Global Tropospheric Chemistry A Plan for Action (1984) / Chapter Skim
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2 A FRAMEWORK
2 A FRAMEWORK
Pages 11-18
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From page 11... ...
Materials injected into the troposphere can undergo chemical reactions, some of which are cyclic and some of which produce a wide range of species that can have chemical and physical properties very different from those of the reactants; such transformations can effectively remove the species from a specific chemical cycle. The distribution of a species in the troposphere will be dependent on source characteristics and on the controlling chemical reactions; however, distributions are also greatly affected by a variety of transport processes that range in scale from that of boundary layer turbulence to that of planetary flow.
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From page 12... ...
At this time, it is not possible to make accurate global estimates of the natural surface sources for such globally important trace gases as CH4, N2O, ammonia (NH3) , carbonyl sulfide (COS)
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From page 13... ...
Cyclones, in combination with upper tropospheric jet streams, also provide an effective mechanism for transporting materials downward from the upper troposphere and lower stratosphere. On the largest scale, thermally driven upward transport in the tropics mixes the troposphere as a whole in that region and transports trace gases from the
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From page 14... ...
Gases such as CH4 and CH3C1, with somewhat shorter tropospheric lifetimes of a few years, are generally well-mixed vertically, and there are only small hemispheric differences. Gases with tropospheric lifetimes of a few months or less (CO, O3, SO2, NO, NO2, HNO3, end reactive hydrocarbons, for A key process in all the biogeochemical cycles is the chemical transformation oftropospheric trace gases into species that are either nonreactive in the troposphere or easily removed by rain or surface deposition.
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From page 15... ...
The key issue here is their chemical composition and deposition rate since these provide essential information on the sources and sinks of various species. It is difficult to develop estimates of global removal rates from local data sets because of the great spatial variability of precipitation events and of the concentration of particles and soluble trace gases.
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From page 16... ...
For gases with surface sources such as NO2 and SO2, dry removal is, however, even more difficult to evaluate globally because local concentration measurements are extremely dependent on the distance of the sampling site from the sources. Deposition velocities, i.e., the ratio ofthe flux of a substance to its mean air concentration at some reference height near the surface, have been measured for O3 over a range of surfaces, and there are some similar data for HNO3, NO2, and SO2.
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From page 17... ...
The right side of the figure lists atmospheric processes affected by the species or environment identified in the rectangles. Triangles denote processes where material flows primarily in one direction; diamonds represent reversible processes: a, sources; b, sinks; c, gas-to-particle conversion; d, sorption; e, deliquescence; f, efflorescence; g, Raoult's equilibrium; h, reaction in concentrated solution droplet; i, nucleation and condensation of water; j, evaporation; k, capture of aerosol by cloud drops; l, reaction in dilute solution; m, rain; n, freezing of supercooled drop by ice nucleus; o, melting; p, direct sublimation of ice on ice nucleus; q, precipitation.
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From page 18... ...
A quantitative understanding of these fundamental processes will enable predictive models of the PART I A PLAN FOR ACTION SUMMARY tropospheric chemical system to be developed and the physical effects of trace substances in the troposphere to be determined. Predictive models will allow the effects of future perturbations ofthe global troposphere to be evaluated.
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