Global Tropospheric Chemistry A Plan for Action (1984) / Chapter Skim
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5 CRITICAL PROCESSES AFFECTING THE DISTRIBUTION OF CHEMICAL SPECIES
5 CRITICAL PROCESSES AFFECTING THE DISTRIBUTION OF CHEMICAL SPECIES
Pages 55-93
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From page 55... ...
HARRISS, AND R DICKINSON THE IMPORTANCE OF BIOLOGICAL SOURCES In recent years we have seen increasingly clear manifestations of the important, even dominant, roles of biological processes as sources of atmospheric chemicals.
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From page 56... ...
In the photochemical control oftropospheric 03, biogenic gases such as CH4, CO, many hydrocarbons, N2O, and possibly NH3 are important players, N2O through its stratospheric production of odd-nitrogen oxides that flow downward into the upper troposphere, and NH3 as a possible NOX source and as an NOX sink. From a climatic point of view, the potential warming effects of several biogenic and anthropogenic trace gases are startling.
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From page 57... ...
Of the elements from biological sources appearing in the atmosphere, the ones most actively cycled are those having several oxidation states within the range of stability of water, and for which at least one reasonably stable gaseous form exists. Under the reducing conditions of anoxic environments (reducing because some biologically oxidizable compound is present and the influx of atmospheric oxygen is limited by a diffusion barrier or other means)
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From page 58... ...
The carbon of atmospheric CH4 contains less ~4C than that of atmospheric CO2. Biological sources are largely "modern," and as noted above, they are large.
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From page 59... ...
To identify biomes of special interest in atmospheric chemistry, Table 5.2 lists about twenty specific, although informally classified, biomes and one process, biomass burning, as the rows of a matrix. The columns of this ~.
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From page 60... ...
The suggested sites include: TABLE 5. 1 Surface Areas, Net Primary Productivity, and Phytomass of Terrestrial Ecosystems of the Biospherea Ecosystem Type 1.
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From page 61... ...
approach is most desirable. This approach would provide boundary layer gradient information on volatiles of interest, which, in turn, would make possible the estimation of emission and absorption rates.
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From page 62... ...
Tropical continental areas, both wet forested regions and drier sites such as savannas, are probably major sources of a variety of trace gases important in tropospheric chemistry. The potential importance of these regions stems from their geographical size, biological productivity, anoxic environments (in wet areas)
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From page 63... ...
Potentially large emissions of gaseous hydrocarbons, N2O, CH4, reduced sulfur compounds, and possibly methylated metals and CO (see below) can be released from tropical forests.
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From page 64... ...
Recent estimates of the annual global area involved in biomass burning range from 3 to 7 x 1 o6 km2, with estimates of the total biomass burned ranging from 4400 to 7000 Tg/yr. Although total biomass burning quantities are probably uncertain to within a factor of 2 or 3 and vary from year to year, they PART II ASSESSMENTS OF CURRENT UNDERSTANDING are almost certainly significant in the global atmospheric carbon cycle and probably in other cycles as well, e.g., oxygen and nitrogen.
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From page 65... ...
Rice Agriculture The potential importance of rice paddies as sources of atmospheric chemicals might not seem obvious. Several ~ - ~ .
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From page 66... ...
Once in the marine boundary layer, DMS is probably oxidized by photochemical processes to produce SO2 with intermediates such as DMSO and methane sulfonic acid. Qualitatively, the concentration of reduced sulfur compounds in surface seawater correlates with PART II ASSESSMENTS OF CURRENT UNDERSTANDING indicators of algal biomass.
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From page 67... ...
To achieve the goal of a better understanding of global tropospheric chemistry requires that both the magnitude of surface sources at their origin and the subsequent transport from the planetary boundary layer to the free troposphere be determined. For certain reactive reduced gas species, removal processes in the boundary layer may restrict the significant influence of a particular species to regional or hemispheric scales (e.g., HIS, CO, and C5He)
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From page 68... ...
Seller (1979~. Biomass burning as a source of atmospheric gases CO, H2, N2O, NO, CH3C1 and COS.
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From page 69... ...
To this end we recommend measurements: 3. To establish, quantify, and explain long-term trends in the concentration and distribution of environmentally important trace gases, especially those that are radiatively active.
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From page 71... ...
This poor image persists in some circles despite the fact that some of the most exciting developments in atmospheric chemistry in recent years have been derived from programs that involved a monitoring effort. The most outstanding example is that of CO2.
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From page 72... ...
program, also monitor other trace gases (such as the halocarbons, CO, and 03~. As of 1981, the WMO network consisted of 100 regional, 12 continental, and 12 baseline stations operating in 49 countries (Figure 5.3~.
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From page 73... ...
Indeed, many of these questions will be addressed by the individual research programs that are focused on specific processes; consequently, this essential information will not be available for some years to come. Faced with these gaps in the knowledge of atmospheric chemistry and meteorology and because of the problems with logistics, we took a pragmatic approach.
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From page 74... ...
Clearly, it should not be necessary to measure N2O very often or at many different stations in order to characterize these trends. At the other extreme, aerosol particles have a residence time of 1 to 2 weeks.
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From page 75... ...
The most impressive and extensive data set on the distribution of a broad range of 75 important atmospheric chemistry species was that obtained in a series of flights made in 1977 as part ofthe Global Atmospheric Measurement Experiment on Aerosols and Gases (GAMETAG)
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From page 76... ...
While paired mountain/surface measurements will provide useful free troposphere/boundary layer data, extensive vertical profile data will require the use of aircraft (except for O3 and water vapor, where sondes can be used)
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From page 77... ...
CRITICAL PROCESSES Turco, R
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From page 78... ...
For the mostpart, the key elements(e.g., carbon, nitrogen, and sulfur) making up these trace gases are in reduced oxidation states.
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From page 79... ...
6, the eventual Mule~steps CxHy-~'H2O \ 79 CO, H2O Multisteps CO HNO~ ~5 3 XO, O2 HCi, H2O FIGURE 5.4 The central role of OH in the oxidation of tropospheric trace gases. fate of the CH3O appears to be the formation of CO via the intermediate product, formaldehyde (CH2O)
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From page 80... ...
most likely classes of compounds that might fit this role are the nonmethane hydrocarbons, NMHC (CxH', where x andy > 1~. In general, the atmospheric chemistry of the NMHCs is far more complex than that of CH4.
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From page 81... ...
There is now some evidence, in fact, that suggests that this species could define a major reservoir for NOX in the "remote" free troposphere. Like the HxOy fast-photochemical cycle, there is now a great abundance of laboratory kinetic data that suggest that the fast NxOy cycle is one of the key photochemical cycles operating in the troposphere.
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From page 82... ...
Thus, whether the coupled HXo'/NxOy cycles define a net sink or source for O3 in the "remote" troposphere stands as one of the major unanswered scientific questions in global tropospheric chemistry. Noncyclic Transformations Earlier, it was noted that the OH radical appears to be the principal species responsible for initiating the oxidation of numerous "reduced" trace gas species.
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From page 83... ...
Traditionally, the approach taken in unraveling this chemistry has involved studies of closed chemical reactor systems in which only two or perhaps three major chemical species are added to solution reactors. (In many respects, these studies have their analogue in gasphase smog chamber investigations.)
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From page 84... ...
Of course, solid-solid and liquid-liquid interfaces are also possible, but are not likely to be of great importance in atmospheric chemistry. The study of interfaces, with their interesting and significant physical and chemical processes, is an old discipline that has recently reawakened.
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From page 85... ...
These reactions might better be termed multiphase rather than heterogeneous because the reactants react in one phase, although some originate from another phase. One special class of heterogeneous reactions is that Direct Emission or Produced by Gas BOX 1 Water Vapor, Atmospheric Trace Gases (NH3, NOx, H2S, SO2'03' Unsaturated Hydrocarbons)
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From page 86... ...
The above discussion makes it quite apparent that the inclusion of heterogeneous processes is essential to PART II ASSESSMENTS OF CURRENT UNDERSTANDING A HO2 ~ HN024 p NO ~ ~a' l i ~ H NO3 ~ p, A H2O2-OHMS 1~ __ NO2 ~ HO2 NO2 A, P I ~ ~-` -- -- _ A' ,>~` \ NO3 ~ ~ N2O5 !
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From page 87... ...
Hamill (1982~. Importance of heterogeneous processes to tropospheric chemistry: studies with a one-dimensional model, in Heterogeneous Atmospheric Chemistry.
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From page 88... ...
are easily scavenged by falling raindrops and are rapidly adsorbed at exposed surfaces. WET DEPOSITION Wet deposition constitutes a very intermittent but highly efficient mechanism for transforming and eventually removing trace gases and aerosol particles from the troposphere.
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From page 89... ...
Scavenging ratios for materials of surface origin are likely to be underestimates if groundlevel air concentrations are used in their derivation, because air concentrations near the surface will generally be greater than those characteristic of the air from which the material is being scavenged by precipitation. Similarly, experimental evaluations of scavenging ratios for substances with sources in the upper troposphere will tend to be too high if ground-level air concentration data are used.
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From page 90... ...
Recent emphasis on precipitation acidity has tended to divert attention from the basic questions of precipitation scavenging of particular trace gases and aerosol particles. High rainfall acidity does not necessarily mean very high concentrations of dissolved trace species in the rain, nor does a pH of 7 mean that the rain is completely free of dissolved material.
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From page 91... ...
The coefficient of proportionality between atmospheric concentrations and dry deposition rates, which is known as the deposition velocity, clearly depends on the meteorological conditions, the chemical nature of the substance in question, and the nature of the surface on which it is being deposited. The term "deposition velocity" suggests an analogy with gravitational settling that is usually incorrect.
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From page 92... ...
The first advocates the use of collecting surfaces and subsequent careful chemical analysis of material deposited on them. The second infers deposition rates from routine measurements of air concentration of the trace gases and aerosol particles of concern and of relevant atmospheric and surface quantities.
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From page 93... ...
1, Precipitation Scavenging. Elsevier, New York, 729 pp.
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