Photochemical Smog

Photochemical Smog: Formation, Causes, Effects, and Control

A Modern Urban Air Pollution Phenomenon – Also Known as Summer Smog or Los Angeles Smog

Introduction

Photochemical smog is a type of air pollution characterized by a brownish haze that forms in the troposphere (lower atmosphere) when sunlight interacts with certain air pollutants like nitrogen oxides (NOₓ) and volatile organic compounds (VOCs). Unlike classical "London smog" (sulfurous/industrial smog), photochemical smog is a secondary pollutant mixture driven by photochemical reactions.

It is most common in warm, sunny, densely populated urban areas with heavy traffic and poor air circulation — famously associated with cities like Los Angeles, Mexico City, Athens, Beijing, and many others.

            Key feature: It typically forms in summer afternoons under clear skies and temperature inversions that trap pollutants near the ground.
        

Formation and Detailed Chemical Mechanism

Photochemical smog forms through sunlight-initiated chain reactions. Primary pollutants (NOₓ and VOCs) are transformed into secondary pollutants (O₃, PAN, aldehydes, aerosols).

Primary Pollutants

NOₓ (NO + NO₂): from high-temperature combustion
VOCs (hydrocarbons, RH): from vehicles, solvents, fuels

Key Reaction Stages

1. Emission & Initial Conversion

\[\mathrm{N_2 + O_2 \xrightarrow{\text{high temp}} 2NO}\]
Some NO oxidizes to NO₂: \[\mathrm{2NO + O_2 \rightarrow 2NO_2}\]

2. Initiation (Photolysis – requires UV sunlight)

\[\mathrm{NO_2 + h\nu \rightarrow NO + O(^3P)}\]

3. Ozone Formation

\[\mathrm{O + O_2 + M \rightarrow O_3 + M}\]

4. Null Cycle (without VOCs – no net O₃ buildup)

\[\mathrm{O_3 + NO \rightarrow NO_2 + O_2}\]

5. VOC-Driven Propagation (Net Ozone Buildup)

VOCs + OH• initiate chains producing peroxy radicals that convert NO → NO₂ without consuming O₃.

\[\mathrm{RH + OH^\bullet \rightarrow R^\bullet + H_2O}\] \[\mathrm{R^\bullet + O_2 \rightarrow RO_2^\bullet}\] \[\mathrm{RO_2^\bullet + NO \rightarrow RO^\bullet + NO_2}\] \[\mathrm{RO^\bullet + O_2 \rightarrow R'CHO + HO_2^\bullet}\] \[\mathrm{HO_2^\bullet + NO \rightarrow OH^\bullet + NO_2}\]

6. Important Secondary Products

PAN formation (eye irritant): \[\mathrm{CH_3C(O)OO^\bullet + NO_2 \rightleftharpoons CH_3C(O)OONO_2\ (PAN)}\]
Termination (HNO₃): \[\mathrm{OH^\bullet + NO_2 + M \rightarrow HNO_3 + M}\]

Reactions peak in warm, sunny afternoons with inversions trapping pollutants.

Causes and Contributing Factors

Factor	Description	Examples
Emissions	High traffic & combustion sources	Vehicles, power plants, refineries
Sunlight	Strong UV radiation needed	Summer, clear skies, latitudes < 45°
Meteorology	Stagnant air, temperature inversion	Valley cities (LA, Mexico City)
Topography	Mountains trap air pollutants	Basin geography

Effects of Photochemical Smog

Human Health

Irritation of eyes, nose, throat
Respiratory problems (asthma aggravation, reduced lung function)
Chronic exposure → increased risk of COPD, cardiovascular issues
PANs and aldehydes are strong eye/respiratory irritants

Environmental & Ecological Impacts

Damage to plants (reduced photosynthesis, leaf necrosis)
Crop yield losses (e.g., soybeans, wheat, cotton)
Material degradation (rubber cracking, paint fading, fabric damage)
Contribution to regional haze and climate effects via aerosols

Ecosystems

Ground-level ozone harms forests, reduces biodiversity, and disrupts ecosystems.

Control and Prevention Strategies

Reduce NOₓ emissions: Catalytic converters, low-NOₓ burners, emission standards
Reduce VOC emissions: Vapor recovery at gas pumps, reformulated gasoline, solvent controls
Vehicle technology: Electric/hybrid vehicles, stricter tailpipe standards
Urban planning: Better public transport, congestion pricing, green belts
Monitoring & alerts: Air quality indices, smog alerts, temporary restrictions
International cooperation: Many cities have dramatically reduced photochemical smog through regulations (e.g., Los Angeles since the 1970s)

Classical Smog vs. Photochemical Smog

While both affect visibility and health, their origins and chemistry are polar opposites.

Feature	Classical (London) Smog	Photochemical (L.A.) Smog
Primary Components	Sulfur dioxide (SO₂) and soot/particulates.	Nitrogen oxides (NOₓ), VOCs, and Ozone (O₃).
Primary Source	Coal and heavy fuel oil combustion.	Vehicular exhaust and industrial emissions.
Weather Conditions	Cool, humid, and foggy (Winter).	Warm, dry, and sunny (Summer).
Chemical Nature	Reducing (High SO₂ concentration).	Oxidizing (High O₃ concentration).
Appearance	Grey/Black thick fog.	Brownish/Yellowish haze.
Main Health Issue	Bronchitis and severe lung irritation.	Eye irritation and asthma aggravation.

Measuring the Threat: The AQI

Environmental agencies monitor photochemical smog primarily by measuring Ground-Level Ozone (O₃) and Nitrogen Dioxide (NO₂). These levels are then converted into the Air Quality Index (AQI).

0-50 (Good): Air quality is satisfactory.
101-150 (Unhealthy for Sensitive Groups): People with asthma may start feeling the effects of ozone.
200+ (Very Unhealthy): Triggers a health alert; everyone may experience effects.

Summary

Photochemical smog represents a classic example of how human activity, combined with natural conditions, can transform relatively simple pollutants into a harmful atmospheric cocktail. While significant progress has been made in controlling it in some regions through technology and policy, it remains a major challenge in rapidly urbanizing and motorizing parts of the world. Addressing photochemical smog requires continued reductions in NOₓ and VOC emissions, smarter urban design, and global cooperation on air quality.

Understanding and controlling photochemical smog is essential for healthier cities and a cleaner troposphere.

Photochemical Smog