Introduction

Life on Earth would not be possible without the electromagnetic energy supplied to us by the Sun or the presence of the thin veil of gas surrounding our planet called the atmosphere. So thin is Earth's atmosphere that if it were compressed to the pressure at the surface, it's height would represent a mere 0.1% of our planet's radius. The atmosphere is composed almost entirely of N₂ and O₂, the former present in such large abundance as a result of emission from volcanic eruptions early in Earth's history and the latter due to the evolution of photosynthetic organisms. Yet it is the trace gases which hold the most interest to atmospheric scientists. Species such as ozone (O₃), NO₂ and OH, for example, which comprise but a tiny fraction of the total number of molecules in the atmosphere, play a disproportionately large role in atmospheric chemistry. As a result of their small abundance, these trace gases, and their impact on the atmosphere as a whole, are very susceptible to perturbations. These perturbations, for the most part the result of anthropogenic emissions, cause depletion of the ozone layer, global warming, acid rain, and an increase in tropospheric ozone.

This is work is relevant to understanding the stratosphere, the atmospheric regime which contains over 90% of global ozone. The stratosphere extends from the tropopause ($\sim 10$ km or 200 mbar) to the stratopause ($\sim 50$ km or 1 mbar). The prefix `strat' means, literally, stratified or layered, which is quite accurate and is a direct result of the temperature structure. Due to heating by ozone, the stratospheric temperature increases with height from a minimum of about 220 K at the tropopause to to a maximum of near 270 K at the stratopause. This temperature inversion (temperature usually decreases with height) enforces a strong vertical stability. Typical vertical motions in the stratosphere are on the order of 0.1 cm/s, much smaller than the 10 m/s horizontal winds (Brasseur and Solomon, 1986). The mixing ratio of ozone and other species in the stratosphere is given in Table 1.

 

Table 1.1: Composition of stratospheric air.

 

Constituent	Mixing Ratio	Note
N2	0.7808	1
O2	0.2095	1
Ar	9.34 $\times 10^{-3}$	1
CO2	3.45 $\times 10^{-4}$	1
CH4	1.6 $\times 10^{-6}$	2
O3	$1\times 10^{-5}$	3
H2O	$6\times 10^{-6}$	4
N2O	$3\times 10^{-7}$	2
NO2	$1\times 10^{-8}$	3
HNO3	$7\times 10^{-9}$	3
OH	$4\times 10^{-10}$	4
BrO	$2\times 10^{-11}$	3
ClO	$1\times 10^{-12}$	5
OClO	$3\times 10^{-13}$	5

1: Constant throughout stratosphere.
2: At tropopause; decreases in stratosphere.
3: Peak mixing ratio in stratosphere.
4: Upper stratosphere.
5: Peak mixing ratio in stratosphere but can be a factor of 10³ larger through heterogeneous processing.

Ozone is one of the most important compounds in the atmosphere as it shields the biosphere from harmful UV-B radiation (280-320 nm). However, over the past ten years, global ozone has been decreasing at a rate of 6%/decade (Solomon, 1997). In particular, each spring over the Antarctic, beginning in 1980 and as first reported by Farman et al. (1985), a `hole' in the ozone layer forms, reducing the total column to about one-third its pre-1980 level. Similarly, each spring in the Arctic, ozone loss of up to 20% has been observed. The main culprit of this ozone destruction has been proven to be, unequivocally, chlorine (Cl). Its source has also been well established: photolysis of chlorofluorocarbons (CFCs), an anthropogenic propellant and coolant which has a lifetime of several years in the troposphere.

In the stratosphere, ozone is constantly being destroyed and reformed. Ozone is formed primarily through,

$$\rm O_2 + h\nu \longrightarrow \rm 2O$$ (1.1)

$$\rm 2( \hspace{0.1in} O + O_2 + M \longrightarrow \rm O_3 + M \hspace{0.1in} )$$ (1.2)

$$% \rm net: \hspace{0.1in} 3O_2 \longrightarrow \rm 2O_3$$ (1.3)

and is destroyed either by photolysis or one of many catalytic cycles. One important catalytic cycles is,

$$\rm X + O_3 \longrightarrow \rm XO + O_2$$ (1.4)

$$\rm XO + O \longrightarrow \rm X + O_2$$ (1.5)

$$% \rm net: \hspace{0.1in} O + O_3 \longrightarrow \rm 2O_2$$ (1.6)

where X is one of NO, OH, Cl, or Br. Note that X was not consumed in the net reaction. This allows a single Cl atom, for example, to destroy up to 10⁵ ozone molecules. Another important catalytic cycle is,

$$\rm OH + O_3 \longrightarrow \rm HO_2 + O_2$$ (1.7)

$$\rm HO_2 + O_3 \longrightarrow \rm OH + 2O_2$$ (1.8)

$$% \rm net: \hspace{0.1in} 2O_3 \longrightarrow \rm 3O_2$$ (1.9)

which is important in the stratosphere as it does not require atomic oxygen.

The chlorine which results from CFC emissions, about a factor of 3-5 larger than from natural sources. The situation is exacerbated as heterogeneous chemistry (chemistry on and in aerosol particles) converts the majority of chlorine from an inactive to an active form. Note that, from Table 1.1, many of the species involved in these catalytic cycles are present in very small amounts.