How much “cleaner” does summer fuel burn?

Well, it's that time of year again: the switchover to "summer" fuel and resulting price increases. This EPA regulation system requires gasoline producers to blend additives into the gasoline that reputedly make it burn cleaner.

What exactly does it mean "burn cleaner" and how much "cleaner" is summer fuel than regular gasoline?

Gasoline, diesel fuel, and ethanol-gasoline blends are adjusted seasonally and geographically to ensure proper starting and performance. Winter blends contain higher levels of high vapor pressure compounds like butane to ensure the fuel is sufficiently volatile to ignite in cold temperatures. This in turn makes the gasoline cheaper to produce as it does not have to be as pure as the summer blend$^{[1]}$.

Conversely, summer blends contain lower levels of high vapor pressure liquids to ensure that the fuel doesn't evaporate away in the heat and pollute the atmosphere. This is why summer blends are marginally cleaner than winter blends. A study on winter versus summer emissions near tunnels form the University of Wisconsin collected the following data on non-methane hydrocarbon emissions$^{[2]}$:

As for fuel efficiency, winter-grade gasoline yields $1.7$% less energy on average than the summer. This may seem like a considerable amount, but energy content can vary widely from station to station in either season from $3.4$-$4.8$%$^{[3]}$.

$^{[1]}$ The Oil Drum
$^{[2]}$Univeristy of Wisconsin, Summer and winter nonmethane hydrocarbon emissions from on-road motor vehicles in the Midwestern United States
$^{[3]}$ EPA, Fuel Economy Impact Analysis of RFG

• Those are pretty huge margins of errory. – Shaka Boom May 4 '16 at 22:12
• It's the only data I could find. I'll keep looking though. – ringo May 4 '16 at 22:13
• @ShakaBoom Wow, the top two, especially. "X ± X, give or take." – SendersReagent May 5 '16 at 0:06