While it's scorching, like it is now (I'm not too far north of you, maybe an hour's ride), the shade will do it some good. Otherwise, Junipers are full sun trees. If you're in Manhattan proper, then you wouldn't really know, but your Juniper is the same species, could even be from the same farms, as the ones used for ground cover by homeowners and commercial landscapers as "ground cover". I say this because they're used as accents, more often than not, on wide open lawns or strip mall borders/edgings, where protection from teh sun is non-existent and, in the case of commercials, barely maintained.
The guy you bought it from likely imported it in mass quantities from Asia, or bought it from someone who did. It's doubtful he as any, not even one, of his own at home. Or, if he does, he rotates his stock through his home. Being Asian means nothing. Asians are no more born with a knowledge of bonsai than are Mexicans born with a knowledge of pinata making or Irish with distilling whiskey or Scots with playing the bagpipes.
Why does it need to be outside? It needs sun, light, lots of it. It needs humidity, more than your average home can provide, which is usually, especially with A/C and heating, drier than the Sahara. It needs air movement, which is virtually non-existent indoors.
Your indoor air may be "better" for you, but you're not a tree. Your air is actually drier and harsher inside than it is outside. Pollution is another story, but likely not an issue at all for your tree. J. procumbens, J. horizontalis, and other Junipers are used frequently all of the way down to zones 8-9, Florida, so no, 90Ã‚Âº won't kill it.
You can mist it if you like, but in very high humidity, like we've been having, it's completely unnecessary. Maybe if you lived in the hot, dry heat of Arizona, Texas, etc., you might consider misting frequently, but not here.
There is absolutely no way a Juniper can be kept alive in your average home. You would need a greenhousem abd not just a mini-indoor greenhouse. The only "indoors" Junipers can tolerate are free-standign greenhouse structures, cold frames, etc.
Yes, it should stay outside for the winter, it NEEDS dormancy.
As linlaoboo suggested, if you want a tree inside, look into appropriate species. Ficus are a good start, and Baby Jade are virtually indestructible.