Here in Atlanta, we have frequent smog alert days. On these days, electronic road signs flash a message that says (from memory):
Smog Alert. Please Fuel after 6 pm.
What difference does it make what time you fuel up?
On hot, humid summer days, any sorts of fumes in the air add to the smog.
Gasoline, carbon monixide, exhaust from any internal combustion engine, barbecues, etc.
Apparently something to do with the way sulight reacts with various polluants, including gasoline fumes.
Ivar, I couldn’t find the details on that link… is there a subpage you could point me too?
I don’t doubt that both of you are right, but I’m looking to see some of the science and logic behind it
I’m also in Atlanta.
I think the question is the TIME. The answer is actually really simple. Later in the day it cools down and the gasoline doesn’t evaporate as quickly. Less gasoline vapor means less smog.
-Beeblebrox
“Anyone who is capable of getting themselves made President should on no account be allowed to do the job.”
Howdy Zaphod.
I can’t buy this argument. How much vapor is really produced fueling up? I don’t know the answer, but when I fuel up, it doesn’t seem like a lot. There’s a hose deep in my gas tank for ~90 seconds, and possibly a few spilt drops when I return the pump. Could this little bit really make such a big difference?
Note the signs say nothing about driving at any any hour, which I would think is a hundred times more important in producing gas fumes than the fueling process.
I also note that there is no mention of the day’s temperature on these signs – though I will look to see if cooler smog days get the same message.
To be honest, I don’t buy the logic either. In my defense, it ain’t mine - it’ the DOT’s. It probably isn’t a major cause of our smog, but the DOT or GRTA is never going to blame DRIVING. That would make far too much sense and force them to listen to the EPA, stop building roads, and invest more in MARTA.
One thing though, people are considerably worried about those gas vapors. Ever notice that all the pumps you go to now have those sleeve things on them? Just a few years ago they were naked. The reason is evaporation.
I personally believe that car exhaust is the chief hazard.
The following stems from an imperfect memory, was not verified, and like everything people read on the internet, should be taken as absolute fact and never questioned:
Trace nitrates in the exhaust combine with the air to form Nitrous Oxide (yes, laughing gas). This in turn catalyzes the formation of ozone which is what is measured for smog alerts. The formation of ozone is considerably speeded by heat. This is why we don’t have smog days in Winter.
I was unable to find a good site for more info but I think “Fuel after 6 pm” is putting a band aid on a sucking chest wound.
-Beeblebrox
“I am so amazingly cool you could keep a side of meat in me for a month. I am so hip I have difficulty seeing over my pelvis.”
That is a very good defense, and thanks for the info.
I guess will hold off to see what else is answered (it’s only been up here for a couple hours).
muttrox, what was your title supposed to be? It got mangled by the software, but I can’t figure out your original intention.
Ever smell a gas station? There’s definitely significant amounts of vapor.
The subject was supposed to be:
“How does fueling up after 6 affect smog?”
Please change it, I don’t know what happened.
Yes, I have smelled many a gas station, however
(1) it smells about the same after 6 pm that I’ve ever noticed (and as of yet, no one has been able to really explain how sunlight or heat makes a difference).
(2) huge abnormal concentrations – I am not denying there is some fume leakage, just the quantity per car.
(3) smell is vastly confounding by fumes from exhaust.
Just remembered - It’s not heat that speeds up the formation of ozone. The reaction requires uv radiation (i.e. sunlight) More sun = more uv rays = more ground level ozone.
Heat does increase the vapor pressure in a fluid, though. The gas station should be emitting more fumes at 2 pm when it’s 90 than at 7 pm when it’s 75. I guess the real question is how do petrochemical vapors produce ozone? I always thought the NO2 was a product of the combustion and wouldn’t be present in the vapor.
Can we get a little help from a chemist?
-Beeblebrox
“I’m a pretty dangerous dude when I’m cornered.”
“Yeah, you go to pieces so fast people get hit by the shrapnel.’”
The only difference I can think of between driving & not fueling vs. driving & fueling is one extra starting up of the car (since you are supposed to turn the engine off while pumping). So let’s ponder: Does the simple act of starting the car produce enough pollutants that we want to limit the number of engine startings during the day?
So all Atlanta needs now are signs that say “Please Don’t Stop at the 7-11 (if you got 'em) on the way home” or “Listen to your mother: come straight home and don’t go anywhere.” 
bump.
PLEASE! No bumps 'til after 6 p.m.!
Oy that was embarrassing.
Uh, 6 a.m., yeah. That’s what I meant.
Cooking with chemistry! –
Famous Amos’s Ozone Recipe!
(this delicious treat is also known as smog in some regions)
Take one part Nitrous Oxides (NOx) from cars or power plants
add one part evaporated hydrocarbons (from evaporated gasoline, exhaust, paint thinner, etc.)
Mix well.
Expose to strong sunlight and cook at 80 degrees F
Yield: one part ozone (O3)
Warning: Do not expose to lungs, eyes, or other sensitive membranes!
[I’m forwarding this recipe 'cause a friend of a friend was charged $1.6 million for it by Neiman Marcus, and…]
Anyway the gist of the recipe is that you need both sunlight and heat to make ozone, so if you have to pollute, doing it at night is better. (that’s also why smog isn’t a problem in the winter). I guess waiting for a cloudy, relatively chilly day to pump gas makes just as much sense, too.
And yes, gasoline does evaporate when pumping it – ever see the shimmery/wavy/mirage-like air just above a fuel pump? I imagine there’s more evaporation when it’s hot, but the real reason not to pump gas during the day is to keep the raw ingredients out of the recipe when the oven is on, so to speak.
If you want a neat smog link, EPA has animated daily ozone maps for the east coast, so you can see how it builds up during the day: http://www.epa.gov/region1/oms/index.html
You can also usually see how it moves east and north with prevailing winds in the northeast (ending up at Acadia on the Maine coast).
Check out June 20 in the archives for a recent high ozone day.
Thank you Quercus!
One followup question: Do the “evaporated hydrocarbons” also occur in normal fuel exhaust (driving)?