Anthropogenic NOX emissions

FIGURE 2.2 Contribution of various sources to total anthropogenic NOx emissions in the United States in 1996 (from EPA, 1999). 

The reason why SIA is higher in urban areas is less obvious as these are secondary aerosols. The observed increment is predominantly caused by more nitrate and sulphate. The reaction of nitric acid and sulphuric acid with the sea-salt aerosol in a marine urbanised environment follows an irreversible reaction scheme. In essence, the chloride depletion stabilises part of the nitrate and sulphate in the coarse mode and may partly explain part of the observed increment. However, it also raises the question how to assign the coarse mode nitrate in the mass closure. The sea salt and nitrate contributions cannot simply be added any more as nitrate replaces chloride. Reduction of NOx emissions may cause a reduction of coarse mode nitrate, which is partly compensated by the fact that chloride is not lost anymore. A reduction would yield a net result of ((N03-C1)/N03 = (62-35)/62=) 27/62 times the nitrate reduction (where the numbers are molar weights of the respective components), and this factor could be used to scale back the coarse nitrate fraction in the chemical mass balance. A similar reasoning may be valid for the anthropogenic sulphate in the coarse fraction. Corrections like these are uncommon in current mass closure studies, and consequences will have to be explored in more detail. 

Once MNT-enabled solar energy becomes the exclusive energy source, problems such as acid rain and smog should not exist. In addition, future vehicles that are constructed from nano-materials, driven by nano-electromechanical systems and powered by hydrogen fuel cells (see Fig. 7) or solar cells (see Fig. 8) should totally eliminate transportation-related S02 and NOx emission. Therefore, the anthropogenic release of S02 and NOx that has assaulted the atmosphere since the Industrial Revolution should be ceased further acidification of the environment and the threat to human health will be relieved [31-33]. 

While O3 is not directly affected by anthropogenic emissions, note that as emissions of hydrocarbons CHa, O, and NOx increase they tend to enhance the rate of O3 production and could eventually enhance tropospheric O3 levels on a global scale as they already have on a local and regional scale. In fact Fishman et al. (1979) have argued on the basis of the observed asymmetry between northern and southern hemispheric ozone that CO and NOx emissions primarily from fossil fuel burning have already led to an increase in O3 levels in much of the northern hemisphere. 

The 1992 U.S. emissions of anthropogenic nitrogen oxides (NOx) are estimated at 23 million tons. Of this amount, approximately 45% were from transportation sources (cars, trucks, etc.) and the remainder from stationary sources. Examples of stationary source emitters include power plants (53%), internal combustion engines (20%), industrial boilers (14%), process heaters (5%), and gas turbines (2%). Total NOx emissions are estimated to have increased 5% since 1983. Stationary sources have accounted for the majority of the increase emissions from mobile sources have remained relatively constant. Approximately 51% of the total NOx emissions are a result of combustion in stationary-sources applications [1]. 

Olivier JGJ, Bouwman AF, Van der Hoek KW, Berdowski JJM. 1998. Global air emission inventories for anthropogenic sources of NOx,NH3 and N2O in 1990. Environmental Pollution 102 135-148. 

Table 2.1 gives an estimate for global-scale natural and anthropogenic emissions of NOx as well as of CO, CH4, and VOC (Muller, 1992). It is seen that biomass burning and biogenic emissions of NO are comparable and together equal to about half of the anthropogenic emissions. 

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

While elevated NO, levels are clearly associated with anthropogenic emissions (see Chapter 2), there are also small concentrations due to natural processes. Hence small concentrations of 03 are also formed via the reactions of natural VOC and NOx discussed later in this chapter. Finally, the periodic intrusion of stratospheric air with its relatively high concentrations of O, provides an additional source of tropospheric ozone. 

In 1971, Johnston suggested that anthropogenic emissions of NOx from a proposed fleet of supersonic transports (SSTs) could cause a reduction in ozone due to the set of chain reactions (10) and (11). At the time, a fleet of 500 SSTs flying seven hours a day in the stratosphere by 1985 was projected, and based on that, Johnston (1971) showed that the emissions would be expected to lead to significant ozone depletion. This was never realized because of the much smaller use of SSTs than projected. However, a subsequent proposal for the development of a high-speed civil transport (HSCT) raised some of the same issues, as discussed in the following section. 

For many years the presence of nitric oxide (NO) and nitrogen dioxide (N02) in the atmosphere has been a cause for concern on account of both the scale of anthropogenic emissions of these compounds and their impacts on health and the environment. There are various atmospheric reactions which cycle NO and N02, and it is therefore convenient to think of the two compounds as a group. By convention the sum total of oxides of nitrogen (i.e. NO + N02) is termed NOx and is expressed as N02 mass equivalents. 

Biogenic VOC and NO emissions are calculated on-line based on land use data, simulated surface temperature and radiation. Anthropogenic emissions of primary pollutants, like NOx, SO2, and hydrocarbons, as well as emissions of primary particulate matter have to be supplied either at hourly intervals or as yearly data from gridded emission inventories. Validation studies with MCCM have shown its ability to reproduce observed meteorological quantities and pollutant concentrations for different conditions and regions of the Earth (Forkel and Knoche 2006 Forkel et al. 2004 Grell et al. 1998, 2000 JazcUevich et al. 2003 Kim and Stockwell 2007 Suppan and Skouloudis 2003 Suppan and Schadler 2004 Suppan 2010). 

A comparison of the various sources of NOx in Table 9-14 shows that the anthropogenic contribution is preponderant. The global production of NOx by lightning discharges is potentially the largest natural source. Its magnitude may be equivalent to that of human-made emissions, but a sufficiently precise quantification is presently not possible. The input of HN03 from the stratosphere and the oxidation of ammonia to NOx are comparatively minor sources. 

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