Seminar on Physics and Chemistry of the Atmosphere (Abstract)

 Kinetic studies of the gas phase I2/O3 photochemistry
 

The evaluation of the relevance of tropospheric iodine chemistry in O₃ depleting cycles and in aerosol formation requires a previous understanding of the kinetics and mechanisms involved. In order to compile such information, the chemical system generated by flash photolysis of I₂ in the presence of O₃ has been studied by using multichannel time-resolved molecular absorption spectroscopy and atomic resonance absorption spectroscopy.

This system was previously used to determine absolute absorption cross sections of IO, IO*, OIO and higher iodine oxides (see Peter Spietz thesis), and as a result of this work the time dependent concentration curves of relevant gas phase species are known. These data have been analysed to derive a mechanism which enables to explain:
-Pressure dependence of the branching ratios of the IO self reaction: the I and OIO yields are relevant in order to determine the ozone destroying potential and the particle formation potential of the IO self reaction.
-Fate of OIO and I₂O₂: the homogeneous aerosol nucleation observed in the MBL. The reaction rates of the reactions considered are obtained by integrating the corresponding set of differential rate equations and by applying optimisation methods.

In this work the reactions IO+OIO+M->I₂O₃+M and OIO+OIO->I₂O₄+M are proposed as the most plausible paths to higher iodine oxides and aerosol formation. Modelling studies performed at the UEA (Norwich) considering rate constants calculated in the context this work are able to reproduce the ammounts of OIO and 'new particles' observed in Mace Head. Theoretical calculations using RRKM Theory fit our experimentally determined presure dependent branching ratios and rate constants very well.