Question

The chemistry of NO in the atmosphere is complicated. NO can destroy ozone, as seen in Chapter 2. But remember from Chapter 1 that NO can react with \(\mathrm{O}_{2}\) to form \(\mathrm{NO}_{2}\). In turn, \(\mathrm{NO}_{2}\) can react in sunlight to produce ozone. Summarize these reactions, noting in which region of the atmosphere they each occur.

Short answer

NO converts to NO2 in the troposphere, which can photodissociate into NO and O, forming ozone.

Step-by-step solution

Understanding the Reaction of NO with O2

NO can react with \( ext{O}_2 \) in the atmosphere to form \( ext{NO}_2 \). This reaction typically occurs in the troposphere, where pollution and vehicle emissions are most common. The chemical equation for this reaction is \[ ext{NO} + ext{O}_2 ightarrow ext{NO}_2. \] This process is a part of urban air pollution dynamics.

Photochemical Reaction of NO2 with Sunlight

When \( ext{NO}_2 \) is present in sunlight, it can photodissociate into NO and a free oxygen atom (O). The reaction occurs in the lower troposphere and can lead to the formation of ozone. The chemical equation for this photodissociation is \[ ext{NO}_2 + ext{sunlight} ightarrow ext{NO} + ext{O}. \] The free oxygen atom can then react with molecular oxygen, \( ext{O}_2 \), to form \( ext{O}_3 \) (ozone).

Understanding the Source of Ozone Formation

The free oxygen atom generated from \( ext{NO}_2 \) photodissociation reacts with \( ext{O}_2 \) to form ozone. This is important in the tropospheric ozone formation. The chemical equation is \[ ext{O} + ext{O}_2 ightarrow ext{O}_3. \] This reaction is significant because it contributes to the formation of ground-level ozone, which impacts air quality and living organisms.

Balancing Ozone Destruction and Formation

NO plays a role in both the destruction and formation of ozone. While \( ext{NO} \) directly participates in reactions that can initially decrease ozone levels by converting \( ext{O}_3 \) to \( ext{O}_2 \), an indirect pathway via \( ext{NO}_2 \) results in the photochemical production of ozone. This complex interplay primarily occurs near the Earth's surface in the urban air environment.

Key concepts

Ozone Formation

Ozone formation in the troposphere is primarily driven by chemical reactions involving nitrogen oxides and sunlight. This process happens close to Earth’s surface, particularly in areas with heavy traffic and industrial activities. The key reaction starts with nitrogen dioxide

• When nitrogen dioxide (\( \text{NO}_2 \)) is exposed to sunlight, it breaks down into nitrogen monoxide (\( \text{NO} \)) and a free oxygen atom (\( \text{O} \)).
• Then, this free oxygen atom quickly reacts with molecular oxygen (\( \text{O}_2 \)) present in the atmosphere to produce ozone (\( \text{O}_3 \)).

Both reactions contribute to photochemical smog, commonly observed as a brownish haze over urban areas. Understanding these reactions is crucial for managing air quality and addressing public health concerns related to smog.

Ozone Destruction

Ozone destruction in the atmosphere is a natural process, often occurring alongside ozone formation. While ozone is essential for blocking harmful UV radiation, it's detrimental at ground level. One prominent destroyer of ozone in the troposphere is (\( \text{NO} \)), which can turn ozone (\( \text{O}_3 \)) back into (\( \text{O}_2 \)). This occurs as:

• Nitric oxide (\( \text{NO} \)) reacts with ozone (\( \text{O}_3 \)), converting it into nitrogen dioxide (\( \text{NO}_2 \)) and oxygen gas (\( \text{O}_2 \)).

Such reactions balance the amount of ozone, preventing excessive accumulation of ground-level ozone, which can be harmful to health and the environment. However, urban pollution tends to tip this balance, causing ozone levels to fluctuate and potentially impact air quality.

Photodissociation

Photodissociation, or photolysis, is a fundamental process in atmospheric chemistry, driven by sunlight's energy. It involves the splitting of chemical compounds by photons, often initiating further reactions. In the context of ozone formation:

• When sunlight strikes a molecule like nitrogen dioxide (\( \text{NO}_2 \)), it absorbs energy, causing it to split into nitric oxide (\( \text{NO} \)) and a free oxygen atom (\( \text{O} \)).

The energy provided by sunlight is crucial for both disrupting chemical bonds and facilitating subsequent reactions that lead to ozone production. This process is sensitive to sunlight intensity and thus varies with time of day and season, influencing patterns of ozone and other pollutants.

Tropospheric Pollution

Tropospheric pollution is a significant environmental issue, mostly affecting the air quality in urban areas. The troposphere is the lowest atmospheric layer, where humans live and breathe. Pollution here is largely due to:

• Emissions from vehicles and industrial activities that release nitrogen oxides (\( \text{NO}_x \)) and volatile organic compounds (VOCs).
• These emissions react in sunlight, leading to the production of ozone and other secondary pollutants.

Ozone in the troposphere, unlike in the stratosphere, doesn’t protect us from UV rays and can harm respiratory health, degrade ecosystems, and damage crops. Thus, controlling pollutants that lead to ozone formation is essential for mitigating air quality issues and protecting public health.