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Numerical Investigation of Tropospheric Halogen Release and Ozone Depletion in the Polar Spring

Cao, Le

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Ozone is one of the most important natural atmospheric constituents in both the stratosphere and the troposphere of the earth. Compared with the ozone in the stratosphere, which is mainly formed via the natural photolytic decomposition of oxygen molecules, the ozone in the troposphere originates from reactions involving volatile organic compounds (VOC) and nitrogen oxides (NOx). The near-surface mixing ratio of ozone varies with the regions. In pristine regions, such as the Arctic and the Antarctic, the natural mixing ratios of ozone in the troposphere range from 30 to 40 nmol/mol on average. About thirty years ago, a rapid destruction of the tropospheric ozone on a time scale of hours to days was observed in polar regions during spring time. Meanwhile, a negative correlation between halogen (e.g. Br and Cl) concentrations and ozone mixing ratios was found, which reveals the involvement of halogen species in the ozone depletion process. It has been presumed that halogen species participate in a variety of chemical reaction cycles, leading to ozone depletion. Moreover, the oxidation of inert halogen ions from aerosol particles, fresh sea ice or snow packs can also speed up the depletion of ozone in the troposphere by releasing Br2 and BrCl from the surfaces of these substrates.

The tropospheric ozone depletion event has broad impacts on the atmospheric chemistry of the polar regions, such as changing the lifetimes of the hydrocarbons and the formation of the aerosol particles. Therefore, in this study, the modeling of tropospheric ozone depletion in polar spring is addressed. The first step is to identify an appropriate chemical reaction mechanism for capturing the temporal evolution of the chemical mixing ratios. Thus, a box model study is conducted, in which the transport of the air is not included and only chemical reactions are considered. Three chemical reaction schemes are investigated in the box model study: a bromine-only reaction scheme, which then is subsequently extended to include nitrogen and chlorine related species. The heterogeneous reaction rates are parameterized by considering the aerodynamic resistance, the reactive surface ratio, β, i.e. the ratio of reactive surface area to total flat ground surface area, and the boundary layer height, Lmix . In the chemical reaction mechanism in which nitrogen and chlorine are not considered, it is found that for β = 1, a substantial ozone decrease occurs after five days and lasts for 40 h for Lmix = 200 m. For about β ≥ 20, the time required for major ozone depletion ([O3 ] < 4 ppb) to occur becomes independent of the height of the boundary layer and approaches the minimum time of two days, 28 h of which are attributable to the induction and 20 h to the depletion time. In the chemical reaction mechanism including the nitrogen containing compounds, it is found that a small amount of NOx may have a strong impact on the ozone depletion rate in the polar regions. During the ozone depletion, a reaction cycle involving BrONO2 hydrolysis is dominant. A critical value of 0.0004 of the uptake coefficient of the BrONO2 hydrolysis reaction at the aerosol and saline surfaces is identified, beyond which the existence of NOx accelerates the ozone depletion event, whereas for lower values, deceleration occurs. In the chlorine related chemical reaction mechanism, the calculation of the time-integrated bromine and chlorine atom concentrations suggests a value on the order of 103 for the [Br] / [Cl] ratio, which shows that chlorine atoms have minor direct influence on ozone depletion.

A sensitivity analysis is conducted in the box model to show the relative importance of each reaction in the mechanism. It is found that during the ozone depletion time, the heterogeneous activation processes related to HOBr become more important due to the high bromine concentration in the air at this time period. Moreover, the critical role of the BrONO2 related reactions in the ozone depletion mechanism is confirmed by the relative NOx concentration sensitivities. On the basis of the sensitivity analysis of the bromine-only chemical reaction mechanism, 27 less important reactions with the lowest sensitivities and four related species are removed from the original bromine only chemical reaction mechanism. As a result, a skeletal mechanism applicable in 3-D simulations is obtained, which consists of 28 reactions among 26 species. The major features, including the evolution and peak values of the mixing ratios of the chemical species, are maintained.

The skeletal chemical reaction mechanism is implemented in a meteorological 3-D model. Large eddy simulation accounts for the turbulence, and the Smagorinsky model is employed as the sub-grid model. Different physical boundaries on the ground are used in the 3-D model: a flat surface on the ground, and a mountain located on the surface. It is found that the vertical turbulent mixing of air parcels occurs below the height of the polar boundary layer, leading to a non-uniform vertical distribution of the chemical species mixing ratios. The ozone is totally consumed near the bottom surface where a strong elevation of the reactive bromine species occurs. Weather conditions such as wind speed, surface topography and the boundary layer stability may affect the boundary layer height, thus influencing the ozone depletion rate.

Item Type: Dissertation
Supervisor: Gutheil, Prof. Dr. Eva
Date of thesis defense: 25 April 2014
Date Deposited: 14 May 2014 07:45
Date: 2014
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institute of Physical Chemistry
Controlled Keywords: Ozone depletion, Halogen release, Detailed chemical reaction mecha- nism, Sensitivity analysis, Skeletal chemical reaction mechanism, Large eddy simulation
Uncontrolled Keywords: Ozonabbau, Halogenfreisetzung, detaillierter chemischer Reaktionsmechanismus, Skelettmechanismus, Grobstruktursimulation
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