These indicated that the oxidizing ability was stronger
than when only air was used, with more active species in the
non-thermal plasma treatment when oxygen was introduced
into the reactor. In addition, the double bond of O2 is more
easily broken than the triple bond of N2, and it was reported
that introducing nitrogen into the discharge system could
decrease the degradation ratio for treatment by pulsed
high-voltage discharge (Bian et al., 2013). There is only 21%
of oxygen in air, while nitrogen makes up 78%. Therefore, the
degradation ratio in air plasma was lower than in oxygen
plasma. In the case of oxygen, more active species would then
be produced (Gumuchian et al., 2014) and the oxidizing ability
was stronger. According to Reactions (3) and (4), the increasing
production of ozone when oxygen was introduced could also
promote the degradation efficiency of pollutants.