[Ice7771]

Hi guys, having a bit of trouble at the moment with something which should be quite basic.

I am being asked to calculate the residence time of methane. The current concentration is 1.74ppm. The total emissions from all sources is 5.0 x 10^11 kg per year. My first thought is to divide the current concentration by the emissions which would give 3.48x10^-12 but this seems like gibberish if there are no discernible units.

I would greatly appreciate any insite into this