Monday, September 17, 2007

The Scientific Method

In my experience and observation, what’s sometimes taught in schools as “The Scientific Method” if fairly rare in the actual practice of science. But I’m going to brag a little about a time when I did get to go through the thing pretty much in the prescribed manner (from the Wikipedia entry on Scientific Method):
  • Define the question
  • Gather information and resources
  • Form hypothesis
  • Perform experiment and collect data
  • Analyze data
  • Interpret data and draw conclusions that serve as a starting point for new hypotheses
  • Publish results

The question we were working on was the photooxidation of toluene (used as a solvent and a component of gasoline) in photochemical smog. It’s a moderately complicated molecule, a benzene ring with a methyl group replacing a hydrogen, and somewhere in its oxidation process, we knew that the ring had to open, and, well, then what?

What I brought to the problem was systems theory and a familiarity with simulation modeling, something that the previous generation of smog researchers had only intermittently. In articular, I was concentrating on the mass flows in the system, which you’d think that chemists would do as a matter of course, but no, they didn’t. In fact, in most generalized photochemical mechanisms that I analyzed, it turned out they didn’t conserve carbon. In once case the non-conservation was so bad that it actually was an infinite carbon generator; the mechanism alone generated more hydrocarbons than did emissions!

So anyway, I was looking to see how toluene oxidized in the atmosphere. Now smog is basically a “slow burn,” with the “burning” mediated by what are called free radicals, in this case hydroxyl (HO), and peroxyl, the simplest being hydroperoxyl (HOO). The HO and peroxyl radicals cycle back and forth in the oxidation process, and one of the byproducts is ozone.

The source of the radicals is partly burned hydrocarbons: aldehydes, ketone, glyoxals, a lot of things having a carbonyl group (C=O) in them. Some of these are emitted by automobiles directly, but the smog process makes during its slow burn. These compounds are photolytic; the break down in the presence of UV light to form free radicals.

The smog process also consumes the primary “fuels,” hydrocarbons and nitrogen oxides (NOx), with the burn essentially ceasing when the system runs out of NOx.

What I was looking at was the “stoichiometry” of ozone formed as a ratio to NOx consumed. It varies with conditions, so I put “stoichiometry” in quotes. While running my numbers for various hydrocarbons, it became pretty clear that toluene produced much less ozone per unit of NOx than did other hydrocarbons.

Whitten had already gotten an ad hoc simulation mechanism for toluene by reacting it with a short-lived species nitrogen trioxide (NO3). We figured that the NO3 was reacting with some oxidation product of toluene, which wasn’t a very big leap, partly because the “ring-opened” compounds would have a lot of very active double bonds (C=C) that were known to react with NO3.

I’ll also mention that my systems analysis strongly suggested that toluene was producing something that photolyzed very rapidly to free radicals. That meant that toluene, if added to other hydrocarbons, would accelerate the oxidation process. But the above-average consumption of NOx should then terminate the reaction at a lower level of ozone than would occur from the other hydrocarbons.

We had an EPA contract that let us suggest smog chamber experiments to the University of North Carolina researchers who had a two-sided, outdoor smog chamber that was perfect for controlled experiments. So I suggested that they load one side of the chamber with a standard mix of NOx and propylene (or propene, or methyl ethane, which are different words for the same stuff), and the other side with the same mix, plus some toluene. I told them that it should form ozone more quickly in the morning but produce an ozone peak that was notably less than
the control side.

It’s been called a “daring prediction.” I don’t remember feeling that daring. It seemed pretty inevitable to me, and I would have been mightily surprised if it hadn’t worked. I’ll never know, because it worked exactly as I predicted.

There were other hydrocarbons known to suppress ozone formation. Some like reactive olefins, simply react directly with ozone, so if there’s enough of them around, they’ll destroy the ozone as it’s being formed. One of them, isoprene, produces some very bizarre looking ozone curves in smog chamber experiments, where the ozone first goes up, then down, then up again after the isoprene has all been destroyed, but while the partly oxidized products of the isoprene still can
continue to react. Continuing my brag, I was the first person to get those double peaks in simulations also.

Another class of hydrocarbons suppressed ozone formation by soaking up free radicals. One is called DEHA (diethylhydroxyamine, if memory serves), and it was touted as a smog palliative for a while, until it was shown to boost smog once its radical absorption property was used up.

What we’d done, however, was to show that there was a new class of compounds that could, under some conditions reduce ozone peaks. It wasn’t anything like a smog palliative, because toluene is nasty stuff, and the things it forms are nastier still. But scientifically, it was very cool research.

Later we published a brief communication on the two-sided experiment alone, and also a paper on our toluene photooxidation mechanism. I don’t think that all the products of toluene oxidation have been identified to this very day, but we got the main features of its mass balance and ozone formation behavior twenty years ago. As they say, it was good enough for smog research.

1 comment:

Blogger said...

New Diet Taps into Innovative Idea to Help Dieters Lose 20 Pounds in Just 21 Days!