for the nonresonant hole burning (NHB) dielectricexperiments in supercooled liquids, is compared to the measurements of the third harmonics P 3 of the polarisation, reported recently in glycerol, close to the glass transition temperature T g [C. Crauste-Thibierge, C. Brun, F. Ladieu, D. L’Hôte, G. Biroli, and J.-P. Bouchaud, Phys. Rev. Lett.104, 165703 (2010)]. In this Model, each Box is a distinct dynamical relaxing entity (hereafter called dynamical heterogeneity (DH)) which follows a Debye dynamics with its own relaxation time τ dh . When it is submitted to a strong electric field, the Model posits that a temperature increase δT dh , depending on τ dh , arises due to the dissipation of the electrical power. Each DH has thus its own temperature increase, on top of the temperature increase of the phonon bath δT ph . Contrary to the “fast” hole burning experiments where δT ph is usually neglected, the P 3 measurements are, from a thermal point of view, fully in a stationary regime, which means that δT ph can no longer be neglected a priori. This is why the version of the Box Model that we study here takes δT ph into account, which implies that the δT dh of the DHs are all coupled together. The value of P 3, including both the “intrinsic” contribution of each DH as well as the “spurious” one coming from δT ph , is computed within this Box Model and compared to the P 3 measurements for glycerol, in the same range of frequencies and temperatures T. Qualitatively, we find that this version of the Box Model shares with experiments some nontrivial features, e.g., the existence of a peak at finite frequency in the modulus of P 3 as well as its order of magnitude. Quantitatively, however, some experimental features are not accounted for by this Model. We show that these differences between the Model and the experiments do not come from δT ph but from the “intrinsic” contribution of the DHs. Finally, we show that the interferences between the 3ω response of the various DHs are the most important issue leading to the discrepancies between the Box Model prediction and the experiments. We argue that this could explain why the Box Model is quite successful to account for some kinds of nonlinear experiments (such as NHB) performed close to T g , even if it does not completely account for all of them (such as the P 3 measurements). This conclusion is supported by an analytical argument which helps understanding how a “space-free” Model as the Box Model is able to account for some of the experimental nonlinear features.
A Karunaharan - One of the best experts on this subject based on the ideXlab platform.
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simulating atmospheric composition over a south east asian tropical rainforest performance of a chemistry Box Model
Atmospheric Chemistry and Physics, 2009Co-Authors: Thomas A M Pugh, A R Mackenzie, C N Hewitt, Ben Langford, Peter Edwards, K L Furneaux, Dwayne E Heard, J R Hopkins, C E Jones, A KarunaharanAbstract:Abstract. Atmospheric composition and chemistry above tropical rainforests is currently not well established, particularly for south-east Asia. In order to examine our understanding of chemical processes in this region, the performance of a Box Model of atmospheric boundary layer chemistry is tested against measurements made at the top of the rainforest canopy near Danum Valley, Malaysian Borneo. Multi-variate optimisation against ambient concentration measurements was used to estimate average canopy-scale emissions for isoprene, total monoterpenes and nitric oxide. The excellent agreement between estimated values and measured fluxes of isoprene and total monoterpenes provides confidence in the overall Modelling strategy, and suggests that this method may be applied where measured fluxes are not available, assuming that the local chemistry and mixing are adequately understood. The largest contributors to the optimisation cost function at the point of best-fit are OH (29%), NO (22%) and total peroxy radicals (27%). Several factors affect the Modelled VOC chemistry. In particular concentrations of methacrolein (MACR) and methyl-vinyl ketone (MVK) are substantially overestimated, and the hydroxyl radical (OH) concentration is substantially underestimated; as has been seen before in tropical rainforest studies. It is shown that inclusion of dry deposition of MACR and MVK and wet deposition of species with high Henry's Law values substantially improves the fit of these oxidised species, whilst also substantially decreasing the OH sink. Increasing OH production arbitrarily, through a simple OH recycling mechanism , adversely affects the Model fit for volatile organic compounds (VOCs). Given the constraints on isoprene flux provided by measurements, a substantial decrease in the rate of reaction of VOCs with OH is the only remaining option to explain the measurement/Model discrepancy for OH. A reduction in the isoprene+OH rate constant of 50%, in conjunction with increased deposition of intermediates and some modest OH recycling, is able to produce both isoprene and OH concentrations within error of those measured. Whilst we cannot rule out an important role for missing chemistry, particularly in areas of higher isoprene flux, this study demonstrates that the inadequacies apparent in Box and global Model studies of tropical VOC chemistry may be more strongly influenced by representation of detailed physical and micrometeorological effects than errors in the chemical scheme.
R Sander - One of the best experts on this subject based on the ideXlab platform.
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halogen activation in the plume of masaya volcano field observations and Box Model investigations
Atmospheric Chemistry and Physics, 2020Co-Authors: R Sander, Julian Rudiger, Alexandra Gutmann, Nicole Bobrowski, Marcello Liotta, Maarten J De Moor, Florian DingerAbstract:Abstract. Volcanic emissions are a source of halogens to the atmosphere. Rapid reactions convert the initially emitted hydrogen halides (HCl, HBr, HI) into reactive species e.g. BrO, Br2, BrCl, ClO, OClO and IO. The activation reaction mechanisms in the plume consume ozone (O3), which is entrained by in-mixed ambient air. In this study, we present observations of the oxidation of bromine, chlorine and iodine during the first 11 minutes after emission, investigating the plume of Santiago Crater of Masaya volcano in Nicaragua. Two field campaigns were conducted, in July 2016 and September 2016. The sum of the reactive species of the respective halogens were determined by gas diffusion denuder sampling followed by GC-MS analysis, while the total amounts of halogens and sulfur amounts were obtained by alkaline trap sampling with subsequent IC and ICP-MS measurements. Both ground and airborne sampling with an unmanned aerial vehicle (including a denuder sampler in combination with an electrochemical SO2 sensor) was performed at different distances from the crater rim. The in-situ measurements were accompanied by remote sensing observations (DOAS). For bromine, the reactive fraction increased from 0.20 ± 0.13 at the crater rim to 0.76 ± 0.26 at 2.8 km downwind, while chlorine showed an increase of the reactive fraction from (2.7 ± 0.7) × 10−4 to (11 ± 3) × 10−4 in the first 750 m. Additionally, a reactive iodine fraction of 0.3 at the crater rim and 0.9 at 2.8 km was measured. No significant increase in BrO / SO2 molar ratios was observed with the estimated age of the observed plume ranging from 1.4 min to 11.1 min. This study presents a comprehensive gas diffusion denuder data set on reactive halogen species and compares BrO / SO2 ratios with the sum of all reactive Br species. With the observed field data, a chemistry Box Model (CAABA/MECCA) enabled the reproduction of the observed progression of the reactive bromine to total bromine ratio. An observed contribution of BrO to the reactive bromine fraction of about 10 % was reproduced in the first minutes of the Model run. The Model results emphasize the importance of ozone entrainment into the plume for the reproduction of the measured reactive bromine formation and the dependence on the availability of HXOY and NOX.
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the community atmospheric chemistry Box Model caaba mecca 4 0
Geoscientific Model Development, 2019Co-Authors: R Sander, A J G Baumgaertner, Sergey Gromov, H Harder, Patrick Jockel, David Cabreraperez, Franziska Frank, J U Groos, Vincent Huijnen, Vlassis A KarydisAbstract:Abstract. We present version 4.0 of the atmospheric chemistry Box Model CAABA/MECCA that now includes a number of new features: (i) skeletal mechanism reduction, (ii) the Mainz Organic Mechanism (MOM) chemical mechanism for volatile organic compounds, (iii) an option to include reactions from the Master Chemical Mechanism (MCM) and other chemical mechanisms, (iv) updated isotope tagging, and (v) improved and new photolysis modules (JVAL, RADJIMT, DISSOC). Further, when MECCA is connected to a global Model, the new feature of coexisting multiple chemistry mechanisms (PolyMECCA/CHEMGLUE) can be used. Additional changes have been implemented to make the code more user-friendly and to facilitate the analysis of the Model results. Like earlier versions, CAABA/MECCA-4.0 is a community Model published under the GNU General Public License.
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the atmospheric chemistry Box Model caaba mecca 3 0
Geoscientific Model Development, 2011Co-Authors: R Sander, A J G Baumgaertner, Sergey Gromov, H Harder, Patrick Jockel, Astrid Kerkweg, Dagmar Kubistin, E Regelin, Hella Riede, Adrian SanduAbstract:Abstract. We present version 3.0 of the atmospheric chemistry Box Model CAABA/MECCA. In addition to a complete update of the rate coefficients to the most recent recommendations, a number of new features have been added: chemistry in multiple aerosol size bins; automatic multiple simulations reaching steady-state conditions; Monte-Carlo simulations with randomly varied rate coefficients within their experimental uncertainties; calculations along Lagrangian trajectories; mercury chemistry; more detailed isoprene chemistry; tagging of isotopically labeled species. Further changes have been implemented to make the code more user-friendly and to facilitate the analysis of the Model results. Like earlier versions, CAABA/MECCA-3.0 is a community Model published under the GNU General Public License.
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simulation of atmospheric mercury depletion events amdes during polar springtime using the mecca Box Model
Atmospheric Chemistry and Physics, 2008Co-Authors: R Sander, Ulrich Poschl, F SlemrAbstract:Atmospheric mercury depletion events (AMDEs) during polar springtime are closely correlated with bromine-catalyzed tropospheric ozone depletion events (ODEs). To study gas- and aqueous-phase reaction kinetics and speciation of mercury during AMDEs, we have included mercury chemistry into the Box Model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere), which enables dynamic simulation of bromine activation and ODEs. We found that the reaction of Hg with Br atoms dominates the loss of gaseous elemental mercury (GEM). To explain the experimentally observed synchronous depletion of GEM and O 3 , the reaction rate of Hg+BrO has to be much lower than that of Hg+Br. The synchronicity is best reproduced with rate coefficients at the lower limit of the literature values for both reactions, i.e. k Hg+Br ≈3×10 −13 and k Hg+BrO ≤1×10 −15 cm 3 molecule −1 s −1 , respectively. Throughout the simulated AMDEs, \chem{BrHgOBr} was the most abundant reactive mercury species, both in the gas phase and in the aqueous phase. The aqueous-phase concentrations of BrHgOBr, HgBr 2 , and HgCl 2 were several orders of magnitude larger than that of Hg(SO 3 ) 2 2− . Considering chlorine chemistry outside depletion events (i.e. without bromine activation), the concentration of total divalent mercury in sea-salt aerosol particles (mostly HgCl 4 2− ) was much higher than in dilute aqueous droplets (mostly Hg(SO 3 ) 2 2− ), and did not exhibit a diurnal cycle (no correlation with HO 2 radicals).
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hydroxyl radicals in the tropical troposphere over the suriname rainforest comparison of measurements with the Box Model mecca
Atmospheric Chemistry and Physics, 2008Co-Authors: Dagmar Kubistin, R Sander, H Harder, M Martinez, M Rudolf, Heiko Bozem, G Eerdekens, H Fischer, C Gurk, T KlupfelAbstract:Abstract. As a major source region of the hydroxyl radical OH, the Tropics largely control the oxidation capacity of the atmosphere on a global scale. However, emissions of hydrocarbons from the tropical rainforest that react rapidly with OH can potentially deplete the amount of OH and thereby reduce the oxidation capacity. The airborne GABRIEL field campaign in equatorial South America (Suriname) in October 2005 investigated the influence of the tropical rainforest on the HOx budget (HOx = OH + HO2). The first observations of OH and HO2 over a tropical rainforest are compared to steady state concentrations calculated with the atmospheric chemistry Box Model MECCA. The important precursors and sinks for HOx chemistry, measured during the campaign, are used as constraining parameters for the simulation of OH and HO2. Significant underestimations of HOx are found by the Model over land during the afternoon, with mean ratios of observation to Model of 12.2 ± 3.5 and 4.1 ± 1.4 for OH and HO2, respectively. The discrepancy between measurements and simulation results is correlated to the abundance of isoprene. While for low isoprene mixing ratios (above ocean or at altitudes >3 km), observation and simulation agree fairly well, for mixing ratios >200 pptV ( Box Model simulations have been performed with the condensed chemical mechanism of MECCA and with the detailed isoprene reaction scheme of MCM, resulting in similar results for HOx concentrations. Simulations with constrained HO2 concentrations show that the conversion from HO2 to OH in the Model is too low. However, by neglecting the isoprene chemistry in the Model, observations and simulations agree much better. An OH source similar to the strength of the OH sink via isoprene chemistry is needed in the Model to resolve the discrepancy. A possible explanation is that the oxidation of isoprene by OH not only dominates the removal of OH but also produces it in a similar amount. Several additional reactions which directly produce OH have been implemented into the Box Model, suggesting that upper limits in producing OH are still not able to reproduce the observations (improvement by factors of ≈2.4 and ≈2 for OH and HO2, respectively). We determine that OH has to be recycled to 94% instead of the simulated 38% to match the observations, which is most likely to happen in the isoprene degradation process, otherwise additional sources are required.