Particulate organic carbon transformations

Transformations and Cycling of Dissolved and Particulate Organic Carbon... 

Particulate organic carbon comprises a large number of compounds having significant variations in volatility as a result, a number of these compounds can be present in both the gas and particulate phases. The ability of such semivolatile compounds to coexist in both phases complicates the distinction between primary and secondary OC. Strictly speaking, secondary OC starts its atmospheric life in the gas phase as a VOC, undergoes one or more chemical transformations in the gas phase to a less volatile compound, and finally transfers to the particulate phase by condensation or nucleation. Therefore the term... 

Figure 6.1. Ecosystem area and soil carbon content to 3-m depth. Lower Panel Global areal extent of major ecosystems, transformed by land use in yellow, untransformed in purple. Data from Hassan et al. (2005) except for Mediterranean-climate ecosystems transformation impact is from Myers et al. (2000) and ocean surface area is from Hassan et al. (2005). Upper Panel Total C stores in plant biomass, soil, yedoma/permafrost. D, deserts G S(tr), tropical grasslands and savannas G(te), temperate grasslands ME, Mediterranean ecosystems F(tr), tropical forests F(te), temperate forests F(b), boreal forests T, tundra FW, freshwater lakes and wetlands C, croplands O, oceans. Data are from Sabine et al. (2004), except C content of yedoma permafrost and permafrost (hght blue columns, left and right, respectively Zimov et al., 2006), and ocean organic C content (dissolved plus particulate organic Denman et al., 2007). This figure considers soil C to 3-m depth (Jobbagy and Jackson, 2000). Approximate carbon content of the atmosphere is indicated by the dotted lines for last glacial maximum (LGM), pre-industrial (P-IND) and current (about 2000). Reprinted from Fischlin et al. (2007) in IPCC (2007). See color insert.

The above mentioned urban air pollution in Asian cities drives the tropospheric chemical reactions. This tropospheric chemistry is dominated by the oxidation of trace atmospheric components, as aresult ofwhich organic compounds such as methane and other hydrocarbons are converted into carbon dioxide and water. The consequences of these chemical transformations are known as photochemical smog (photosmog) and the associated problem of ground level ozone. Here we should consider also the effects of particulate matter, one of the major pollutants of urban air in Asia. 

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

The analysed data on carbon content and also on content of organic N and P in dissolved and particulate state at the end of all the experiments illustrate the stability of OM. The experiments simulate accumulation of resistant insoluble and soluble organic compounds in reservoirs and soils. These compounds appear as the result of decomposition of dead living matter and its living excretions. The processes are not only responsible for transformation of initial OM (oxidation, polymerisation, condensation), but also for synthetic processes, caused by bacterial activity (Kononova, 1963), and this is the water humus of planktonic origin. 

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