The atmosphere is a "soup" of many gasses that are constantly interacting. Air chemistry is concerned with the chemical composition and chemical reaction that occur in the atmosphere, both natural and anthropogenic (human induced). It is also concerned with how human activities may be changing the chemical and physical characteristics of the atmosphere. There are a number of critical environmental issues associated with a changing atmosphere, including photochemical smog, global climate change, toxic air pollutants, acidic deposition, and stratospheric ozone depletion. A great deal of research and development activity is aimed at understanding and hopefully solving some of these problems is underway. Much of the anthropogenic (human) impact on the atmosphere is associated with our increasing use of fossil fuels as an energy source - for things such as heating, transportation, and electric power production. Photochemical smog/tropospheric ozone is one serious environmental problem associated with burning fossil fuels.

A link to air chemistry terms:

Gases are naturally given off by volcanoes, plants and animals, not all of which are healthy. Plants give off oxygen gas (O₂), Animals give of carbon Dioxide (CO₂) and methane (CH₂). Gas release from volcanoes is mainly H₂O, but includes significant amounts of CO₂, sulfur dioxide (SO₂), chlorine (Cl), florine (F), nitrogen (N), carbon monoxide (CO), and Hydrogen sulfide (H₂S). In general the evidence shows that the earth-atmosphere system has maintained a balance of gases for many millions of years. Although there are instances where many deaths have occurred due to gasses given off by volcanic activity.

The principal pollutants produced by industrial, home heating and traffic sources are:

When fossil fuels (e.g., gasoline) are burned, a variety of pollutants are emitted into the earth's troposphere, i.e., the region of the atmosphere in which we live - from ground level up to about 15 km. Two of the pollutants that are emitted are hydrocarbons (e.g., unburned fuel) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which the NO is converted to nitrogen dioxide (NO2). NO2 is a brown gas and at sufficiently high levels can contribute to urban haze. However, a more serious problem is that NO2 can absorb sunlight and break apart to produce oxygen atoms that combine with the O2 in the air to produce ozone (O3). Ozone is a powerful oxidizing agent, and a toxic gas. In North America elevated levels of tropospheric ozone cause several billion dollars per year damage to crops, structures, forests, and human health. It is believed that the natural level of ozone in the clean troposphere is 10 to 15 parts-per-billion (ppb). Because of increasing concentrations of hydrocarbons and NO in the atmosphere, scientists have found that ozone levels in "clean air" are now approximately 30 ppb. A principal activity of atmospheric chemists is to study and determine how we might reverse this trend.

Fossil fuels are composed mostly of carbon. When they are burned this carbon is released into the atmosphere, mainly in the form of carbon dioxide, CO2. We currently emit roughly 5 billion tons of carbon into the atmosphere each year. As a result, there has been a steady increase in global atmospheric levels of CO2. This increase in CO2 (along with other gases including methane, ozone, and CFCs) presents a problem, because these gases are "greenhouse" gases, that is they absorb infrared radiation (i.e., "heat") that is radiated out from the earth. Thus, heat that would otherwise be lost to space is trapped in the atmosphere, leading to increased temperatures. Climatologists have predicted that, as a result of increasing concentrations of greenhouse gases in the atmosphere, the earth's temperature will increase by about 3 C by the year 2030. This will result in significant changes in local climate, in some areas leading to loss of arable land, and an increase in sea level with associated coastal flooding. In addition, global warming may exacerbate the photochemical smog problem. Hundreds of atmospheric scientists are employed worldwide to study the magnitude and implications of this problem, and potential solutions.

In both developed and rapidly industrializing countries, the major historic air pollution problem has typically been high levels of smoke and SO₂ arising from the combustion of sulfur-containing fossil fuels such as coal for domestic and industrial purposes. Smogs resulting from the combined effects of black smoke, sulphate/acid aerosol and fog have been seen throughout Northern European cities for centuries, and still occur in many parts of the developing world.

In economically developed countries, however, this problem has diminished over recent decades as a result of changing fuel-use patterns; the increasing use of cleaner fuels such as natural gas, and the implementation of effective smoke and emission control policies. These long-term changes in air pollution climate are also seen, often occurring very rapidly, in many developing countries.

In both developed and developing countries, the major threat to clean air is now posed by traffic emissions. Petrol- and diesel-engine motor vehicles emit a wide variety of pollutants, principally carbon monoxide (CO), oxides of nitrogen (NO_x), volatile organic compounds (VOCs) and particulates, which have an increasing impact on urban air quality.

In addition, photochemical reactions resulting from the action of sunlight on NO2 and VOCs from vehicles leads to the formation of ozone, a secondary long-range pollutant, which impacts in rural areas often far from the original emission site. Acid rain is another long-range pollutant influenced by vehicle NOx emissions.

In all except worst-case situations, industrial and domestic pollutant sources, together with their impact on air quality, tend to be steady-state or improving over time. However, traffic pollution problems are worsening world-wide. The problem may be particularly acute in developing countries with dramatically increasing vehicle fleets, infrastructural limitations, poor engine/emission control technologies and limited provision for maintenance or vehicle regulation.