There is formation of ${\mathrm{H}}_2{\mathrm{O}}_2$ in the upper atmosphere 

$\begin{array}{l}{\mathrm{H}}_2\mathrm{O}+\mathrm{O}\to 2\mathrm{OH}\to {\mathrm{H}}_2{\mathrm{O}}_2\\ \mathrm{\Delta H}=72{\text{ kJ mol}}^{-1}{\mathrm{E}}_{\mathrm{a}}=77{\text{ kJ mol}}^{-1}\end{array}$

${\mathrm{E}}_{\mathrm{a}}$ for the bimolecular recombination of two OH radicals to form ${\mathrm{H}}_2\mathrm{O}$ and O is :