concentrations were used to eliminate the unwanted effects of the intrinsic contaminants of the compressed air on the concentration of generated particles from so 2 during the evaluation tests for the gac holders instead of total particle concentrations correlation equations between so 2 and particle volume concentrations were also studied and the net particle number and volume concentrations n p net and v p net respectively according to so 2 are shown in fig2 a the correlations of both net particle concentrations according to so 2 changed from non linear to linear relationships over 50 ppb the non linearity in the so 2 range lower than 50 ppb may come from the particle transport loss on tubing walls and complicated reactions for particle generations thereby providing the non linearity at low levels of so 2 the net number and volume size distributions at different so 2 levels are depicted in fig2 b and 2c respectively all net number size distributions in fig2 b had two peaks the lower mode could only be measured down to the lower detection limit of smps 140 i e 2 5 nm but obviously had a lower modal diameter which is the peak of the size distribution the second broad peak showed modal diameters ranging from 5 to 10 nm for 1 to 200 ppb respectively the hidden particles below the smps detection limit may also contribute to the non linearity of the net particle number concentrations according to the so 2 concentrations in addition the increasing modal diameter of the second broad peak in fig2 b shows that nucleation and condensation processes occurred simultaneously in conversion.