Anthropogenic emissions global sources

Forests can act as sources of some of the trace gases in the atmosphere, such as hydrocarbons, hydrogen sulfide, NO, and NH3. Forests have been identified as emitters of terpene hydrocarbons. In 1960, Went (10) estimated that hydrocarbon releases to the atmosphere were on the order of 108 tons per year. Later work by Rasmussen (11) suggested that the release of terpenes from forest systems is 2 x 10 tons of reactive materials per year on a global basis. This is several times the anthropogenic input. Yet, it is important to remember that forest emissions are much more widely dispersed and less concentrated than anthropogenic emissions. Table 8-2 shows terpene emissions from different types of forest systems in the United States. 

If all parts of the livestock production lifecycle are included, livestock are estimated to account for 18% of global anthropogenic emissions [57]. According to Gill and coworkers [57], apart from enteric fermentation and manure management, the other sources of GHG emission from livestock and related activities are fossil fuels used during feed and fertilizer production and transport of processed animal products. 

TABLE 2.1 Global Emission Estimates for CO, NOx, CH4, and VOC from Both Anthropogenic and Natural Sources (in Tg/yr)u... 

The atmospheric deposition rate in this midcontinental area, which has increased by a factor of about 3.7, suggests that natural Hg concentrations were only about 25% of modern levels. Current estimates of recent increases in global atmospheric Hg support this conclusion, and indicate that increased anthropogenic Hg emissions, rather than enhanced removal by atmospheric oxidants, are responsible for elevated Hg deposition. Moreover, the increase appears to be relatively uniform across our study area, implying regional if not global sources for the Hg falling on these remote sites. 

On a global scale, natural emissions of reduced sulfur compounds account for about 50% of the total sulfur flux into the atmosphere (1-3). Hence, it is important to understand the natural sulfur cycle in order to establish a "base line" for assessing the significance of anthropogenic perturbations (primarily SO2 emissions). Dimethylsul-fide (DMS) is the predominant reduced sulfur compound entering the atmosphere from the oceans (4-9), and DMS oxidation represents a major global source of S(VI). The atmospheric oxidation of DMS can be initiated by reaction with either OH or NO3. In marine environments, however, NO3 levels are typically very low and DMS is destroyed primarily by OH ... 

Table 28. The most important anthropogenic emission sources of mercury to the atmosphere on the global level in 1983 (After Nriagu and Pacina, 1988).

We can compare the natural and anthropogenic emissions of lead in the global cycle. Table 32 shows the natural sources and Table 33 demonstrates the major anthropogenic ones. 

According to the results of measurements carried out in the ocean (Seiler, 1974) the carbon monoxide concentration in near surface marine layers is 5 x 10 s mil 1 on an average. This water concentration would be in equilibrium with a surface air CO level of 2.5 ppm. However, the carbon monoxide concentration in air over the ocean surface is between 0.04 ppm and 0.20 ppm, which means that the ocean water is supersaturated with CO. It follows from these data that the ocean is a CO source, the global strength of which is about six times less than the total anthropogenic emission (Table 7). 

Hydrocarbons are known to be released from anthropogenic sources, from vegetation and soils, and from the oceans. Duce et al. (1983) have reviewed available emission rates. These refer almost entirely to bulk emissions and not to individual compounds, because information on the distribution of hydrocarbons being emitted is in many cases incomplete. Ehhalt et al. (1986) have prepared a more detailed global source estimate for the C2-C5 hydrocarbons. Source distributions for the light hydrocarbons are now reasonably well known. The following presentation includes information on source distributions as far as possible. 

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