ChemistryChapter 7: Nitrogen, Sulphur and Metals
7.1 Photochemical Smog
Photochemical Smog:
Explanation level
Introduction
Photochemical smog occurs when sunlight reacts with specific chemicals in the air. It is a major type of air pollution found in modern cities and industrial areas.
One of the most important steps in photochemical smog formation is the reaction of nitrogen oxides (NO and NO2) in the atmosphere with unburned hydrocarbons in the presence of sunlight. This reaction leads to the formation of various secondary pollutants, including peroxyacetyl nitrate (PAN).
This section explains what photochemical smog is and describes the chemical reactions involved in its formation.
Definition
Photochemical Smog: Photochemical smog occurs when sunlight reacts with specific chemicals in the air, particularly nitrogen oxides (NO and NO2) and unburned hydrocarbons, leading to the formation of secondary pollutants including peroxyacetyl nitrate (PAN).
In simple words: Photochemical smog is a type of air pollution caused by the reaction of sunlight with nitrogen oxides and hydrocarbons in the atmosphere.
Main Idea
- Photochemical smog forms when sunlight reacts with nitrogen oxides and unburned hydrocarbons.
- One of the most important steps in photochemical smog formation is the reaction of nitrogen oxides (NO and NO2) with unburned hydrocarbons in the presence of sunlight.
- This reaction leads to the formation of various secondary pollutants, including peroxyacetyl nitrate (PAN).
- PAN is a major component of photochemical smog.
- PAN is a stable molecule at lower temperatures and can travel long distances in the atmosphere.
- It acts as a reservoir for NOx and organic radicals, releasing them under conditions of higher temperatures and sunlight, thereby contributing to the formation and persistence of photochemical smog.
Core Concept
| Component | Role |
|---|---|
| Nitrogen oxides (NO and NO2) | Primary pollutants from vehicles and industry |
| Unburned hydrocarbons (VOCs) | Primary pollutants from vehicles and industry |
| Sunlight (UV radiation) | Provides energy for reactions |
| Ozone (O3) | Secondary pollutant |
| Peroxyacetyl nitrate (PAN) | Major component of photochemical smog |
Key point: Photochemical smog is formed by the reaction of nitrogen oxides and hydrocarbons in the presence of sunlight, producing secondary pollutants like PAN.
Detailed Explanation
The Chemical Reactions Involved in the Formation of PAN
The source describes six key reactions involved in the formation of peroxyacetyl nitrate (PAN):
1. Formation of Nitrogen Dioxide (NO2)
What happens: Nitric oxide (NO) is emitted from vehicles, industrial processes, and other combustion sources. NO reacts with oxygen (O2) in the atmosphere to form nitrogen dioxide (NO2).
Equation: 2NO(g) + O2(g) UV Radiations→ 2NO2(g)
Key point: NO from combustion sources is oxidized to NO2 in the atmosphere.
2. Photolysis of Nitrogen Dioxide (NO2)
What happens: NO2 absorbs sunlight (particularly UV light) and undergoes photodissociation to form nitric oxide (NO) and a free oxygen atom (O).
Equation: NO2(g) UV Radiations→ NO(g) + [O](g)
Key point: NO2 breaks down in sunlight to form NO and a highly reactive oxygen atom.
3. Formation of Ozone (O3)
What happens: Nascent [O] or atomic oxygen is highly reactive and reacts with atmospheric O2 to form O3.
Equation: O2(g) + [O](g) UV Radiations→ O3(g)
Key point: Atomic oxygen reacts with O2 to form ozone.
4. Reaction of Hydrocarbons
What happens: Unburned hydrocarbons (often referred to as volatile organic compounds or VOCs) in the atmosphere can react with nascent oxygen in the presence of UV light to form a variety of intermediate organic radicals.
Equation: Hydrocarbons(g) + [O](g) → RCO•(g)
Key point: Hydrocarbons react with atomic oxygen to form organic radicals.
5. Formation of Peroxyacyl Radicals
What happens: These organic radicals react with molecular oxygen (O2) to form peroxy acyl radicals (RC(O)O2•).
Equation: RCO•(g) + O2(g) → RC(O)O2•(g)
Key point: Organic radicals react with O2 to form peroxy acyl radicals.
6. Formation of PAN
What happens: The peroxy acyl radicals (RC(O)O2•) react with NO2 to form peroxyacetyl nitrate (PAN).
Equation: RC(O)O2•(g) + NO2(g) → RC(O)O2NO2(g)
Key point: Peroxy acyl radicals react with NO2 to form PAN.
Properties and Behaviour of PAN
- PAN is a stable molecule at lower temperatures.
- It can travel long distances in the atmosphere.
- It acts as a reservoir for NOx and organic radicals.
- It releases them under conditions of higher temperatures and sunlight.
- This contributes to the formation and persistence of photochemical smog.
Key point: PAN is a stable molecule that can travel long distances and later release NOx and organic radicals, contributing to photochemical smog.
Summary Table
| Step | Reaction | What Happens |
|---|---|---|
| 1 | 2NO + O2 → 2NO2 | NO is oxidized to NO2 |
| 2 | NO2 UV→ NO + [O] | NO2 breaks down to NO and atomic oxygen |
| 3 | O2 + [O] → O3 | Ozone is formed |
| 4 | Hydrocarbons + [O] → RCO• | Organic radicals are formed |
| 5 | RCO• + O2 → RC(O)O₂• | Peroxy acyl radicals are formed |
| 6 | RC(O)O₂• + NO2 → RC(O)O₂NO₂ | PAN is formed |
Intermediate Level
Steps in the formation of PAN:
1. Formation of NO2: 2NO + O2 UV→ 2NO2
2. Photolysis of NO2: NO2 UV→ NO + [O]
3. Formation of Ozone: O2 + [O] UV→ O3
4. Reaction of Hydrocarbons: Hydrocarbons + [O] → RCO•
5. Formation of Peroxyacyl Radicals: RCO• + O2 → RC(O)O2•
6. Formation of PAN: RC(O)O2• + NO2 → RC(O)O2NO2
Key point: PAN is formed by the reaction of peroxy acyl radicals with NO2. PAN is a stable molecule that can travel long distances and later release NOx and organic radicals.
Important Points
- - Photochemical smog occurs when sunlight reacts with specific chemicals in the air.
- - One of the most important steps in photochemical smog formation is the reaction of nitrogen oxides (NO and NO2) with unburned hydrocarbons in the presence of sunlight.
- - This reaction leads to the formation of various secondary pollutants, including peroxyacetyl nitrate (PAN).
- - PAN is a major component of photochemical smog.
- - PAN is a stable molecule at lower temperatures and can travel long distances in the atmosphere.
- - PAN acts as a reservoir for NOx and organic radicals, releasing them under conditions of higher temperatures and sunlight.
- - This contributes to the formation and persistence of photochemical smog.
Remember
- - Photochemical smog = sunlight + NOx + hydrocarbons.
- - PAN = peroxyacetyl nitrate.
- - PAN is a major component of photochemical smog.
- - PAN is stable at lower temperatures.
- - PAN can travel long distances.
- - PAN releases NOx and organic radicals at higher temperatures.
- - PAN contributes to the formation and persistence of photochemical smog.
- - The six reactions:
- 1. 2NO + O2 → 2NO2
- 2. NO2 UV→ NO + [O]
- 3. O2 + [O] → O3
- 4. Hydrocarbons + [O] → RCO•
- 5. RCO• + O2 → RC(O)O₂•
- 6. RC(O)O₂• + NO2 → PAN
Common Mistakes
| Mistake | Correction |
|---|---|
| Thinking photochemical smog is the same as industrial smog | Photochemical smog is formed by sunlight reacting with NOx and hydrocarbons; industrial smog is mainly from burning coal |
| Thinking PAN is a primary pollutant | PAN is a secondary pollutant formed in the atmosphere |
| Thinking PAN is unstable at all temperatures | PAN is stable at lower temperatures and can travel long distances |
| Forgetting the role of sunlight | Sunlight (UV radiation) provides the energy for the reactions |
| Thinking ozone is always beneficial | Ozone is beneficial in the upper atmosphere but harmful at ground level |
| Confusing NO and NO2 | NO is nitric oxide; NO2 is nitrogen dioxide |
| Forgetting that hydrocarbons are involved | Unburned hydrocarbons (VOCs) are essential for PAN formation |
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Review the key facts, formulas and checks before you move on.7.1 Photochemical Smog
Sticky Notes · Quick Revision
Key Point 1
- Photochemical smog = sunlight + NOx + hydrocarbons.
Key Point 2
- **PAN** = peroxyacetyl nitrate.
Key Point 3
- PAN is a major component of photochemical smog.
Key Point 4
- PAN is stable at lower temperatures.
Key Point 5
- PAN can travel long distances.
Key Point 6
- PAN releases NOx and organic radicals at higher temperatures.
Key Point 7
- PAN contributes to the formation and persistence of photochemical smog.
Key Point 8
- Six reactions:
Key Point 9
- 1. 2NO + O2 → 2NO2
Key Point 10
- 2. NO2 UV→ NO + [O]
Key Point 11
- 3. O2 + [O] → O3
Key Point 12
- 4. Hydrocarbons + [O] → RCO•
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