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What is Atmospheric Chemistry?

| The Chemistry of Atmospheric Pollutants | What is Global Warming

Atmospheric chemists are interested in understanding the chemical composition of the natural atmosphere, the way gases, liquids, and solids in the atmosphere interact with each other and with the earth's surface and associated biota, and how human activities may be changing the chemical and physical characteristics of the atmosphere. This latter question is currently a driving force behind the growing need for atmospheric chemists in Canada. 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. All of these issues affect Canadians, and a great deal of research and development activity 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. The importance of Atmospheric Chemistry has been acknowledged by awarding the Nobel Prize in Chemistry 1995 to the Atmospheric Scientists P. Crutzen, M. Molina and F. S. Rowland.

Photochemical Smog/Tropospheric Ozone

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 (45 million/per year in Ontario), 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.

Stratospheric Ozone Depletion

The "ozone layer" is a region of relatively high ozone concentration (at about 25 km altitude) in the stratosphere, which is a layer of the atmosphere between 15 and 50 km. The ozone is produced from interactions between the energetic ultra violet (UV) light reaching the stratosphere and molecular oxygen. The presence of the ozone layer in the stratosphere is vital to life at the surface since the ozone layer absorbs UV light that would otherwise reach the surface and cause damage to both animal and plant life. However, the ozone layer is becoming chemically perturbed due to the presence of chlorofluorocarbons (CFCs) in the stratosphere. CFCs are used in air conditioners and as cleaning and blowing agents in the chemical industry. These compounds are chemically inert, and they ultimately diffuse upward to the stratosphere. In the stratosphere, because of the presence of the higher energy UV light, these compounds can absorb the light and decompose to produce chlorine atoms which then participate in a chain reaction in which ozone in the ozone layer is destroyed. In 1985 an "ozone hole" was discovered over Antarctica which appears every year in October. The amount of ozone in the stratosphere over Antarctica has decreased to only half the natural level. This large decrease is localized over Antarctica due to the very cold temperatures, but there appears to have been a global decrease in the "natural" abundance of stratospheric ozone of 3-5%. This problem of stratospheric ozone depletion is a very complex and challenging problem, that will likely have a significant impact on human activities for decades to come. Atmospheric chemists are needed to study the processes that occur in the stratosphere, and to study the atmospheric impact of the candidate replacement compounds (e.g., HCFCs).

Toxic Air Pollutants

As a result of a variety of human activities (e.g., agriculture, transportation, industrial processes) a large number of different toxic pollutants are emitted into the atmosphere. Among the chemicals that may pose a human health risk are pesticides, PCBs, polycyclic aromatic hydrocarbons (PAHs), dioxins, and volatile organic compounds (e.g., benzene, carbon tetrachloride). Many of the more environmentally persistent compounds (e.g., PCBs) have been measured in Arctic wildlife and, for example, in tissues of the local Inuit population. Because of the enormous variety of toxic pollutants present in the air that we breathe, it is an enormously challenging task to determine the human health risks from exposure to this mixture. Scientists are needed in this field to measure the atmospheric concentrations of these species, to identify and quantify the sources of these pollutants, and to determine their environmental fate. This has been identified as a priority area by Canadian Environment Ministries.