8/19/2019 Chemisty of Air

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The study of atmospheric chemistry as a scientific discipline goes back to the eighteenth century,

when the principal issue was identifying the major, chemical components of the atmosphere,

nitrogen, oxygen, water, carbon dioxide, and the nobel gases. In the late ninetheenth and early

twentieth centuries attention turned of air by volume (1 mol per mole!. "e now know that the

atmospheric contains a myriad of trace species some at levels as low 1 part per trillion parts of 

air. The role od trace species is disproportionate to their atmospheric abundance# they are

responsible for phenomena ranging for urban photochemical smog, to acid desposition, to

stratospheric o$one depletion, to potential climate change.%oreover , the composition of the

atmosphere is changing& analysis of air trapped in ice cores revals a record of striking increases

in the long' lived so'calleed greenhouse gases, carbon dioxide()*!, methane (+!, and nitrous

oxide (-*)!. "ithin the last century, concentration of tropospheric o$one () !, sulfate (/)*'

!,

and carbonaceous aerols in the -orthem +emisphere have increased significantly. There is

evidence that all these changes are alerting the basic chemistry of the atmosphere.

0tmospheric chemistry occurs within a fabric of profoundly complicated atmospheric

dynamics.The results of this coupling od dynamics and chemistry are often unexpected. "itness

the uniue combination of dynamical forces that lead to a wintertime polar vortex over 0ntartica,

with the concomitant formation od polar stratospheric clouds that serve as sites for 

heterogeneous chemical reactions involving ahlorine compaunds resulting from anthrogenic

chlorofluorocarbons' all leading to the near total depletion of stratospheric o$one over the /outh

2ole each spring# witness the nonlinear, and counterintutive, dependence of the amout of o$one

generated by reactions involving hydrocarbons and oxides od nitrogen (-)3! at the urban and

regional scale' although both hydrocarbons and -)3 are o$one precursors, situations exist where

continious emission of more and more -)3 actually leads to less o$one.

The cemical constitutens of the atmosphere do not go through theit life cycles independently# the

cycles of the various species are linked together in a complex way.Thus a perturbation of the one

component can lead to significant,and nonlinear, changes to other components and to feedbacks

that can amplify or damp the original perturbation.In many respects , at one both the most

important and the most paradoxical trace gas in the atmosphere is o$one ()!.