Friday, November 02, 2012

Adventures in ozoneland: down the rabbit-hole.

Eli was looking for something science like to write about when he came across the above titled paper by Neil Donahue.  Now some, not Eli to be sure, could resist. 

The transformation of SO2 to SO3 and thence by reaction with water (H2O) to sulfuric acid (H2SO4) is the key step in forming sulfate aerosols, which, in turn can reflect and absorb considerable sunlight, or act as cloud condensation nuclei again, increasing the Earth's albedo.  Serious (well this is Rabett Run) consideration has been given to the idea of counteracting greenhouse gas warming by injecting SO2 into the atmosphere, although this would accelerate acidification of lakes and oceans.

For some time atmospheric chemists have waved their hands over the oxidation process

SO2 --> majic --> SO3

It was well known and accepted that the hydroxyl radical, HO could drive the reaction

but there is not enough HO in the atmosphere to match the observed sum of SO3 and H2SO4.

The alkene precursors are stuff we spew into the atmosphere, but also emission of VOCs (volatile organic compounds) from trees and other vegetation.

Another mystery has long been the existence of the Crigee Intermediate (aka CI).  Over 50 years ago Rudolph Crigee proposed that ozonolysis (react ion with ozone where a carbon carbon double bond is replaced by a double bond between a carbon and an oxygen atom) of alkenes (stuff with a double bond between two carbon atoms) initially formed a biradical (a molecule with two unpaired electrons).  Everyone accepts the mechanism, there is a huge amount of work consistent with its existence, see the Donahue paper above,  but it had never been directly observed and rate constants were guessed at, but not measured, or at best inferred

R is organiker speak for anything else.  The dots indicate the positions of the unpaired electrons.  Having two of these makes the CI very reactive, thus hard to isolate.

Both of these mysteries have been solved thanks to the work of groups at the University of Helsinki and the Combustion Research Facility at Sandia (Livermore).  There is an explosion of papers appearing in the literature.

In 2008 Taatjes, Osborn and colleagues at Sandia working on the Advanced Light Source at Lawrence Berkeley National Laboratory detected CH2O2. It turned out to have a sharp ionization edge (e.g. you see nothing until you reach the ionization energy and then there is a sharp rise to a plateau at 10 eV (see the figure below on the left).

They knew of the Timonen group's work at UH which showed that the reaction of CH2I with O3 produced I atoms and CH2O2 although it was not clear what the isomers of CH2O2 were.  In January of this year the Sandia group published a paper in Science that showed that the isomer produced was the Crigee, and they used that reaction to produce CH2O2 to measure it's reactions with NO, NO2 and SO2.  The results were confounding.  The reaction with NO was a hundred times slower than had been previously estimated, but the reactions with NO2 and SO2 were much faster (see the figure below on the right).

As they pointed out, this meant that NO3 and SO3 would be produced efficiently by reactions with the CI and the effects on atmospheric chemistry would be significant where there were enough alkene precursors.  How significant, well as significant as the oxidation of SO2 by OH, and for NO3 it would account for up to a 20% increase.

Mauldin, et al (there are about 10 Als) from UH discussed the implications in "A new atmospherically relevant oxidant of sulphur dioide", a paper which appeared in Nature in August (paywall).  They have a nice diagram of the reaction mechanism

The red stuff on the left side is the new set of reaction.  The CI is originally created with a great deal of energy, but losses much by collisions with stuff like nitrogen or oxygen.  The stabilized CI, then reacts on a longer time scale with sulfur dioxide.  (see this for an explanation of collisional stabilization).  Boy, et al (12 Als) also UH, validate this mechanism against in situ measurements at stations in Germany and Finland and, as a result we have a much improved understanding of the formation of sulfate aerosols.

1 comment:

Hank Roberts said...

check those links, some don't work.

The paper is: