Tropospheric
ozone can act both as a direct greenhouse gas and as an indirect
controller of greenhouse gas lifetimes. As a direct greenhouse
gas, it is thought to have caused around one third of all the
direct greenhouse gas induced warming seen since the industrial
revolution.
Tropospheric ozone is a particularly difficult greenhouse gas
to keep track of, due to its short lifespan and the fact that
ozone concentrations can vary hugely from place to place. The
current estimate for global tropospheric ozone is 370 million
tonnes, equivalent to about 50 parts per billion.
The largest net source of tropospheric ozone is influx from the
stratosphere. Large amounts of ozone are also produced in the
troposphere by photochemical reactions, the amounts increasing
with high levels of air pollution.
Much of this ozone, though, is balanced by photochemical ozone
destruction. One positive side to tropospheric ozone is its effect
on the atmospheric lifetimes of some greenhouse gases. The break
down of tropospheric ozone in sunlight leads to the production
of hydroxyl (OH) radicals, these help to mop up some other greenhouse
gases, like methane, and so
lessen their global warming potential. Another important sink
for tropospheric ozone is uptake by plants.
Human Impact
Increasing emission of atmospheric pollutants, such as those
from biomass and fossil fuel burning has led to large increases
in tropospheric ozone concentrations in the last 100 years. As
noted previously, a high rate of photochemical ozone production
relies on high concentrations of other atmospheric pollutants,
such as those released in large quantities by the activities of
man. Increased concentrations in ozone, and the compounds which
lead to its formation, can be detected many miles downwind of
large cities.
Potential for control
Our potential for control of tropospheric ozone lies in reducing
the levels of atmospheric pollution arising from man-made sources,
such as biomass burning, industry and transport. Though exact
data on changes in tropospheric ozone concentrations are difficult
to obtain, and short term trends hard to identify, tropospheric
ozone remains a powerful greenhouse gas whose concentration has
undoubtedly been elevated by man.