Answer:
Dr. M and Ishikawajima-shibaura machinery Co.,Ltd. kindly answered this question.
 
  
It would be ozone that killed the bacteria.
 
Oxygen is dissociated and oxygen atoms are generated when high voltage electricity is discharged at the tips of tungsten electrodes. The oxygen atoms so formed react with the surrounding oxygen thus creating ozone.
 
*To give details, it is thought that the hydroxy radical etc. rather than ozone itself will kill bacteria.  Hydroxy radicals etc. are generated from ozone by reacting with H₂O in air.  They then attack the surface of bacteria and kill them.
   
Singlet oxygen or superoxide might be generated, however, their lifetimes are quite short and their concentrations are very low in the atmosphere.  In fact, it is very difficult to measure these molecules. Such a low concentration is not likely to contribute to killing the bacteria.
  
On the other hand, it is easy to measure ozone since it is stable as long as it is not reduced.
 
In fact, an instrument for ozone disinfection is available commercially (see picture on right). This instrument generates ozone from oxygen in air using the method called "silent discharge method". The ozone emitted from the instrument kills bacteria in air and on the wall. Since extra ozone in air is broken down by the instrument, no ozone remains in the air after it works.
 
 
Acknowledgement
We wish to again thank Dr. M and Ishikawajima-shibaura machinery Co.,Ltd. for their answer.

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