The exhaust gases of cars and trucks are major sources of nitrogen oxides, as are the emissions from electrical power generation plants. Nitrogen oxides are at least partially responsible for several types of air pollution. Nitrogen dioxide lends its color to the reddish-brown haze we call smog. Photodissociation of nitrogen dioxide by sunlight produces nitric oxide and ozone in the troposphere, which is another component of smog. A series of chemical reactions transform Volatile Organic Compounds VOCs into substances that combine with nitrogen dioxide to produce PAN Peroxyacytyl nitrate , yet another element in smog.
Nitrogen dioxide in the air also reacts with water vapor to form nitric acid, one of the types of acid in acid rain. Nitric oxide concentration in unpolluted air is around 0. In smog, the concentration rises twenty-fold to about 0. Although nitrogen oxides have gained dubious distinction as pollutants, they are also used beneficially in some industrial processes. Nitric oxide is manufactured on a large scale, and is subsequently used to make nitric acid HNO3. To create nitric oxide for industrial uses, chemists combine ammonia NH3 with oxygen O2 , releasing water H2O as a byproduct.
Nitrogen compounds derived from nitric acid are used to create chemical fertilizers, explosives, and other useful substances. Skip to main content. The Carbon Cycle The element carbon is a part of seawater, the atmosphere, rocks such as limestone and coal, soils, as well as all living things. Carbon moves from the atmosphere to plants.
In the atmosphere, carbon is attached to oxygen in a gas called carbon dioxide CO2. Through the process of photosynthesis, carbon dioxide is pulled from the air to produce food made from carbon for plant growth. Carbon moves from plants to animals. Through food chains, the carbon that is in plants moves to the animals that eat them.
Animals that eat other animals get the carbon from their food too. Carbon moves from plants and animals to soils. When plants and animals die, their bodies, wood and leaves decays bringing the carbon into the ground. Some is buried and will become fossil fuels in millions and millions of years. Carbon moves from living things to the atmosphere. The long carbon cycle involves the long-term storage of carbon. Marine organisms, such as shellfish and phytoplankton, build their shells by combining calcium with carbon.
When they die they accumulate on the ocean floor. Over millions of years, these organisms become compressed and become carbon-rich sedimentary rock. This carbon is usually stored in rocks for around million years. Oceanic crust, containing sedimentary rock, is subducted causing the crust to melt.
CO2 that was stored in the rock is released into the atmosphere through volcanic eruptions. Sedimentary rocks near the surface release CO2 as they are chemically weathered. The basic chemical reaction looks like this:. In all four processes, the carbon dioxide released in the reaction usually ends up in the atmosphere.
The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop.
It is as if the Earth is breathing. The ebb and flow of the fast carbon cycle is visible in the changing seasons. As the large land masses of Northern Hemisphere green in the spring and summer, they draw carbon out of the atmosphere.
This graph shows the difference in carbon dioxide levels from the previous month, with the long-term trend removed.
This cycle peaks in August, with about 2 parts per million of carbon dioxide drawn out of the atmosphere. In the fall and winter, as vegetation dies back in the northern hemisphere, decomposition and respiration returns carbon dioxide to the atmosphere.
On Earth, most carbon is stored in rocks and sediments, while the rest is located in the ocean, atmosphere, and in living organisms. These are the reservoirs, or sinks, through which carbon cycles. Carbon is released back into the atmosphere when organisms die, volcanoes erupt, fires blaze, fossil fuels are burned, and through a variety of other mechanisms. Humans play a major role in the carbon cycle through activities such as the burning of fossil fuels or land development.
As a result, the amount of carbon dioxide in the atmosphere is rapidly rising; it is already considerably greater than at any time in the last , years.
What is the carbon cycle? Carbon is the chemical backbone of all life on Earth.
0コメント