4.2.3.2 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 |
-
|
-
|
4.8
|
4.9
|
8.3
|
8.6
|
8.6
|
2.3
|
4.6
|
13.7
|
| Biomass burning |
-
|
-
|
5.6
|
3.3
|
1.7
|
8.6
|
4.3
|
1.8
|
2.8
|
1.8
|
| Vegetation |
503
|
124
|
4.0
|
4.1
|
2.5
|
8.6
|
8.6
|
-
|
-
|
-
|
| Oceans |
-
|
-
|
0.8
|
1.1
|
-
|
1.6
|
1.4
|
-
|
-
|
-
|
|
| TARb |
Total
|
Isoprene
|
Terpene
|
Acetone
|
|
|
|
|
|
|
|
| Fossil fuela |
161
|
|
|
|
|
|
|
|
|
|
| Biomass burning |
33
|
|
|
|
|
|
|
|
|
|
| Vegetation |
377
|
220
|
127
|
30
|
|
|
|
|
|
|
|
| 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
|
|
|
|
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.
