Geoscience Reference
In-Depth Information
326
Northern hemisphere
Global
Southern hemisphere
324
322
320
318
Figure 10.4 Global and
hemispheric trends in
concentration of atmospheric
N
2
O estimated from direct
measurements.
316
314
1998
2000
2002
2004
2006
2008
2010
2012
Year
The global nitrogen budget is studied to understand atmospheric N
2
O
values. Nitrous oxide is released from many different sources due to biological
activity in soils and water—for example, microbial activity in tropical forests.
Important anthropogenic sources are related to soil and manure management
in agricultural practice, sewage treatment, and fossil fuel combustion. See
chapter 12
for more details.
TROPOSPHERIC OZONE (O
3
)
Ozone in the stratosphere absorbs ultraviolet solar radiation (see
chapter 4
),
protecting life on the planet from these destructive wavelengths. But in the tropo-
sphere, O
3
is a pollutant that can damage living tissue, and it is a greenhouse gas.
The largest natural source of tropospheric ozone is downward migration from
the stratosphere; lightning also generates O
3
by breaking down molecular oxy-
gen. Ozone is produced by a host of human activities, including biomass burn-
ing, fossil fuel combustion, and the use of chemical solvents. Even photocopying
and laser printing produce O
3
. These activities release nitrogen oxides (NO
x
,
or NO and NO
2
, not to be confused with N
2
O), carbon monoxide (CO), and
volatile organic compounds
(VOCs) into the atmosphere, where they support
the photochemical production of O
3
. Ozone breaks down in sunlight, and one
product of this photodissociation is the highly reactive OH radical, which helps
remove CH
4
from the atmosphere. Uptake by plants is also an important sink.
Tropospheric O
3
varies significantly on all space and time scales, so produc-
ing an accurate measurement of its average levels would require an extensive
observing network. Because such a network does not exist, global O
3
levels are
estimated using computer models, and the validity of these models is tested
against the relatively few measurements available. The most recent estimates
take into account increases in tropospheric O
3
due to its anthropogenic emis-
sion as well as from compounds that react in the atmosphere to produce O
3
.
Negative tendencies in tropospheric O
3
due to the depleted ozone source in
the stratosphere are also included in the estimates. Locally, in situ ozone mea-
surements can be used to evaluate trends. A consistency among direct mea-
surements of tropospheric ozone levels has not been established; some local
measurements show increases at rates as high as 1.4% per year, while measure-
ments in other areas find steady values.
