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Other Direct Greenhouse Gases - Tropospheric Ozone

high altitude cloudsTropospheric 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.


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