A numerical method t o solve the set of differential equations which describes the chemical development in polluted air is presented. The photochemical lifetimes, continuously monitored for all compounds, determine how the integrations are performed at all times. Components with lifetimes less than 10% of the time step, which is taken as 30 sec, are assumed to be in photochemical equilibrium, while compounds with photochemical lifetimes greater than 100 times the time step are computed according to Euler's method. All other components are calculated from the exponential solution of the continuity equation. The computational accuracy may be improved by iteration on components assumed to be determined by the instant values of other components. The convergence of the iteration is speeded up by ordering the short-lived compounds in a hierarchical sequence. Since computational errors connected with QSSA methods are difficult to assess, comparison with an automatic scheme is necessary. Our method has been compared with Gear's method for a range of model mixtures of hydrocarbons, nitrogen oxides, and air, thought to cover most conceivable situations of atmospheric pollution. The agreement with Gear's method is within a few percent for all components all the time, in most cases even within 1%. No accumulation of deviations occur during long-term integrations (e.g., 48 hr with day and night shiftings), and differences which appear during periods with strong concentration gradients (e.g., after sunrise) vanish when the activity has culminated. The method presented here is considerably more efficient than Gear's method, with respect to both computer time and storage.

Very little progress has been made in this century in the determination of the nature of noctilucent clouds. Two theories have been proposed. Most writers assume that the clouds consist of dust particles, either of cosmic or terrestrial origin, but they do not exclude the possibility that noctilucent clouds are ordinary clouds, i.e. that they consist, at least partly, of water substance (ice). The latter point of view has especially been defended in Russian literature.

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A steady state two-dimensional model of the stratospheric ozone layer is presented. Chemical reactions involving oxygen, hydrogen, and nitrogen are considered along with the effect of a parameterized, two-dimensional transport, by mean motion and by eddies. The model is in reasonable agreement with observed ozone data. T h e reduction of ozone from emission of NO, from high flying aircraft is studied, assuming a fleet of 500 supersonic aircraft in transatlantic operation, flying at given altitudes. T h e effect is found to depend critically upon flight level. For mid-latitude, summer, the ozone column density is reduced by 0.3% for a flight level of 18 km. For flight levels of 23 and 28 km the reduction is 2 and 5 % , respectively. Accordingly, the increase in u.v. radiation amounts to approximately 0.6, 4, and 10% for the same flight levels.