A fresh cut is a minimum of one inch off the trunk bottom. And the tree ABSOLUTELY MUST be in water within an hour of the cut. The timeline is incremental: half hour is better than an hour, 15 minutes is better than half an hour, 5 minutes is better than 10 minutes. Once air gets suck up into the stem, those particular cells can no longer conduct water.
Water stress closes the "breathing holes" in the needles and slows the production and emanation of scents.
Also, scents in general are much more volatile (disperse more quickly) as temperature increases. Think of how pine pitch hardens when cold. When the temperature increases, any such scent is going to initially "explode" until it reaches an equilibrium with the ambient temperature. This explains your in-car experience. And of course, standing in the bag, the scent is confined to as small area, and so stays "concentrated".