PAN

PAN is fascinating stuff if you’re an air geek, and it’s maybe interesting to other sort of people. PAN is the acronym of peroxyacetyl nitrate. It’s got two parts to it, peroxyacetyl:

CH3CO-OO*

And nitrogen dioxide:

-NO2

The asterisk (*) on the peroxyacetyl is one of the conventions used for indicating that it is a radical; it has an unpaired electron that plays well with others, especially if they also have an unpaired electron.

Now a bit of history, in an attempt to lose anyone that I haven’t already lost with the chemical formulae.

Los Angeles was known to have a smog problem even before WWII, but during and after the war it got much worse, partly because of the massive expansion of oil refineries, and the attendant expansion of automobile travel. L.A. smog was known to be different from “London smog,” in that the L.A. sort was oxidizing, and London’s was reducing. Ozone was identified as a major component of L.A. smog, but the ozone alone couldn’t account for “plant bronzing,” damage with a characteristic yellow-brown splotches on the leaves of plants. A guy by the name of Haagen-Smit (mentioned in a magical incantation in SunSmoke), managed to replicate the plant damage by using the product of some smog chamber reactions, but could not identify the compound that was responsible.

Some researchers at the Franklin Institute in Philadelphia (Stevens, Hanst, Doerr, and Scott), used a technique called long-path infrared spectroscopy on smog chamber products and spotted a set of IR bands that were particularly strong in the results of a biacetyl-NOx run. They dubbed the responsible agent, “Compound X.” Compound X turned out to be PAN, and how cool is that?

In the mid-1970s, PAN was discovered to thermally decompose, i.e. at elevated temperatures, it rapidly changed back to a peroxyacetyl radical and nitrogen dioxide. That made everything much more interesting, because PAN gets formed early in the day, when it’s cooler, then, as the air warms, it can decompose and feed radicals and NOx back into the smog formation system, producing more ozone. The thermal behavior of PAN is one of the reasons why smog is worse on hot days. PAN can also assist in the long range transport of oxidizing smog, serving as sort of an ozone storage system.

The thing is that PAN and its constituents/products form a steady-state at constant temperature, with PAN existing in balance with peroxyacetyl and NO2. Change the temperature and the balance changes. At higher temperatures, PAN decays and if there is still sunlight around, ozone goes up. But this process is dominated by the behavior of peroxyacetyl radicals.

If NO2 were the only thing that peroxyacetyl could react with, this wouldn’t happen. But peroxyacetyl also reacts with nitric oxide (NO), and that is one of the reactions whereby ozone is generated, by converting NO to NO2, which then photolyzes to ozone (note: the entire system is ‘way complicated, which is why I spent 20 years studying it). By the same token, if something reduces the amount of peroxyacetyl, relative to other peroxy radicals, then PAN concentrations decline, NO2 comes back into the system, and ozone can increase.

Peroxyacetyl radicals also react with other radicals, and that alters the balance. In the early 1980s, looking over the set of chemical reactions we had available, I decided that the cross-reactions between radicals were set too low. Fortunately, there was a paper by a fellow named Addison that had measured them higher that the generally accepted values, so I used Addison’s numbers. I can still remember the combination of excitement and satisfaction that came when Addison’s numbers led to a simulation that just nailed the PAN decay data. Since then, rate constants have been measured that are even higher than Addison’s; when I used the new, higher still numbers, the results was almost exactly the same. There seems to be a point of diminishing returns, a gating function, call it what you will. Once you get above the critical numbers, there is little additional effect.

So even without my own insights into PAN decay, mostly the result of my paying attention to that particular problem, it would only have been a few years until the problem was solved by better measurements, and correct PAN decay would have been achieved in simulations anyway.

On the other hand, there were several features of the system, such as the specific products of some of the radical-radical reactions that have not been addressed to this very day, to the best of my knowledge, and, nearly as I can tell, no one is looking at those problems and no progress is being made. Sometimes the great grinding engines get it and sometimes they don’t. There’s room for a ton of lessons here, I’m just not sure what they all are.