The concentration in parts per million (ppm) is defined as the number of molecules of the gas of interest per million molecules of air. In this case, we have 10 molecules of SO2 gas and 125000 molecules of air. So, the concentration of SO2 gas in ppm is:

$$Concentration (ppm) = \frac{10 \ molecules \ SO_2}{125000 \ molecules \ air} \times 10^6 = 80 ppm$$

Therefore, the concentration of SO2 gas in the air is 80 ppm.