Lead, biosphere cycling

As Barr et al. (2003) pointed out, the importance of such emissions is determined mainly by their impact on the three processes taking place in the atmosphere. The first consists in that such NMHCs as isoprene form in the course of carboxylization in plants and contribute much thereby to the formation of biospheric carbon cycle. The second process is connected with NMHCs exhibiting high chemical activity with respect to such main oxidants as hydroxyl radicals (OH), ozone (03), and nitrate radicals (N03). Reactions with the participation of such components result in the formation of radicals of alkylperoxides (R02), which favor efficient transformation of nitrogen monoxide (NO) into nitrogen dioxide (N02), which favors an increase of ozone concentration in the ABL. Finally, NMHC oxidation leads to the formation of such carbonyl compounds as formaldehyde (HCHO), which stimulates the processes of 03 formation. Finally, the oxidation of monoterpenes and sesquiterpenes results in the intensive formation of fine carbon aerosol with a particle diameter of <0.4 pm... 

For example, in the carbon cycle consider the balance between terrestrial photosynthesis and respiration-decay. If the respiration and decay flux to the atmosphere were doubled (perhaps by a temperature increase) from about 5200 x 1012 to 10,400 x 1012 moles y-l, and photosynthesis remained constant, the CO2 content of the atmosphere would be doubled in about 12 years. If the reverse occurred, and photosynthesis were doubled, while respiration and decay remained constant, the CO2 content of the atmosphere would be halved in about the same time. An effective and rapid feedback mechanism is necessary to prevent such excursions, although they have occurred in the geologic past. On a short time scale (hundreds of years or less), the feedbacks involve the ocean and terrestrial biota. As was shown in Chapter 4, an increase in atmospheric CO2 leads to an increase in the uptake of CO2 in the ocean. Also, an initial increase in atmospheric CO2 could lead to fertilization of those terrestrial plants which are not nutrient limited, provided there is sufficient water, removal of CO2, and growth of the terrestrial biosphere. Thus, both of the aforementioned processes are feedback mechanisms that can operate in a positive or negative sense. An increased rate of photosynthesis would deplete atmospheric CO2, which would in turn decrease photosynthesis and increase the oceanic evasion rate of CO2, leading to a rise in atmospheric CO2 content. More will be said later about feedback mechanisms in the carbon system. 

The migration of chemical elements in a thermodynamically open system does by no means lead to chaos. The direction of mass transport is kept under control by a large variety of diverse equilibrium processes. On the other hand, the main feature of the migration cycles of the biosphere, i.e. their openness, depends on this equilibrium. 

This chapter also serves as an environmental road map, laying out the physical, physicochemical and chemical means by which lead is transported and mobilized in and out of environmental compartments that serve as lead contact points for human and ecological populations. Environmental compartment cycling of lead until recently was a largely unknown cluster of phenomena, and the multimedia impact of lead emissions on the larger biosphere was little understood and even misunderstood. This chapter is not intended to be encyclopedic, but it focuses on data eventually useful for human health risk assessment and regulatory initiatives. 

In conclusion, accurate and precise measurements of lead, including environmental and human lead burdens relative to natural levels, lead cycling and metabolism, and lead biomolecular speciation, are required to fully evaluate the problem of lead contamination and toxicity in the biosphere. Current data demonstrate that environmental lead levels are often more than 10-fold and sometimes more than 10,000-fold higher than natural levels. Although regulation of many lead-containing products in some industrialized countries has resulted in a reduction in some lead discharges into the biosphere, the demonstrated persistence of contaminant lead indicates that environmental lead levels will remain substantially elevated for a protracted time. 

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