Air Pollution

Objectives

Give the major air pollutants Discuss the sources and effects of air pollution Study major large scale secondary pollution problems: Photochemical smog Acid deposition Ozone depletion Note: Global warming is in its own section

Preknowledge

To understand the structure of the atmosphere see the section Atmosphere

The Atmosphere

The description of the Earth's Atmosphere is now in its own section: Atmosphere

Air pollution

Definitons

Air pollution  

Presence of chemical(s) in the atmosphere in sufficient quantity to cause harm to humans, other organisms, or materials

Air pollutants 

Chemicals which cause air pollution

Types

Primary air pollutants 

Pollutants emitted directly

Secondary air pollutants 

pollutants created from reactions between primary air pollutant and other chemicals in the air

Standards

Standards are the maximum quantity of an air pollutant that is legally allowed. Specific standards vary from country to country. There are two ways to define standards.

Air Quality Standards 

maximum allowed concentration of a pollutant in the air. Note this is total concentration present in an area, regardless of its source(s). This is usually used for high volume air pollutants, such as sulfur dioxide or particulates.

Emission Standards 

maximum amount (mass) of a pollutant allowed to be released into the air from a given source. This is used for lower volume air pollutants (but may be more toxic) with only a few sources; for example, benzene.

Major air pollutants

Primary pollutants

• Sulfur dioxide
• Nitrogen dioxide
• Particulate matter and aerosols
• Carbon monoxide
• Lead
• Mercury
• Hydrocarbons
• Chlorinated solvents
• Bezene
• Polynuclear aromatic hydrocarbons
• Asbestos
• Tar/Tobacco smoke
• Formaldehyde
• Radon
• Dioxins
• Pesticides

Secondary pollutants

• Acid deposition
• Ozone
• Photochemical smog

Other

• Ozone depleting chemicals

Sulfur dioxide and acid deposition

Sulfur dioxide - sources

• burning of sulfur-containing compounds including coal and oil

S + O₂ → SO₂

• smelting of metal ores

Cu₂S (ore) + O₂ → 2Cu + SO₂

• Pulp and paper production

• Sulfuric acid manufacturing

Doe Run's lead smelter in Herculaneum, Missouri, USA

Direct effect of sulfur dioxide

• Absorbed by moist respiratory tissue causing restriction of airway

- especially severe in people with asthma

Acid deposition

Sulfur Dioxide can combine with water in the atmosphere to form sulfuric acid:

2SO₂ + O₂ + 2H₂O → 2H₂SO₄

Sulfuric acid can removed from the air by rain ("acid rain") or by attaching to dust particles ("dry deposition")

Effects of acid deposition

• Destruction of concrete and stone structures

The sulfuric acid converts calcium carbonate to calcium sulfate which is more soluble: CaCO₃ + H₂SO₄ → CaSO₄ + 2H⁺ + CO₃²⁺

Acid rain events on a stone monument

• Acidification of forests, rivers, and lakes

Some lakes have natural buffering, but many do not.

Acidified water causes death of fish and other organisms, due to both the lower pH directly and the leaching of metal, especially aluminum. It also causes damage to trees and other plants.

Acid rain damage on woodlands in Czech Republic

Nitrogen dioxide, ozone, and smog

Nitrogen oxides

Nitrogen can react with oxygen to form nitrogen oxides

• N₂O (nitrous oxide) - not toxic but important for global warming

• NO (nitric oxide) - important precursor of ozone and smog

• NO₂ (nitrogen dioxide) - important primary pollutant

• N₂O₃, N₂O₄, N₂O₅ - these are very rare in the atmosphere

NO and NO₂ are together known as NO_x (read as nox)

Nitrogen dioxide

At low temperature combustion nitrogen does not combine with oxygen:

CH₄ + O₂ + N₂ → CO₂ + H₂O + N₂

However; at high temperatures, such as in a burner or internal combustion engine, Nitric oxide (NO) and Nitrogen dioxide (NO₂) are formed :

CH₄ + O₂ + N₂ → CO₂ + H₂O + NO

NO + O₂ → NO₂

About half the second reaction occurs in the burner/engine, the rest occurs in the air

Nitrogen dioxide - health effects

• lung irritation

• increases susceptibility to lung disease

The flask contains nitrogen dioxide. It is this that gives the characteristic brownish color to smog

Ozone

Nitrogen dioxide reacts with sunlight to form nitric oxide and atomic oxygen. This oxygen reacts quickly with molecular oxygen (O₂) to form ozone. Ozone can then react with nitric oxide to regenerate nitrogen dioxide.

NO₂ + hν → NO + O

O + O₂ → O₃

O₃ + NO → NO₂ + O₂

Note The ozone formed here is tropospheric ozone ("bad ozone"), not to be confused with stratospheric ozone ("good ozone")

Ozone - effects

Health Effects:

• lung problems

• eye irritation

Environmental Effects:

• damage to plants and trees

Ozone damage to plants. The top row shows normal plants, the bottom row show plants exposed to ozone.

Photochemical smog

Ozone is extremely reactive and reacts with any organic chemicals in the air. The result of this reaction between tropospheric ozone and organic chemicals is a "toxic soup" called photochemical smog

Photochemical smog includes NO_x, ozone, peroxyacyl nitrates (PANs), aldehydes, and other organic chemicals.

Effects

• Lung disease
• Visibility

Smog in Kuala Lumpur, Malaysia

Thermal inversions

Under certain conditions a layer of warm air can occur above cold air. This causes any pollutants to be trapped and not dispersed.

Smoke rising in Lochcarron, Scotland is stopped by an overlying layer of warmer air

Winter smoke in Shanghai

A good animation of thermal inversion can be found here.

Particulate matter (PM) and aerosols

Definition

Particulate Matter 

Small solid and liquid particles which remain suspended in the atmosphere.

Particulate matter can be divided into two size ranges: 10 - 2.5 μm (PM₁₀) and < 2.5 μm (PM_2.5).

burning field producing particulate matter (smoke)

Sources

• dust from construction, agriculture, roads, etc.
• burning of solid and liquids -- especially from coal, wood, and diesel fuel
• fires, especially forest fires and agricultural fields

Effects

• The small particles can get trapped in the lining of the lung causing irritation, inflammation, or cancer

• PM_2.5 is much more dangerous as it gets deeper into the lung.

view of the human lung showing where different size particulate matter affects different areas

Examples:

• Inflammation

• Silicosis (silica dust)

• Black Lung disease (coal dust)

• Cancer

• Asbestos

• Polynuclear aromatic hydrocarbons (PAH)

• Tar from incomplete combustion and tobacco smoke

• Soot (or black carbon) - very fine particles of pure carbon formed by incomplete combustion of carbon-based fuels

• Health effect - contains many carcinogens

• Global warming effect - unlike most PM, which reflects light, black carbon absorbs light - heating it up and heating the atmosphere

Asbestos micrograph showing the fibers

Toxic air pollutants

Carbon monoxide (CO)

Car exhaust contains carbon monoxide (note CO is colorless, the white color is steam)

Source:

complete combustion (burning) of hydrocarbons gives carbon dioxide:

CH₄ + 2O₂ → CO₂ + 2H₂O

incomplete combustion of hydrocarbons gives carbon monoxide:

CH₄ + 1.5O₂ → CO + 2H₂O

Effects:

Carbon monoxide binds with hemoglobin in the blood to form carboxyhemoglobin. This means the blood cannot carry oxygen to the cells.

The physical effects include lightheadness, headache, confusion, and dizziness to unconsciousness and death

Heme - the oxygen carrying part of hemoglobin. Both oxygen and carbon monoxide bind to the Fe(II) ion

Lead

Lead

Lead is a soft, dense metal. It has with a fairly low melting and boiling points, which means it can be present in air exhaust streams easily. It is highly poisonous.

Sources:

• lead paints (often children will chew on old, peeling paint)
• lead smelters
• lead-acid batteries, such as those in cars
• leaded gasoline (banned in many countries but still produced)

Effects:

• memory loss
• learning difficulties (especially in children)
• nervous system damage
• damage to bones and kidneys
• accumulative poison (i.e. the concentration in the body continually increases over time)

Mercury

Mercury is a highly toxic, liquid metal. Since it is a liquid at room temperature it becomes a gas easily. The form methylmercury is especially toxic.

Sources:

• burning of coal

Most coal contains trace amounts of mercury. But with the large amounts of coal that are burned, the total amount of mercury released is significant.

• small-scale gold mining

Small-scale ("artisanal") mining uses mercury. Mercury combines with gold to form an amalgam, which is very heavy and therefore settles out. The amalgam is then burned to release the mercury and leave only the gold. This released mercury can then be breathed in by the workers and deposited in nearby waters.

Effects:

• mental effects ("mad-hatter" disease)
• kidney disease
• neurological disease (Minamata disease)
• accumulative poison
• biomagnification - this is the accumulation through the food chain as shown below. For mercury this is especially important in fish.

Action

As a response to problems of mercury poisoning, the Minamata Convention on Mercury was adopted in 2013. It controls production and export of mercury, which will be banned by 2025.

Mercury amalgam from small-scale gold mining. This will be heated to evaporate the mercury

biomagnification of mercury in food chain

Volatile organic compounds (VOC's)

Definition:

• Organic compounds which rapidly evaporate.

Examples:

• benzene - a carcinogen which can cause leukemia
• gasoline
• chlorinated solvents
• ethanol and other alcohols
• acetone (main component of nail polish)
• pesticides - especially of concern are organophosphate and organochlorine pesticides

Effects:

toxicity varies widely depending on substance, for example

• eye, nose, and throat irritation
• dizziness and headaches
• damage to liver, kidney, and nervous system

Indoor air pollution

Indoor air pollution is air pollutants found in your home, office, etc.

Examples:

• formaldehyde (from building materials)
• household cleaners
• pesticides
• tobacco smoke
• radon

Radon

Source:

• Naturally radioactive gas. A decay product of radium (which is a decay product of uranium). Uranium and radium can be found naturally in some rocks especially granite. Radon can then enter houses built on such rocks.

Effects:

• lung cancer (due to alpha radiation)

How radon can enter buildings

Ozone depletion

Mechanism

As stated above, the stratosphere contains a layer of ozone which protects the Earth from dangerous ultraviolet radiation.

Chlorinated and brominated hydrocarbons can travel unreacted up to the stratosphere. The most important of these are the chlorofluorocarbons (CFC's).

CCl₃F + hν → CCl₂F + Cl

This free chlorine then reacts with the ozone.

Cl + O₃ → ClO + O₂

ClO + O → Cl + O₂

---

net O₃ + O → O₂ + O₂

This depletes the amount of ozone present.

Ozone depleting chemcials (ODC)

• chlorofluorocarbons (CFC's)

• hydrochlorofluorocarbons (HCFC's)

• bromochlorofluorocarbons (Halons)

• methyl bromide, carbon tetrachloride, methyl chloroform

Montreal Protocol

Ozone "hole" in 2006. Blue and purple have the least ozone and green, yellow, and red have the most.

In the 1980's, it was found the ozone layer was thinning. In 1987 a treaty called the Montreal Protocol was signed.

It phases out the use of most ozone depleting chemicals.

Deadlines:

• CFCs, carbon tetrachloride, methyl chloroform - banned as of 2010
• Halons - banned as of 2010, except for use as a fire suppressant on submarines and aircraft
• Methyl bromide - banned as of 2015
• HCFCs - to be phased out by 2030 (99.5% reduction by 2020)

The ozone layer has been stabilized and concentrations of ozone in the stratosphere are now increasing.

The Montreal Protocol is often considered the most successful environmental treaty ever.

Activity

Print this form listing different air pollutants. Then research their effects and fill out the blanks on the form. Be sure to include references