There's an Unbearable Chemical Smell Hovering Over Parts of Houston, and Experts Are Worried.

[hunter]

38. isobutane has an (R+M)/2 octane rating of 100...

... and butane's contribution to the fuel's overall vapor pressure is considerably reduced in solution with heavier hydrocarbons.

In the bad-old days before strict environmental regulations gasoline might contain up to 10% cheap butane, contributing 5 p.s.i. to the overall vapor pressure of the mix.

A similar percentage of higher octane aromatics and alkenes would be added to an inexpensive stew of lower octane hydrocarbons. (This article says aromatics are limited to 1% in Europe today.) I remember gasolines that smelled like straight toluene.

I knew a chemist who'd retired from Shell. He was wary of no-name gasoline brands and told me that Shell brand gasoline had to smell like Shell brand gasoline, not paint thinner, alcohol, or anything else.