Many climate processes are controlled by the levels of trace gases in the atmosphere, rather than the major constituents. These gases are present in very low amounts, i.e. a few molecules in one million or even one billion air molecules. To describe this, we often use the unit ppm (parts per million) so a trace gas with a concentration of 1 ppm means that there is just one molecule of the gas in every 1,000,000 air molecules (the more scientific unit is 1 Ámol mol-1, we will talk more about atmospheric gas concentration units later).
Levels of carbon dioxide, a very important greenhouse gas have increased from 280 ppm in preindustrial times to about 385 ppm now (2008) and predictions are that these concentrations will continue to rise due to human activities, the most important of which is fossil fuel combustion. Two other important greenhouse gases are methane (1.8 ppm) and ozone (varying around about. 0.04 ppm). In addition, there are thousands of organic and inorganic gases which are emitted into the air from plants (imagine the smell of flowers) or during industrial procedures (think about solvents) or are formed during chemical processes in the atmosphere. These gases all play a part in the complex chemistry which goes on in the lowest layer of the atmosphere, the troposphere.