Here some sequential screenshots showing the product of AIR volume fraction multiplied by the O2 mass fraction in the air (Air.O2.Mass Fraction*Air.volume Fraction). On the very right are reported the AIR volume fractions corresponding to the first and to the last result.
Where the dispersion of air in water is present it is clearly visible a slightly change in the shade of the blue: from dark (first 2 screenshot) to lighter blue (the rest). Going from ~Air.O2.Mass Fraction*Air.volume Fraction=0.233*0.15=0.035 to ~1.00*0.15=0.15. On the top, where the AIR volume fraction is 1.00 it is clear the change in color from roughly 0.233 to 0.00 and then to 1.00.
I will calculate a the volume fraction at the sparger using the absolute pressure given by the solver at the sparger throat location and set the corresponding values rather than using the zero gradient option. I will also run a transient simulation hoping to see only 0.233 everywhere.