Reducing Air Pollutants (College Board AP® Environmental Science): Study Guide

Methods of reducing air pollutants

• Methods to reduce air pollutants include regulatory practices, conservation practices, and alternative fuels

• These strategies help in:

  • Reducing the sources of pollution

  • Promoting cleaner technologies

  • Encouraging sustainable living

Conservation

• Reducing energy usage is key to reducing air pollutants

• Methods include:

  • Carpooling and mass transit release fewer pollutants

  • Combine this with more compact cities and now you are traveling even less

  • Make the cities greener with more energy-efficient buildings, green roofs, and improved municipal waste management

Alternative fuels

• Reducing pollution from the combustion of fossil fuels is one reason for finding alternative fuels

• Cleaner alternative energy sources like biofuels, solar, wind, hydro, and hydrogen reduce greenhouse emissions and eliminate particulates from combustion

• Many buildings can be retrofitted for wind and solar and reduce dependence on the power grid

Regulations

• To combat air pollution, numerous laws and procedures have been put into place

The Kyoto Protocol (1997)

• This was signed by UN members and aims to reduce global warming-causing greenhouse gas emissions

• Human CO₂ emissions were the most likely cause of global warming, the committee said

• The protocol listed reducing carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HCFs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆)

The Paris Agreement (2016)

• The goal is to limit global temperature rise to below 2°C of the pre-industrial average

• This requires adapting and mitigating anthropogenic carbon dioxide

The United States' Clean Air Act (CAA)

• This US federal legislation governs fixed and mobile air pollution

• The legislation mandates states to prepare strategies to meet and maintain National Ambient Air Quality Standards (NAAQS) for specified pollutants

Montreal Protocol

• This international convention ends production and consumption of ozone-depleting substances to safeguard the ozone layer

• Many are also greenhouse gases, so limiting their use can help combat climate change and improve air quality

European Union's Air Pollution Thematic Strategy

• This approach is a comprehensive plan to lower air pollution in the EU and its member states

• It consists of steps to cut emissions from a range of sources, including households, transportation, industry, and agriculture

Beijing Action Plan

• This is a strategy created by the Beijing administration to deal with the air pollution issues in the city

• It restricts car use, closes polluting factories, and promotes the use of clean energy

Vapor recovery nozzle

• A vapor recovery nozzle is an air pollution control device on a gasoline pump that prevents fumes such as benzene (which is a known carcinogen) from escaping into the atmosphere when fueling a motor vehicle

• There is a separate tube inside the filling nozzle that captures vapors and returns them to an underground storage tank beneath the gas station

• The nozzle reduces VOCs, which contribute to smog and irritate the respiratory tract

Catalytic converter

• Catalytic converters are devices fitted to gasoline and diesel vehicles along with kerosene heaters to reduce harmful emissions

• They contain a series of transition precious metal catalysts, such as platinum and rhodium

• The metal catalysts are in a honeycomb within the converter to increase the surface area available for reaction

• The system is attached to the exhaust system and converts toxic gases like CO, NOₓ, and hydrocarbons into less toxic compounds like CO₂, O₂, N₂, and water

• In many countries, it is compulsory for vehicles to have catalytic converters

Wet and dry scrubbers

• Companies must reduce smokestack pollution under EPA requirements

• Scrubbers remove building exhaust gases and particles

• Wet scrubbers

  • These pipe exhaust gases through a water-sprayed chamber

  • Particulates and dissolved gases are then filtered from the liquid

  • After cleaning (scrubbing), the exhaust is emitted

  • There is a sludge collection system that traps polluted water for later disposal

• Dry scrubbers

  • Exhaust gases are neutralized or converted by dry reagent scrubbers

  • The converted materials are recovered from the gas stream with the cleaned gases being emitted

  • Calcium oxide is a common dry scrubber additive which reacts with SO₂ to form calcium sulfite

Flue gas desulfurization (FGD)

• FGD is important because SO₂ emissions are a health and environmental hazard

• FGD systems can remove up to 95% of SO₂ from flue gases

• FGD is the main way to reduce sulfur dioxide emissions from coal-fired power stations

  • Emissions are passed into a scrubbing chamber where

    • They are sprayed with a wet slurry of calcium oxide and calcium carbonate 

    • The calcium compounds react with sulfur dioxide to produce calcium sulfate

    • The cleaned gas is then emitted from the chimney

Flue gas desulfurization

The scrubber sprays a lime slurry over the waste gases to remove 90 - 95% of the sulfur dioxide

Electrostatic precipitators

• Coal-fired power stations use electrostatic precipitators and scrubbers to remove dust and smoke from emission gases to control air pollution

• It works by using a high-voltage screen that filters waste gases so particles are ionized

• Gases are passed across positively and negatively charged electrodes, creating ion particles in the gas stream

• Collector plates attract the ionized particles and remove them from the gas stream into a collection hopper for disposal in landfills

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Diagram showing the method of electrostatic precipitation