| MadSci Network: Environment & Ecology |
The ozone is nearly all gone by sunset, and completely dissipated within ten minutes afterward. In photochemical smog, we have a very complicated series of chemical reactions involving a large number of different chemical substances. Detailed computer models include as many as several hundred chemicals interacting in several hundred reactions. Ozone is quite a reactive material. It is produced in the polluted air mass from the action of light on nitrogen dioxide, the brown gas that typically gives smog its colour NO2 + light (blue, or near ultraviolet) --> NO + O O + O2 + (N2 or O2) --> O3 + (N2 or O2) It is continually being produced, and is rapidly being destroyed, so it builds up to a steady concentration so long as both NO2 and blue light are around. But when the sun gets lower in the sky, and the sunlight weakens and is scattered away by the smog particles, the ozone concentration starts to fall. It responds quickly -- response time roughly ten minutes, though it does depend on the detail of the smog composition and transport behaviour. Ozone reacts with organic materials in the polluted air. Among other things it helps to turn less toxic hydrocarbon gases into more toxic and less volatile oxygenated compounds -- aldehydes, ketones, acids, peroxy compounds. There are many discussions of photochemical smog in elementary chemical textbooks, but I do not know of one that gives quite the information that you want. For an advanced reference and discussion, try R.P. Wayne "Chemistry of Atmospheres" 2nd Ed., Clarendon (Oxford) 1991. Pages 252-263. Look especially at Figure 5.9.
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