Figure 1 Selected nonmethane hydrocarbons emitted from natural sources.
(2E)-Hexenal Leaf aldehyde typical Arrhenius temperature kinetics with species-dependent temperature optima that range from 36 to 40°C (Guenther et al., 1993). Long-term factors that influence isoprene emission rates include light and temperature conditions in which leaves develop, water and nutrient availability, and disease (Monson et al., 1994; Lerdau and Throop, 2000; Anderson et al, 2000; Harley et al, 1994).
Although plants may lose a significant fraction of fixed carbon to isoprene production, it is not known if the production and emission of isoprene serves an adaptive role in plant tissues. One hypothesis is that isoprene protects photosynthetic apparatus against damage from exposure to high temperature and light intensity (Sharkey, 1997). Another possibility is that isoprene scavenges reactive oxidants inside the leaf that can damage plant tissues (Harley et al, 1999).
Over 90% of total isoprene fluxes are from canopy foliage (Guenther, 1999). Emissions from bacteria and fungi in soils and ground cover foliage of mosses and ferns make up the bulk of the remaining natural source strength (9%). Mammals and marine algae and anthropogenic sources (automobile emissions and industrial processes) each contribute less 1% of the global isoprene flux. Although oxidation in the atmosphere is the primary sink for isoprene, microbial consumption in soils is a small net sink as well (Cleveland and Yavitt, 1998).
Typical surface layer mixing ratios of isoprene in the summer range from less than 1 ppbv in a Colorado pine forest (Goldan et al, 1993) to 8ppbv in the tropical rain forest (Rasmussen and Khalil, 1988) and can be as high as 20ppbv in rural forests in the southeastern United States (Hagerman et al, 1997). Isoprene ambient concentrations are highest in the summer months and typically show strong diurnal patterns where concentrations sharply increase after sunrise to a maximum in the afternoon and fall to zero at night (Fehsenfeld et al, 1992). This pattern can be explained by the dependence of isoprene emission rates on both temperature and light.
Factors that influence ambient isoprene mixing ratios include emission rates, season, stability of the atmosphere, origin of air masses, and oxidation capacity of the atmosphere (Steinbrecher, 1997). Mixing ratios typically decrease rapidly with altitude since isoprene reacts rapidly in the troposphere with both OH and ozone (e.g., Helmig et al, 1998). Some isoprene has been found in the free troposphere, but only in very low and variable amounts.
Monoterpenes (Ci0H16) are a class of structurally diverse compounds produced by over 46 families of flowering plants (e.g., mint, composite, and citrus families), almost all conifers, and some species of liverworts (Banthorpe and Charlwood, 1980; Adam et al, 1996). The accumulation and emission of these compounds directly defends plant tissues against herbivores and pathogens, indirectly defends plant tissues by attracting predators of herbivores, and attracts floral pollinators [reviewed in Langenheim (1994)]. The array of over 1000 different monoterpene structures includes acyclic, monocyclic, and bicyclic forms that can be simple hydrocarbons (e.g., a-pinene, /i-pinene, myrcene, ¿-3-carene, limonene, /?-cymene) or oxygenated derivatives (e.g., 1-8 cineole, linalool, camphor) (Fig. 1). The specific monoterpenes produced and emitted from each species is under tight genetic control, and typically only a few monoterpenes dominate the emissions profile of each species.
Monoterpenes, like isoprene, are synthesized in chloroplasts of specialized tissues by the glyceraldehyde-3-phosphate pathway (Lichtenthaler et al, 1997). Two 5C "isoprene" units condense to form a 10C precursor, which is transformed into the myriad of monoterpenes by cyclization reactions catalyzed by the enzymes monoterpene cyclases (Gershenzon and Croteau, 1991). Monoterpenes typically accumulate in storage structures in plant tissues such as glandular trichomes (mints), resin cysts and ducts (conifers), or cavities (eucalypts).
Release to the atmosphere of these stored pools is dependent upon both volatilization and diffusion processes. Plant foliage is the largest source of monoterpene emissions (over 90% of the total global flux) (Guenther, 1999). The remaining fluxes are from woody tissues, buds, cones, and flowers.
In conifers, total foliar monoterpene emission rates vary from 0.01 to 10 pg/g dw h. Emission of monoterpenes is a diffusive process controlled primarily by the influence of needle temperature on monoterpene vapor pressure and monoterpene concentration in the resin ducts and the diffusive resistance of the tissue to volatile losses (Tingey et al, 1991). Other controls over the emission rates of stored pools include leaf age (Lerdau et al, 1997), phenology (Fukui and Doskey, 1998; Cao et al, 1997; Lerdau et al, 1995), herbivory (Litvak and Monson, 1998; Litvak et al, 1999), relative humidity (Dement et al, 1975), foliar moisture (Lamb et al, 1985), and water stress (Yani et al, 1993).
Atmospheric monoterpene concentrations in four rural sites in the southeastern United States ranged from 0.32 to 0.63 ppbv in the summer, and from 0.125 to 0.19 ppbv in the winter (Hagerman et al, 1997). Maximum summer ambient concentrations of total monoterpenes were 0.80 ppbv above a lodgepole pine forest in Colorado (Roberts et al, 1983) and 0.38 above a ponderosa pine plantation in the Sierra Nevada (Lamanna and Goldstein, 1999). Clear diurnal patterns in ambient concentrations of monoterpenes are not as pronounced as those observed for isoprene, but concentrations are often highest at night and lowest during the day (e.g., Lamanna and Goldstein, 1999; Hagerman et al, 1997). Both vertical mixing and chemical loss are important controllers of ambient monoterpene concentrations. In clean air, vertical mixing and dispersion are the most important factors (Hewitt et al, 1995). Because monoterpenes are still emitted at night when atmospheric conditions are relatively stable, concentrations often increase after sunset until the breakdown of these conditions in the morning.
Recent evidence also indicates some species (e.g., Holm oak Quercus ilex, Norway spruce Picea abies, Pinus pinea, and Acer saccharinum) produce and emit monoterpenes that do not accumulate in pools (Steinbrecher et al, 1993; Loreto et al, 1996; Staudt et al, 1997). These monoterpenes are emitted at relatively high rates in a light- and temperature-dependent manner very similar to that observed for isoprene and are sensitive to water stress (Bertin and Staudt, 1996), and phenology (Staudt et al, 1997). Many questions remain concerning the physiological and ecological controls over light-dependent production and emission of monoterpenes as well as the specific roles these compounds play in plant tissues.
The aromatic />cymene (l-methyl-4-isopropyl-benzene) is the only volatile arene emitted from vegetation. Trace fluxes of /;-cymene have been measured from conifers, sage, and eucalyptus but together are equivalent to only 1% of the estimated global monoterpene source strength (Fehsenfeld et al, 1992).
Substantial quantities of ethene, propene, and butenes, are released annually from automobiles, industry, and biomass burning (estimated at 10 Tg/yr). However, atmospheric measurements of alkenes made in remote areas that are not impacted by urban or industrial emissions suggest the presence of biogenic sources of light alkenes as well (Lamanna and Goldstein, 1999; Goldstein et al, 1996; Heikes et al, 1996a; Rudolph, 1997).
Emissions from terrestrial ecosystems, particularly plant tissues, make up the bulk of the total global emissions of ethene from natural sources (Sawada and Totsuka, 1986). Ethene functions as a hormone in plant tissues that triggers growth and developmental processes including seed germination, flowering, fruit ripening, senescence, and growth regulation [reviewed in Abeles et al. (1992)]. In addition, ethene is a well-known stress indicator and may play a role in triggering plant defense mechanisms. The amino acid L-methionine is enzymatically converted to ethene in a two-step process involving the intermediate 1-aminocyclo-propane-l-carboxylate (ACC) [reviewed in Fall (1999)]. Production and emission rates of ethene vary with species, tissue type, and phenology and are significantly induced in response to wounding, air pollution, insect and pathogen attack, drought, waterlogging, high and low temperatures, and gamma radiation [reviewed in Abeles et al. (1992)]. Global estimates of ethene fluxes from undisturbed canopy foliage are 2 to 4 Tg/yr (Table 1; Rudolph, 1997).
Ethene is also emitted in small quantities from soil microorganisms. Fluxes are correlated with the organic matter content in soil and on a global scale are 2.6 to 3.7 Tg/yr (Rudolph, 1997). Fluxes of ethene, propene, butene, and acetylene have been measured from wetlands but are insignificant on a global scale.
Due to the short lifetimes, measurement difficulty, and wide variety of sources of ethene and propene, ambient concentrations of these compounds are variable. Mean summertime emission rates of ethene, propene, and 1-butene from a deciduous forest in the northeastern United States were 2.6, 1.1, and 0.4 x 1010 molecules/cm-2 s, respectively (Goldstein et al., 1996). In this forest, biogenic emissions of propene and 1-butene exceeded the anthropogenic emissions, while biogenic emissions of ethene were equivalent to 50% of emissions from anthropogenic sources. Maximum ambient concentrations above the forest were 0.2, 0.95, and 0.08 ppbv for propene, ethene, and 1-butene, respectively (Goldstein et al., 1996). Lamanna and Goldstein (1999) also observed a local biogenic source for ethene and propene in measurements above a Sierra Nevada ponderosa pine plantation where ambient concentrations of these compounds varied between 0.18 and 0.45 ppbv.
Photochemical degradation of dissolved organic carbon (DOC) released by marine algae results in an estimated global emission rate of 5 Tg/yr for ethene, propene, butenes, and acetylene from ocean surface water [reviewed in Rudolph (1997)]. Fluxes vary seasonally, increase with DOC and light intensity (particularly shorter wavelengths), and depend strongly on DOC, biological activity of the algae, and wind speed (drives the exchange at the air-sea interface) (Ratte et al., 1995). Fluxes are inferred from a combination of atmospheric measurements, seawater measurements, air-sea exchange rates and photochemical models, and encompass large uncertainties. In the remote marine boundary layer and free troposphere over the South Atlantic and western Indian Oceans, ethene and propene concentrations were less than 20 and 6ppt, respectively (Heikes et al., 1996a).
A C5 alcohol, 2-methyl-3-buten-2-ol (MBO), was recently identified in air samples taken in a Colorado pine forest in concentrations higher than isoprene (up to 3.5 ppbv; Goldan et al., 1993). It is now known that MBO is emitted at relatively high rates from many pine species that grow predominantly in the western United States (up to 70 ng C/g h). Fluxes of MBO, like isoprene, are both light and temperature dependent, suggesting that MBO is emitted immediately following production rather than stored in specialized structures (Harley et al., 1998). Although the production mechanism of MBO in plant tissues is not well known, there is some evidence that it is derived from a 5C precursor of isoprene (Fall, 1999).
Most of the nonreactive other NMVOC flux in Table 1 is contributed by methanol (Guenther et al., 1995). Methanol fluxes measured from leaves are comparable to isoprene and monoterpenes and vary from 0.2 to 40 |ig C/h g dry weight (MacDonald and Fall, 1993; Nemecek-Marshall et al., 1995). Emission rates of methanol are highest in young leaves and vary with phenology, leaf damage, and stomatal conductance (Nemecek-Marshall et al., 1995; Fukui and Doskey, 1998). Significant fluxes of methanol and ethanol have also been measured from decaying plant material. Warneke et al. (1999) estimate that globally, emissions from decaying plant material alone could account for 18 to 40 Tg of methanol per year.
Methanol was one of the most abundant VOCs detected above a pine forest canopy in the rural southeastern United States, with summertime mixing ratios of 10 to 20 ppbv (Goldan et al., 1995). Like isoprene, ambient methanol mixing ratios in these studies varied diurnally and peaked in the midafternoon, suggesting that at least in these rural forested areas, methanol was derived primarily from biogenic sources. At a rural site in Colorado, maximum summertime ambient mixing ratios were 6 ppbv (Goldan et al., 1997). Relatively high concentrations of methanol have been detected in the free troposphere, particularly in the northern midlatitudes (0.6 to 0.8 ppbv in northern areas and 0.4 ppbv in southern areas) (Singh et al., 1995). In addition to direct emissions from vegetation, sources of atmospheric methanol include fossil fuel use, biomass burning, and tropospheric production.
Other nonterpenoid alcohols emitted from many agricultural crops, grasses, pastures and forest trees include 3Z-hexenol (leaf alcohol), ethanol, methyl propanol, butanol, and octanol (Isidorov et al., 1985; Arey et al., 1993; Macdonald and Fall, 1993; König et al., 1995; Puxbaum, 1997; Kirstine et al., 1998; Helmig et al., 1999). Production of many of these alcohols, particularly leaf alcohol and ethanol, varies with phenology and is triggered by physical injury and environmental stress (Kirstine et al., 1998; MacDonald et al., 1989). Fluxes of alcohols and other oxygenated VOC's released during the process of crop harvesting may be large enough to have a short-term influence on local air quality (Karl et al., 2001).
Many aldehydes and ketones that are detected in the atmosphere, e.g., acetaldehyde (ethanal), formaldehyde, propanal, butanal, acetone, and butenone, have both anthropogenic and biogenic sources. The dominant sources of these species are fossil fuel combustion, biomass burning, and photochemical oxidation of man-made and natural hydrocarbons, but direct emissions from a variety of forest trees, shrubs, grasses, ferns and mosses occur as well (Isidorov et al., 1985; MacDonald and Fall, 1993; Kotzias et al., 1997; Fukui and Doskey, 1998).
Acetone in plant tissues is produced through fatty acid oxidation [reviewed in Fall (1999)]. Small acetone fluxes have been measured from live plant foliage, decaying vegetation, and seeds and buds of many conifer species, suggesting at least some of the acetone measured in forest canopies and the free troposphere is contributed by natural sources (Fukui and Doskey, 1998; Warneke et al, 1999; Kotzias et al, 1997; MacDonald and Fall, 1993). Warneke et al. (1999) estimated that on a global scale, decaying vegetation emits 6 to 8 Tg of acetone annually.
Acetone is one of the most abundant oxygenated species in the remote atmosphere (Singh et al, 1995). In the free troposphere over the Pacific Ocean, Singh et al. (1995) measured acetone concentrations that range from 0.5 ppbv in the northern latitudes to 0.25 ppbv in the southern latitudes.
Singh et al. (1995) estimated that direct biogenic emissions account for 21% of the total global acetone source. Like methanol, acetone was one of the most abundant VOCs measured above several rural forested areas in Alabama (4 to 7 ppbv; Goldan et al, 1995). Summertime acetone mixing ratios in the Sierra Nevada above a ponderosa pine plantation ranged from 1.5 to 8 ppbv (Lamanna and Goldstein, 1999). At this site, biogenic sources (primarily oxidation of the alcohol MBO) accounted for 45% of the acetone concentrations that exceeded background levels (Goldstein and Schade, 1999). In remote and rural forested regions in Europe, ambient surface acetone concentrations varied between 0.2 and 2.2 ppbv (Solberg et al, 1996). Solberg et al. (1996) observed a strong seasonal dependence of acetone mixing ratios at these sites where summertime maximum acetone concentrations are correlated with high concentrations of biogenic VOC precursors.
The most common aldehydes directly released from the tissues of many plants are 2E-hexenal (also called leaf aldehyde) and other C6 aldehydes from the hexenal family (Hatanaka et al, 1987; Arey et al, 1993; Fukui and Doskey, 1998; Kirstine et al, 1998). In undisturbed tissues, hexenal aldehyde emission rates are small (1.0 to 27 ng/gdwh) (Konig et al, 1995). Hexenals function as antibiotics in plant tissues, however, and emission increases in response to physical wounding, herbivory, and pathogen attack, suggesting that current estimates are low.
Formaldehyde and acetaldehyde are directly emitted from plant foliage at relatively low rates (0.2 to 1 pg/gdwh) (Kesselmeier et al, 1997). In most areas, photochemical oxidation of isoprene and other biogenic VOC precursors emitted from vegetation is a more important biogenic source of these compounds than direct emission (e.g., Fried et al, 1997).
Typical background mixing ratios of formaldehyde are 0.1 to 0.15 ppbv (Heikes et al, 1996b). In a rural site in Colorado, the midday background formaldehyde mixing ratio was 1.17 (Fried et al, 1997). In rural areas in Europe, Solberg et al. (1996) observed a seasonal pattern in formaldehyde ambient mixing ratios similar to acetone, where concentrations are highest in the summer (1.3 to 5.9 ppbv), compared to the rest of the year (0.4 to 2.4 ppbv). Arlander et al. (1990) report a latitudinal distribution of formaldehyde from measurements taken over the Pacific Ocean. Maximum mixing ratios (between 0.6 and 0.8 ppbv) of formaldehyde during this cruise were seen between 20°N and the equator, reflecting the latitudinal distribution of both anthropogenic and biogenic alkene precursors.
Atmospheric mixing ratios of formic and acetic acid typically range from 0.02 to l .9 in remote and marine locations to l to 16 ppbv in urban polluted areas [reviewed in Khare et al. (1999)]. Sources of these organic acids include fossil fuel combustion, biomass burning, direct emissions from formicine ants, soils and plant foliage, and photochemical production in the atmosphere through isoprene and monoterpene oxidation. Direct emissions of both acids have been measured from the European species Quercus ilex and Pinus pinea, tropical trees in the Amazon, and savanna soils (Kesselmeier et al, 1997; Talbot et al, 1990; Sanhueza and Andreae, 1991). Though the precise biosynthetic mechanisms of organic acids in plant tissues is unknown, acetic acid is formed through lipid metabolism, and formic acid is a by-product of carbohydrate and CI metabolism [reviewed in Fall (l999)]. In soils, microbial activity is the likely organic acid source.
Vertical profiles and observed seasonal, diurnal, and latitudinal patterns of organic acid concentrations in both precipitation and gas-phase measurements support a significant biogenic source of these acids in rural midlatitude continental, tropical continental, and marine locations (Khare et al, 1999). For example, mixing ratios over the Amazon Basin, and temperate forests in eastern United States were higher during the growing season and the afternoon than in the winter and at night or in the early morning (Keene and Galloway, 1988; Talbot et al, 1988, 1990). Although photochemical production of precursors emitted from vegetation is considered to be the dominant biogenic source of organic acids, direct emission by soils and vegetation can be important in rural areas in the eastern United States and in the Amazon rainforest (Andreae et al, 1988; Talbot et al, 1990, 1995). Formic and acetic acid budgets in marine atmospheres suggest the presence of a natural source as well (Arlander et al, 1990; Heikes et al, 1996a).
3 REGIONAL AND GLOBAL DISTRIBUTION OF BIOGENIC NMVOC EMISSIONS
To understand the impact biogenic NMVOCs have on tropospheric chemistry, reliable emission estimates at local, regional, and global scales are necessary. Flux measurements made at a variety of scales are the primary means of both developing and evaluating these emission estimates. The techniques used to measure these fluxes are reviewed in Guenther et al. (1996). Enclosure methods are used to estimate fluxes on small scales including from a single leaf, branch, or whole tree. These measurements are a particularly good way to quantify species-specific basal emission rates (or the capacity to emit NMVOCs under a standard set of environmental conditions). Tower-based micrometeorological techniques are used to directly measure canopy-scale fluxes of NMVOCs on diurnal, seasonal, and annual time scales. These techniques include eddy covariance, relaxed eddy accumulation (REA), surface layer gradient, and tracer methods. Finally, sampling systems on tethered balloons and aircraft are used to construct vertical mixing ratio profiles and calculate surface fluxes on scales of tens to hundreds of kilometers using
4 SUMMARY AND CONCLUSIONS 117
eddy accumulation, REA, mixed-layer mass balance, and mixed-layer gradient methods.
To construct inventories, basal emission rates from a wide range of vegetation classes are modified by instantaneous changes in both temperature and light intensity using algorithms developed by Guenther et al. (1993), multiplied by estimates of foliar density of each vegetation class, and aggregated to give flux estimates on regional and global scales (Lamb et al., 1987; Guenther et al., 1995; Guenther, 1997). Large uncertainties are associated with these inventories, however, due to gaps in our knowledge of (l) the contribution of nonfoliar emissions, (2) physiological and ecological controls over emissions from plants, (3) specific emission factors from a wider variety of plants and ecosystems, particularly of nonterpenoid NMVOCs, and (4) detailed data on coverage of ecosystem type, foliage density, surface temperatures, and radiation properties (Steinbrecher, 1997).
These inventories are useful for identifying where, on a regional basis, biogenic contributions to total NMVOC fluxes are particularly relevant due to vegetation type, foliar density, and ambient temperature patterns. For example, in urban areas such as Los Angeles, biogenic NMVOCs contribute a relatively small fraction to the total VOC emissions (Benjamin et al., 1997). In Atlanta and remote rural areas, however, biogenic sources, during the summer months especially, can dominate the total VOC emission profile (Geron et al., 1995; Hagerman et al., 1997). In North America as a whole, and in Norway, Sweden, and Finland, biogenic emissions of VOCs exceed anthropogenic emissions (Guenther et al., 1995; Simpson et al., 1995). In Italy, biogenic emissions account for 50% of the total VOCs emitted (Simpson et al., 1995).
On a global scale, Guenther et al. (1995) derived estimates for emissions of isoprene (420TgC/yr), monoterpenes (l30TgC/yr), and other reactive VOCs (280TgC/yr). As expected due to the influence of light intensity and temperature on emissions, biogenic fluxes show seasonal as well as latitudinal differences (Guenther et al., 1995; Guenther, 1999). Drought deciduous forests and savannas in the tropics contributed half of all the global VOCs from biogenic sources in this estimate. Other woodlands, crops, and shrublands contributed 10 to 20% of these fluxes. Crops, in particular, were high emitters of VOCs other than isoprene and monoterpenes.
Relative to isoprene, trends in the global distribution of monoterpene and other NMVOC fluxes are hard to find. The high reactivity, spatial and temporal variability in source strengths, and uncertainties in reliable identification and quantification of these species have contributed to large variability in observed ambient mixing ratios. Thus, although measurements of these species have been made, considerable work is necessary to truly understand the global distribution of these reactive VOCs in the atmosphere.
A variety of nonmethane hydrocarbons are released from natural sources, particularly plant foliage, in quantities sufficient to alter production of tropospheric ozone, organic acids and nitrates, PAN, OH, and CO. Isoprene and monoterpenes together dominate NMVOC fluxes from many species, ecosystems, regions, and on a global scale. Although the mechanisms of production, emission, and degradation in the atmosphere are fairly well known for isoprene and monoterpenes, uncertainties such as why only certain plants produce these compounds and detailed distributions of the vegetation sources remain.
Uncertainties are largest for VOCs other than isoprene and monoterpenes. Nonterpenoid hydrocarbons contribute an estimated 45% of the total biogenic VOC global fluxes. To make more reliable estimates of the source strength and atmospheric impacts of these hydrocarbons, a better understanding of the biological and ecological factors that control spatial and temporal variability in these fluxes is needed.
Current NMVOC inventories rely on empirical models based only on the response of emissions to temperature, light, and foliar density. Given that emissions of isoprene, monoterpenes, and many of the other VOCs are influenced by a whole suite of physiological and ecological factors, using these inventories to predict emissions in response to disturbances, land-use change, and/or climate change is risky. An important aspect of future biogenic VOC research is to incorporate a more mechanistic understanding of VOC production and emission into emission inventories (Monson et al., 1995). In this way inventories will be able to extrapolate emission rates and the impacts of these emissions on atmospheric chemistry across complex ecological gradients in both space and time.
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