Chchch

CH3CCH2CH3 Methylethylketone Acrolein

Table 2.9 lists a number of organic species found in the atmosphere.

TABLE 2.9 Atmospheric Organic Species

Type of Compound

General Chemical Formula

Examples

Alkanes

R—H

CH4, methane

CH3CH3, ethane

Alkenes

R,C=CR2

CH2=CH2, ethene or ethylene

CH3—CH=CH2, propene

Alkynes

RC=CR

HC=CH, acetylene

Aromatics

C6R6 (cyclic)

C6H6, benzene

C6H5(CH3), toluene

Alcohols

R—OH

CH3OH, methanol

CH3CH2OH, ethanol

Aldehydes

R—CHO

HCHO, formaldehyde

CH3CHO, acetaldehyde

Ketones

RCOR

CH3C(0)CH3, acetone

Peroxides

R—OOH

CH3OOH, methylhydroperoxide

Organic acids

R—COOH

HC(0)0H, formic acid

CH3C(0)0H, acetic acid

Organic nitrates

R—0N02

CH30N02, methyl nitrate

CH3CH20N02, ethyl nitrate

Alkyl peroxy nitrates

ro2no2

CH302N02, methyl peroxynitrate

Acylperoxy nitrates

R— C(0)00N02

CH3C(0)02N02,

peroxyacetyl nitrate (PAN)

Biogenic compounds

c5h8

CH2=C(CH3)— CH=CH2, isoprene

CioH16

a-pinene, P-pinene

Multifunctional species

CH3C(0)CH0, methylglyoxal

CH2(OH)CHO, glycolaldehyde

Source-. Adapted from Brasseur et al. (1999).

CH2(OH)CHO, glycolaldehyde

Source-. Adapted from Brasseur et al. (1999).

2.4.2 Methane

Methane is the most abundant hydrocarbon in the atmosphere. Table 2.10 summarizes the global sources of CH4, which are estimated at 598 Tg(CH4) yr '. Methane is removed from the atmosphere through reaction with hydroxyl radicals (OH) in the troposphere, estimated at 506Tg(CH4)yr_1, and by reaction in the stratosphere, estimated at 40Tg(CH4)yr~\ Microbial uptake in soils contributes an estimated 30 Tg(CH4) yr"1 removal rate. The estimated imbalance of +22 Tg(CH4) yr"1 between the current sources and sinks of CH4 in Table 2.10 indicates that methane is accumulating in the atmosphere.

Atmospheric CH4 concentrations have changed considerably over time. Figure 2.4 shows CH4 mixing ratios over the past 1000 years. CH4 has increased from a preindustrial mixing ratio near 700 ppb to a present-day value of 1745 ppb. All the data points are based on CH4 in air bubbles trapped in ice cores in Antarctica, with the exception of the solid curve, which is based on atmospheric measurements made at Cape Grim, Tasmania.

TABLE 2.10 Estimates of the Global CH, Budget (in Tg CH4 yr ' ) and Values Adopted by IPCC (2001)

Reference: Base Year:

Fung et al. (1991) 1980s

Lelieveld et al. (1998) 1992

Houweling et al. (1999)

Mosier et al. (1998a) 1994

Olivier et al. (1999) 1990

IPCC (2001) 1998

Natural sources Wetlands Termites Ocean Hydrates Anthropogenic sources Energy Landfills Ruminants Waste treatment Rice agriculture Biomass burning Other Total source

Sinks Soils

Tropospheric OH Stratospheric loss Total sink

20 10 5

75 40 80

100 55

10 450

97 35 90"

40 587

489 46 535

225b 145

20 20

15 15

110 89

40 73

115 93

40 40

Imbalance (Trend)

30 30 510 40 580

25-54 34

60 23

30 506 40 576

"Waste treatment included under ruminants. fcRice included under wetlands.

850-

6501-

] Antarctica Greenland

Cape Grim, Tasmania

1000

1200

1400

1600

1800

2000

FIGURE 2.4 Methane mixing ratios over the last 1000 years as determined from ice cores from Antarctica and Greenland (IPCC 1995). Different data points indicate different locations. Atmospheric data from Cape Grim, Tasmania, are included to demonstrate the smooth transition from ice core to atmospheric measurements.

2.4.3 Volatile Organic Compounds

The term volatile organic compounds (VOCs) is used to denote the entire set of vapor-phase atmospheric organics excluding CO and C02. VOCs are central to atmospheric chemistry from the urban to the global scale.

As an illustration of the variety of organic compounds identified in the atmosphere, Table 2.11 lists the median concentrations of the 25 most abundant nonmethane organic species measured in the 1987 Southern California Air Quality Study.

2.4.4 Biogenic Hydrocarbons

Vegetation naturally releases organic compounds to the atmosphere. In 1960, Went (1960) first proposed that natural foliar emissions of VOCs from trees and other vegetation could have a significant effect on the chemistry of the Earth's atmosphere.

Measurements in wooded and agricultural areas coupled with emission studies from selected individual trees and agricultural crops have demonstrated the ubiquitous nature of biogenic emissions and the variety of organic compounds that can be emitted. Table 2.12 shows the chemical structures of some of the common biogenic hydrocarbons. Each of the compounds shown in Table 2.12 is characterized by an olefinic double bond that renders the molecule highly reactive in the atmosphere, with the result that the lifetimes of these molecules tend to be quite short.

Isoprene (2-methyl-l,3-butadiene, C5H8) is unique among the biogenic hydrocarbons in its relationship to photosynthetic activity in a plant. It is emitted from a wide variety of mostly deciduous vegetation in the presence of photosynthetically active radiation, exhibiting a strong increase in emission as temperature increases. Not only do the isoprene and terpenoid emissions vary considerably among plant species, but the biochemical and biophysical processes that control the rate of these emissions also appear to be quite

TABLE 2.11 Median Mixing Ratio of the 25 Most Abundant Nonmethane Organic Compounds Measured in the Summer 1987 Southern California Air Quality Study

Median Mixing Ratio in Parts per Billion of Carbon"

Ethane 27.1

Ethene 22.3

Acetylene 17.3

Propane 56.0

Propene 7.8

Butane 42.0

Pentane 24.0

2-Methylpentane 16.0

3-Methylpentane 11.8 Hexane 10.8 Methylcyclopentane 10.1 Benzene 17.0 3-Methylhexane 7.4 Heptane 6.0 Methylcyclohexane 7.0 Toluene 49.1 Ethylbenzene 7.6 m, p-Xylenes 25.2 o-Xylene 10.0 1,2,4-Trimethylbenzene 8.2 Formaldehyde 9.1 Acetaldehyde 14.8 Acetone 22.4

"Parts per billion of carbon (ppbC) is the parts per billion of carbon atoms in the molecule. It is simply the volume mixing ratio of the compound multiplied by the number of carbon atoms in the molecule. Source: Lurmann and Main (1992).

distinct. Isoprene emissions appear to be a species-dependent byproduct of photosynthesis, photorespiration, or both; there is no evidence that isoprene is stored within or metabolized by plants. As a result, isoprene emissions are temperature- and light-dependent; essentially no isoprene is emitted without illumination. By contrast, terpenoid emissions seem to be triggered by biophysical processes associated with the amount of terpenoid material present in the leaf oils and resins and the vapor pressure of the terpenoid compounds. As a result, terpene emissions do not depend strongly on light (and they typically continue at night), but they do vary with ambient temperature. The dependence of natural isoprene and terpenoid emissions on temperature can result in a large variation in the rate of production of biogenic VOCs over the course of a growing season. An analysis of emissions data by Lamb et al. (1987) indicates that an increase in ambient temperature from 25 to 35°C can result in a factor of 4 increase in the rate of natural VOC emissions from isoprene-emitting deciduous trees and in a factor of 1.5 increase from

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TABLE 2.12 Organic Compounds Emitted by Vegetation"

Isoprene

Camphene

2-Carene a-Pinene

Sabinene

A3-Carene a-Terpinene d-Limonene y-Terpinene

Myrcene

Teripinolene

Ocimene

ß-Phellandrene a-Phellandrene p-Cymene

"In the simplified molecular structures here bonds between carbon atoms are shown. Vertices represent carbon atoms. Hydrogen atoms bonded to the carbons are not explicitly indicated.

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terpene-emitting conifers. Thus, all other factors being equal, natural VOC emissions are generally highest on hot summer days.

Biogenic hydrocarbon emission rates from individual plant species have been estimated experimentally by placing small plants or branches in enclosures and measuring the accumulation of emitted compounds. Extensive emission rate measurements have been reported for a relatively limited number of compounds: isoprene and a number of the dominant monoterpenes. Isoprene does appear to be the dominant compound emitted from vegetation.

There have been a number of efforts to compile inventories of biogenic hydrocarbon emissions [e.g., see Lamb et al. (1987, 1993) and Guenther et al. (1995)]. Emission rate measurements from individual plant species are used with empirical algorithms that account for temperature and, for isoprene, light effects to scale up to entire geographic regions, based on land-use data and biomass density factors. Biomass density, expressed in g m-2 of area, is required to convert individual plant species emission rates, expressed in mgg-1 min to emission fluxes, in mgm-2 min-1, which are then multiplied by vegetation class land coverage, in m2, to yield total emissions. Uncertainties in these inventories are large; Lamb et al. (1993) estimated total U.S. biogenic hydrocarbon emissions ranging from 29 to 51 Tg yr1.

On a global scale, the largest biogenic hydrocarbon emissions occur in the tropics, with isoprene being the dominant emitted compound. These result from a combination of high temperatures and large biomass densities. During summer months, the maximum flux of biogenic hydrocarbon emissions in the southeastern United States is predicted to be as large as that in the tropics. An estimate of global biogenic VOC emissions appears in Table 2.13. On a global basis, biogenic hydrocarbon emissions far exceed those of anthropogenic hydrocarbons.

2.4.5 Carbon Monoxide

The global sources and sinks of CO are given in Table 2.14. Methane oxidation (by OH) is a major source of CO, as are technological processes (combustion and industrial processes), biomass burning, and the oxidation of nonmethane hydrocarbons. Uncertainties in each of these estimated sources are large. It is estimated that about two-thirds of the CO comes from anthropogenic activities, including oxidation of anthropogenically derived CH4. The major

TABLE 2.13 Global Biogenic VOC Emission Rate Estimates by Source and Class of Compound, Tg yr 1

Source

Isoprene

Monoterpenes

ORVOC"

Total VOC*

Woods

372

95

177

821

Crops

24

6

45

120

Shrub

103

25

33

194

Ocean

0

0

2.5

5

Other

4

1

2

9

Total

503

127

260

1150

"Other reactive biogenic VOCs (ORVOC).

''These totals include additional nonreactive VOCs not reflected in the columns to the left. Source: Guenther et al. (1995).

"Other reactive biogenic VOCs (ORVOC).

''These totals include additional nonreactive VOCs not reflected in the columns to the left. Source: Guenther et al. (1995).

TABLE 2.14 Estimates of Global Tropospheric CO Budget (in Tg(CO) yr"1) and Values Adopted by IPCC (2001)

Hauglustaine Bergamaschi Reference: et al. (1998) et al. (2000) WMO (1998) IPCC (2001)

Sources

TABLE 2.14 Estimates of Global Tropospheric CO Budget (in Tg(CO) yr"1) and Values Adopted by IPCC (2001)

Hauglustaine Bergamaschi Reference: et al. (1998) et al. (2000) WMO (1998) IPCC (2001)

Sources

Oxidation of CH4

795

800

Oxidation of isoprene

268

270

Oxidation of terpenes

136

~0

Oxidation of industrial NMHC

203

110

Oxidation of biomass NMHC

30

Oxidation of acetone

20

Subtotal in situ oxidation

881

1402

1230

Vegetation

100

150

Oceans

49

50

50

Biomass burning

768

500

700

Fossil and domestic fuel

641

500

650

Subtotal direct emissions

1219

1458

1150

1550

Total sources

2100

2860

Surface deposition 190

OH reaction 1920

Sinks

Surface deposition 190

OH reaction 1920

sink for CO is reaction with OH radicals, with soil uptake and diffusion into the stratosphere being minor routes.

Tropospheric CO mixing ratios range from 40 to 200 ppb. Carbon monoxide has a chemical lifetime of 30-90 days on the global scale of the troposphere. Measurements indicate that there is more CO in the Northern Hemisphere than in the Southern Hemisphere; the maximum values are found near the surface at northern midlatitudes. In general, the CO mixing ratio decreases with altitude in the Northern Hemisphere to a free tropospheric average value of about 120 ppb near 45°N. In the Southern Hemisphere CO tends to be more nearly uniformly mixed vertically with a mixing ratio of about 60 ppb near 45°S. Seasonal variations have been established to be about ± 40% about the mean in the Northern Hemisphere and ± 20% about the mean in the Southern Hemisphere. The maximum concentration is observed to occur during the local spring and the minimum is found during the late summer or early fall.

2.4.6 Carbon Dioxide

Because of its overwhelming importance in global climate, we will delay consideration of C02 until Chapter 22, where we address the global cycle of C02.

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