[en] The oxidation of hydrocarbons in the troposphere under NO_x rich conditions is associated with ozone formation via conversion of NO to NO₂ in reactions with HO₂/RO₂ in the subsequent photolysis of NO₂. The extent of NO/NO₂ conversion is hydrocarbon specific and changes with i.e. chain length in the case of n-alkanes. In the present project a laser spectrometric techniques has been developed which allows the direct determination of NO/NO₂ conversion by measurements of OH and NO₂ growth in the oxidation of individual hydrocarbons. This technique has been applied to all n-alkanes up to n-hexane. It is found that for the simplest alkanes (CH₄, C₂H₆, C₃H₈) ΔNO₂/Δh.c. equals 2.0. For all larger alkanes ΔNO₂/Δh.c. increases. These results provide the basis for the derivation of parameterized, lumped mechanistic expressions for the description of ozone formation in chemical/dynamical modells as well as the delineation of ozone formation potentials of individual hydrocarbons. (orig.). 19 refs

[en] Evidence is presented that rate measurements over a very wide temperature range (300 to 2000⁰K) for some reactions involving the OH radical show distinct non-Arrhenius behavior. An attempt to fit the available data empirically leads to k₁ = 10⁶¹⁹T²¹³ exp(-1233 K/T) cm³/mols and k₂ = 10⁸⁰T¹⁶ exp(-1660⁰K/T) cm³/mols for the reactions with a finite activation barrier around room temperture ((1) OH + CH₄ → CH₃ + H₂O and (2) OH + H₂ → H₂O + H) and k₃ = exp(24.98 + 9.2 x 10^-4T) cm³/mols and k₄ = exp(27.1 + 1.5 x 10^-3T) cm³/mols for reactions without substantial temperature dependence around 300⁰K ((3) OH + CO → CO₂ + H and (4) OH + OH → H₂O + O). The extent of non-Arrhenius behavior may plausibly be explained by means of the total reactive cross section, for which various forms are derived. A more direct explanation, however, is provided from measurements of the vibrational rate enhancement. It is shown that for OH + H₂, vibrational excitation of hydrogen produces sufficient enhancement of the rate constant kappa/sub OH + H₂(ν = 1)//K/sub OH + H₂(ν = 0)// = 1.5 (/sub -0.5/sup +1.0/) 10² at 298⁰K to account for the Arrhenius graph curvature of the overall thermal rate constant. 7 figures, 41 references

[en] The causes of stratospheric ozone depletion are exclusively man-made. Chlorofluorocarbons and, to a smaller extent, halones interfere with the chain mechanisms of photochemical ozone depletion. They thereby lower the concentration of stationary ozone. Contrary to expectations, compensatory effects consisting in enhanced ozone formation in lower layers do not occur. Heterogeneous processes at ice particles during the cold polar night contribute particularly to ozone loss. Reactions at these particles activate chlorine-containing storage compounds in such a way that the light of the rising sun in spring leads to enhanced release of compounds containing radicals which deplete ozone. This phenomenon is well documented for the events during the formation of the ozone hole over the Antarctic, but the winterly north polar region, too, shows similar instances of coupling between meteorological conditioning and chlorine-induced ozone depletion. (orig./EF)

[en] Time and again allegations are made that jet aircraft, which travel at high altitude, destroy the stratospheric ozone layer of our earth and contribute to the greenhouse effect. The current state of knowledge on atmospheric chemistry permits to give a more differentiated judgement of the situation. (orig.)

No abstract available

[en] Hydrogen formation in the reaction of O(¹D) atoms with water has been determined via the branching ratio k₂/k₁, where (1) O(¹D) + H₂O → 20H and (2) O(¹D) + H₂O → H₂ + O₂, by direct measurement of the yields of H₂ and OH in the flash photolysis of O₃/H₂O/He mixtures. k₂/k₁ is found to be 0.01 (+0.005, -0.01) at 298 K

[en] According to present knowledge the causes of depleting the stratospheric ozone layer are exclusively anthropogenic. CFC's and (to smaller extent) halons enter the chain mechanisms of photochemical ozone loss with the result that its steady state concentration is reduced. Ozone depletion in the stratosphere has a number of consequences for the chemistry and the dynamics of the stratosphere. More severe, however, is its potential consequence to humans, animals and plants due to an increase in UV-B intensity near the surface. Current model studies predict, that the observed ozone change in mid-latitudes of the northern hemisphere should have resulted in a change of the UV-B-dose by as much as +7%/decade. (orig./DG)

[en] Chemistry of the stratosphere is largely the chemistry of gas phase free radical species, the existence of which is the result of the thermodynamic non-equilibrium maintained by the presence of sun light. For the stratosphere (15-50 km), the most important photochemical active region is that between 200-300 nm, involving O₂, O₃ and a small number of other source gases as the main photochemical sources of free radicals. The majority of these source gases are of natural origin. However, the addition to the atmosphere of long-lived anthropogenic trace gases such as chlorofluorocarbons, CCl₄, N₂O and halons have produced additional free radicals, the appearance of which modifies the background chemistry of the stratosphere and has led to significant changes in atmospheric composition. The most noteable effects being the decrease in concentration of ozone in the global stratosphere and the dramatic temporal loss of polar stratospheric ozone during the ozone hole event over Antarctica. (orig.)

No abstract available

[en] Emissions from hydrogen fueled aircraft engines include large concentrations of radicals such as NO, OH, O and H. We describe the result of modelling studies in which the evolution of the radical chemistry in an expanding and cooling plume for three different mixing velocities is evaluated. The simulations were made for hydrogen combustion engines at an altitude of 26 km. For the fastest mixing conditions, the radical concentrations decrease only because of dilution with the ambient air, since the time for chemical reaction is too short. With lower mixing velocities, however, larger chemical conversions were determined. For the slowest mixing conditions the unburned hydrogen is converted into water. As a consequence the radicals O and OH increase considerably around 1400 K. The only exception being NO, for which no chemical change during the expansion is found. The concentrations of the reservoir molecules like H₂O₂, N₂O₅ or HNO₃ have been calculated to remain relatively small. (orig.)