Research Projects

Organizational Units

Journal Issue


The release of Br atoms from photolyzable bromine species, such as $Br_{2}$, is responsible for the almost complete destruction of ground-level ozone observed in the Arctic after polar sunrise, and is likely to play a role in the partial destruction of ozone observed in the marine boundary layer at mid-latitudes. To investigate the mechanism of the reactions of $O_{3}$ with NaBr aerosol, experiments were carried out at room temperature and atmospheric pressure in a 561 L aerosol chamber above the deliquescence point of NaBr aerosol. Fourier transform infrared spectroscopy was used to measure the concentrations of $O_{3}$ and molecular bromine concentrations were monitored using atmospheric pressure chemical ionization mass spectrometry. A computer kinetics model, including gas and aqueous phase chemical reactions, gas and aqueous phase diffusion, and mass transfer between the liquid aerosol droplets and the gas phase, was used to evaluate the mechanism for bromine production. Experimental results are not reproduced well by known gas phase and aqueous phase bromine chemistry alone, and thus, a reaction occurring at the air-water interface between gaseous ozone and aqueous bromide ion to produce $Br_{2}$ via an $O_{3}\cdots Br^{-}$ surface complex is proposed. With the inclusion of this interface reaction, the model satisfactorily reproduces experimental results. While there is no direct spectroscopic evidence of the surface complex, molecular dynamics simulations provide further support for the proposed heterogeneous reaction mechanism. They show that $O_{3}$ strongly prefers to reside at the interface rather than in the bulk solution, and that it makes frequent and long enough contacts with bromide ion for the surface reaction to be feasible. The atmospheric implications will be discussed.


$^{a}$Support by NSF Collaborative Research in Chemistry grant is gratefully acknowledged.
Author Institution: School of Physical Sciences, Department of Chemistry, University of California; School of Physical Sciences, Department of Chemistry, EPRI; The Henry Samueli School of Engineering, Department of Mechanical and Aerospace Engineering, University of California; School of Physical Sciences, Department of Chemistry, University of California