Base level can be Vanilla. When everything is perfect and terror-free, we will be overwhelmed by the smell of Vanilla-scented candles. Enormous buckets of vanilla scented body sprays will be dumped on cities in the manner of helicopters dropping water on forest fires.
Moving up one will be bacon. This will ensure that if the terror level does rise moderately, everyone in the United States will become grossly fat and that vegetarians will fight fervently to keep the level down.
One above that, everything will smell like clove cigarettes, and rising one further, everything will have that "new car smell," maybe. Or burning hair. I'm undecided.
I'm also unclear as to what the smell of "ultimate terror" would be. I'm debating between "Limburger cheese," "The New York City Subway Systems," or "cat shit."
All suggestions are appreciated.