FIGURE 2.3 NOt emissions in million tons of equivalent N02 for the period 1970 to 1986 for Asia, Europe, North America, and the USSR (from Hameed and Dignon, 1992).

of North America and Europe have decreased or leveled off, those from Asia and the USSR increased significantly, a trend that has continued. Figure 2.4a shows the geographical pattern of the emission flux of NOx in Asia in 1987 (Akimoto and Narita, 1994). Clearly, Japan and China are major contributors to the flux of NOx in this region, with the City of Tokyo having the highest emission flux rate.

There are also significant natural sources of oxides of nitrogen, in particular nitric oxide, which is produced by biomass burning as well as by soils where nitrification, denitrification, and the decomposition of nitrite (N02) contribute to NO production. Figure 2.4b, for example, shows the relative emission rates for biogenically produced NO in the United States in 1990 (EPA, 1995).

Another important natural source is NOx produced by lightning, with recent estimates in the range of 10-33 Tg yr~1 as N02 (Flat0y and Hov, 1997; Price et al., 1997a, 1997b; Wang et al., 1998). By comparison to the estimated emissions from biomass burning and continental biogenic sources (Table 2.1), it is seen that lightning is quite important.

There is also some NO produced from the oxidation of NH3 by photochemical processes in oceans and by some terrestrial plants (e.g., Wildt et al., 1997).

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

Nitrous oxide (N20, "laughing gas") is also produced by biological processes and, to a lesser extent, by anthropogenic processes (see Chapter 14.B.2c). While

Biogenic NO emissions
FIGURE 2.4 (a) Pattern of 1987 annual emission flux of NOA. in Asia (in units of millimoles as N per m2 per year) (from Akimoto and Narita, 1994). (b) Estimated relative rates of biogenic emissions of NO in the United States in 1990 (from EPA, 1995).

biomass burning is the major anthropogenic source, there are a variety of smaller sources, including motor vehicles. Interestingly, the NzO emissions from current catalyst-equipped cars appear to be higher than from noncatalyst-equipped vehicles (Berges et al., 1993), but total NOx emissions are much lower. N20 is inert in the troposphere and is ultimately transported to the stratosphere, where it acts as a major source of NOx (see Chapters 1 and 12).

2. Volatile Organic Compounds (VOC)

Historically, organics in the troposphere have been measured as non-methane hydrocarbons (NMHC). As

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

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