Friday, October 30, 2009

Diesel Exhaust is a Weighty Matter

Scientific American has this online article -

- the point of which is to point out the enormous amounts of pollutants produced by idling truck engines, and that New York City has a law regarding this that really ought to be enforced. I have little doubt that this really is a significant source of pollutions, but this statement made me raise my eyebrows:

"Idling buses, cars and trucks may not seem like a big deal, but in New York City they spew out as much pollution as nine million diesel trucks driving from the Bronx to Staten Island, according to the Environmental Defense Fund. That’s roughly 130,000 tons of carbon dioxide, 940 tons of nitrogen oxide, 24 tons of soot particles, and 6,400 tons of carbon monoxide each year"

Can that be right? That's a lot of trucks making the trip. I'm a statistician, and I wonder about such things like the accuracy of statistics like this. Watching TV on a Friday night, I started doing so some Googling and back-of-the-envelope calculations during the commercials.

The claim of the article: 130,000 + 940 + 24 + 6,400 = 137364 of pollutants released each year, that's 274,728,000 pounds. The distance between the Bronx and Staten Island is 33.9 miles (so says Google Maps), and in 9 million trips that works out to 305,100,000 miles. 274,728,000 pounds divided by 305,100,000 miles is 0.9 pounds of pollutants per mile.

The Economy of Diesel trucks: The average big diesel truck pulling a load gets about 5.5 miles per gallon of fuel (so says Wiki Answers), or 0.182 gallons of diesel per mile. Diesel weighs about 7.2 pounds per gallon (in cold weather yet, so says, so 0.182 gallons/mile times 7.2 lbs/gallon is about 1.31 pounds of diesel per mile driven.

Now 0.9 lbs/mile of pollutants divided by 1.31 ponds per mile of diesel work out to 0.6873, or about 70% of the total mass of the diesel fuel converted to pollution. At this point, I ran into difficulty finding a source for the actual composition of diesel exhaust. 70% might be reasonable; after all, the mass of the fuel has to go somewhere.

But wait, I've made a mistake - most of the 130,000 tons of carbon dioxide is oxygen, without looking up the molecular weights of carbon and oxygen, AT LEAST two-thirds of that mass is coming from the atmosphere and not the diesel fuel. 70% now seems more reasonable.

There is another possible error, which I don't know how to resolve. In my reading I discovered (lost the link, sorry) that idling diesel engines are relatively clean, but produce heavy pollution when under a load (you see this on the road all the time). Therefore the number of idling vehicles must be really huge to make of this difference. This is also quite possible; NYC is a BIG place, but the article does not offer any information about how many vehicles this might be.

I would assume that if NYC has a law that trucks should not idle for more than a minute, there must be some good evidence somewhere to back this up. The original claim seems to come from the Environmental Defense Fund, but I'm too tired for more digging tonight. This has been an interesting bit of fact checking, except that I ran into a wall with limited knowledge of chemistry. Maybe I should ask a Chemist?

[UPDATE, 11/01/2009]
I received this response to my question at About.Chemistry, Sean writes:
the average chemical formula for diesel is C12H23. with that said, the mass of a carbon atom is 12.01 g, and hydrogen is 1.008. so, mathematically, diesel has the molecular weight, on average of 167.304 grams per mole of fuel.
the weight of the oxygen atom is 15.99g (mostly rounded to 16g), so carbon dioxide is 44.01 grams per mole.
in general, this relates to something of the sort:
2 moles of C12H23 +  O2 gas in excess -makes-> 12moles CO2+ 12moles CO+ 23moles H2O. as the formula for the burning of the diesel, if it was a very complete and clean reaction, however, we all know that's never the case. ;.;
as for the 130,000 tons of CO2, that comes to
 117 934 016 200g of CO2

and of that gram mass, 27% is carbon, while 73% is oxygen. that's what...
31842184374 grams of carbon and 86091831826 grams of oxygen.
however, diesel is more of a blend of things and not just the carbon and hydrogen, which pretty much takes all that I have written and makes it almost useless. In my research, though, I've seen more about the fact that sulfur is present in the fuel than the carbon emissions, as that will inhibit the use of catalytic filters to scrub the exhaust clean, as in most vehicles.
also, in response to your lost link, I dug this up
which says that idling is bad for diesel because it produces more exhaust via incomplete combustion.
anywho, I'm not too advanced in chemistry, so forgive me if I supplied you with random nonsense, I was trying to think of a way to equate the mass of fuel to pollutants produced, but as I can't find an exact formula for what the reactions are this is the best I think I can do. I'm hoping someone else can chime in from here and make more sense of things, and of anything, I wish you luck with your search.

Thanks Sean!

After some more digging I turned up an abstract and a paper on wind tunnel experiments (Part 1, Part 2) that are related, but none seem to be the origin of the original statistics. I might have more luck searching from work tomorrow.
-- OR --
I could ask the author of the SciAm article, Mr Brett Israel. Would that make it too easy?

[Further update 12/09/2009]
I never heard back from the author and I've pretty much given up. Sean's comment about catalytic converters is something else I had not thought of that might affect the results. So many windmills to tilt at, and so little time.

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