Question

What is the mechanism of photochemical-smog formation? Discuss in brief.

Short answer

Photochemical smog forms when nitrogen oxides and volatile organic compounds in the atmosphere react under sunlight. The sunlight breaks down these gases, releasing nitrogen and oxygen atoms that further react with VOCs, leading to their oxidation. This produces peroxy radicals and nitrogen dioxide. The radicals react with nitrogen oxide and other compounds to form ozone and other harmful airborne particles.

Step-by-step solution

Identify the initial pollutants

The process of photochemical smog formation starts with the emission of nitrogen oxides (NOx) and volatile organic compounds (VOCs) into the atmosphere. These pollutants are often released during fossil fuel combustion, for example, from car exhausts and industrial processes.

Reaction initiation

When these pollutants are released into the sunlit atmosphere, they interact with sunlight. Ultraviolet (UV) light from the sun breaks the nitrogen oxide molecules into individual nitrogen and oxygen atoms.

Oxidation of VOCs

The free nitrogen and oxygen atoms can then react with VOCs present in the atmosphere, leading to their oxidation. This reaction produces peroxy radicals and nitrogen dioxide.

Formation of ozone and toxic substances

The peroxy radicals can react with nitrogen oxide to form nitrogen dioxide and ozone. In addition, these radicals can react with other compounds in the air to create additional pollutants, including secondary organic aerosols and toxic substances such as peroxyacetyl nitrate.

Key concepts

Nitrogen Oxides (NOx)

Understanding the role of nitrogen oxides in the formation of photochemical smog is essential. Nitrogen oxides (NOx) are a group of gases mainly composed of nitric oxide (NO) and nitrogen dioxide (NO2). These gases are typical air pollutants produced from vehicle emissions, power plants, and other combustion processes. When NOx is released into the atmosphere, it doesn't just linger inertly; it interacts with other atmospheric constituents under the influence of sunlight.

Key aspects of NOx in photochemical smog formation include their ability to undergo photolysis, where UV light splits nitrogen dioxide back into nitric oxide and an oxygen atom. Following that, the free oxygen atom often combines with diatomic oxygen (O2) to form ozone (O3). This sequence is a critical step in photochemical smog creation, leading to increased concentrations of ozone at ground level, which can be harmful to respiratory health and the environment.

Volatile Organic Compounds (VOCs)

Volatile organic compounds, or VOCs, are another piece of the photochemical smog puzzle. These are a large group of carbon-containing chemicals that evaporate readily into the air at room temperature. Sources of VOCs include motor vehicle exhaust, industrial emissions, and chemical solvents, as well as natural sources such as plants.

Once in the atmosphere, VOCs can undergo a variety of chemical reactions. In the presence of sunlight, VOCs are more prone to react with oxygen species, forming reactive organic radicals that contribute to the smog formation process. These reactions usually lead to the creation of more complex and potentially more harmful secondary pollutants. Understanding and controlling VOC emissions is pivotal in managing smog and its effects on human health and the environment.

Ultraviolet (UV) Light Reactions

Ultraviolet (UV) light from the Sun initiates the chemical reactions necessary for photochemical smog formation. When UV light enters the Earth's atmosphere, it can break down chemical bonds in pollutants like nitrogen dioxide. This process, called photodissociation, creates free radicals which are highly reactive atoms or molecules with unpaired electrons.

The subsequent reactions between these radicals and other air pollutants are a major driving force behind the complex chemistry of smog formation. For instance, the reaction between a free oxygen atom and ambient oxygen results in ozone creation, one of the key players in smog. Thus, the amount of UV light and the intensity of sunlight are critical factors that influence the rate and extent of smog formation, which explains why smog is often worst on sunny days.

Ozone and Secondary Pollutants

Ozone (O3) is a major component of photochemical smog. While ozone high in the Earth's atmosphere protects us from harmful UV radiation, at ground level it's a pollutant with serious health implications. It's not emitted directly but formed when primary pollutants, like NOx and VOCs, react under the influence of sunlight.

Besides ozone, these reactions lead to secondary pollutants such as peroxyacetyl nitrate (PAN) and secondary organic aerosols (SOAs). These secondary pollutants are formed from the initial reactions of primary pollutants and, once formed, can be transported over large areas. They contribute to the overall toxicity and opacity of photochemical smog, affecting air quality and visibility. Effective smog management must therefore account for both the control of primary pollutants and the processes leading to secondary pollutant formation.