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Moon.
How fast would this suck up all of our atmosphere? Hours? Days?
Probably years, if not more. XKCD's "What If?" has a nice explanation on a similar question, but in that case it was a portal at the bottom of the mariana trench and the question was about draining the oceans. The answer is that you probably wouldn't notice in all your lifetime.
I remember that video, but I‘m not sure it is comparable — air might flow faster than water [citation needed] and there might be less of it [citation needed].
I'm pretty sure there is much more air. At least volume wise. Sphere volume goes up by radius cubed.
there's more water than air (mass-wise).
Above you, atmosphere scale height is 8 km, with a density of 1.25 kg / m³.
That makes 10^4 kg of air above your head per m² surface area.
To get the same mass of water per surface area, it would only have to be about 10 m deep. But the ocean is kilometers deep in most places.
Unless the pressure differential causes it to escape the moons gravity well it would probably stabilize at some point before dumping the whole atmosphere, right?
Pretty sure the game theorists channel on youtube did that one several years back. It's been a minute since I watched it but a search should pull it for you.
This one? https://youtu.be/0Hwjmek0Uvw It more addresses survivability in space, but unfortunately not the „drain earth of its atmosphere“ thing. Then again, in game the portal was closed quickly again.
Easy solution for that, you fire the earth portal on a wall inside some sort of specially constructed airlock.
That hard part would be getting the moon portal in a useful place, you'd probably need to go up there and find a good solid cliff face or something. On the plus side having done that you could be home again in a few seconds.
As long as I'm the first one through, the rest is set dressing.