Two additional considerations: 1) these basic parameters have already been infered (but not closely detailed) in previous writing in this setting. 2) Core requirement is that I have an atmosphere with sufficient oxygen in it (preferably oxy/nitro mix) that an unprotected human can survive in it, even if in great discomfort, for some period of time (at least in more temperate locations on the globe).
Question: I need pockets of toxic gases that are trapped in lowland areas, ground-hugging like fog. I presume held down by combo of atmospheric pressure and their own molecular weight. The q is, what kinds of gases should these be? Obviously this ought to be some kind/s of volcanic gases, but which ones in particular? I don't know what a volcano emits that lends itself to this behavior in these conditions, as opposed to blending harmlessly with the dominant atmosphere.
Research done: I've read up on basic atmospheric science (with textbooks) but it is earth-normative of course, and I don't know enough atmospheric science and/or gas physics to extrapolate soundly into this kind of setting. Hence my question to the group mind.
Bonus questions: given parameters above and whatever variables you advise be added, what color will the sky be? Musings about color/temp of primary star and necessary orbital distance from same also welcome.