3.8: Biogeochemical reactions in the stratosphere
Ozone is present in the earth's atmosphere at all altitudes from the surface up to at least \(100 \mathrm{~km}\). The bulk of the ozone resides in the stratosphere with a maximum ozone concentration of \(5 \times 10^{12}\) molecule \(\mathrm{cm}^{-3}\) at about \(25 \mathrm{~km}\). In the mesosphere \((>60 \mathrm{~km}) \mathrm{O}_3\) densities are quite low and are not discussed in the present report. Although \(\mathrm{O}_3\) concentrations in the troposphere are also less than in the stratosphere, ozone plays a vital role in the atmospheric chemistry in this region and also affects the thermal radiation balance in the lower atmosphere.
Atmospheric ozone is formed by combination of atomic and molecular oxygen.
\[
\mathrm{O}+\mathrm{O}_2+\mathrm{M} \rightarrow \mathrm{O}_3+\mathrm{M}
\]
where \(\mathrm{M}\) is a third body required to carry away the energy released in the combination reaction. At altitudes above approximately \(20 \mathrm{~km}\) production of \(\mathrm{O}\) atoms results almost exclusively from photodissociation of molecular \(\mathrm{O}_2\) by short wavelength ultraviolet radiation \((\lambda<243 \mathrm{~nm})\) :
\[
\mathrm{O}_2+\mathrm{h} \nu \rightarrow \mathrm{O}+\mathrm{O}
\]
At lower altitudes and particularly in the troposphere, \(\mathrm{O}\) atom formation from the photodissociation of nitrogen dioxide by long wavelength ultraviolet radiation is more important:
\[
\mathrm{NO}_2+\mathrm{h} \nu \rightarrow \mathrm{NO}+\mathrm{O}
\]
Ozone itself is photodissociated by both UV and visible light:
\[
\mathrm{O}_3+\mathrm{h} \nu \rightarrow \mathrm{O}_2+\mathrm{O}
\]
but this reaction together with the combination reaction (1) only serves to partition the 'odd oxygen' species between \(\mathrm{O}_{\text {and }} \mathrm{O}_3\). The production processes (2) and (3) are balanced by chemical and physical loss processes. Until the \(1950 \mathrm{~s}\), chemical loss of odd oxygen was attributed only to the reaction:
\[
\mathrm{O}+\mathrm{O}_3 \rightarrow \mathrm{O}_2+\mathrm{O}_2
\]
originally proposed by S. Chapman (1930). It is now known that ozone in the stratosphere is removed predominantly by catalytic cycles involving homogeneous gas phase reactions of active free radical species in the \(\mathrm{HO}_{\mathrm{x}}, \mathrm{NO}_{\mathrm{x}}, \mathrm{ClO}_{\mathrm{x}}\) and \(\mathrm{BrO}_{\mathrm{x}}\) families:
\[
\begin{array}{l}
\mathrm{X}+\mathrm{O}_3 \rightarrow \mathrm{XO}+\mathrm{O}_2 \\
\mathrm{XO}+\mathrm{O} \rightarrow \mathrm{X}+\mathrm{O}_2 \\
\hline \text { net: } \quad \mathrm{O}+\mathrm{O}_3 \rightarrow 2 \mathrm{O}_2
\end{array}
\]
where the catalyst \(\mathrm{X}=\mathrm{H}, \mathrm{OH}, \mathrm{NO}, \mathrm{Cl}\) and \(\mathrm{Br}\). Thus these species can, with varying degrees of efficiency, control the abundance and distribution of ozone in the stratosphere. Assignment of the relative importance and the prediction of the future impact of these catalytic species is dependent on a detailed understanding of the chemical reactions which form, remove and interconvert the active components of each family.
Physical loss of ozone from the stratosphere is mainly by dynamical transport to the troposphere where further photochemically driven sources and sinks modify the ozone concentration field. Ozone is destroyed at the surface of the earth and so there is an overall downward flux in the lower part of the atmosphere. Physical removal of ozone and other trace gaseous components can also occur in the precipitation elements and on the surface of atmospheric aerosols. Since most of the precursor and sink molecules for the species catalytically active in ozone removal in the stratosphere are derived from or removed in the troposphere, global tropospheric chemistry is a significant feature of overall atmospheric ozone behavior.
\(\mathrm{HO}_{\mathrm{x}}\) Chemistry
There have been relatively few changes recently in the kinetics data base for \(\mathrm{HO}_{\mathrm{x}}\). The principal catalytic cycle for odd-oxygen destruction within the \(\mathrm{HO}_{\mathrm{x}}\) family is:
(I)
\[
\text { net: } \quad \frac{\mathrm{O}+\mathrm{HO}_2 \rightarrow \mathrm{OH}+\mathrm{O}_2}{\mathrm{O}+\mathrm{O}_3 \rightarrow 2 \mathrm{O}_2}
\]
\[
\begin{array}{l}
\mathrm{OH}+\mathrm{O}_3 \rightarrow \mathrm{HO}_2+\mathrm{O}_2 \\
\frac{\mathrm{O}+\mathrm{HO}_2 \rightarrow \mathrm{OH}+\mathrm{O}_2}{\mathrm{O}+\mathrm{O}_3 \rightarrow 2 \mathrm{O}_2}
\end{array}
\]
Depending on altitude the following cycles may also become important:
(II)
\[
\begin{array}{l}
\mathrm{HO}_2+\mathrm{O}_3 \rightarrow \mathrm{OH}+2 \mathrm{O}_2 \\
\text { net: } \\
\text { III } \\
2 \mathrm{O}_3 \rightarrow 3 \mathrm{O}_2 \\
\mathrm{O}+\mathrm{OH} \rightarrow \mathrm{H}+\mathrm{O}_2 \\
\mathrm{H}+\mathrm{O}_2+\mathrm{M} \rightarrow \mathrm{HO}_2+\mathrm{M} \\
\mathrm{O}+\mathrm{HO}_2 \rightarrow \mathrm{OH}+\mathrm{O}_2 \\
\text { net: } \\
2 \mathrm{O} \rightarrow \mathrm{O}_2 \\
\end{array}
\]
2.1.3 \(\mathrm{NO}_{\mathrm{x}}\) Chemistry
Odd nitrogen species are important in the stratosphere because they are involved in catalytic cycles which directly destroy \(\mathrm{O}_3\),
\[
\begin{array}{l}
\mathrm{NO}+\mathrm{O}_3 \rightarrow \mathrm{NO}_2+\mathrm{O}_2 \\
\mathrm{NO}_2+\mathrm{O} \rightarrow \mathrm{NO}+\mathrm{O}_2 \\
\hline \mathrm{O}+\mathrm{O}_3 \rightarrow 2 \mathrm{O}_2
\end{array}
\]
and
\[
\begin{array}{l}
\mathrm{NO}+\mathrm{O}_3 \rightarrow \mathrm{NO}_2+\mathrm{O}_2 \\
\mathrm{NO}_2+\mathrm{O}_3 \rightarrow \mathrm{NO}_3+\mathrm{O}_2 \\
\text { net: } \mathrm{NO}_3+\mathrm{h} \nu \rightarrow \mathrm{NO}+\mathrm{O}_2 \\
\hline \mathrm{O}_3 \rightarrow 3 \mathrm{O}_2
\end{array}
\]
32
STRATOSPHERIC CHEMISTRY
The first of these two cycles is much more important than the second. Even though all the above reactions have been studied in the laboratory, there exist some uncertainties in the values for the rate coefficient for reaction (22) at stratospheric temperatures and the quantum yield for \(\mathrm{NO}_{\text {in }} \mathrm{NO}_3\) photolysis. In addition to their involvement in direct \(\mathrm{O}_3\) destruction, \(\mathrm{NO}_{\mathrm{x}}\) species play crucial roles in the partitioning of odd hydrogen and odd chlorine into various forms. The rates of conversion of \(\mathrm{HO}_2\) to \(\mathrm{OH}\) and \(\mathrm{ClO}\) to \(\mathrm{Cl}\) are determined by the reactions involving \(\mathrm{NO}\),
\[
\begin{array}{c}
\mathrm{HO}_2+\mathrm{NO} \rightarrow \mathrm{OH}+\mathrm{NO}_2 \\
\mathrm{ClO}+\mathrm{NO} \rightarrow \mathrm{Cl}+\mathrm{NO}_2
\end{array}
\]
and those involving \(\mathrm{O}\left({ }^3 \mathrm{P}\right)\), i.e., \(\mathrm{O}+\mathrm{HO}_2 \rightarrow \mathrm{OH}+\mathrm{O}_2\) and \(\mathrm{O}+\mathrm{ClO} \rightarrow \mathrm{Cl}+\mathrm{O}_2\). Thus, these reactions in conjunction with reactions of \(\mathrm{OH}\) and \(\mathrm{Cl}\) with \(\mathrm{O}_3\), control the ratios \(\left[\mathrm{HO}_2\right] /[\mathrm{OH}]\) and \([\mathrm{ClO}] /[\mathrm{Cl}]\). Both reactions 26 and 27 are well characterized. \(\mathrm{NO}_{\mathrm{x}}\) species are also involved in sequestering \(\mathrm{HO}_{\mathrm{x}}\) species in temporary reservoirs e.g.:
and those involving \(\mathrm{O}\left({ }^3 \mathrm{P}\right)\), i.e., \(\mathrm{O}+\mathrm{HO}_2 \rightarrow \mathrm{OH}+\mathrm{O}_2\) and \(\mathrm{O}+\mathrm{ClO} \rightarrow \mathrm{Cl}+\mathrm{O}_2\). Thus, these reactions in conjunction with reactions of \(\mathrm{OH}\) and \(\mathrm{Cl}\) with \(\mathrm{O}_3\), control the ratios \(\left[\mathrm{HO}_2\right] /[\mathrm{OH}]\) and \([\mathrm{ClO}] /[\mathrm{Cl}]\). Both reactions 26 and 27 are well characterized. \(\mathrm{NO}_{\mathrm{x}}\) species are also involved in sequestering \(\mathrm{HO}_{\mathrm{x}}\) species in temporary reservoirs e.g.:
\[
\begin{array}{l}
\mathrm{OH}+\mathrm{NO}_2 \stackrel{\mathrm{M}}{=} \mathrm{HNO}_3 \\
\mathrm{HO}_2+\mathrm{NO}_2 \stackrel{\mathrm{M}}{\rightarrow} \mathrm{HO}_2 \mathrm{NO}_2 \\
\end{array}
\]
The above processes have been thoroughly investigated and their rate coefficients are quite well established. The photolysis of \(\mathrm{NO}_2\), reaction (3), serves as the major source of odd oxygen in the troposphere. The absorption cross section for \(\mathrm{NO}_2\) and the quantum yield for \(\mathrm{O}\) atom production are still somewhat uncertain, as are their temperature dependences.
In addition to the above mentioned reactions, the majority of reactions involving \(\mathrm{NO}_{\mathrm{x}}\) that are important in understanding stratospheric chemistry are well characterized. In the following section, we will discuss only the problem areas and areas where significant new data have been recently reported.
\(\mathrm{N}_2 \mathrm{O}\) is the major source of \(\mathrm{NO}_{\mathrm{x}}\) in the stratosphere. The predominant path for \(\mathrm{N}_2 \mathrm{O}\) destruction is photolysis. Its reaction with \(\mathrm{O}\left({ }^1 \mathrm{D}\right)\) contributes only \(2 \%\) to \(\mathrm{N}_2 \mathrm{O}\) destruction but is currently assumed to be the main \(\mathrm{NO}_{\mathrm{x}}\) production mechanism. Therefore, the possibility of \(\mathrm{N}_2 \mathrm{O}\) photolysis to give \(\mathrm{NO}+\mathrm{N}\) needs to be very carefully assessed. Even if such a pathway constitutes only \(1 \%\) of the total \(\mathrm{N}_2 \mathrm{O}\) photolysis rate, it could be equal to the \(\mathrm{O}\left({ }^1 \mathrm{D}\right)+\mathrm{N}_2 \mathrm{O}\) source [for each \(\mathrm{N}_2 \mathrm{O}\) photolyzed to give \(\mathrm{NO}+\mathrm{N}\), one more molecule of \(\mathrm{NO}\) is produced due to the reaction of \(\mathrm{N}\) with \(\mathrm{O}_2\) or \(\mathrm{O}_3\) ].
The majority of \(\mathrm{O}\left({ }^1 \mathrm{D}\right)\) produced by \(\mathrm{O}_3\) is physically deactivated to \(\mathrm{O}\left({ }^3 \mathrm{P}\right)\). The thermal rate coefficients for the reaction/deactivation of \(\mathrm{O}\left({ }^1 \mathrm{D}\right)\) by atmospheric gases \(\mathrm{N}_2, \mathrm{O}_2, \mathrm{O}_3, \mathrm{CO}_2, \mathrm{Ar}, \mathrm{N}_2 \mathrm{O}, \mathrm{H}_2 \mathrm{O}\) and \(\mathrm{CH}_4\) are well defined (NASA evaluation). However, the yield of \(\mathrm{NO}\) due to the \(\mathrm{O}\left({ }^1 \mathrm{D}\right)+\mathrm{N}_2 \mathrm{O}\) reaction in the stratosphere is uncertain by as much as \(30 \%\). This uncertainty is partly due to the combined errors in the measured values of all the rate coefficients for \(O\left({ }^1 \mathrm{D}\right)\) removal reactions, and is partly due to the possibility that the branching ratio of \(\mathrm{O}\left({ }^1 \mathrm{D}\right)+\mathrm{N}_2 \mathrm{O}\) reaction to yield \(\mathrm{NO}\) (as opposed to \(\mathrm{N}_2\) and \(\mathrm{O}_2\) ) changes with the kinetic energy of \(O\left({ }^1 \mathrm{D}\right)\). Since \(O\left({ }^1 \mathrm{D}\right)\) produced by ozone photolysis is translationally hot, and since the \(\mathrm{O}\left({ }^1 \mathrm{D}\right)+\mathrm{N}_2\) and \(\mathrm{O}\left({ }^1 \mathrm{D}\right)+\mathrm{O}_2\) quenching rates are temperature dependent, the uncertainty of the atmospheric rate of \(\mathrm{O}\left({ }^1 \mathrm{D}\right)+\mathrm{N}_2 \mathrm{O} \rightarrow 2 \mathrm{NO}\) reaction branch is further enhanced if translationally hot \(O\left({ }^1 \mathrm{D}\right)\) reacts differently than thermal \(O\left({ }^1 \mathrm{D}\right)\). Therefore, experiments designed to measure NO production under stratospheric conditions which do not rely on the accuracy of the individual reaction rates need to be carried out.
Currently, all stratospheric \(\mathrm{N}_2 \mathrm{O}\) is assumed to be produced at the ground level and transported into the stratosphcre. However, local production of \(\mathrm{N}_2 \mathrm{O}\) due to reactions such as \(\mathrm{N}_2\left(A^3 \Sigma\right)+\mathrm{O}_2\) and \(\mathrm{OH}\left(\mathrm{A}^2 \Pi\right)\) \(+\mathrm{N}_2\) cannot be ruled out (Zipf, Prasad 1982). If such reactions occur in the mesosphere they could influence the stratospheric \(\mathrm{NO}_{\mathrm{x}}\) budget by downward transport of \(\mathrm{N}_2 \mathrm{O}\).
The main known process which removes \(\mathrm{NO}_{\mathrm{x}}\) from the stratosphere is transport of long lived species such as \(\mathrm{HNO}_3\), but a small amount of \(\mathrm{NO}_{\mathrm{x}}\) loss occurs through the \(\mathrm{N}+\mathrm{NO}\) and \(\mathrm{N}+\mathrm{NO}_2\) reactions in the upper stratosphere. The latter reaction may produce \(\mathrm{N}_2 \mathrm{O}\) as a major product. Kinetic data for reaction of \(\mathrm{N}\) with \(\mathrm{NO}\) are reasonably well established, but the rate constant for reaction with \(\mathrm{NO}_2\) is only reliable to within a factor of 3 at room temperature and its temperature dependence has not been established.
Removal of odd-hydrogen in the lower stratosphere occurs mainly by the reaction of \(\mathrm{OH}\) with nitric acid and peroxynitric acid:
\[
\begin{array}{c}
\mathrm{OH}+\mathrm{HNO}_3 \rightarrow \mathrm{H}_2 \mathrm{O}+\mathrm{NO}_3 \\
\mathrm{OH}+\mathrm{HO}_2 \mathrm{NO}_2 \rightarrow \mathrm{H}_2 \mathrm{O}+\mathrm{NO}_2+\mathrm{O}_2
\end{array}
\]
Changes in the recommended rate coefficients for these reactions have previously resulted in significant revisions of the calculated ozone column. The existence of a negative temperature dependence for the \(\mathrm{OH}+\mathrm{HNO}_3\) reaction is now well established and confirmation of the small pressure dependence may help explain some of the divergence between results of the kinetics studies in different laboratory systems (NASA, 1985). The equally important \(\mathrm{OH}+\mathrm{HO}_2 \mathrm{NO}_2\) reaction is not as well characterized, either with regard to the temperature dependence or the reaction products. New data have been reported recently for the temperature and pressure dependence of the \(\mathrm{HO}_2 \mathrm{NO}_2\) formation reaction (Sander and Peterson, 1984):
\[
\mathrm{HO}_2+\mathrm{NO}_2+\mathrm{M} \rightarrow \mathrm{HO}_2 \mathrm{NO}_2+\mathrm{M}
\]
The rate constant for stratospheric conditions is about \(40 \%\) lower than previously recommended. The products and temperature dependence of the photodissociation of \(\mathrm{HO}_2 \mathrm{NO}_2\) are still not established and the equilibrium constant for \(\mathrm{HO}_2 \mathrm{NO}_2\) formation is not reliably known. These gaps in the data base lead to some uncertainty in the description of peroxynitric acid behavior in the lower stratosphere and the troposphere.
The possibility of formation of an isomer of nitric acid in the recombination reaction of \(\mathrm{OH}\) with \(\mathrm{NO}_2\) reaction (28) has also been considered. Such an isomer, if more reactive than \(\mathrm{HONO}_2\), would serve to reduce the effective rate of nitric acid formation. To date no firm evidence has been found for a complication of this kind in the kinetics of the \(\mathrm{OH}+\mathrm{NO}_2\) reaction. \(\mathrm{NO}_2\) and \(\mathrm{NO}\) have been made. The reliability of the data base for these reactions is now greatly improved.
\[
\begin{array}{c}
\mathrm{NO}_3+\mathrm{NO} \rightarrow 2 \mathrm{NO}_2 \\
\mathrm{NO}_3+\mathrm{NO}_2+\mathrm{M} \rightarrow \mathrm{N}_2 \mathrm{O}_5+\mathrm{M}
\end{array}
\]
The equilibrium constant for the formation in the latter reaction of the important temporary reservoir species \(\mathrm{N}_2 \mathrm{O}_5\) has also been measured directly in several studies, but there remains some uncertainty in this quantity.
2.1.4 \(\mathrm{ClO}_{\mathrm{x}}\) Chemistry
The principal odd oxygen destruction cycle involving \(\mathrm{ClO}_{\mathrm{x}}\) is:
\[
\begin{array}{ll}
& \mathrm{Cl}+\mathrm{O}_3 \rightarrow \mathrm{ClO}+\mathrm{O}_2 \\
& \mathrm{O}+\mathrm{ClO} \rightarrow \mathrm{Cl}+\mathrm{O}_2 \\
\hline \mathrm{O}+\mathrm{O}_3 \rightarrow 2 \mathrm{O}_2
\end{array}
\]
However, in large parts of the stratosphere, the conversion of \(\mathrm{ClO}\) to \(\mathrm{Cl}\) occurs mainly by coupling with \(\mathrm{NO}_x\) :
\[
\mathrm{NO}+\mathrm{ClO} \rightarrow \mathrm{NO}_2+\mathrm{Cl}
\]
In this case the sequence: reaction (34) followed by reaction (27) does not destroy odd oxygen, because \(\mathrm{NO}_2\) is rapidly photolyzed, reaction (3).
The main sink of active chlorine species is the reaction
\[
\mathrm{Cl}+\mathrm{CH}_4 \rightarrow \mathrm{HCl}+\mathrm{CH}_3
\]
Excerpted from
V.A. Mohnen, W. Chameides K.L. Demerjian D. H. Lenschow J.A. Logan R.J. McNeal, S.A. Penkett, U. Platt, U. Schurath, P. da Silva Dias. Tropospheric Chemistry, in Atmospheric Ozone 1985: Assessment of Our Understanding of the Processes Controlling Its Present Distribution and Change, World Meteorological Organization. Accessed November 2023 at https://csl.noaa.gov/assessments/ozone/1985/report.html