On the “Ozone Deficit Problem”: What Are Ox and HOx Catalytic Cycles for Ozone Depletion Hiding?

Abstract

Studies on the role of vibrational excitation in the reactants for the O2+O2, OH+O2, and HO2+O2 reactions show that they can be important sources of ozone in the stratosphere, particularly at conditions of local thermodynamic disequilibrium. The results suggest that the Ox and HOx cycles commonly viewed as catalytic sinks of ozone may actually lead to its production, and hence help to clarify the “ozone deficit problem”. This paper also presents an explanation for the general overestimation of the OH abundance in the upper stratosphere and lower mesosphere through standard HOx chemistry.

Are Vibrationally Excited Molecules a Clue for the “O3 Deficit Problem” and “HOx Dilemma” in the Middle Atmosphere?

“…First, they find support on consistent theoretical studies 4,11,19,[35][36][37][38]72,73,81 of the involved elementary chemical reactions. In fact, the rate coefficients for some reactions involving vibrationally excited OH and O 2 were even taken from trajectory calculations carried out for such reactions.…”

“…This may be a significant observation because, as pointed out in the Introduction, diatomic molecules such as O 2 (V′′) near the dissociation limit are known to require 10 5 -10 6 collisions to de-excite in the presence of inert species such as Ar atoms. 20 In addition, reactions involving such vibrationally excited diatomic molecules have been shown 4,37,38 to occur significantly faster than the corresponding vibrational relaxation processes, with the same being true for the reactions [photolysis of O 3 , and reaction 6] through which they are produced in the middle atmosphere. Thus, vibrationally excited oxygen molecules, as well as hydroxyl radicals, should be abundant in the middle atmosphere, although only LTD modeling simulations and the solution of the involved master equations in their full complexity may allow a proper answer to the questions raised above (see sections 3.2 and 3.3).…”

“…The y ) 1 mechanism has been shown in section 2 to generally favor HO 2 formation over OH. Indeed, despite the fact that the reaction 6 has a rate constant nearly 3 orders of magnitude larger than (4) …”

“…[41][42][43][44][45][46][47][48][49] It should be noted that these theoretical dynamics studies employed either our own single-valued global potential energy surface 50 obtained from the DMBE (double many-body expansion 51-53 ) method for ground triplet O 4 or reduced-dimensionality single-valued ab initio surfaces, 41,47 and hence the relevance of nonadiabatic processes in explaining the above experimental results remains an open question. 22,49 However, dynamics studies 4,11 have shown that odd oxygen can be formed if the O x mechanism is reformulated to involve two vibrationally excited molecules, although this second-order process can occur only if two such species have a chance of colliding with each other.…”

“…Note that k 13 (V′ ) 9, T) can be a factor 4-fold or more larger than k 13 (V′ ) 4, T) [i.e., 1 order of magnitude larger than k 6 (T)] at the temperatures of interest in the mesosphere. 4 We open here a small parenthesis to summarize the input data used for the calculations. For the ozone number density versus altitude, we have employed the SAGE (stratospheric aerosol and gas experiment) data from the vertical distribution of ozone reported by Randell et al 79 from analyses of satelite, ground-based, and balloon measurements.…”