Time-Resolved Measurements and Master Equation Modelling of the Unimolecular Decomposition of CH3OCH2

The rate coefficient for the unimolecular decomposition of CH3OCH2,k(1), has been measured in time-resolved experiments by monitoring the HCHO product. CH3OCH2 was rapidly and cleanly generated by 248 nm excimer photolysis of oxalyl chloride, (ClCO)(2), in an excess of CH3OCH3, and an excimer pumped dye laser tuned to 353.16 nm was used to probe HCHO via laser induced fluorescence. k(1)(T,p) was measured over the ranges: 573-673 K and 0.1-4.3 x 10(18) molecule cm(-3) with a helium bath gas. In addition, some experiments were carried out with nitrogen as the bath gas. Ab initio calculations on CH3OCH2 decomposition were carried out and a transition-state for decomposition to CH3 and H2CO was identified. This information was used in a master equation rate calculation, using the MESMER code, where the zero-point-energy corrected barrier to reaction, Delta E-0,E-1, and the energy transfer parameters, x T-n, were the adjusted parameters to best fit the experimental data, with helium as the buffer gas. The data were combined with earlier measurements by Loucks and Laidler (Can J. Chem. 1967, 45, 2767), with dimethyl ether as the third body, reinterpreted using current literature for the rate coefficient for recombination of CH3OCH2. This analysis returned Delta E-0,E-1 = (112.3 +/- 0.6) kJ mol(-1), and leads to k(1)(infinity)(T) = 2.9 x 10(12) (T/300)(2)(.5) exp(-106.8 kJ mol(-1)/RT). Using this model, limited experiments with nitrogen as the bath gas allowed N-2 energy transfer parameters to be identified and then further MESMER simulations were carried out, where N-2 was the buffer gas, to generate k(1)(T,p) over a wide range of conditions: 300-1000 K and N-2 = 10(12) -10(25) molecule cm(-3). The resulting k(1)(T,p) has been parameterized using a Troe-expression, so that they can be readily be incorporated into combustion models. In addition, k(1)(T,p) has been parametrized using PLOG for the buffer gases, He, CH3OCH3 and N-2.Peer reviewe

Towards Making Videos Accessible for Low Vision Screen Magnifier Users

People with low vision who use screen magnifiers to interact with computing devices find it very challenging to interact with dynamically changing digital content such as videos, since they do not have the luxury of time to manually move, i.e., pan the magnifier lens to different regions of interest (ROIs) or zoom into these ROIs before the content changes across frames. In this paper, we present SViM, a first of its kind screen-magnifier interface for such users that leverages advances in computer vision, particularly video saliency models, to identify salient ROIs in videos. SViM\u27s interface allows users to zoom in/out of any point of interest, switch between ROIs via mouse clicks and provides assistive panning with the added flexibility that lets the user explore other regions of the video besides the ROIs identified by SViM. Subjective and objective evaluation of a user study with 13 low vision screen magnifier users revealed that overall the participants had a better user experience with SViM over extant screen magnifiers, indicative of the former\u27s promise and potential for making videos accessible to low vision screen magnifier users

OH yields from the CH3CO+O-2 reaction using an internal standard

Laser flash photolysis of CH3C(O)OH at 248 nm was used to create equal zero time yields of CH3CO and OH. The absolute OH yield from the CH3CO + O2 (+M) reaction was determined by following the OH temporal profile using the zero time OH concentration as an internal standard. The OH yield from CH3CO + O2 (+M) was observed to decrease with increasing pressure with an extrapolated zero pressure yield close to unity (1.1 ± 0.2, quoted uncertainties correspond to 95% confidence limits). The results are in quantitative agreement with those obtained from 248 nm acetone photolysis in the presence of O2

Kinetics of the Gas Phase Reactions of the Criegee Intermediate CH2OO with O3 and IO

The kinetics of the gas phase reactions of the Criegee intermediate CH2OO with O3 and IO have been studied at 296 K and 300 Torr through simultaneous measurements of CH2OO, the CH2OO precursor (CH2I2), O3, and IO using flash photolysis of CH2I2/O2/O3/N2 mixtures at 248 nm coupled to time-resolved broadband UV absorption spectroscopy. Experiments were performed under pseudo-first-order conditions with respect to O3, with the rate coefficients for reactions of CH2OO with O3 and IO obtained by fitting to the observed decays of CH2OO using a model constrained to the measured concentrations of IO. Fits were performed globally, with the ratio between the initial concentration of O3 and the average concentration of IO varied in the range 30 to 700, and gave kCH2OO+O3 = (3.6 ± 0.8) × 10-13 cm3 molecule-1 s-1 and kCH2OO+IO = (7.6 ± 1.4) × 10-11 cm3 molecule-1 s-1 (where the errors are at the 2σ level). The magnitude of kCH2OO+O3 has a significant effect on the steady state concentration of CH2OO in chamber studies. Atmospheric implications of the results are discussed

HONO Measurement by Differential Photolysis

Nitrous acid (HONO) has been quantitatively measured in situ by differential photolysis at 385 and 395 nm, and subsequent detection as nitric oxide (NO) by the chemiluminescence reaction with ozone (O3). The technique has been evaluated by Fourier transform infrared (FT-IR) spectroscopy to provide a direct HONO measurement in a simulation chamber and compared side by side with a long absorption path optical photometer (LOPAP) in the field. The NO-O3 chemiluminescence technique is robust, well characterized, and capable of sampling at low pressure, whilst solid-state converter technology allows for unattended in situ HONO measurements in combination with fast time resolution and response

Effect of pretreatment of rats with carbon tetrachloride on tolerance development

To evaluate the effect of pretreatment of rats with CCl4 on tolerance development to subsequent exposures, rats were exposed 25-7500 ppm of CCl4 vapor or given CCl4 orally, 3.25 mg/g body weight. Forty-eight hours later these rats were reexposed to a normally lethal 7500 ppm CCl4 concentration. Mortality, blood clotting time, serum bilirubin, serum and liver triglycerides, and CCl4 and CHCl3 concentrations in tissues and in the whole rat were used as a measure of the response to CCl4 exposure. Conversion of CCl4 to CHCl3 in vitro was also determined in normal and pretreated animals. It was found that animals exposed to 4000 ppm for 6 hr or given 3.25 mg/g of CCl4 orally developed tolerance to subsequent normally lethal CCl4 exposures. Serum bilirubin levels in animals protected by CCl4 pretreatment were significantly lower than in those receiving only a single exposure to CCl4. Blood clotting times in protected animals were normal, while in those not protected by pretreatment, they were greatly prolonged. Serum triglyceride levels in protected animals were normal whereas they were considerably depressed in the nonprotected animals. Liver triglycerides were elevated in both protected and nonprotected, but were not as high in the protected animals. Carbon tetrachloride conversion to CHCl3 was slower in protected than in the non-protected animals. Tolerance to CCl4 developed in animals whose ability to metabolize CCl4 had been depressed by pretreatment with the same compound. This suggests that the toxicity of CCl4 is related to its metabolic pathway or to the production of toxic intermediates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32734/1/0000103.pd

Reanalysis of Rate Data for the Reaction CH₃ + CH₃ → C₂H₆ Using Revised Cross Sections and a Linearized Second-Order Master Equation

Rate coefficients for the CH3 + CH3 reaction, over the temperature range 300-900 K, have been corrected for errors in the absorption coefficients used in the original publication ( Slagle et al., J. Phys. Chem. 1988 , 92 , 2455 - 2462 ). These corrections necessitated the development of a detailed model of the B̃(2)A1' (3s)-X̃(2)A2″ transition in CH3 and its validation against both low temperature and high temperature experimental absorption cross sections. A master equation (ME) model was developed, using a local linearization of the second-order decay, which allows the use of standard matrix diagonalization methods for the determination of the rate coefficients for CH3 + CH3. The ME model utilized inverse Laplace transformation to link the microcanonical rate constants for dissociation of C2H6 to the limiting high pressure rate coefficient for association, k∞(T); it was used to fit the experimental rate coefficients using the Levenberg-Marquardt algorithm to minimize χ(2) calculated from the differences between experimental and calculated rate coefficients. Parameters for both k∞(T) and for energy transfer ⟨ΔE⟩down(T) were varied and optimized in the fitting procedure. A wide range of experimental data were fitted, covering the temperature range 300-2000 K. A high pressure limit of k∞(T) = 5.76 × 10(-11)(T/298 K)(-0.34) cm(3) molecule(-1) s(-1) was obtained, which agrees well with the best available theoretical expression

Direct measurements of OH and other product yields from the HO2 + CH3C(O)O2 reaction

The reaction CH3C(O)O2 +HO2 → CH3C(O)OOH+O2 (Reaction R5a), CH3C(O)OH + O3 (Reaction R5b), CH3 + CO2 + OH + O2 (Reaction R5c) was studied in a series of experiments conducted at 1000 mbar and (293±2) K in the HIRAC simulation chamber. For the first time, products, (CH3C(O)OOH, CH3C(O)OH, O3 and OH) from all three branching pathways of the reaction have been detected directly and simultaneously. Measurements of radical precursors (CH3OH, CH3CHO), HO2 and some secondary products HCHO and HCOOH further constrained the system. Fitting a comprehensive model to the experimental data, obtained over a range of conditions, determined the branching ratios α(R5a) = 0.37±0.10, α(R5b) = 0.12±0.04 and α(R5c) = 0.51±0.12 (errors at 2σ level). Improved measurement/model agreement was achieved using k(R5) = (2.4±0.4)×10−11 cm3 molecule−1 s−1, which is within the large uncertainty of the current IUPAC and JPL recommended rate coefficients for the title reaction. The rate coefficient and branching ratios are in good agreement with a recent study performed by Groß et al. (2014b); taken together, these two studies show that the rate of OH regeneration through Reaction (R5) is more rapid than previously thought. GEOS-Chem has been used to assess the implications of the revised rate coefficients and branching ratios; the modelling shows an enhancement of up to 5 % in OH concentrations in tropical rainforest areas and increases of up to 10 % at altitudes of 6–8 km above the equator, compared to calculations based on the IUPAC recommended rate coefficient and yield. The enhanced rate of acetylperoxy consumption significantly reduces PAN in remote regions (up to 30 %) with commensurate reductions in background NOx

The Essential Role for Laboratory Studies in Atmospheric Chemistry

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines