There is no way to remove the poison gas, but there are places high enough to drop down to in human form (ie the top of Lovis' tower). The top of the wall and tower building for the quarry are above the gas, which is where you need to go first for the sigil. Broken Valley has basically become a flying fortress now, and there are a couple headquarters there, with various anti-dragon shields to be brought down.
BTW, this was posted in a BD forum, rather than D2:ED.