Tertiary ozone maximum

Das tertiäre Ozonmaximum (Beschreibung z.Zt. nur in englisch verfügbar)

The spatio-temporal behavior of the ozone-mixing ratio in the upper mesosphere/mesopause region under nearly polar night conditions is not completely understood or reproduced by models thus far. Based on our sophisticated 3D-model of the dynamics and chemistry of the middle atmosphere particularly designed to investigate the extended mesopause region, the spatio-temporal structure of this phenomenon is examined and discussed in terms of its chemistry. The most marked feature, reproduced by our model, is a pronounced ozone maximum around 70 km, also called the tertiary ozone maximum, and a strong drop of the mixing ratios around 80 km.  This feature was found by means of ground-based microwave measurements at mean latitude at Lindau (51.66° N) and high latitude at ALOMAR (69.29° N) during the night in the winter season. It was, however, absent during the daytime hours. The calculations by our model bring evidence that the enhanced ozone values occur in a latitudinal band close to the polar night terminator. It is confined both to a height range of approximately 66 to 77 km and to a certain latitudinal range which changes with the season according to the change of the polar night terminator. For the constant latitude, two annual maxima of the ozone mixing ratio occur nearly symmetrically to the winter solstice. The higher the latitude, the more distant these maxima appear. The theoretical analysis of this phenomenon also brought evidence that ozone is formed under nearly grazing incidence conditions of radiation. The radiation dissociating water vapor is absorbed almost completely. This leads to reduction of the formation of hydrogen radicals which destroy the odd oxygen. However, some radiation is still producing atomic oxygen. This atomic oxygen is converting into ozone during the short twilight periods close to the polar night terminator.

Figure (model results) exhibits the altitude-season section of ozone at 67.5° N which is close to the ALOMAR (69.19° N) latitude. The figure displays two relatively isolated regions of ozone enhancement around 72 km. The first one starts in September and reaches its largest values in the beginning of November. The second one finishes at the end of March and its maximum occurs in mid February. The winter solstice values are only slightly enhanced. At the upper part of the figure the so-called second ozone maximum in the mesopause region can be seen but is not permanently present. During the entire year the 80 km domain is characterized by the lowest mixing ratios. At the lower part of the figure, a clear annual variation occurs with maximum values during summer solstice. The tertiary ozone maximum has a height extension of approximately 5 km. The early winter peak (October-November) is located somewhat higher (about 2 km) than the late winter one (January-March).

The impact of the enhanced ozone concentration on the aeronomic state seems to be small because all reactions slow down under condition of small radiation, meaning the response time of the system increases. However, ozone also determines the composition of the hydrogen radicals reducing atomic hydrogen. Perhaps large ozone concentration can cool the air a little locally by infra red emission like CO2.

midnight ozone

Calculated contour plot of the seasonal section of the midnight ozone mixing ratio [ppmv] at 67.5° N for solar minimum condition. Figure shows the maximum around 70 to 71 km. The upper maximum above 80 km during the winter season results from downward transport of atomic oxygen from the thermosphere within the polar vortex.