Geoscience Reference
In-Depth Information
produced in the ground from the decomposition of fos-
silized carbon. The resulting natural gas ,which con-
tains more than 90 percent pure methane, often leaks
to the air or is harnessed and used for energy. Methane
also leaks from storage under permafrost and in the deep
ocean as temperatures rise (Section 12.3.2.4). Methane
is also produced during biomass burning, fossil fuel and
biofuel combustion, and atmospheric chemical reac-
tions. Its sinks include chemical reaction; dissolution
into surface water; deposition to sea ice, snow, soil,
vegetation, and structures; and consumption by methan-
otrophic bacteria. The e -folding lifetime of tropospheric
methane against loss by chemical reaction, its major
loss process, is 8 to 12 years, which is slow in compar-
ison with the lifetimes of other organic gases. Because
methane is relatively insoluble, its dissolution rate in
water is low. Approximately 80 percent of the methane
in the air today is biogenic in origin; the rest originates
from fuel combustion and natural gas leaks.
Table 3.10. Sources and sinks of atmospheric ozone
Sources
Sinks
Chemical reaction of
O(g) with O 2 (g)
Photolysis
Atmospheric reaction in gas
phase and on surfaces
Dissolution in surface water
Deposition to sea ice, snow, soil,
vegetation, and structures
“good” when it is in the stratosphere and “bad” when
it is in the boundary layer, ozone molecules are the
same in both cases. In the United States, it is one of the
six criteria air pollutants whose outdoor mixing ratios
require control under CAAA70. It is also regulated in
most countries.
3.6.5.1. Sources and Sinks
Table 3.10 summarizes the sources and sinks of ozone.
Ozone is not emitted. Its only source in the air is chem-
ical production. Sinks of ozone include chemical reac-
tion; dissolution in surface water; and deposition to
sea ice, snow, soil, vegetation, and structures. Because
ozone is relatively insoluble, its dissolution rate is rela-
tively low.
3.6.4.2. Mixing Ratios
Methane's average mixing ratio in the troposphere is
near 1.85 ppmv, which is an increase from about 0.8
ppmv in the mid-1800s (Ethridge et al., 1992). Its tropo-
spheric mixing ratio increased steadily due to increased
biomass burning, fossil fuel and biofuel combustion,
fertilizer use, and landfill development until the 1990s,
when they leveled off through much of the 2000s. They
began to increase again in 2007 (Figure 12.8b). Mixing
ratios of methane are relatively constant with height in
the troposphere, but decrease in the stratosphere and
above due to its increase in chemical loss with increas-
ing height. At 25 km, methane's mixing ratio is about
half that in the troposphere.
3.6.5.2. Mixing Ratios
In the background troposphere, ozone mixing ratios are
20 to 40 ppbv near sea level and 30 to 70 ppbv at higher
altitudes. In urban air, ozone mixing ratios today range
from
<
10 ppbv at night to 550 ppbv during afternoons
in the most polluted cities of the world, with typical val-
ues of 70 to 150 ppbv during moderately polluted after-
noons. Indoor ozone mixing ratios are almost always
less than are those outdoors. In the stratosphere, peak
ozone mixing ratios are 10 to 12 ppmv.
3.6.4.3. Health Effects
Methane has no direct harmful human health effects at
typical outdoor or indoor mixing ratios.
3.6.5.3. Health Effects
Symptoms of ozone health problems start with head-
ache (
3.6.5. Ozone
Ozone [O 3 (g)] is a relatively colorless gas at typical
mixing ratios. It appears faintly purple when its mix-
ing ratio is high because it weakly absorbs green wave-
lengths of visible light and transmits red and blue, which
combine to form purple. Ozone exhibits an odor when
its mixing ratio exceeds 0.02 ppmv. In urban smog
or indoors, it is an air pollutant that harms humans,
animals, plants, and materials. In the stratosphere, its
absorption of UV radiation provides a protective shield
for life on Earth. Although ozone is considered to be
250 ppbv), and then
advance to shortness of breath, cough, and breathing
discomfort (
>
150 ppbv) and chest pain (
>
300 ppbv). Ozone decreases lung func-
tion for people who exercise steadily for more than an
hour while exposed to
>
300 ppbv. Over the long term,
ozone causes respiratory illness, the aging of lung tis-
sue, and lung disease, all of which can be fatal. The
elderly, sick, asthmatics, those with chronic bronchitis,
those with emphysema, and young children are most
susceptible to the corrosive effects of ozone on the
>
 
Search WWH ::




Custom Search