ReviewGas-phase tropospheric chemistry of biogenic volatile organic compounds: a review
Introduction
It is now well recognized that a wide variety of volatile non-methane organic compounds (referred to hereafter as biogenic volatile organic compounds (BVOCs)) are emitted into the atmosphere from vegetation (Guenther et al (1995), Guenther et al (2000); Fall, 1999; Fuentes et al., 2000; Geron et al., 2000). Table 1 lists a subset of the total number of BVOCs observed as plant emissions, chosen to be representative of the organic compound classes involved and including the dominant emissions. Guenther et al. (1995) have estimated that 1150×1012 g carbon (1150 Tg C) year−1 of BVOCs are emitted worldwide. As discussed in detail elsewhere (see, for example, Fall, 1999; Fuentes et al., 2000), the emission rates of BVOCs are, in general, dependent on temperature and light intensity. Although there are large uncertainties in the magnitude of emission rates of individual (and total) BVOCs, a recent estimate for North America (Guenther et al., 2000) suggests that of an estimated 84 Tg C year−1 of BVOC emissions, 30% are isoprene, 25% terpenoid compounds (see Fig. 1, Fig. 2 for structures of selected C10 and C15 BVOCs), and 40% are non-terpenoid compounds including methanol, hexene derivatives, and 2-methyl-3-buten-2-ol.
Emission inventories of BVOCs and of anthropogenic non-methane organic compounds (NMOCs) indicate that on regional and global scales the emissions of BVOCs exceed those of anthropogenic compounds, by a factor of ∼10 worldwide and a factor of ∼1.5 for the USA (Lamb et al (1987), Lamb et al (1993); World Meteorological Organization, 1995). Because of the higher atmospheric reactivity of most BVOCs compared to many anthropogenic NMOCs [calculated lifetimes of BVOCs are typically a few hours or less (see Table 1) compared to a few days for most anthropogenic NMOCs (Atkinson and Arey, 1998; Atkinson, 2000)], BVOCs are calculated to play a dominant role in the chemistry of the lower troposphere and atmospheric boundary layer (Fuentes et al., 2000).
In the presence of NO emitted from combustion sources (mainly anthropogenic and exemplified by vehicle exhaust in an urban area such as Los Angeles, CA) and, to a lesser extent, from soils, atmospheric reactions of BVOCs lead to the formation of O3 and other manifestations of photochemical air pollution (National Research Council, 1991). The only significant formation route of O3 in the troposphere is the photolysis of NO2:Organic peroxy (RO2•) radicals and HO2 radicals formed during the photooxidations of biogenic and anthropogenic NMOCs react with NO to form NO2:whose photolysis then leads to net O3 formation through reactions (1) and (2).
Even for such a highly urbanized area as Los Angeles, CA, the estimated summer-day BVOC emissions are 125–140 ton/day (Benjamin et al., 1997), which is ∼15% of the estimated 2000 summertime anthropogenic NMOC emissions of 937 ton/day and ∼33% of the 413 ton/day of NMOC emissions calculated to be the upper value allowable if the 120 ppbv Federal National Ambient Air Quality Standard (NAAQS) for O3 (1-h average) is to be achieved in the Los Angeles air basin (South Coast Air Quality Management District, 2002). BVOCs therefore make the attainment of the NAAQS for O3 more difficult utilizing only control of anthropogenic NMOC emissions, and it may be necessary in many parts of the USA, and indeed in many urban areas worldwide, to also control anthropogenic NOx emissions from, for example, fossil-fueled power plants and gasoline- and diesel-fueled vehicles. Region-specific strategies utilizing VOC and/or NOx controls in the USA must now, of course, be reexamined in the light of the new Federal 80 ppbv, 8-h average ozone standard. For example, while the 120 ppbv, 1-h standard was violated in the Los Angeles air basin on 40 and 36 days in 2000 and 2001, respectively, the corresponding violations of the 80 ppbv, 8-h standard were 111 and 100 days (South Coast Air Quality Management District, 2002).
Section snippets
Tropospheric loss processes for BVOCS
As with other volatile organic compounds (Atkinson, 2000), the potential removal and transformation processes for BVOCs are wet and dry deposition, photolysis, reaction with the hydroxyl (OH) radical, reaction with the nitrate (NO3) radical, and reaction with ozone (O3). Reaction with chlorine (Cl) atoms may also be important in, for example, coastal areas (Oum et al., 1998). For most BVOCs, dry and wet deposition is probably of minor importance, though these physical removal processes could be
Lifetimes of biogenic organic compounds in the troposphere
Rate constants for the gas-phase reactions of many of the BVOCs emitted from vegetation with OH radicals, NO3 radicals and O3 have been measured. These rate constants can be combined with assumed ambient tropospheric concentrations of OH radicals, NO3 radicals and O3 to calculate the BVOC lifetime (time for decay of the BVOC to 1/e of its initial concentration) with respect to each of these loss processes (as shown in Table 1 for selected BVOCs). The data in Table 1 indicate that many of the
Reaction mechanisms and products
The initial reactions of OH radicals, NO3 radicals and O3 with NMOCs (including BVOCs) have been elucidated over the past two decades (see, for example, Atkinson (1997a), Atkinson (2000); Calvert et al., 2000) and the reactions of BVOCs have been previously reviewed by Atkinson and Arey (1998) and Calogirou et al. (1999a). For the BVOCs listed in Table 1 there are two general reaction mechanisms: (1) addition to CC bonds by OH radicals, NO3 radicals and O3 and (2) H-atom abstraction from C–H
Conclusions
Emissions of non-methane organic compounds from vegetation are to a large extent composed of compounds containing reactive CC bonds. All BVOCs react with OH radicals and many also react rapidly with NO3 radicals and O3 and have calculated lifetimes in the troposphere of a few hours or less. While the kinetics of the gas-phase reactions of biogenic NMOCs with OH radicals, NO3 radicals and O3 appear to be reasonably well understood, the products formed from these reactions and the detailed
Acknowledgements
The authors gratefully thank the National Science Foundation (Grant No. ATM-9909852) and the University of California Agricultural Experiment Station for support.
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Also Department of Chemistry, University of California, Riverside, CA 92521, USA. Tel.: +1-909-787-4191; fax: +1-909-787-5004.