Global Tropospheric Chemistry A Plan for Action (1984) / Chapter Skim
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7 TROPOSPHERIC CHEMICAL CYCLES
7 TROPOSPHERIC CHEMICAL CYCLES
Pages 101-140
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From page 101... ...
2, gives an annual increment of about 0.3 percent. Industrial nitrogen fixation and the use of legumes have about equaled "natural" nitrogen f~xation, and sulfur from fossil fuel combustion and mining activities has about equaled the natural sources of atmospheric sulfur.
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From page 102... ...
~ . I Land A - als | 1 1 Subduction Wedge _: / PART II ASSESSMENTS OF CURRENT UNDERSTANDING | Atmosphere | Ocean Plate 1.
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From page 103... ...
Although some pool sizes are OXYGEN SEO AC1 SEO SER SO OW ICE CW FEO COM PA RTM E NT SU LFU R S04 PW AO AW S I 01 ASO ASU ASH COMPARTMENT known with reasonable accuracy, others are accurate only to a factor of 2 or more. Numeric values are also presented in Table 7.
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From page 104... ...
Biological Response to Fossil Fuel Combustion Products, in Global Chemical Cycles and Their Alterations by Man, Werner Stumm, ed. Dahlen Konferenzen, Berlin, pp.
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From page 105... ...
The sulfur cycle: man's contributions are compared to natural sources of sulfur compounds in the atmosphere and oceans. Science 175:587-596.
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From page 106... ...
On the global average, 1 .0 m of water per year moves from the surface to the atmosphere and falls again as precipitation. The water vapor at wet interfaces is maintained at the saturation vapor pressure by equilibrium between the wet surface and its immediately adjacent molecular boundary layers.
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From page 107... ...
TRANSPORT AND DISTRIBUTION Water vapor moves from the surface, through the planetary boundary layer, and then into the free atmosphere, where it is redistributed horizontally and vertically by atmospheric wind systems until it undergoes gas-to-droplet conversion. On a global average, a column of air holds about 27 kg/m2 of water.
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From page 108... ...
These generally occur on a horizontal scale with a fine structure of the order of 1 km and a PART II ASSESSMENTS OF CURRENT UNDERSTANDING large-scale organization of the order of 10 to 100 km. Layered precipitation systems are driven by upward motions in large-scale atmospheric wind systems forced by other modes of atmospheric instability.
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From page 109... ...
These different approaches all arrive at a cross-tropopause flux in the range 3-12 x 10~° molecules per square centimeter per second. In situ photochemical production occurs both in the polluted boundary layer and in the free troposphere as a whole.
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From page 110... ...
Recent analyses of some regional boundary layer data in the equatorial Pacific strongly supports the existence of a photochemical removal process with an effective column removal rate of the order of 1-2 x 10~i molecules per square centimeter per second. This removal rate is much larger than the estimated surface deposition flux and is needed to explain the extremely low O3 mixing ratios (5 to 10 ppb)
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From page 111... ...
Recently, a unification of the transport and photochemical theory was proposed in which photochemical production occurred primarily in the upper troposphere with the precursor NOX being injected from the stratosphere. Ozone destruction would then dominate in the lower troposphere where NOX was very low.
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From page 112... ...
Estimating the ozone budget in the boundary layer by use of aircraft measurements of ozone eddy flux and mean concentration.
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From page 113... ...
The observed global increases of N2O indicate that the global nitrogen cycle is not in a perfectly balanced steady state. SOURCES Emissions from fossil fuel burning (primarily hightemperature combustion)
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From page 114... ...
Because NH3 is readily absorbed by surfaces such as water, plants, soil, and atmospheric aerosol particles, its residence time should be very short in the lower troposphere. In fact, NH3 emitted from vegetation-covered soil may have little chance of escaping to the atmosphere.
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From page 115... ...
If N2O5 or NO3 is scavenged by aerosol particles as indicated by recent field measurements of NO3, they would 115 constitute a significant sink for odd nitrogen and a major source for HNO3 or particulate NO3 . The photochemical transformations among oddnitrogen species discussed above are summarized in Figure 7.4.
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From page 116... ...
The global nitrogen cycle, in Nitrogen, Phosphorus, arid Sulfur: Global Cycles. SCOPE Report 7.
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From page 117... ...
Experiments in coastal waters TABLE 7.4 Approximate Tropospheric Concentration Range of Selected Sulfur Compounds in Unpolluted Air Atmospheric Concentrationsa (ng/m3) Location H2S DMS CS2 COS SO2 Ocean boundary layer Temperate continental boundary layer Tropical coastal boundary layer Free troposphere < 5-150 20-200 1 00-9000 PD < 2-200 PD PD < 2(PD)
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From page 118... ...
Combustion processes are also thought to be a significant global source of COS. These processes include biomass burning, fossil fiael burning, and high-temperature industrial processes involving sulfur compounds.
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From page 119... ...
In situ photochemical production from COS and CS2 precursors is the most likely source of H2S measured in remote ocean air. Atmospheric concentrations of H2S in continental air are highly variable, resulting from a complex interaction of factors determining ground emissions, in situ photochemical production, and atmospheric lifet~me.
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From page 120... ...
Evaporation of cloud droplets may produce small sulfate-rich aerosol particles that subsequently act as cloud condensation nuclei. If the transport and reaction mechanisms mentioned in this paragraph are active over large areas of the nonurban troposphere, they contribute to explanations for acid rain in remote oceanic regions and higher SO2 in the free troposphere than in underlying ocean boundary layer air.
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From page 121... ...
(1982~. Exchange of sulfur between the free troposphere, marine boundary layer, and the sea surface..
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From page 122... ...
THE CYCLES OF REACTIVE CARBON COMPOUNDS Sources Major direct sources of CO appear to be primarily anthropogenic, e.g., fossil fuel use and tropical biomass burning. The latter source is still poorly quantified in terms of spatial and seasonal variations.
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From page 123... ...
. Existing speculations on the total source strength for POC are based on rough estimates of the tropospheric POC distribution 123 and the mean tropospheric residence time of aerosol particles (4 to 7 days)
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From page 124... ...
Carboxylic acids, particularly formic and acetic acids, are a common constituent of both aerosol particles and rain and have also been observed in the vapor phase. There is evidence that these weak organic acids PART II ASSESSMENTS OF CURRENT UNDERSTANDING can contribute a significant fraction of the free acidity of precipitation at locations far from urban areas.
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From page 125... ...
Aqueous photochemistry involving OH and O3 may play a particularly important role in transforming POC, as indicated by the presence of a variety of oxygenated compounds in POC. Dry deposition processes involving the sea, soil, and vegetation are also potentially significant sinks.
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From page 126... ...
A detailed assessment of chemical ir~teractions in atmospheric aerosol particles is hampered by a scarcity of relevant rate data. BIBLIOGRAPHY Andreae, M
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From page 127... ...
and S containing compounds, Chapter 3 in The Major Biogeochemical Cycles and Their Interactions.
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From page 128... ...
Also, for each halogen there are volatile inorganic and organic species. In the liquid phase, there are dissolved halides and halide ions, and there are halide condensates on aerosol particles.
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From page 129... ...
Generally, the C1- content of aerosol particles decreases with distance from oceans; isolated observations of higher chloride in continental aerosol particles have been attributed to large intrusions of marine air or to pollution sources. In rainfall, there is generally more chloride (up to 10 mg/1)
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From page 130... ...
Although its vertical and latitudinal distributions are not known, HC1 is probably most concentrated (1 to 2 ppbv) in the marine boundary layer, where its residence time is perhaps 4 days.
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From page 131... ...
Aged aerosol particles do gather gaseous iodine to increase the I-/C1- ratio further, and if so, what are the sources of Ig that allow this? Fluorine sources, especially those for Fg, are similar to those for chlorine.
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From page 132... ...
Equilibrium-type data such as vapor pressures, even when available, are not necessarily valid when complicated multiphase, multiconstituent mixtures are to be considered. Homogeneous aqueous-phase reactions and transformations are potentially very important in clouds, rain, and water-coated aerosol particles, for example, with halogens as oxidizing agents, but virtually no research has been performed on this topic.
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From page 133... ...
Many, if not most, trace elements are present entirely in the particulate phase and are not directly involved in gas-particle conversion processes or other aspects of gasphase tropospheric chemistry. Being primarily present in aerosol particles, their tropospheric residence times are of the order of days to a few weeks, and there has been relatively little effort to evaluate global-scale changes in their distribution caused by human activity.
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From page 134... ...
DISTRIBUTION A growing data base is developing on the trace element composition of aerosol particles in remote regions. Reasonably good data are available over short time periods from the boundary layer in both polar regions and over the Atlantic and Pacific Oceans.
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From page 135... ...
Duce (1983~. Vapor phase and particulate selenium in the marine atmosphere.
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From page 136... ...
The subject of aerosol particles is somewhat anomalous in the general context of this section on cycles since all the other subjects focus on specific chemical species. Indeed, in most cases, the atmospheric aerosol is the end product of many of the chemical processes acting on the aforementioned species.
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From page 137... ...
The rates of production and the physical and chemical characteristics of sea-salt aerosol are important for a number of reasons. For example, salt spray may play an important role in transporting trace metals and organic materials from the ocean into the atmosphere and in absorbing or reacting with gaseous species in the marine boundary layer.
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From page 138... ...
a 138 PART II ASSESSMENTS OF CURRENT UNDERSTANDING TABLE 7.8 Estimates of Global Particle Production from Natural and Man-Made Sources ~ ~ o6 tons/yr) After Study of Marl 's Impact on Climate ( 1 9 7 ~ ja Other < 20,um < 6 lamb Estimatesa Man-Made Direct particle production Transportation 1.8 Stationary fuel sources 9.6 Industrial processes 12.4 Solid waste disposal 0.4 Miscellaneous 5.4 Subtotal 29.6 37-110 10-90 6-54 54 126 Particles formed from gases Converted sulfates 200 110 130-200 Converted nitrates 35 23 30-35 Converted hydrocarbons 15 27 15-90 Subtotal 250 160 175-325 270 Total man-made 280 269 185-415 396 Natural Direct particle production Sea salt Windblown dust Volcanic emissions Meteoric debris 500 250 25 o 1095 300 180 60-360 100-500 60-300 4 0.02-0.2 25-150 15-90 Forest fires 5 146 3-150 Subtotal 780 1610 428- 1100 1730 Particles formed from gases Converted sulfates 335 37-365 130-200 Converted nitrates 60 600-620 140-700 160 Converted hydrocarbons 75 182-1095 75-200 154 Subtotal 470 2080 345-1100 1319 Total natural -1250 3690 773-2200 3049 Grand Total - ~1530 3959 958-2615 3445 1000-2000 70 60-360 128 + 64 200 + 100 4.2 1-10 0.02-0.2 aFor references see Bach (1976)
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From page 139... ...
Of particular interest are the possible effects on the urban and regional scale where the magnitude of the anthropogenic sulfur sources is dramatically higher for example, in the eastern United States, where it is 10 times that of natural sources. The assessment of the impact of man on weather or climate is difficult for a number of reasons.
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From page 140... ...
Most important in this category are the sulfur species that are injected into the stratosphere, where they are oxidized to sulfuric acid droplets, which have a residence time of years.
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