For some reason Im not thinking and I don't understand this. If I have a concentration of 1000 ppbv in an airmass and another concentration of 1500 ppbv in another one, when I put them together, what will be the concentration?

It may be that your problem is that you don't have enough information. You have to know the sizes, or at least the relative sizes, of the airmasses.

Suppose the airmasses were, both, 1 billion units (whatever units you want). Then one airmass will contain 1000 units of whatever material you are talking about and the other will contain 1500 units. Together they contain 1000+ 1500= 2500 units in a total of 2 billion units of airmass. That's 2500/2= 1250 PPBV.

On the other hand, if one airmass were very, very small compared to the other, it's "contribution" would be negligible and the concentration would be essentially the concentration of the other airmass. Depending on the relative sizes, the concentration in the combined airmasses could be anything from 1000 to 1500 PPBV.

Assuming the two airmasses are equal in volume, then it is, like I said above, 1250 PPBV.

Thanks, ur right. I just wasn't thinking! One of those days.