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The ozone depletion events (ODEs) in the springtime Arctic have beeninvestigated since the 1980s. It is found that the depletion of ozone ishighly associated with an auto-catalytic reaction cycle, which involves mostlythe bromine-containing compounds. Moreover, bromide stored in varioussubstrates in the Arctic such as the underlying surface covered by ice andsnow can be also activated by the absorbed HOBr. Subsequently, this leads toan explosive increase of the bromine amount in the troposphere, which is called the“bromine explosion mechanism”.  In the present study, a reaction scheme representing the chemistry of ozonedepletion and halogen release is processed with two different mechanismreduction approaches, namely, the concentration sensitivity analysis and theprincipal component analysis. In the concentration sensitivity analysis, theinterdependence of the mixing ratios of ozone and principal bromine specieson the rate of each reaction in the ODE mechanism is identified. Furthermore,the most influential reactions in different time periods of ODEs are alsorevealed. By removing 11 reactions with the maximum absolute values ofsensitivities lower than 10 %, a reduced reaction mechanism of ODEs isderived. The onsets of each time period of ODEs in simulations using theoriginal reaction mechanism and the reduced reaction mechanism are identicalwhile the maximum deviation of the mixing ratio of principal bromine speciesbetween different mechanisms is found to be less than 1 %.  By performing the principal component analysis on an array of the sensitivitymatrices, the dependence of a particular species concentration on acombination of the reaction rates in the mechanism is revealed. Redundantreactions are indicated by principal components corresponding to smalleigenvalues and insignificant elements in principal components with largeeigenvalues. Through this investigation, aside from the 11 reactionsidentified as unimportant in the concentration sensitivity analysis,additionally nine reactions were indicated to contribute only little to thetotal response of the system. Thus, they can be eliminated from the originalreaction scheme. The results computed by applying the reduced reactionmechanism derived after the principal component analysis agree well withthose by using the original reaction scheme. The maximum deviation of themixing ratio of principal bromine species is found to be less than 10 %, whichis guaranteed by the selection criterion adopted in the simplificationprocess. Moreover, it is shown in the principal component analysis thatO(<sup>1</sup>D) in the mechanism of ODEs is in quasi-steady state, which enables afollowing simplification of the reduced reaction mechanism obtained in thepresent study

atmospheric chemistry and physics

Derivation of the reduced reaction mechanisms of ozone depletion events in the Arctic spring by using concentration sensitivity analysis and principal component analysis