No matter which weather setting I choose under Weather Themes in FSX i get polluted air. I can choose clear skies, and I also get scattered clouds. I have opus as the weather engine. I also checked REX 4 , but don't think that's the problem. Is there a simple way to disconnect OPUS without un-installing ?

Try making sure Live Weather is disabled in Opus, then after starting a flight, press Ctrl-; to reload. For some reason Opus wants to inject weather when it connects to FSX even if the live weather update is turned off, but after reloading the flight it will not do it again.

I tried that, so far no luck.