In the context of ecology and environmental science, the word emission generally refers to a release of a substance or energy to the environment. Often, emissions refer to substances or energy that are ecological stressors, and can potentially cause deleterious changes.
Emission sources are of two broad types, point and diffuse. Point sources of emission are discrete and spatially localized. A volcano is a natural example of a point sources of emission of gases and particulates to the atmosphere. Some point sources associated with human activities are automobiles, chimneys of homes, smokestacks of power plants and smelters, and aquatic discharge pipes for chemical effluent and waste heat of factories. In contrast, diffuse emissions occur over large areas from many, often indistinct sources. For example, although each is a small point source, the large numbers of houses and vehicles in cities collectively represent a large, diffuse source of emissions of pollutants into the atmosphere.
Emitted energy can be of two types, heat or electromagnetic. Heat is a type of kinetic energy, involving the vibration of atoms or molecules. The more vigorously these are vibrating, the greater the heat content of a substance. Discharges of warm water from power plants and factories are examples of the emission of heat.
Electromagnetic energy is the energy of photons, which are entities that have properties of both particles and waves. Electromagnetic energy is divided into spectral components on the basis of wavelength, and includes radio waves, infrared, visible (so-called because it is perceptible by the human eye), ultraviolet, x rays, and gamma radiation. Electromagnetic energy is generally emitted by a point source, whose emission spectrum can involve one or more of the categories just noted.
Emissions of materials can be from diffuse or point sources. Human activities result in emissions of many chemicals that are important pollutants that cause environmental damages. Some of the most important of these emitted materials are gases such as sulfur dioxide, oxides of nitrogen (a mixture of nitric oxide and nitrogen dioxide), and carbon dioxide, particulates containing metals and organic compounds, and vapors of hydrocarbons, mercury, and other chemicals.