AP: Lesson - Introduction to Air Pollution (Topic 7.1) 📖

⏳ Estimated Reading Time: 11 - 13 minutes

Learning Objective

Identify the sources and effects of air pollution.

 

Introduction to Air Pollution

Air pollutants can be gas or particle contaminants that find their way into the atmosphere. Sources of air pollution can be stationary such as pollution from a power plant or mobile like pollution from vehicles. Combustion, grinding, construction, and evaporation are a few of the release mechanisms for air pollutants. Once in the air, pollutants can cause health problems, damage human-made materials, and degrade ecosystems.

The number of pollutants entering the air and the amount of space into which the pollutants are dispersed determine air pollutant levels. So, smaller areas with large amounts of pollutants entering the atmosphere will be more polluted than larger areas with smaller amounts of air pollutants entering the atmosphere.  Usually this means that urban areas suffer more from air pollution than rural areas.  If there are mechanisms in place that remove pollutants from the atmosphere, then that can lower the air pollution levels in those locations.

 

Fossil Fuel Combustion

Fossil fuel combustion is a process that involves burning fossil fuels such as coal, oil, and natural gas to produce energy. This process is widely used to generate electricity, power vehicles, and heat buildings. However, the combustion of fossil fuels comes with health and environmental consequences due to the release of various pollutants into the atmosphere. The graph below shows the number of deaths from air pollution in 2015. While the United States didn't have the most deaths, we still had over 100,000 deaths:

But why is burning fossil fuels a health and environmental problem? As we learned in the last module, when fossil fuels are burned in the presence of oxygen, they react with the oxygen molecules and produce carbon dioxide and water as the main products, along with other pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter (also known as suspended particulate matter or SPM), which are tiny particles left over from the burning process. The reaction also releases a large amount of heat. The general equation for the combustion of a hydrocarbon fuel is:

The nitrogen oxides contribute to the formation of ozone, which is a key component of photochemical smog (we will learn more about this in a future lesson). Ozone, when present at ground level, can have harmful effects on human health and the environment. Additionally, nitrogen oxides can react with other compounds in the atmosphere to form nitric acid, which can then fall back to the Earth's surface as acid rain.

In addition to the nitrogen oxides, the combustion of fossil fuels also produces other pollutants like carbon monoxide (CO), hydrocarbons, and particulate matter (PM). Carbon monoxide is a poisonous gas that can be harmful to both humans and animals when inhaled in large quantities. Hydrocarbons are organic compounds that can contribute to the formation of smog and are associated with respiratory issues. Particulate matter consists of tiny particles suspended in the air that can be inhaled into the lungs, causing respiratory problems and reducing air quality.

 

Essential Knowledge

The combustion of fossil fuels releases nitrogen oxides into the atmosphere. They lead to the production of ozone, formation of photochemical smog, and convert to nitric acid in the atmosphere, causing acid rain. Other pollutants produced by fossil fuel combustion include carbon monoxide, hydrocarbons, and particulate matter.

 

This image features a power plant with tall stacks, from which dark smoke is billowing against a cloudy sky. Superimposed on the smoke are blue circles containing chemical symbols for various pollutants such as lead (Pb), nickel (Ni), sulfur dioxide (SO2), iron (Fe), arsenic (As), and selenium (Se), indicating the types of pollutants that may be emitted from such facilities.
Burning fossil fuels produces SPM and SO2.

Coal combustion is a type of fossil fuel combustion that is especially harmful to the environment. Burning coal releases carbon dioxide (CO2), sulfur dioxide (SO2), heavy metals such as mercury and lead, and particulates into the atmosphere. For example, a typical 1000-megawatt (MW) coal power plant burns 8,000 tons of coal per day, releasing 20,000 tons of CO2 and 800 tons of SO2, which is as much CO2 as all of the cars and trucks in the United States.

 

Essential Knowledge

Coal combustion releases air pollutants including carbon dioxide, sulfur dioxide, toxic metals, and particulates.

 

Primary vs. Secondary Pollutants

There are 6 primary pollutants, which are pollutants released directly into the atmosphere.

  1. Particulate Matter (PM): released during industrial processes, grinding, construction, and combustion
  2. Volatile Organic Carbons (VOCs): released during the combustion of fossil fuels; primarily by automobiles
  3. Carbon Monoxide (CO): released during the combustion of fossil fuels; primarily by automobiles
  4. Nitrogen Dioxide (NO2): released during the combustion of fossil fuels; primarily by vehicles
  5. Sulfur Dioxide (SO2): released during the combustion of fossil fuels; primarily by power plants
  6. Lead: released during the combustion of lead-containing fuel or paint

Secondary pollutants are formed when primary pollutants chemically react with other elements in the atmosphere. Any of these eventually get deposited back to Earth.

The four major secondary pollutants are:

  1. Ozone (O3): formed in a reaction between VOCs and NOx
  2. Peroxyacetyl nitrates (PANs): formed in a reaction between VOCs and NOx
  3. Sulfuric acid: formed when sulfur compounds are burned and a contributor to acid rain
  4. Nitric acid: formed when nitrogen reacts to high temperatures and is a contributor to acid rain

Ozone is a particularly important secondary pollutant. Pre-industrial concentrations of ozone in the troposphere, the lower layer of the atmosphere, were 10-15 ppb. Now, concentrations of ozone in the atmosphere are 20-50 ppb. Ozone is a photochemical oxidant, which means that it is formed from reactions that need sunlight to proceed (more about this in the next lesson!).

Ozone in the lower atmosphere is considered bad (ozone pollution) and ozone in the stratosphere is considered good (in the ozone layer) hence the saying: “Ozone: good up high, bad nearby.” 

 

Essential Knowledge

Air pollutants can be primary or secondary pollutants.

 

Air Pollution Legislation

Air pollution impacts all life on Earth. Even organisms that don’t breathe air can eventually be impacted by air pollutants that end up in soil and water. To that end, we have recognized that air pollution should be prevented whenever and wherever possible.

In the United States, the Environmental Protection Agency (EPA) sets and enforces air pollution laws, regulations, standards, and rules. The Clean Air Act 💨 (and its subsequent amendments and modifications) was the first federal law to regulate air emissions, reducing and controlling air pollution by setting standards designed to protect the health and welfare of the public. Since the inception of the original Clean Air Act 💨 in the 1960s, air pollution in the United States has significantly declined. Find the United States on the graph below. How much has air pollution declined since 1990 (when the Clean Air Act 💨 was last amended)?

National Ambient Air Quality Standards (NAAQS)

The Clean Air Act 💨 authorized the Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) for each of the six criteria pollutants.

These are the maximum allowable levels of six common and harmful air pollutants, also known as criteria pollutants, that protect public health and welfare. 

The six criteria pollutants are:

Ozone (O3) (click to reveal)
      • What is it?
        • A gas that forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react with sunlight.
        • Secondary Pollutant
      • Health Effects
        • Ozone can cause respiratory problems and reduce lung function.
      • Community Effects
        • Ozone can damage crops and other vegetation.

 

Particulate Matter (PM) (click to reveal)
      • What is it?
        • A mixture of solid and liquid particles that vary in size, composition, and origin.
        • PM can be divided into two categories: fine particles (PM2.5), which are 2.5 micrometers or smaller, and coarse particles (PM10), which are between 2.5 and 10 micrometers.
        • Primary Pollutant
      • Health Effects
        • PM can penetrate deep into the lungs and cause various health effects, such as asthma, bronchitis, and heart attacks.
      • Community Effects 
        • PM can also reduce visibility and damage buildings and monuments.

 

Carbon Monoxide (CO) (click to reveal)
      • What is it?
        • A colorless, odorless gas that is produced by incomplete combustion of fossil fuels, such as gasoline, diesel, and wood. 
        • Primary Pollutant
      • Health Effects
        • CO can reduce the amount of oxygen in the blood and affect the cardiovascular and nervous systems. CO can be especially harmful for people with heart disease, anemia, or respiratory problems.
      • Community Effects 
        • CO is a greenhouse gas.

 

Lead (Pb) (click to reveal)
      • What is it?
        • A metal that is emitted from various sources, such as ore and metal processing, waste incineration, and lead-acid batteries. 
        • Lead used to be added to fuel to reduce engine “knock”, a condition in which internal combustion engines run rough. Lead also added octane to gasoline which helps prevent seal damage in engines.
        • Primary Pollutant
      • Health Effects
        • Lead can accumulate in the body and cause neurological damage, especially in children. Lead can also affect the blood, kidneys, and reproductive system.
        • In the early 1970s, the EPA banned the use of lead in fuel, resulting in a dramatic decrease in airborne lead.
      • Community Effects 
        • Lead persists in the environment and can accumulate in soils and sediments.

 

Sulfur Dioxide (SO2) (click to reveal)
      • What is it?
        • A gas that is emitted from coal and oil combustion, industrial processes, and volcanic eruptions. 
      • Health Effects
        • SO2 can also irritate the respiratory system and aggravate asthma and bronchitis.
      • Community Effects 
        • SO2 can react with other substances in the air to form acid rain, which can harm aquatic life, forests, and crops.

 

Nitrogen Dioxide (NO2) (click to reveal)
      • What is it?
        • A gas that is emitted from vehicles, power plants, and other combustion sources. 
      • Health Effects
        • NO2 can affect the respiratory system and increase the risk of respiratory infections.
      • Community Effects 
        • NO2 can contribute to the formation of ozone and acid rain.

 

These six pollutants have maximum ambient air levels according to the CAA 💨. The EPA sets the NAAQS based on the latest scientific evidence, and the states are responsible for developing and implementing plans to achieve and maintain the NAAQS within their jurisdictions.

The CAA 💨 also established a primary standard, which is based on presumed highest levels that can be tolerated by humans without ill effects. Each of these pollutants has both an exposure level and a time of exposure that applies to it and it is the job of the EPA to make sure that these six pollutants stay below the level and time standards.

Primary standards especially protect children, the elderly, and people with asthma and other breathing issues. 

The EPA also developed the Air Quality Index (AQI) as an index for reporting air quality. The index ranges from 0 to 500 with 0 being great 😀 and 500 being extremely bad 😢, so the higher the number, the greater human health concern.

Air Quality Index (AQI) Chart: This color-coded chart represents different levels of air quality based on the AQI, serving as a visual guide for understanding air quality conditions and their impact on health.
AQI basics for ozone and particle pollution.

Index levels below 50 mean that air quality is not a concern and any number 300 or over means that air quality can harm human health.

Anyone can check the Air Quality Index in real time by navigating to the EPA’s AirNow website Links to an external site. and entering their zip code. Here's an example of the Air Quality Index for Atlanta in early March 2024:

A digital display shows the current air quality in Atlanta, GA at 12 PM EST Mar 4, with a NowCast AQI of 35 for PM2.5 particles.
AQI for Atlanta, Georgia at noon on March 4th, 2024.

 

Why doesn’t the EPA set standards for ALL potential air pollutants rather than just for six?

Besides the increased cost of monitoring ALL pollutants, the EPA has determined through scientific studies that the six criteria pollutants have the potential to cause the greatest harm to humans and the environment. By controlling these six major air pollutants, many other air pollutants have declined as well. With continued study, monitoring, and enforcement, it is hoped that the downward trend in air pollution will continue.

 

 

Essential Knowledge

Air quality can be affected through the release of sulfur dioxide during the burning of fossil fuels, mainly diesel fuels.

Through the Clean Air Act 💨, the Environmental Protection Agency (EPA) regulated the use of lead, particularly in fuels, which dramatically decreased the amount of lead in the atmosphere.

 

AP Exam Tip

It is important to know the general idea behind the CAA 💨.

You should know the primary and secondary air pollutants and why they have this designation.

 

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