---
title: "Photochemical smog and its reactions"
subject: "Environment and Pollution Control"
chapter: 1
key: "semester_7/environment_and_pollution_control:photochemical-smog-reactions"
source: https://www.bctengineer.com/q/semester_7/environment_and_pollution_control/photochemical-smog-reactions
site: BCT Engineering
---

# Photochemical smog and its reactions

Environment and Pollution Control · Chapter 1: Air pollution

- Asked in: year not given, 8 marks
- Video: [Photochemical Smog: How it Happens? Ozone, Nitric Acid, Aldehydes, Peroxyacyl Nitrates (PANs)](https://www.youtube.com/watch?v=i3pfxTx5N2s) — Examrace (UPSC, NET, NCERT, ICSE ...)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/environment_and_pollution_control/photochemical-smog-reactions
- Whole chapter: https://www.bctengineer.com/md/semester_7/environment_and_pollution_control/chapter-1

## Question

What is photochemical smog? Explain the photochemical reactions involved in its formation, with the favourable conditions, the typical daily variation of pollutants and its effects.

## Short answer

Photochemical smog is a brownish oxidising haze from sunlight reacting with NOx and hydrocarbons. NO2 photolysis forms NO and O, producing ozone; VOCs yield peroxy radicals that oxidise NO to NO2 without consuming O3, leading to O3 and PAN build-up. Favourable conditions are heavy traffic, strong sun and inversion. Ozone peaks afternoon, causing eye irritation, plant damage and reduced visibility.

## Answer

Photochemical smog is a brownish haze formed when sunlight acts on nitrogen oxides and volatile organic compounds (VOCs) from vehicle exhaust. It was first observed in Los Angeles, so it is also called Los Angeles smog. It contains ozone, PAN and aldehydes (an oxidising mixture).

#### Favourable conditions
- Heavy traffic (source of $\text{NO}_x$ and hydrocarbons)
- Strong sunlight and warm weather
- Low wind speed and a temperature inversion that traps pollutants

#### Reactions
1. NO is formed in engines: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$, and some oxidises to $\text{NO}_2$:
$$
2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2
$$
2. Photolysis of $\text{NO}_2$ by sunlight ($\lambda < 400$ nm):
$$
\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}
$$
3. Atomic oxygen reacts with oxygen to form ozone:
$$
\text{O} + \text{O}_2 + M \rightarrow \text{O}_3 + M
$$
4. Ozone reacts back with NO:
$$
\text{O}_3 + \text{NO} \rightarrow \text{NO}_2 + \text{O}_2
$$
Steps 2-4 alone give a steady state with no net ozone build-up.
5. Hydrocarbons (RH) react with OH and O radicals to form peroxy radicals ($\text{RO}_2$):
$$
\text{RH} + \text{OH} \rightarrow \text{R} + \text{H}_2\text{O}, \qquad \text{R} + \text{O}_2 \rightarrow \text{RO}_2
$$
6. Peroxy radicals oxidise NO to $\text{NO}_2$ without destroying ozone, so ozone accumulates:
$$
\text{RO}_2 + \text{NO} \rightarrow \text{RO} + \text{NO}_2
$$
7. Organic radicals combine with $\text{NO}_2$ to give **peroxyacyl nitrates (PAN)** and aldehydes (HCHO).

#### Daily variation
```text
conc.
 |  NO        NO2          O3
 |  /\      /    \       /  \
 | /  \____/      \____/    \___
 +--------------------------------> time
   6am     10am      1pm     6pm
```
NO peaks in the morning rush hour, then $\text{NO}_2$ rises and ozone peaks around noon to early afternoon.

#### Effects
- Eye irritation (PAN, aldehydes), cough, chest pain, and reduced lung function.
- Plant damage (leaf spotting) and crop loss from ozone and PAN.
- Cracking of rubber, and damage to paints and fabrics.
- Reduced visibility.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Photochemical smog needs which combination?
   A. SO2, smoke and fog with cold weather
   B. NOx and VOCs with strong sunlight, low wind and inversion
   C. CO2 and water vapour at night
   D. Particulates only, in rain
   Answer: B. Sunlight acting on NOx and hydrocarbons from vehicles, with low wind and a temperature inversion, forms the smog. (Key point: Raw materials and conditions)

2. What is the key sunlight-driven step that starts ozone formation?
   A. NO2 + hv -> NO + O
   B. O3 + NO -> NO2 + O2
   C. N2 + O2 -> 2NO
   D. RO2 + NO -> RO + NO2
   Answer: A. Photolysis of NO2 gives atomic oxygen, which combines with O2 to form ozone. (Key point: Photolysis of NO2)

3. Why does ozone accumulate when hydrocarbons are present?
   A. Peroxy radicals oxidise NO to NO2 without consuming ozone
   B. Hydrocarbons produce O atoms directly
   C. Hydrocarbons absorb ozone
   D. They remove NO2 from air
   Answer: A. Without hydrocarbons O3 + NO reverses the formation (steady state). Peroxy radicals convert NO to NO2 without destroying O3, so ozone builds up. (Key point: Role of hydrocarbons)

4. In the typical daily pattern, which peaks first and which peaks around noon to early afternoon?
   A. Ozone first, NO at noon
   B. NO2 first, NO at noon
   C. NO first (morning rush hour), ozone around noon
   D. All peak together at midnight
   Answer: C. NO peaks in the morning traffic, NO2 then rises, and ozone peaks around noon to early afternoon. (Key point: Daily variation)

## Related in Environment and Pollution Control

- [Particulate pollutants: sources and effects](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aparticulate-pollutants-sources-effects) — Environment and Pollution Control, chapter 1
- [Typical noise levels and effects of noise](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Anoise-levels-places-effects) — Environment and Pollution Control, chapter 6
- [Indoor air pollutants and their sources](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aindoor-air-pollutants-sources) — Environment and Pollution Control, chapter 3
- [Sources and effects of SO2, NOx and CO](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Agaseous-pollutants-so2-nox-co) — Environment and Pollution Control, chapter 1
- [Water pollution: sources and effects](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Awater-pollution-sources-effects) — Environment and Pollution Control, chapter 4
- [Stratospheric ozone depletion mechanism and effects](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aozone-depletion-mechanism-effects) — Environment and Pollution Control, chapter 7

## Related in other subjects

- [Air pollutants: effects and remedies](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aair-pollutants-effects) — Engineering Chemistry, chapter 3
- [Classification of air pollutants; sources, effects](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aair-pollutants-classified) — Engineering Chemistry, chapter 3
- [Water pollution, pollutants and adverse effects](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-effects) — Engineering Chemistry, chapter 3
- [Soil pollutants, effects and remedies](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asoil-pollutants-effects-remedies) — Engineering Chemistry, chapter 3, asked 5×