ATMOSPHERIC AND IONOSPHERIC RESPONSE TO TRACE GAS PERTURBATIONS THROUGH THE ICE-AGE TO THE NEXT CENTURY IN THE MIDDLE ATMOSPHERE .2. IONIZATION

A global two-dimensional meridional ion composition model of the middl e 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 h eights. Changes in the neutral parameters for this study are taken fro m 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 do ubled 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 atmos phere. The fall-off height of the fractional abundance of water cluste r 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 sS km; above this height it starts to decrease sharply wi th height. When a scenario with doubled CO2, with CH4 and business-as- usual (BAU) (for CFCs and N2O) is considered in the stratospheric regi on, 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. (C) 1997 Elsevier Science Ltd.