Photochemical Smog and Its Components

Photochemical smog is a complex mixture of pollutants that forms when nitrogen oxides and hydrocarbons react in the presence of sunlight. This section delves into its components and formation process.

Definition: Photochemical smog is a type of air pollution that occurs when sunlight reacts with nitrogen oxides and volatile organic compounds in the atmosphere.

The two primary components of photochemical smog are:

1. Nitrogen oxides $NOx$: These are formed when nitrogen and oxygen in the air react during high-temperature processes, such as combustion in car engines. They are also released from fertilizers, as nitrogen is a key component.
2. Hydrocarbons: These are released when fuel is only partially burned in an improperly functioning engine. They are also emitted during the extraction of hydrocarbons, often involving the burning of natural gas rather than allowing it to escape.

Highlight: The source of nitrogen oxides is primarily from human activities, particularly the burning of fossil fuels in vehicles and industrial processes.

The effects of photochemical smog are wide-ranging and severe:

• NOx can increase the risk of respiratory infections such as colds, flu, and bronchitis, and exacerbate existing health problems like asthma.
• Hydrocarbons can cause eye irritation, breathing difficulties, asthma, emphysema, and an increased risk of heart attacks.

Example: Los Angeles is notorious for its photochemical smog, which is exacerbated by the city's geography and high vehicle usage.

The photochemical smog formula is complex, involving multiple chemical reactions. However, a simplified representation can be:

NO2 + sunlight → NO + O O + O2 → O3 $ozone$ Hydrocarbons + NOx + sunlight → Peroxyacetyl nitrates $PANs$ + other pollutants

Understanding how photochemical smog is formed is crucial for developing effective control strategies.

Control Methods for Air Pollution

This section explores various techniques to mitigate air pollution, focusing on methods to reduce emissions of sulphur dioxide, nitrogen oxides, and hydrocarbons.

Flue gas desulphurization is a key method for reducing sulphur dioxide emissions. There are two main approaches:

1. Wet flue gas desulphurization: Flue gases are passed through a bed of crushed calcium carbonate. Sulphur dioxide reacts with the calcium carbonate, producing solid calcium sulphate.
2. Dry flue gas desulphurization: Hydrated lime is used to absorb the SO2, producing calcium sulphide which is then disposed of in landfills or used to make cement.

Vocabulary: Flue gas - The gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator.

For reducing nitrogen oxide emissions, flue gas denitrification can be used in power stations. This process involves injecting ammonia or urea, which converts NOx into harmless nitrogen gas.

Highlight: Reducing peak temperatures in any form of combustion will decrease NOx formation, offering another strategy for emission control.

To control hydrocarbon emissions, several methods are employed:

1. Catalytic converters: These devices oxidize hydrocarbons to CO2 and H2O.
2. Vapor collection and incineration: Gases are collected and condensed at filling stations, then returned to the main fuel supply.
3. Activated carbon filters: Vapors can be passed through an activated carbon filter where hydrocarbons are absorbed onto carbon particles.

Example: Modern vehicles are equipped with catalytic converters as a standard feature to reduce harmful emissions.

Improving combustion efficiency is another crucial strategy. This involves operating boilers with an optimum amount of excess air to ensure complete combustion of fuels.

Carbon Monoxide and Smog

This section discusses carbon monoxide pollution and different types of smog, including their formation and effects.

Carbon monoxide $CO$ is a dangerous pollutant produced when carbon-based materials are burned with insufficient oxygen, such as in the incomplete combustion of hydrocarbons in vehicle engines.

Definition: Carbon monoxide is a colorless, odorless gas that can be lethal in high concentrations due to its ability to interfere with oxygen transport in the body.

The effects of carbon monoxide on humans are severe:

• It binds to hemoglobin, preventing it from carrying oxygen from the lungs to body tissues.
• In high concentrations, it can cause brain damage and potentially death.

Control methods for carbon monoxide emissions include:

1. Exhaust catalytic converters: These devices oxidize CO to CO2.
2. Improved combustion efficiency: Ensuring complete combustion of fuels reduces CO production.

Highlight: While CO2 is also a pollutant, the increase in CO2 emissions from oxidizing CO is very small compared to other sources of CO2 emissions.

Smog is a type of air pollution that can take different forms:

1. Traditional smog: Forms on cloud-free nights when the ground cools rapidly, causing moist air above to condense into water droplets.
2. Photochemical smog: Results from the reaction of nitrogen oxides and hydrocarbons in the presence of sunlight, forming peroxyacetyl nitrates $PANs$ and other pollutants.

Example: The infamous London "pea soup" fogs of the past were a form of traditional smog, while modern-day Los Angeles often experiences photochemical smog.

Factors contributing to photochemical smog include:

• Time: Rush hour traffic is a major source of NOx and hydrocarbons.
• Topography: Valleys are more susceptible due to decreased air flow.
• Meteorological conditions: Temperature inversions increase the likelihood of photochemical smog developing.

Sewage Pollution and Additional Control Methods

This final section covers sewage pollution and additional control methods for various forms of pollution.

Sewage pollution is a significant environmental issue, consisting of:

1. Dissolved organic material $e.g., bacteria, urine, soap$
2. Particulate organic waste $e.g., food debris$
3. Inorganic materials $e.g., plastics$

Highlight: Proper sewage treatment is crucial for protecting water resources and public health.

Additional control methods for various pollutants include:

1. For nitrogen pollution from agriculture: More precise use of fertilizers, targeting nitrogen application to where it's needed. Urea sprays as alternatives to supplement fertilizers when plants can't take up enough nitrogen.
2. For sulphur pollution: Natural gas desulphurization: Hydrogen sulphide is removed from natural gas after extraction to prevent corrosion damage. Crude oil desulphurization: Sulphur compounds are removed from oil during distillation using molybdenum catalysts. Coal desulphurization: Sulphur can be removed by washing and steaming crushed coal.

Example: The use of precision agriculture techniques, including GPS-guided fertilizer application, can significantly reduce nitrogen pollution from farmland.

These control methods, when implemented effectively, can significantly reduce the environmental impact of various pollutants and contribute to improved air and water quality.

Acid Rain and Its Impacts

Acid rain is a significant environmental issue with far-reaching consequences. This section explores the causes and impacts of acid rain on both living and non-living entities.

Definition: Acid rain is precipitation that has become acidic due to atmospheric pollution, primarily from the combustion of fossil fuels and emission of photochemical smog.

The effects of acid rain on buildings and monuments are particularly severe. Limestone structures are especially vulnerable, as the acidic rain dissolves their surface layer. Metal structures such as railways, water pipes, and overhead power lines are also susceptible to damage from acid rain.

Example: The deterioration of historic buildings and monuments in cities with high pollution levels is a visible consequence of acid rain.

The acid rain effects on the environment extend to living organisms as well. Lichens, which are composite organisms formed from algae and fungi, are particularly sensitive to acidic conditions. Their size, health, and abundance can serve as indicators of sulphur dioxide levels in the environment.

Highlight: The impact of acid rain on soil pH and nutrient availability can lead to toxic levels of copper and zinc, further affecting plant and animal life.

The acid rain effects on humans are primarily indirect, through damage to the environment and food sources. However, direct effects can include:

1. Protein damage: Low pH can denature proteins in cell membranes and inhibit enzyme action.
2. Damage to exoskeletons: Invertebrates with calcium-based exoskeletons may die as acids dissolve these protective structures.
3. Respiratory issues: When combined with sulphur dioxide, acid rain can exacerbate breathing difficulties and increase the frequency of respiratory problems.

Vocabulary: Denaturation - The process by which proteins lose their structure, potentially leading to loss of function.