Atmospheric and ionospheric response to trace gas perturbations through the ice age to the next century in the middle atmosphere. Part II - Ionization

Abstract

A global two-dimensional meridional ion composition model of the middle atmosphere is used to examine the effect of changing concentrations of several greenhouse gases on the overall distribution of ionization for this region, along with a steady state calculation for the upper heights. Changes in the neutral parameters for this study are taken from the companion article (Beig and Mitra, (1997). It has been predicted that there are several sensitive signals of man-made perturbations in the middle atmospheric ionization. In the mesospheric region, for a doubled CO₂ scenario, we find that the total ionization density does not change appreciably and the maximum variation is found to be around 15% at about 70 km. However, the distribution of individual ions shows a considerable variation (up to about 100%) throughout the middle atmosphere. The fall-off height of the fractional abundance of water cluster ions is higher for 2050 A.D., suggesting domination of these ions up to greater heights. The concentration of water cluster ions increases below about 85 km; above this height it starts to decrease sharply with height. When a scenario with doubled CO₂, with CH₄ and business-as-usual (BAU) (for CFCs and N₂O) is considered in the stratospheric region, it is found that only one family of negative ions, called NO₃-core ions, is dominant instead of two in the normal case. Simulations are also made through the ages since the last ice age. Results indicate a reverse trend as compared to the above.