ELI5: Why do scientists describe large quantities of gas, as a weight; i.e volcano X produces 200,000 tonnes of CO2 per day.

I'm an engineer and have a good grasp of units and measures (too much time up the pub), but tonnes of a gas throw me. Fathoming the difference between 1,000 tonnes of CO2 and 200,000 almost means nothing, as the former is already an unimaginable amount.

Also, how would helium be measured?