A large variety of nonmethane hydrocarbons (NMHCs) are found throughout the troposphere. The atmospheric abundances of these diverse species range from below detection limits (1 -- 10's of ppt) to the ppb level and higher. The NMHCs are often conveniently lumped into the categories of alkanes, alkenes, aromatics and biogenically produced compounds including isoprene and terpenes. Oxidation of NMHCs produces a variety of oxygenated products including aldehydes, ketones, dicarbonyls, alcohols, phenols, peroxides, organic acids and organic nitrates. The chemistry of these intermediate compounds is imprecisely known, however laboratory and modeling studies are helping to quantify and reduce some of the uncertainties (Altshuller, 1991; Thompson and Stewart,1991; Paulson and Seinfeld, 1992; Paulson et al., 1992; Montzka et al., 1993).
Nonmethane hydrocarbons generally react with OH such that an
increase in their concentration will decrease OH concentrations. However,
as these species are oxidized, they also produce odd-hydrogen species
and organic radicals, so that the emissions of NMHCs contribute to
an increase in the concentration of peroxy radicals (Cantrell et al.,
1993) and odd-hydrogen radicals as a whole. When HO
and RO
(the symbol R refers to a hydrocarbon fraction) concentrations are increased,
in the presence of sufficient NO, the formation of NO
is increased.
This leads to an increased production of ozone after photolysis
of NO
:
NMHC + OHRO
+ other products
RO+ NO
NO
+ RO
NO+ hv
NO + O
O + O+ M
O
+ M.
(In these equations the term hv indicates that the reaction
involves the absorption of a photon.) Reactions of RO and RO
radicals
with NO
produce organic nitrates. The oxidation of NMHCs can lead to
the formation of acetyl radicals (RCO
) which can react with NO
to
form PAN. Organic nitrates, peroxy nitrites, and PAN may carry
reactive nitrogen to remote locations.
The alkenes, isoprene and terpenes can also react with O
directly leading to the removal of O
. Laboratory studies have shown
that the reaction of isoprene with O
can directly produce both OH and
O(3P) (the electronic ground state of atomic oxygen) with yields of 0.68
and 0.45, respectively (Paulson and Seinfeld, 1992). To the extent that
this occurs in the ambient atmosphere it will decrease the rate of
O
destruction by the O
/isoprene reaction and would decrease the OH
sink from the OH/isoprene reaction. At night the reaction of
alkenes, isoprene and terpenes with NO
is also thought to
be significant.
The lifetimes of the nonmethane hydrocarbons vary from a few minutes
to several months for some of the lighter alkanes. These latter species
can be transported long distances from their sources and thereby act as
a tracer for continental sources (see e.g. Parrish et al., 1992).
Isoprene and other biogenic hydrocarbons on the other hand can
be significant in the formation of O
on local to regional scales (McKeen
et al., 1991; Rosille et al., 1991; Chameides et al., 1992).
Emissions of nonmethane hydrocarbons derive from fossil fuel burning, industrial and evaporative sources, biomass burning (including fuel wood, agricultural wastes, forest, and savannah burning), emissions by plants, and oceanic sources. Anthropogenic sources are estimated to be between 100 and 150 Tg/yr (Piccot et al., 1992; Singh and Zimmerman 1992; Prather et al., 1994). Of this total Singh and Zimmerman (1992) estimate that 58 Tg/yr are from industrial and fossil fuel sources with the reminder from biomass burning. Piccot et al. (1992) estimate 67 Tg/yr from industrial and fossil fuel sources and 43 Tg/yr from biomass burning. Additional estimates for the NMHC source from biomass burning are significantly lower. Laursen et al. (1992) estimate only 17 Tg/yr.
Emissions of reactive NMHCs by plants are wide spread and important in the global budget of NMHCs. Plants mainly emit isoprene and terpenes but a variety of oxygenated species have also been identified (Fehsenfeld et al., 1992). On a global basis the sources of isoprene range from 350 to 450 Tg/yr (Singh and Zimmerman 1992). Terpenes and emissions of other hydrocarbons are about as large as those of isoprene (Singh and Zimmerman 1992). Considerable laboratory and field work has been done to narrow the uncertainties in these biogenic estimates (Arey et al., 1991; Guenther et al., 1991; Guenther et al., 1993; Lerdau et al., 1994). Another possible major source of NMHCs, the ocean, also needs better quantification. The ocean source of propane and ethane has been estimated at between 36 and 82 Tg/yr (Singh and Zimmerman, 1992). Singh and Zimmerman (1992) have compiled separate emissions estimates for individual NMHCs from the most important sources. These estimates, though uncertain, are for the most part consistent with observations of the individual species and their known atmospheric lifetimes. These conclusions are subject to change and clarification, however, since the data base on which these estimates are derived is extremely limited. One hydrocarbon for which the emissions estimates are known to be in error is propane. The estimated sources of propane from natural gas emission, biomass burning, and oceans are too small (by about a factor of four) to account for its concentration (Singh and Zimmerman, 1992).
It has recently been recognized that oxygenated hydrocarbon sources may also be important for tropospheric chemistry. Acetone was found to be the dominant nonmethane organic species outside the Arctic planetary boundary layer (Singh et al., 1994). Concentrations in the Arctic may be explained by a combination of oxidation of propane, isobutane and isobutene as well as by direct emission from plants and through biomass burning. (Singh et al., 1994) estimate a global source of 40-60 Tg/yr. Organic alcohols have also been measured from biogenic sources (Goldan et al., 1993; MacDonald and Fall, 1993). Although estimates of their global source have not been made, they may contribute significantly to tropospheric photochemistry through the production of aldehydes (MacDonald and Fall, 1993). Acidity in rainwater in remote regions can be dominated by formic and acetic acids (Talbot et al., 1992; Moody et al., 1991). These acids have a strong continental biogenic source (see e.g. Klemm et al. 1994), but oceanic (Moody et al. 1991) and biomass burning (Lefer et al., 1994) sources cannot be ruled out.