The question of whether smog is the same as emissions is common, but the answer is definitively no. Emissions are the initial substances released into the atmosphere. Smog is the visible, hazy atmospheric condition that results when these substances undergo chemical reactions, making it the complex, often harmful, end product. Understanding the distinction requires examining the source materials and the process that links them.
Emissions: The Source of Pollutants
Emissions are defined as the release of gaseous or particulate substances from a source into the air. These sources are largely categorized as primary, meaning they directly discharge pollutants into the environment. Major sources include the combustion of fossil fuels in vehicle tailpipes, industrial stacks, and the burning of wood or other organic materials.
Primary pollutants released include nitrogen oxides (\(\text{NO}_\text{x}\)), sulfur dioxide (\(\text{SO}_\text{2}\)), carbon monoxide (\(\text{CO}\)), and volatile organic compounds (VOCs). Particulate matter (PM), consisting of tiny solid or liquid particles suspended in the air, is also a significant primary emission. \(\text{NO}_\text{x}\) and \(\text{SO}_\text{2}\) are especially important because they serve as the chemical building blocks for smog.
Smog: The Atmospheric Result
Smog is a visible, hazy form of air pollution that blankets urban and industrial areas. It is a mixture containing both primary pollutants and secondary pollutants, which are compounds formed after primary emissions react in the atmosphere. The most abundant secondary pollutant in modern smog is ground-level ozone (\(\text{O}_\text{3}\)).
There are two main types of smog, distinguished by their chemical composition. Industrial smog is characterized by high concentrations of sulfur dioxide and particulates from coal burning. Photochemical smog, often associated with warm, sunny climates, is a brown haze dominated by ground-level ozone and nitrogen compounds.
How Emissions Create Smog
The transformation of primary emissions into smog requires specific atmospheric conditions and chemical interactions. Photochemical smog, the most common type today, is a complex process driven by solar energy. This process begins when nitrogen dioxide (\(\text{NO}_\text{2}\)), a component of \(\text{NO}_\text{x}\) emissions, absorbs ultraviolet radiation from the sun. This absorption breaks the \(\text{NO}_\text{2}\) molecule apart, releasing a highly reactive oxygen atom.
This free oxygen atom immediately combines with molecular oxygen (\(\text{O}_\text{2}\)) to form ground-level ozone (\(\text{O}_\text{3}\)). Volatile organic compounds (VOCs) are also involved, reacting with nitrogen oxide to prevent the ozone from being naturally depleted. The accumulation of this ozone, alongside other secondary pollutants, creates the visible brown haze.
A meteorological phenomenon known as a temperature inversion often intensifies smog episodes. Normally, air temperature decreases with altitude, allowing pollutant-laden air near the surface to rise and disperse. During an inversion, a layer of warmer air settles above cooler air near the ground. This warmer upper layer acts like a lid, trapping the emissions and the resulting smog close to the surface, allowing pollutant concentrations to build up.