Biogenic Nonmethane Hydrocarbons

MARCY E. LITVAK

1 INTRODUCTION

Nonmethane volatile organic compounds (NMVOCs) are emitted from a wide variety of both anthropogenic and biogenic sources. Major anthropogenic sources of NMVOCs include combustion of fossil fuels, solvent evaporation and biomass burning, while direct emissions from plants are the largest biogenic source. Over 90% of the total NMVOCs entering the atmosphere are biogenic (Guenther et al., 1995; Miiller, 1992). Recent estimates of the upper limit of global NMVOC emissions from biogenic sources range from 1000 to 1500TgC/yr (1 Tg= 1012g), an amount equivalent to the total methane flux from both biogenic and anthropogenic sources (Guenther et al., 1995).

In the atmosphere, NMVOCs are typically very reactive (lifetimes range from minutes to days) and play significant roles in many aspects of atmospheric chemistry. NMVOCs are a key component of the photochemical processes that form ozone and other secondary products in the planetary boundary layer (Fehsenfeld et al., 1992). The other products produced include organic acids, organic nitrates, aerosols, acetone, formaldehyde, and carbon monoxide (Kasting and Singh, 1986; Trainer et al., 1987; Chameides et al., 1988; Jacob and Wofsy, 1988; Andreae et al., 1988; Fehsenfeld et al., 1992). These products are relevant in that they can contribute to both air pollution and climate change. Ozone is not only a potent greenhouse gas but can impact human health and plant productivity. Organic nitrates such as PAN (peroxy-acetyl nitrate) are phytotoxic, an important component of urban smog, and also provide a mechanism for transporting reactive nitrogen (NO and N02, together referred to as NOx) over large distances (Sillman and Samson, 1995). Organic

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aerosol particles scatter light at all visible wavelengths, which creates haze and decreases visibility (Andreae and Crutzen, 1997; Pandis et al., 1991). Finally, NMVOCs and carbon monoxide are considerably more reactive toward the hydroxy 1 radical (OH) than is methane. Increased levels of CO and NMVOCs therefore can significantly suppress OH concentrations and thus the oxidative capacity of the troposphere, resulting in a longer atmospheric lifetime for methane (Brasseur and Chatfield, 1991; Fehsenfeld et al., 1992).

In many localized areas, biogenic hydrocarbons play a dominant role in generating tropospheric ozone locally and in rural areas downwind from these urban centers. In Atlanta, Georgia, which has a high density of NMVOC-emitting plant species, Geron et al. (1995) estimated that with current NOx and biogenic hydrocarbon emissions, even if anthropogenic hydrocarbon emissions were reduced to zero, ozone levels would be above the National Ambient Air Quality Standard (NAAQS). In rural areas long distances from polluted plumes, anthropogenic VOCs are so diluted that isoprene and terpenes emitted from vegetation alone are enough to sustain ozone production (Roselle et al., 1991; Hagerman et al., 1997).

The list of NMVOCs emitted from biogenic sources includes well over 1000 compounds. In many ecosystems, isoprene and monoterpenes are the predominant biogenic hydrocarbons emitted, and these compounds account for over half of the total global NMVOC fluxes from biogenic sources (Table 1). However, recent

TABLE 1 Major Biogenic Methane and NMVOC Sources, Source Strength, and Atmospheric Lifetimes"

Estimated

Primary

Annual Global

Reactivity

Natural

Emission

in Atmosphere

voc

Sources

(TgC)

(lifetime in days)

Methane

Wetlands, rice paddies

319^112

4000

Isoprene

Plants

175-503

0.2

Monoterpenes

Plants

127^180

0.1-0.2

Ethene

Plants,

soils,

8-25

1.9

oceans

Other reactive VOCs

Plants

~ 260

<1

(e.g., acetaldehyde, formaldehyde MBO, hexenal family)

Other less reactive Plants, —260 >1

VOCs (e.g., soils methanol, ethanol, formic acid, acetic acid, acetone)

(e.g., acetaldehyde, formaldehyde MBO, hexenal family)

Other less reactive Plants, —260 >1

VOCs (e.g., soils methanol, ethanol, formic acid, acetic acid, acetone)

"Adapted from Fall (1999). Data are derived from Singh and Zimmerman (1992), Conrad (1995), Guenther et al. (1995), Andreae and Crutzen (1997), and Rudolph (1997).

studies have emerged indicating that many other hydrocarbons, particularly oxygenated VOCs, also provide a significant contribution to the total biogenic NMVOC flux (Isidorov et al., 1985; Arey et al., 1991; Winer et al., 1992; König et al., 1995; Helmig et al., 1999). Quantitative measurements of most of these "other" NMVOCs are scarce because of the wide variety of sources and difficulty in reliable identification and quantification of these compounds.

In this chapter, the major classes of hydrocarbons and their oxygenated derivatives emitted to the atmosphere from biogenic sources are reviewed. Information is also provided on ambient mixing ratios, regional and global distribution, and the primary controlling factors over emissions of these classes of biogenic NMVOC's. Of the large group of biogenic NMVOCs that are highly reactive in the atmosphere (have lifetimes of less than one day), this chapter will mainly focus on isoprene, monoterpenes, ethene, propene, butene, acetaldehyde, formaldehyde, 2-methyl-3-buten-2-ol (MBO), and the hexenal family compounds (hexenylacetate, 2-hexenal, 3-hexenol, and hexanal). The nonreactive biogenic NMVOCs (lifetimes of more than one day) covered here include methanol and ethanol, acetone, ethane, and acetic and formic acid. Emissions of alkanes (e.g. ethane, propane, butane) from terrestrial and oceanic natural sources are very low (Lindskog, 1997; Guenther et al., 1994) and are not covered here.

2 BIOGENIC NMVOCs Isoprene

Isoprene (2-methyl-l,3-butadiene) was first recognized as an emission from plant tissues in the late 1950s (Sanadze, 1991) (Fig. 1). Until recently, it was thought that isoprene was synthesized by the mevalonic acid pathway. It is now known that isoprene is produced in chloroplasts by the glyceraldehyde-3-phosphate pathway, in both an enzyme-dependent (catalyzed by the enzyme isoprene synthase) and nonenzymatic manner (Lichtenthaler et al., 1997). Release to the atmosphere is instantaneous following synthesis, and is the result of simple diffusion of isoprene through cell membranes into the intercellular air spaces and out of pores on the leaf surface, called stomata.

Isoprene emission rates vary among species from 0.1 to 70 |ig/g dwh. Not all plants have the ability to produce and emit significant amounts of isoprene. A compilation of species-level isoprene emission screenings from over 800 species of higher plants indicate that, in general, most isoprene emitters are woody deciduous species, although some ferns, vines, and other herbaceous species also emit significant amounts of isoprene (Harley et al., 1999). Phylogenetic patterns are hard to find, as many plant families that contain isoprene emitters, contain nonemitters as well. High isoprene emitting species have been found in the genera Quercus (oaks), Populus (aspen and poplars), and Liquidambar (sweetgum).

Leaf temperature and light intensity are the primary environmental controllers of short-term (hours to days) changes in isoprene production and emission rates from plant foliage (Guenther et al., 1993; Sharkey et al., 1999). Isoprene emissions show

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