Important effects of relative humidity on the formation processes of iodine oxide particles from CH3I photo-oxidation.

In this work, laboratory chamber experiments of gas-phase methyl iodide photolysis in the presence of ozone at three relative humidity conditions were performed to study the formation and physico-chemical properties of iodine oxide particles. The obtained results revealed significant morphological changes of iodine oxide particles that were observed to depend on relative humidity. The formed iodine oxide particles under dry conditions were supposed to be agglomerates of fine hygroscopic crystals. On the other hand, a humid atmosphere was observed to favor the formation of isomeric, tetragonal and orthorhombic hygroscopic crystals potentially composed of HIO3 likely formed from progressive hydration of iodine oxide clusters. This process leads to a release of molecular iodine, I2, which may indicate a potential role of I2O4 in the particles' evolution processes. The obtained results on the iodine oxides' behavior are important to the nuclear power plant safety industry since many of the organic iodides that may be released during a major nuclear power-plant accident contain radioactive isotopes of iodine that are known to have lethal or toxic impacts on human health.

Theoretical chemical ionization rate constants of the concurrent reactions of hydronium ions (H3 O+ ) and oxygen ions (O 2 + ) with selected organic iodides.

Short chain volatile iodinated organic compounds (VIOCs) are of great importance in many fields that include atmospheric chemistry, agriculture, and environmental chemistry related to nuclear power plant safety. Proton-transfer-reaction mass spectrometry (PTR-MS) allows for fast, sensitive, and online quantification of VIOCs if the chemical ionization (CI) reaction rate coefficients are known. In this work, the theoretical CI rate coefficients for the reactions of hydronium ions (H3 O+ ) and oxygen ions (O 2 + ) with selected atmospherically important short chain VIOCs are determined. The neutral CH3 I, CH2 I2 , C2 H5 I, iso-C3 H7 I, n-C3 H7 I, n-C4 H9 I, 2-C4 H9 I, n-C5 H11 I, 2-C5 H11 I, and 3-C5 H11 I have been chosen because these compounds are of atmospheric and environmental importance in the field of safety of nuclear plant reactors. Theoretical ion-molecule collision rate coefficients were determined using the Su and Chesnavich theory based on parametrized trajectory calculations. The proton affinity, ionization energy, dipole moment, and polarizability values of the neutral molecules were determined from density functional theory and coupled-cluster calculations. The newly calculated rate constants facilitate the use of the CI mass spectrometry in the atmospheric quantification of selected VIOCs.

Evaluation of the reaction artifacts in an annular denuder-based sampler resulting from the heterogeneous ozonolysis of naphthalene.

The heterogeneous ozonolysis of naphthalene adsorbed on XAD-4 resin was studied using an annular denuder technique. The experiments involved depositing a known quantity of naphthalene on the XAD-4 resin and then measuring the quantity of the solid naphthalene that reacted away under a constant flow of gaseous ozone (0.064 to 4.9 ppm) for a defined amount of time. All experiments were performed at room temperature (26 to 30 °C) and atmospheric pressure. The kinetic rate coefficient for the ozonolysis reaction of naphthalene adsorbed on XAD-4 resin is reported to be (10.1 ± 0.4) × 10(-19) cm(3) molecule(-1) s(-1) (error is 2σ, precision only). This value is five times greater than the currently recommended literature value for the homogeneous gas phase reaction of naphthalene with ozone. The obtained rate coefficient is used to evaluate reaction artifacts from field concentration measurements of naphthalene, acenaphthene, and phenanthrene. The observed uncertainties associated with field concentration measurements of naphthalene, acenaphthene, and phenanthrene are reported to be much higher than the uncertainties associated with the artifact reactions. Consequently, ozone reaction artifact appears to be negligible compared to the observed field measurement uncertainty results.

Temperature-dependent kinetics study of the reactions of OH with C2H5I, n-C3H7I, and iso-C3H7I.

Flash photolysis (FP) coupled with resonance fluorescence (RF) was used to measure the absolute rate coefficients for the reactions of OH(X(2)Π) radicals with C(2)H(5)I (k(1)), n-C(3)H(7)I (k(2)), and iso-C(3)H(7)I (k(3)) at temperatures between 297 and 372 K in 188 Torr of He; this represents the first temperature-dependent kinetics studies for the title reactions. The experiments involved time-resolved RF detection of the OH (A(2)Σ(+) → X(2)Π transition at λ = 308 nm) radicals following FP of H(2)O/C(2)H(5)I/He, H(2)O/n-C(3)H(7)I/He, and H(2)O/iso-C(3)H(7)I/He mixtures. The OH(X(2)Π) radicals were produced by FP of H(2)O in vacuum-UV at wavelengths λ > 120 nm. Decays of OH radicals in the presence of C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I were observed to be exponential, and the decay rates were found to be linearly dependent on the C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I concentrations. The results are described by the following Arrhenius expressions (units of cm(3) molecule(-1) s(-1)): k(1)(297-372 K) = (5.55 ± 3.20) × 10(-12) exp[-(830 ± 90) K/T], k(2)(300-370 K) = (1.65 ± 0.90) × 10(-11) exp[-(780 ± 90) K/T] and k(3)(299-369 K) = (7.58 ± 3.70) × 10(-12) exp[-(530 ± 80) K/T]. Reported errors in E/R and in the pre-exponential factors are 2σ random errors, returned by the weighted (by 1/σ(2)) least-squares fits to the kinetic data. The implications of the reported kinetic results for understanding both atmospheric and nuclear safety interests of C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I are discussed.

Kinetic and mechanistic study of the reactions of O(1D2) with HCN and CH3CN.

A laser flash photolysis-resonance fluorescence (LFP-RF) technique is employed to investigate the kinetics and mechanism of the reactions of O((1)D(2)) with HCN [reaction (1)] and CH(3)CN [reaction (2)] as a function of temperature over the range 193-430 K. The experiments involve time-resolved RF detection of O((3)P(J)) or H((2)S(1/2)) following LFP of O(3)/X/He mixtures (X=HCN or CH(3)CN), some of which also contain N(2), H(2), and/or NO(2). Measured rate coefficients for total removal of O((1)D(2)) by HCN and CH(3)CN are well-described by the following Arrhenius expressions (units are 10(-10) cm(3) molecule(-1) s(-1)): k(1)(T)=1.08exp(+105/T) and k(2)(T)=2.54exp(-24/T). Temperature-dependent product yields of O((3)P(J)), k(1a)/k(1) and k(2a)/k(2) are well-described by the following Arrhenius-type expressions: k(1a)/k(1)=0.150exp(+200/T) and k(2a)/k(2)=0.0269 exp(+137/T). The H((2)S(1/2)) yield from reaction (2) is found to be 0.16±0.03 independent of temperature (200-423 K). Large 298 K yields of H((2)S(1/2)), 0.68±0.12 produced per O((1)D(2)) destroyed by HCN, are observed for reaction (1). However, observed kinetics suggest that only about half of detected H((2)S(1/2)) is generated as a primary product of the O((1)D(2))+HCN reaction, with the remainder generated via a fast secondary reaction. The implications of the reported kinetic and mechanistic results for understanding the atmospheric chemistry of HCN and CH(3)CN are discussed.