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6 Includes animal, microbial, and foliage emissions. ' Expressed as N02.

6 Includes animal, microbial, and foliage emissions. ' Expressed as N02.

discussed elsewhere in this book, methane oxidizes relatively slowly in the troposphere and hence on a short-term basis (e.g., hours) does not contribute significantly to ozone formation compared to other organ-ics (with some exceptions, e.g., CFCs). As a result, in terms of smog formation in urban and regional areas, methane has been excluded from consideration and controls have focused on the suite of larger organics.

However, with the recognition that a variety of organics (e.g., aldehydes) and not just hydrocarbons are important in the troposphere, alternative terminologies have been used to describe organics other than methane. These include the most commonly used term VOC (volatile organic compounds) as well as ROG (reactive organic gases) and NMOC (non-methane organic compounds). We use the term VOC, recognizing that certain volatile species such as CFCs are not included.

Non-Road Engines and Vehicles

Non-Road Engines and Vehicles

FIGURE 2.5 Contribution of various sources to total anthropogenic VOC emissions in the United States in 1996 (from EPA, 1997).

FIGURE 2.5 Contribution of various sources to total anthropogenic VOC emissions in the United States in 1996 (from EPA, 1997).

Figure 2.5 shows the distribution of anthropogenic sources of VOCs in the United States in 1996, a total of 19 X 106 short tons, or 17 Tg (EPA, 1997). Figure 2.6 shows VOC emissions in Europe (Friedrich and Obermeier, 1999).

Over the past decade, it has become clear that, on a global scale, biogenic processes also release substantial quantities of reactive hydrocarbons such as isoprene and a-pinene, in addition to methane and other organics, including oxygenated species such as methanol, 2-methyl-3-buten-2-ol, hexenol, acetone, and formic and acetic acids (see, for example, Fehsenfeld et al., 1992; Golden et al., 1993; Monson et al., 1995; Singh et al., 1995; Fall, 1999; Guenther, 1999); see Chapter 6.J.I.

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