Environmental Perturbations that Cause Structural Changes in the SNARE Protein SNAP-25

Digitalitzat per Artypla

Kinetics and spectroscopy of atmospherically important molecules and their degradation products

The present research will explore the impact that both natural and anthropogenic activity has on both the upper and lower atmosphere. The impact on the upper atmosphere, specifically the ozone layer, from the introduction of chlorine and bromine containing compounds is a well characterized event that leads to the removal of ozone, and production of BrO radicals and O2. Few mechanisms have been proposed for converting the BrO radical into ozone scavenging Br atoms. This work proposes two mechanisms and their accompanying kinetics that would convert BrO radicals into photolytically labile BrOH. The proposed mechanism and accompanying kinetics also help to explain the correlation that is observed between ambient ozone and formaldehyde concentrations in the Arctic. Due to the destructive nature of bromine to the ozone layer, the sources of bromine atoms in the stratosphere has received considerable experimental and theoretical attention. Missing from previous studies is an evaluation of the degradation mechanism of CHBr3 and the subsequent products from the decay of CHBr3 under atmospheric conditions. The kinetics for the decay of CHBr3 under atmospheric conditions as well as the spectroscopy and photochemistry of the reaction products will be reported in this work. In addition, the kinetics and spectroscopy of the degradation products of methyl formate will be presented in this work. Recommendations for further research as it pertains to the impact of hydrocarbon release in the troposphere as well as their impact on the stratosphere will be presented in the final sections of this thesis

Local and Regional Contributions to Tropospheric Ozone Concentrations

The Wasatch Front in Utah, USA is currently a non-attainment area for ozone according to the Environmental Protection Agency’s (EPA) National Ambient Air Quality Standards (NAAQS). Nitrogen oxides (NOx = NO2 + NO) and volatile organic compounds (VOCs) in the presence of sunlight lead to ozone formation in the troposphere. When the rate of oxidant production, defined as the sum of O3 and NO2, is faster than the rate of NOx production, a region is said to be NOx-limited and ozone formation will be limited by the concentration of NOx species in the region. The inverse of this situation makes the region VOC-limited. Knowing if a region is NOx-limited or VOC-limited can aid in generating effective mitigation strategies. Understanding the background or regional contributions to ozone in a region, whether it be from the transport of precursors or of ozone, provides information about the lower limit for ozone concentrations that a region can obtain with regulation of local precursors. In this paper, measured oxidant and NOx concentrations are analyzed from 14 counties in the state of Utah to calculate the regional and local contributions to ozone for each region. This analysis is used to determine the nature of the atmosphere in each county by determining if the region is VOC- or NOx-limited. Furthermore, this analysis is performed for each county for the years 2012 and 2022 to determine if there has been a change in the oxidative nature and quantify the regional and local contributions to ozone over a 10-year period. All studied counties—except for Washington County—in Utah were found to be VOC-limited in 2012. This shifted in 2022 to most counties being either in a transitional state or being NOx-limited. Local contributions to ozone increased in two major counties, Cache and Salt Lake Counties, but decreased in Carbon, Davis, Duchesne, Uinta, Utah, Washington, and Weber Counties. Generally, the regional contributions to oxidant concentrations decreased across the state. A summertime spike in both regional and local contributions to oxidants was seen. Smoke from wildfires was seen to increase the regional contributions to oxidants and shift the local regime to be more NOx-limited

Detection of Sulfur Dioxide by Broadband Cavity-Enhanced Absorption Spectroscopy (BBCEAS)

Sulfur dioxide (SO2) is an important precursor for the formation of atmospheric sulfate aerosol and acid rain. We present an instrument using Broadband Cavity-Enhanced Absorption Spectroscopy (BBCEAS) for the measurement of SO2 with a minimum limit of detection of 0.75 ppbv (3-σ) using the spectral range 305.5–312 nm and an averaging time of 5 min. The instrument consists of high-reflectivity mirrors (0.9985 at 310 nm) and a deep UV light source (Light Emitting Diode). The effective absorption path length of the instrument is 610 m with a 0.966 m base length. Published reference absorption cross sections were used to fit and retrieve the SO2 concentrations and were compared to fluorescence standard measurements for SO2. The comparison was well correlated, R2 = 0.9998 with a correlation slope of 1.04. Interferences for fluorescence measurements were tested and the BBCEAS showed no interference, while ambient measurements responded similarly to standard measurement techniques

Temperature‐Dependent Rate Coefficients and Theoretical Calculations for the OH+Cl 2 O Reaction

International audienc

Experimental and ab Initio Study of the HO_2·CH_3OH Complex: Thermodynamics and Kinetics of Formation

Near-infrared spectroscopy was used to monitor HO_2 formed by pulsed laser photolysis of Cl_2−O_2−CH_3OH−N_2 mixtures. On the microsecond time scale, [HO_2] exhibited a time dependence consistent with a mechanism in which [HO_2] approached equilibrium via HO_2 + HO_3OH^M⇆_M HO_2·CH_3OH (3, −3). The equilibrium constant for reaction 3, Kp, was measured between 231 and 261 K at 50 and 100 Torr, leading to standard reaction enthalpy and entropy values (1 σ) of Δ_rH°_(246K) = −37.4 ± 4.8 kJ mol^(-1) and Δ_rS°_(246K) = −100 ± 19 J mol^(-1) K^(-1). The effective bimolecular rate constant, k_3, for formation of the HO_2·CH_3OH complex is 2.8^(+7.5)_(-2.0)·10^(-15)·exp[(1800 ± 500)/T] cm^3 molecule^(-1) s^(-1) at 100 Torr (1 σ). Ab initio calculations of the optimized structure and energetics of the HO_2·CH_3OH complex were performed at the CCSD(T)/6-311++G(3df,3pd)//MP2(full)/6-311++G(2df,2pd) level. The complex was found to have a strong hydrogen bond (D_e = 43.9 kJ mol^(-1)) with the hydrogen in HO_2 binding to the oxygen in CH_3OH. The calculated enthalpy for association is Δ_rH°_(245K) = −36.8 kJ mol^(-1). The potentials for the torsion about the O_2−H bond and for the hydrogen-bond stretch were computed and 1D vibrational levels determined. After explicitly accounting for these degrees of freedom, the calculated Third Law entropy of association is Δ_rS°_(245K) = −106 J mol^(-1) K^(-1). Both the calculated enthalpy and entropy of association are in reasonably good agreement with experiment. When combined with results from our previous study (Christensen et al. Geophys. Res. Lett. 2002, 29; doi:10.1029/2001GL014525), the rate coefficient for the reaction of HO_2 with the complex, HO_2 + HO_2·CH_3-OH, is determined to be (2.1 ± 0.7) × 10^(-11) cm^3 molecule^(-1) s^(-1). The results of the present work argue for a reinterpretation of the recent measurement of the HO_2 self-reaction rate constant by Stone and Rowley (Phys. Chem. Chem. Phys. 2005, 7, 2156). Significant complex concentrations are present at the high methanol concentrations used in that work and lead to a nonlinear methanol dependence of the apparent rate constant. This nonlinearity introduces substantial uncertainty in the extrapolation to zero methanol