I agree with David, and think you're probably better off getting a quality copy made, and properly storing the original. Even if it is possible to neutralize the acid content without damaging the paper, sunlight & humidity with still have an effect on it eventually.
Personally, I don't worry too much about hanging original ads etc on the wall, but just take precausions to keep them away from direct or bright sunlight & drafts, and have them mounted with acid free glue & mounts etc.
All of my ads have come from magazine though, so I probably would copy a newspaper ad because newspaper yellows alot faster with exposure to light than better quality papers.
You could always go the "fine art" route, and limit it's time on the wall, and use UV meters to measure & monitor sunlight & flurescent light, but making a copy would be easier
