@idefix37, thanks for clarifying the real meaning of mg/m2 measure unit. Now it has sense for me due to this explanation:

"It is the amount of SO2 in a column from ground to the top of atmosphere expressed per square meter."

Thanks!

Is it normal to show a gas concentration in mg/m²?
Shouldn't it be expressed in mg/m³?
Right now, via app on Android this gas concentration is being expressed in mg/m²