Question

1. Under the conditions of low oxygen atom concentration, the radical HOO can react reversibly with NO₂ to produce a molecule of HOONO₂.

1. Deduce why the addition of nitrogen oxides to the lower stratosphere could lead to an increase in the steady state ozone concentration as a consequence of this reaction.
2. Deduce how the addition of nitrogen oxides to the middle and upper stratosphere could decrease the ozone concentration there as a consequence of other reactions.

Answer 1

a. Temperature in the stratosphere varies from -56^o C to -2^o C. At the lower stratosphere more of HOONO₂ is there due to lower temperature. If more NO₂ is inserted at the lower stratosphere, more HOO radical is coupled with NO₂ thereby leaving less HOO free. As HOO is a ozone depleting substance(ODS), removal or capturing of HOO helps increasing ozone concentration.

b. At the middle or higher Stratospheric altitude because of NO and NO₂ a set of catalytic ozone degradation occurs especially at an altitude of 24 km.

                               NO₂ + h$\nu$ (UV) $\rightarrow$ NO + O              

                               O₃ + O $\rightarrow$ 2 O₂

This reaction is responsible for the ozone depletion and the cause of such reaction at a higher altitude is UV irradiation is increasing with the altitude.