Climate models continue to throw up numerous
paradoxes and anomalies in the predicted concentrations of ozone and thus
the impact of its depletion on the environment and life on earth. Now, a
Portuguese team has analysed the state of play and reckons our views of
the photochemistry of this gas are at odds with what actually happens at
different levels in the atmosphere. Stretched oxygen and odd hydrogen
species are suggested that offer a clue to explain the so-called "ozone
deficit problem" and the "HOx
dilemma" in the upper stratosphere and mesosphere.Chemist Antonio Varandas of the University of Coimbra in Portugal points out that the words "ozone" and "hole" have all but melded themselves to each other because of the mass media interest in the so-called hole in the ozone layer. Ozone is densest at an altitude of between 25 and 30 km, but the ozone layer that envelopes our planet and protects us from ultraviolet radiation actually extends well into the mesosphere up to 85 km. At this altitude, the chemistry of ozone, attains, what Varandas refers to as, its "greatest simplicity".
But, simple in appearance does not necessarily mean simple to understand. Paradoxically, he confesses, it is in the mesosphere that there remain several important mysteries about the chemistry of ozone that are yet to be unravelled. The clues studies of this seemingly simple ozone chemistry might yield about its behaviour could lead to a clearer vision as to how to deal with that ozone hole.
Ozone is generally considered to form in the same straightforward photochemical way throughout the atmosphere - dioxygen is split by light and the two "O" moieties combine with other dioxygen molecules to form the familiar O<sub>3</sub> system. In contrast, when it comes to ozone (odd-oxygen) destruction, it is location that is all important. Its degradation is most complex over the poles where winds, ice clouds, and low temperatures promote its reaction with halogen and nitrogen species. As Varandas goes on to explain, however, in the upper stratosphere and mesosphere, ozone chemistry seems mostly to be controlled by catalytic reactions involving odd hydrogen HOx species; x = 0, hydrogen; 1, hydroxyl; 2, hydroperoxyl. The HO<sub>x</sub> species being derived from the photochemical oxidation of water.
This chemical simplicity appeared to fit the early atmospheric models but,
says Varandas, despite this simplicity, the models of the 1980s severely
underpredicted the amount of O3
to be observed at these high altitudes. The 50-60% shortcoming soon became
known as the "ozone deficit problem". Advancements in experimental methods
during the mid-1990s, ironically, led to additional conflicts between
theory and results with some teams consistently reporting an ozone
deficiency while others came up with novel ozone sources to explain their
finding higher than anticipated levels. As if to make complicate matters
further, the so-called "HO<sub>x</sub> dilemma" would soon emerge from
between the clouds.Conventional models were failing atmospheric scientists experimenting in the mesosphere too, leading to holes in their theories of how HOx photochemistry proceeds in the middle atmosphere. Hydroxyl measurements at 50 to 80 km were showing a 25-30% lower than predicted concentrations. All kinds of fudge factors were included in the kinetic analysis of the myriad species involved, with little resolution of either the ozone deficit or the HOx dilemma.
Varandas and his colleagues have now come up with a radical (pardon the pun) new theory that could reconcile both the deficit and the dilemma by demonstrating their interrelatedness. Their proposal suggests new mechanisms for the formation of ozone that is based on stretched odd hydrogen and oxygen molecules under the hypothesis of non-local thermodynamic equilibrium or as he calls it local thermodynamic disequilibrium (LTD). This being the occupation of quantum states at odds with the Boltzmann statistics appropriate at conditions of thermal equilibrium, he explains.
The crucial premise in Varandas' proposal is therefore the abundance of high vibrationally excited species in the middle atmosphere in accordance with the LTD hypothesis. Reactions such as Reactions A and B: