Here is my personal take on the setting, not official or anything but I thought I would share as it seems relevant right now.

The Sky:
The sky would still be the same blue that we recognize, maybe a bit darker but in general it would be the same color. This is because the sky's color has more to do with the angle of the sun's rays than anything else. It would of course vary based on the time and other factors but all in all I imagine it would look more or less the same. Gases are mostly clear so the new gas wouldn't really look like much. The water cycle would probably still be fully functional, so there would be the white billowy clouds you are used to although I imagine each cloud type being much more "extreme" and more well defined. The real question is does this new gas change to a liquid at temperatures found here on earth? If so there could be a whole new cycle of this new gas, which would have it's own clouds and weather patterns which brings me to:

Weather:
Would most likely be the same but much more extreme, hotter summers and colder winters. All your normal global warming stuff. If the new gas turned into a liquid at earth like temperatures there could be independent events. Fog would probably be much more common along with smog due to temperature inversions. The air would be a bit more humid as well and I could see, especially in the desert, you waking up each morning to a nice layer of frost on everything.

Plants:
Larger plants would most likely die off, they tend to be less adaptable to sudden changes in climates than smaller plants. The soil wouldn't really be affected by most of this stuff however what really matters is the corrosive properties of the new gas. Just because it can damage flesh and lungs doesn't mean it would affect plants much or at all. I can't think of any plants in particular but after 16 years I could see annual plants evolving a very strong resistance to this new gas considering the high selective pressure it would have exerted combined with their relatively short generations.

Buildings:
Buildings would be mostly gone, even without severe atmospheric changes buildings tend to require constant upkeep to prevent them from falling apart. This wouldn't happen right away but 16 years after everyone left them to rot most wood beams would be in advanced stages of rot(as bacteria and fungi would develop a resistance to the new gas even faster than the plants). Steel buildings would be around and pretty sturdy. The more extreme temperatures would slowly cause them to break as the steel shrank and expanded over time but all in all there isn't much that occurs in nature that would destroy steel. Any glass or other stuff would most likely be shattered or warped.

I can't think of anything other factors and I could have missed a bunch of stuff that would cause or prevent decay but this is my best guess.