Kinetic study of the reactions iodine monoxide + hydroperoxo and iodine monoxide + nitrogen dioxide at 298 K

Products of Criegee intermediate reactions with NO2: experimental measurements and tropospheric implications

The reactions of Criegee intermediates with NO₂ have been proposed as a potentially significant source of the important nighttime oxidant NO₃, particularly in urban environments where concentrations of ozone, alkenes and NO_x are high. However, previous efforts to characterize the yield of NO₃ from these reactions have been inconclusive, with many studies failing to detect NO₃. In the present work, the reactions of formaldehyde oxide (CH₂OO) and acetaldehyde oxide (CH₃CHOO) with NO₂ are revisited to further explore the product formation over a pressure range of 4–40 Torr. NO₃ is not observed; however, temporally resolved and [NO₂]-dependent signal is observed at the mass of the Criegee–NO₂ adduct for both formaldehyde- and acetaldehyde-oxide systems, and the structure of this adduct is explored through ab initio calculations. The atmospheric implications of the title reaction are investigated through global modelling.

Ab Initio Characterization of (CH3IO3) Isomers and the CH3O2 + IO Reaction Pathways

The geometries, harmonic vibrational frequencies, relative energetics, and enthalpies of formation of (CH(3)IO(3)) isomers and the reaction CH(3)O(2) + IO have been investigated using quantum mechanical methods. Optimization has been performed at the MP2 level of theory, using all electron and effective core potential, ECP, computational techniques. The relative energetics has been studied by single-point calculations at the CCSD(T) level. Methyl iodate, CH(3)OIO(2), is found to be the lowest-energy isomer showing particular stabilization. The two nascent association minima, CH(3)OOOI and CH(3)OOIO, show similar stabilities, and they are considerably higher located than CH(3)OIO(2). Interisomerization barriers have been determined, along with the transition states involved in various pathways of the reaction CH(3)O(2) + IO.

Determination of the Rate Coefficients for the Reactions IO + NO2 + M (Air) → IONO2 + M and O(3P) + NO2 → O2 + NO Using Laser-Induced Fluorescence Spectroscopy

Laser-induced fluorescence spectroscopy via excitation of the A2pi(3/2) <-- X2pi(3/2) (2,0) band at 445 nm was used to monitor IO in the presence of NO2 following its generation in the reactions O(3P) + CF3I and O(3P) + I2. Both photolysis of O3 (248 nm) and NO2 (351 nm) were used to initiate the production of IO. The rate coefficients for the thermolecular reaction IO + NO2 + M --> IONO2 + M were measured in air, N2, and O2 over the range P = 18-760 Torr, covering typical tropospheric conditions, and were found to be in the falloff region. No dependence of k1 upon bath gas identity was observed, and in general, the results are in good agreement with recent determinations. Using a Troe broadening factor of F(B) = 0.4, the falloff parameters k0(1) = (9.5 +/- 1.6) x 10(-31) cm6 molecule(-2) s(-1) and k(infinity)(1) = (1.7 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) were determined at 294 K. The temporal profile of IO at elevated temperatures was used to investigate the thermal stability of the product, IONO2, but no evidence was observed for the regeneration of IO, consistent with recent calculations for the IO-NO2 bond strength being approximately 100 kJ mol(-1). Previous modeling studies of iodine chemistry in the marine boundary layer that utilize values of k1 measured in N2 are hence validated by these results conducted in air. The rate coefficient for the reaction O(3P) + NO2 --> O2 + NO at 294 K and in 100 Torr of air was determined to be k2 = (9.3 +/- 0.9) x 10(-12) cm3 molecule(-1) s(-1), in good agreement with recommended values. All uncertainties are quoted at the 95% confidence limit.

Evaluating Data for Atmospheric Models, an Example: IO + NO2 = IONO2

Data for the title reaction have been fit to the different formalisms used by the NASA and IUPAC data evaluation panels. The data are well represented by either formalism. Reported values for the bond dissociation energy at 0 K, D0(IO-NO2) vary from about 95 to 135 kJ mol(-1), with uncertainty ranges of about 20 kJ mol(-1). Master equation/RRKM methods were employed in an attempt to reconcile these values with the data. This was possible within reasonable bounds and suggests a value in the neighborhood of 150 kJ mol(-1). As always, there are sufficient assumptions and unknowns in such an attempt, that this value is somewhat uncertain, but the true value is not expected to be too far from this result. Thus, it is possible to evaluate data of the type addressed here in a manner reasonably consistent with the basic understanding of pressure dependent rate coefficients for use in atmospheric or other models of "engineering" problems. There are, however, strict limits on our ability to know specific details. It is possible that true anharmonicity corrections that include stretch-bend interactions as well as effects due to averaging rotational contributions could combine to lower this value by as much as 10 kJ mol(-1). In addition collision and energy transfer parameters are somewhat uncertain.

Computational Studies of (HIO3) Isomers and the HO2 + IO Reaction Pathways

The geometries, harmonic vibrational frequencies, relative energetics, and enthalpies of formation of (HIO3) isomers have been examined using quantum mechanical methods. At all levels of theory employed, MP2, B3LYP, and CCSD(T), the lowest energy structure is found to be the HOIO2 form, which shows particular stability. The two isomers HOOOI and HOOIO are closely located at the CCSD(T) level of theory. The higher energy structure is HIO3. The interisomerization transition states have been determined, along with the transition states involved in the various pathways of the reaction HO2 + IO.

The role of halogen species in the troposphere

While the role of reactive halogen species (e.g. Cl, Br) in the destruction of the stratospheric ozone layer is well known, their role in the troposphere was investigated only since their destructive effect on boundary layer ozone after polar sunrise became obvious. During these 'Polar Tropospheric Ozone Hole' events O(3) is completely destroyed in the lowest approximately 1000 m of the atmosphere on areas of several million square kilometres. Up to now it was assumed that these events were confined to the polar regions during springtime. However, during the last few years significant amounts of BrO and Cl-atoms were also found outside the Arctic and Antarctic boundary layer. Recently even higher BrO mixing ratios (up to 176 ppt) were detected by optical absorption spectroscopy (DOAS) in the Dead Sea basin during summer. In addition, evidence is accumulating that BrO (at levels around 1-2 ppt) is also occurring in the free troposphere at all latitudes. In contrast to the stratosphere, where halogens are released from species, which are very long lived in the troposphere, likely sources of boundary layer Br and Cl are autocatalytic oxidation of sea salt halides (the 'Bromine Explosion'), while precursors of free tropospheric BrO and coastal IO probably are short-lived organo-halogen species. At the levels suggested by the available measurements reactive halogen species have a profound effect on tropospheric chemistry: In the polar boundary layer during 'halogen events' ozone is usually completely lost within hours or days. In the free troposphere the effective O(3)-losses due to halogens could be comparable to the known photochemical O(3) destruction. Further interesting consequences include the increase of OH levels and (at low NO(X)) the decrease of the HO(2)/OH ratio in the free troposphere.