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Content archived on 2024-05-14

Coupled Bromine Chemistry Affecting Stratospheric Ozone

Deliverables

The strategy for investigation of heterogeneous reactions has been to investigate uptake of single gas-phase reactants onto surfaces with and without addition of a second reactant. In this way the adsorption characteristics of the individual reactant molecules could be defined and the physico-chemical data derived (the Henry constants, H, the liquid and gas-phase diffusion coefficients D(l) and D(g), and the liquid phase rate coefficient, k{II}) used to help in parameterisation of the reaction rates for use in atmospheric models. Two types of condensed phase were investigated. Aqueous sulphuric acid was presented in the liquid phase both with and without doping with other components. Solid water ice was prepared either by freezing or vapour deposition, and could also be doped. Reactivity was determined from the measurement of the uptake coefficient of gas-phase species and investigation of products formed in the heterogeneous reactions. The project focused on the following reactions involving bromine species: HOBr + HCl_BrCl + H(2)O HOCl + HCl_CI(2) + H(2)O BrONO(2) + H(2)O_HOBr + HNO(3) HOBr + HBr - Br(2) + H(2)O HOCl + HBr - BrCl + H(2)O. All of the reactions investigated occur rapidly on both cold liquid sulphuric acid and solid water ice substrates, at temperatures typical of the polar wintertime stratosphere. At higher temperatures (>20S K) reactivity declines due to the limitation in the adsorption of HCl and HOBr in the absence of a reaction partner on the ice substrates. There remains some unresolved conflict between the interpretation of the results from conventional flow reactors and Knudsen reactors on both substrates. The heterogeneous loss rate of HOCl and HOBr in cold sulphuric acid solution in typical stratospheric conditions via the HOX + HCl reaction was parameterised over the temperature range 180- 240 K. The uptake coefficient of HOBr reaches value of 1 at about equal to 203 K. Even at moderate temperature (210 K), the lifetime of HOBr due to the heterogeneous loss on background aerosol is about one week, while the loss rate of HOCl is factor of 30 smaller. At very low temperature (smaller than 192 K, PSC conditions), the loss rate increases rapidly for both reactions due to the increasing surface area. An atmospheric lifetime of HOX of less than one hour is obtained.
During the time frame of COBRA, new data on gas-phase, photochemical and heterogeneous reactions of bromine species involved in stratospheric chemistry became available. Chemical models were used to assess the impact of these data on bromine chemistry and ozone depletion. First, photochemical box models were used to study each individual reaction. 1) HOBr + HCl - BrCl + H(2)O A new parameterisation of this heterogeneous reaction in liquid sulphate aerosol, developed by Partner 6, was run efficiently in the photochemical box model. The reaction rate is significantly faster than previously recommended at temperatures greater than 200 K. In the course of implementing this new parameterisation, the code that calculates the composition of liquid aerosols [28] was necessarily scrutinised. The diffusion coefficient used for both HOBr and HOCl was found to be a factor of 1000 times slower than it should have been. The implications of this error are currently being investigated by the authors of the code. This code is widely used in the numerical modelling community. BrO + HO(2) - HOBr + O(2) The rate of this bimolecular reaction has recently been re-evaluated. The revised rate affects the ozone budget especially in the mid-latitude lower stratosphere (quantified in 3-D model). BrO + NO(2) + M - BrONO(2) + M The rate of this termolecular reaction has been found to be approximately 25% slower at stratospheric temperatures that modify the partitioning of bromine species further in favour of BrO during daytime. HOBr + hv - Br + OH An increase in the photolysis rate of up to 25% at high solar zenith angles leads to a more rapid increase in OH and HO(2) at sunrise. Three-dimensional chemical transport model results: The effect of the new rates has been to modify the partitioning of Br(y) in favour BrO. Consequently, this has increased the importance of bromine in determining the distribution of ozone throughout the extra tropics. Overall the new rates have led to a 5% decrease in ozone abundances by the end of the simulations. Model results: The partitioning of bromine species and the consequent effect on ozone loss has important implications for the future evolution of ozone. In order to understand the future evolution of ozone, there is a need to accurately partition ozone loss between the chemical mechanisms (i.e., the catalytic ozone loss cycles) controlling the ozone distribution.
The main gas-phase reactions of bromine species in the stratosphere are now quite well described in terms of their kinetics and mechanisms, the remaining uncertainties being confined to reactions of minor importance. An exception is the BrO + HO(2) reaction which still has an uncertainty of approximately (+/-) 30%, and requires further clarification. In general it is important to refine and improve the accuracy of these reactions, especially of the product channels, in order to define the partitioning between the bromine species and hence interpret measurements of reactive bromine in the stratosphere, where at present only BrO and HBr are actually measured.

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