Exotic Kinetics: Oscillating Reactions in the Atmosphere

c. Michelle M. Francl, 2004

Full module available as PDF from the author at mfrancl@brynmawr.edu

keywords: chemical kinetics • atmospheric chemistry • chaos

Primary Literature Reference: This module is based on J. Phys. Chem. A 2001, 105, 11212-11219. "Steady State Instability and Oscillation in Simplified Models of Tropospheric Chemistry" by Mark R. Tinsley and Richard J. Field.

Background:

The two graphs at the right show the time dependence of the concentrations of two species in a reaction. We can look at the top one and immediately classify the behavior as zeroeth order kinetics, but the bottom defies our usual classification schemes. How do such exotic behaviors arise? Are they rarities or commonplace? Many biological processes are clearly oscillatory, and at the most fundamental level are run by chemical reactions that therefore must oscillate as well. Larger reaction systems, such as the atmosphere, can also exhibit complex behavior, for example, the seasonal development of a "hole" in the trophospheric ozone layer over the poles. More systems probably exhibit these complex behaviors than do not. A half-century ago, researchers were not convinced that chemical reactions could exhibit oscillatory or other complex behaviors. The assumption was that the concentrations of species either decayed or increased monotonically, except for intermediates, which reached a maximum sometime during the reaction, then decayed to zero. In the 1950s, two Russian chemists discovered a reaction which clearly cycled from reactant to products multiple times before finally reaching equilibrium. The Belousov-Zhabotinsky reaction is spectacular to watch, flashing various colors, and has a complex autocatalytic mechanism. These reactions, while fascinating in and of themselves, opened the door to the exploration of "exotic kinetics," reactions that exhibit oscillatory or even chaotic patterns. Alan Turing, a computer scientist, proposed that such reactions could be used to produce regular patterns, suggesting that these sorts of reactions could give rise to biomorphogenesis - such as the stripes on a zebra.

Sample Critical Reading Questions:

• 4. How is the troposphere defined? What altitude range does is occupy? What are typical pressures and temperatures?

Sample Problem:

• 7. Using the mechanism designated M1, apply the steady state approximation to ozone and solve for an expression for the concentration of ozone. Estimate (to an order of magnitude) the steady state concentration of ozone expected for this model when the concentrations of NO and NO-2 are on the order of 109.5 and that of HO-2 is O(107). How does this compare to what model M1 produces.

Culture of Chemistry Feature:

Profile of Prof. Joseph Francisco