Secondary Air Pollutants
Tropospheric Ozone Creation
Nitrogen dioxide is released through the combustion of fossil fuels, with factories and automobiles being the primary source of its emission.
In the presence of sunlight, nitrogen dioxide (NO2) undergoes a chemical reaction and splits into nitric oxide (NO) and a lone oxygen atom. Energy provided by the sunlight results in the free oxygen reacting with atmospheric oxygen (O2) in order to form ozone (O3). On longer, warmer days, tropospheric ozone will continue to build up.
In the absence of sunlight, such as at night, these reactions stop. Ozone reacts with the nitric oxide, forming oxygen and nitrogen dioxide. Ozone destruction is the name of this process, which occurs daily.
Photochemical Smog
The natural process mentioned above can be disrupted by human activities. When certain VOCs are present, they can react with nitric oxide to form photochemical oxidants. This prevents the NO from being able to react with ozone when the sun goes down. The build-up of ozone and photochemical oxidants results in photochemical smog, which can result in the burning of the eyes, respiratory irritation, aggravation of existing respiratory conditions, and pained breaths due to inflamed airways.
What is a temperature inversion and how does it affect smog?
Normally, warm air rises in the troposphere. When it comes to air pollution and smog, this is beneficial as it allows the pollutants to rise and disperse, rather than being concentrated near all the people.
It is possible for air to defy these normal rules. In the winter, solar radiation warms up the ground and the air above it. At night, the ground can rapidly lose heat, meaning the air directly above it does as well, resulting it the bottom air being colder than the warmer air above it that had rose earlier in the day. Thermal inversions, which is what this phenomenon is known as, can act to trap the bottom cold air close to the ground, also trapping the pollutants.
Acid Deposition
Normal rainwater is slightly acidic, due to carbon dioxide combining with the water and forming a weak acid. However, it can become even more acidic due to air pollution.
Nitrogen oxides and sulfur oxides, when released into the atmosphere, undergo a variety of reactions with oxygen and water vapor. This results in the formation of the secondary air pollutants nitric acid (HNO3) and sulfuric acid (H2SO4), which will dissociate into H+ ions and either sulfate or nitrate ions. Acid deposition is when the water in precipitation (which need not be rain, you can have acid snow, acid hail, etc.) is mixed with these secondary pollutants, resulting in even more acidic water.
This can bring organisms outside their range of tolerance, acidify the soil or water of ecosystems, and make toxic metals more soluble. Habitats where the bedrock is limestone are not as affected, due to limestone neutralizing acids.
