Fig 2 . Energetic for the formation of CI by the reaction of C2H4 with O3. The optimized geometries of the intermediates and transition states were also shown. Bond lengths are in Å.
CI-formation takes place via ozonolysis of C2H4. First step is 1,3-dipolar cycloaddition of ozone to the double bond of ethane leading to primary ozonide or 1st molozonide, 1 . The reaction is found to be exergonic by 75.3 kcal/mol. In 2nd step, the carbonyls (as 1,3-compounds) again undergo 1,3-cycloaddition, leading to the formation of secondary ozonide 2 (more stable than primary ozonide). The 3rd step is the breaking of C-O bonds to generate the Criegee intermediate (carbonyl oxides) and H2CO which involves a barrier of 25.1 kcal/mol. Overall, the formation of CI is exergonic suggesting the high rate of the reaction between C2H4 and O3.
We then turned our attention to investigate all the possible reaction channels of some acids with CI. The choice of the acids namely, hydrochloric acid, formic acid and nitric acid for the reaction with CI is stimulated by their high abundance in atmosphere.
3.3 Reactions between hydrochloric acids and CI : For the co-reactant HCl, two reaction channels are considered one involving the attack of H atom of HCl to the O atom of CI while the other one involves concomitant attack of H of HCl to the terminal O of CI and Cl of HCl to the C atom of CI resulting in the formation of a cyclic transition state TS-2 (Fig 3). The energy profile diagram is shown in Fig 3.