Question

Weather conditions affect the smog equilibrium in the atmosphere. What happens to the nitrogen dioxide concentration on (a) hot, sunny days and (b) cool, overcast days?

Short answer

On hot, sunny days, NO2 decreases; on cool, overcast days, NO2 remains higher.

Step-by-step solution

Understanding Smog Formation

Smog is primarily formed from nitrogen dioxide (NO2) and volatile organic compounds in the presence of sunlight. NO2 can be photolyzed by sunlight to produce nitric oxide (NO) and atomic oxygen, which can then form ozone, a key component of smog.

Analyzing Hot, Sunny Days

On hot, sunny days, the increased sunlight and higher temperatures accelerate the photolysis of NO2, leading to more ozone formation. As a consequence, the concentration of nitrogen dioxide in the atmosphere will decrease because it is being used up in the photochemical reactions that produce ozone.

Analyzing Cool, Overcast Days

On cool, overcast days, there is reduced sunlight, which slows down the photolysis of NO2. This results in less ozone formation and as such, the concentration of nitrogen dioxide remains higher in the atmosphere due to its lower consumption in photochemical reactions.

Key concepts

Nitrogen Dioxide

Nitrogen dioxide (NO₂) is a significant air pollutant and plays a key role in the formation of smog. It is a reddish-brown gas with a characteristic sharp, biting odor and is primarily produced from burning fossil fuels. NO₂ can have harmful effects on human health, such as respiratory problems.

During the day, NO₂ is photolyzed by sunlight, a process which leads to the formation of ozone. This means that the more sunlight there is, the more NO₂ gets broken down. When it breaks down, it forms nitric oxide (NO) and a free oxygen atom. The free oxygen atom can then react with molecular oxygen (O₂) to form ozone (O₃).

• NO₂ sources include vehicle emissions, industrial activity, and electricity generation.
• It contributes to the brownish atmosphere sometimes seen in polluted urban settings.
• NO₂ is harmful as it can lead to ground-level ozone formation and acid rain.

Photolysis

Photolysis is a chemical process where molecules are broken down by photons, or particles of light. In the context of smog formation, photolysis plays an essential role in transforming nitrogen dioxide (NO₂) into reactive substances that lead to ozone production.

The reaction is highly dependent on the presence of sunlight. More sunlight means more photolysis of NO₂, which in turn enhances the chemical reactions leading to smog. On a hot, sunny day, photolysis of NO₂ happens rapidly, reducing its concentration in the atmosphere as it's converted into other chemicals. Conversely, on cloudy or overcast days with less sunlight, the rate of photolysis drops, leading to higher concentrations of NO₂ in the air.

• Photolysis is crucial for the initial breakdown and transformation of pollutants.
• The process is a main driver for daytime atmospheric chemistry.
• Factors like weather, time of year, and atmospheric conditions can influence the rate of photolysis.

Ozone Formation

Ozone (O₃) is an important component of the Earth's atmosphere, and its formation is closely tied to the photolysis of nitrogen dioxide. Once NO₂ is exposed to sunlight, it breaks down into nitric oxide (NO) and a free oxygen atom. This highly reactive oxygen atom can then combine with oxygen molecules (O₂) to produce ozone.

Higher levels of ozone can be observed on sunny days due to increased photolysis activity, leading to more NO₂ breaking down and subsequently forming more ozone. However, while ozone is beneficial higher up in the atmosphere where it protects us from harmful UV radiation, ground-level ozone is a significant component of urban smog and is harmful to human health.

• Ozone formation is a photochemical process that requires both sunlight and pollution.
• It is a secondary pollutant, not emitted directly but formed in the atmosphere.
• Ozone can cause respiratory problems and exacerbate conditions such as asthma.