Working Group I: The Scientific Basis

Other reports in this collection Volatile organic compounds (VOC)

Volatile organic compounds (VOC), which include non-methane hydrocarbons (NMHC) and oxygenated NMHC (e.g., alcohols, aldehydes and organic acids), have short atmospheric lifetimes (fractions of a day to months) and small direct impact on radiative forcing. VOC influence climate through their production of organic aerosols and their involvement in photochemistry, i.e., production of O3 in the presence of NOx and light. The largest source, by far, is natural emission from vegetation. Isoprene, with the largest emission rate, is not stored in plants and is only emitted during photosynthesis (Lerdau and Keller, 1997). Isoprene emission is an important component in tropospheric photochemistry (Guenther et al., 1995, 1999) and is included in the OxComp simulations. Monoterpenes are stored in plant reservoirs, so they are emitted throughout the day and night. The monoterpenes play an important role in aerosol formation and are discussed in Chapter 5. Vegetation also releases other VOC at relatively small rates, and small amounts of NMHC are emitted naturally by the oceans. Anthropogenic sources of VOC include fuel production, distribution, and combustion, with the largest source being emissions (i) from motor vehicles due to either evaporation or incomplete combustion of fuel, and (ii) from biomass burning. Thousands of different compounds with varying lifetimes and chemical behaviour have been observed in the atmosphere, so most models of tropospheric chemistry include some chemical speciation of the VOC. Generally, fossil VOC sources have already been accounted for as release of fossil C in the CO2 budgets and thus we do not count VOC as a source of CO2.

Given their short lifetimes and geographically varying sources, it is not possible to derive a global atmospheric burden or mean abundance for most VOC from current measurements. VOC abundances are generally concentrated very near their sources. Natural emissions occur predominantly in the tropics (23°S to 23°N) with smaller amounts emitted in the northern mid-latitudes and boreal regions mainly in the warmer seasons. Anthropogenic emissions occur in heavily populated, industrialised regions (95% in the Northern Hemisphere peaking at 40°N to 50°N), where natural emissions are relatively low, so they have significant impacts on regional chemistry despite small global emissions. A few VOC, such as ethane and acetone, are longer-lived and impact tropospheric chemistry on hemispheric scales. Two independent estimates of global emissions (Ehhalt, 1999; and TAR/OxComp budget based on the Emission Database for Global Atmospheric Research (EDGAR)) are summarised in Table 4.7a. The OxComp specification of the hydrocarbon mixture for both industrial and biomass-burning emissions is given in Table 4.7b.

Table 4.7(a): Estimates of global VOC emissions (in TgC/yr) from different sources compared with the values adopted for this report (TAR).
Ehhalt (1999) Isoprene (C5H8) Terpene (C10H16) C2H6 C3H8 C4H10 C2H4 C3H6 C2H2 Benzene (C6H6) Toluene (C7H8)
Fossil fuela
Biomass burning
TARb Total Isoprene Terpene Acetone
Fossil fuela
Biomass burning
a Fossil includes domestic fuel.a Fossil includes domestic fuel.
b TAR refers to recommended values for OxComp model calculations for the year 2000.
Table 4.7(b): Detailed breakdown of VOC emissions by species adopted for this report (TAR).
  Industrial Biomass burning
Species wt% #C atoms wt% #C atoms
Alcohols 3.2 2.5 8.1 1.5
Ethane 4.7 2.0 7.0 2.0
Propane 5.5 3.0 2.0 3.0
Butanes 10.9 4.0 0.6 4.0
Pentanes 9.4 5.0 1.4 5.0
Higher alkanes 18.2 7.5 1.3 8.0
Ethene 5.2 2.0 14.6 2.0
Propene 2.4 3.0 7.0 3.0
Ethyne 2.2 2.0 6.0 2.0
Other alkenes, alkynes, dienes 3.8 4.8 7.6 4.6
Benzene 3.0 6.0 9.5 6.0
Toluene 4.9 7.0 4.1 7.0
Xylene 3.6 8.0 1.2 8.0
Trimethylbenzene 0.7 9.0 - -
Other aromatics 3.1 9.6 1.0 8.0
Esters 1.4 5.2 - -
Ethers 1.7 4.7 5.5 5.0
Chlorinated HC's 0.5 2.6 - -
Formaldehyde 0.5 1.0 1.2 1.0
Other aldehydes 1.6 3.7 6.1 3.7
Ketones 1.9 4.6 0.8 3.6
Acids 3.6 1.9 15.1 1.9
Others 8.1 4.9    
wt% values are given for the individual VOC with the sums being: industrial, 210 Tg(VOC)/yr, corresponding to 161 TgC/yr; and biomass burning, 42 Tg(VOC)/yr, corresonding to 33 TgC/yr.

One of the NMHC with systematic global measurements is ethane (C2H6). Rudolph (1995) have used measurements from five surface stations and many ship and aircraft campaigns during 1980 to 1990 to derive the average seasonal cycle for ethane as a function of latitude. Ehhalt et al. (1991) report a trend of +0.8%/yr in the column density above Jungfraujoch, Switzerland for the period 1951 to 1988, but in the following years, the trend turned negative. Mahieu et al. (1997) report a trend in C2H6 of -2.7 ± 0.3%/yr at Jungfraujoch, Switzerland for 1985 to 1993; Rinsland et al. (1998) report a trend of -1.2 ± 0.4%/yr at Kitt Peak, Arizona for 1977 to 1997 and -0.6 ± 0.8%/yr at Lauder, New Zealand for 1993 to 1997. It is expected that anthropogenic emissions of most VOC have risen since pre-industrial times due to increased use of gasoline and other hydrocarbon products. Due to the importance of VOC abundance in determining tropospheric O3 and OH, systematic measurements and analyses of their budgets will remain important in understanding the chemistry-climate coupling.

There is a serious discrepancy between the isoprene emissions derived by Guenther et al. (1995) based on a global scaling of emission from different biomes, about 500 TgC/yr, and those used in OxComp for global chemistry-transport modelling, about 200 TgC/yr. When the larger isoprene fluxes are used in the CTMs, many observational constraints on CO and even isoprene itself are poorly matched. This highlights a key uncertainty in global modelling of highly reactive trace gases: namely, what fraction of primary emissions escapes immediate reaction/removal in the vegetation canopy or immediate boundary layer and participates in the chemistry on the scales represented by global models? For the isoprene budget, there are no measurements of the deposition of reaction products within the canopy. More detail on the scaling of isoprene and monoterpene emissions is provided in Chapter 5. Although isoprene emissions are likely to change in response to evolving chemical and climate environment over the next century, this assessment was unable to include a projection of such changes.

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