Structure and Composition of the Present-Day Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 64

1999). In the background troposphere, CO(g) mixing
ratios generally vary from 50 to 150 ppbv.

Table 3.9. Sources and sinks of atmospheric
methane

Sources Sinks

3.6.3.3. Health Effects
Exposure to 300 ppmv of CO(g) for one hour causes a
headache. Exposure to 700 ppmv for one hour causes
death. From 1999 to 2004, CO(g) poisoning caused
about 500 unintentional U.S. deaths per year. Many
of these deaths were due to indoor CO(g) buildup
from leaking indoor combustion heaters combined with
poor ventilation. CO(g) poisons by dissolving in blood
and replacing oxygen as an attachment to hemoglobin
[Hb(aq)], an iron-containing compound. The conver-
sion of O 2 Hb(aq) to COHb(aq) (carboxyhemoglobin)
causes suffocation. CO(g) can also interfere with O 2 (g)
diffusion in cellular mitochondria and with intracellular
oxidation (Gold, 1992). For the most part, the effects of
CO(g) are reversible once exposure to CO(g) is reduced.
However, following acute exposure, individuals may
still express neurological or psychological symptoms
for weeks or months, especially if they become tem-
porarily unconscious (Choi, 1983).

Methanogenic bacteria
Natural gas leaks during
fossil fuel mining and
transport
Leaks from deep ocean and
from under permafrost
Fossil fuel, biomass, and
biofuel combustion
Atmospheric chemical
reaction

Atmospheric chemical
reaction
Dissolution in surface
water
Deposition to sea ice, snow,
soil, vegetation, and
structures
Methanotrophic bacteria

3.6.4. Methane
Methane [CH 4 (g)] is the most reduced form of car-
bon in the air. It is also the simplest and most abun-
dant hydrocarbon and organic gas. Methane is a green-
house gas that absorbs thermal-IR radiation twenty-five
times more efficiently, molecule for molecule, than does
CO 2 (g), but mixing ratios of carbon dioxide are much
higher than are those of methane. Because its e -folding
lifetime against chemical breakdown is so long (8-12
years), methane hardly enhances ozone formation in
photochemical smog. In contrast, because it builds up
in the background troposphere and breaks down slowly
over time, it is an important contributor to global tro-
pospheric ozone. In the stratosphere, methane has lit-
tle effect on the ozone layer because other sources of
ozone are much larger, but its chemical decomposition
provides one of the few sources of stratospheric water
vapor.

3.6.4.1. Sources and Sinks
Table 3.9 summarizes the sources and sinks of atmo-
spheric methane. Methane is produced in anaero-
bic environments, where methanogenic bacteria con-
sume organic material and excrete methane (Reaction
2.4). Ripe anaerobic environments include rice pad-
dies (Figure 3.15); landfills; wetlands; and the digestive
tracts of cattle, sheep, and termites. Methane is also

Figure 3.15. Rice paddies, such as this one in the
town of Sapa, Vietnam, produce not only an
important source of food, but also methane gas.
C

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