Air Pollution

Mobile Sources

The Thin Shell Called the Atmosphere

• Earth’s atmosphere is a thin layer of gas held around the surface by gravity.

• 90% of the atmosphere’s mass is within 15 km of the earth’s surface– Earth's radius is about

6400 km – The atmosphere is like a

layer of paint on a basketball

Composition of the Atmosphere

Origin of the Atmosphere• Earth’s primitive atmosphere consisted mainly of

CO2 released by volcanic eruptions• About 3.5 billion years ago, photosynthesizing

algae (stromatolites) began to release oxygen which steadily increased over time.

• This was the first global air pollution crisis because oxygen was poisonous to life forms that existed before photosynthesis.

• Lead to major extinction event 2.2 billion years ago (Great Oxygen Crises).

Origin of the Atmosphere• Volcanic eruptions also release trace amounts NH3

(ammonia). • N2 is formed from the reaction between NH3 and

oxygen in the atmosphere.• The high percentage of nitrogen in today’s

atmosphere is because N2 is not chemically active with other molecules in the atmosphere.

• Conversely, the amount of O2 in the atmosphere is limited because it is very reactive with other molecules in Combustion, Decomposition, and Respiration

What is an Air Pollutant?

• An air pollutant is anything in the atmosphere that is hazardous to life or the environment

• Primary pollutant – one that is emitted directly into the atmosphere by the source

• Secondary pollutant – one that not emitted directly, but is generated through chemical reactions over time from material emitted by the source

Air Pollutants SourcesPrimary Pollutants

Secondary Pollutants

Sources

Natural

Stationary

CO CO2

SO2 NO NO2

VOCsParticles

SO3

HNO3 H2SO4O3 PANs

Mobile

NO2

Air Pollution

• Natural sources of air pollution are sources not caused by people or their activities. – An erupting volcano emits particulate matter

and gases– Forest and prairie fires can emit large

quantities of pollutants– Plants and trees emit hydrocarbons– Dust storms can create large amounts of

particulate matter.

Human Air Pollution

• A mobile source of air pollution refers to a source that is capable of moving under its own power.

• A stationary source of air pollution refers to an emission source that does not move (i.e., power plants, chemical and manufacturing industries).

Air Pollution Standards• The Clean Air Act (1970), which was last

amended in 1990, requires EPA to set National Ambient Air Quality Standards (NAAQS) for wide-spread pollutants from numerous and diverse sources considered harmful to public health and the environment.

• EPA has set NAAQS for six principal pollutants, which are called "criteria" pollutants (see next slide). – Units of measure for the standards are parts per

million (ppm) by volume, milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (µg/m3).

Table 8-5, p. 268

Air Pollution Standards

• The Clean Air Act established two types of national air quality standards.

• Primary Standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly.

• Secondary Standards set limits to protect public welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings.

The Good News

• National air quality levels measured at thousands of monitoring stations across the country have shown improvements over the past 20 years for all six principal pollutants.

• Since 1970, aggregate emissions of the six principal pollutants have been cut 48 percent.

The Good News

The Not So Good News• Despite this progress, about 160 million tons of

pollution are emitted into the air each year in the United States.

• Approximately 146 million people live in counties where monitored air in 2002 was unhealthy at times because of high levels of at least one of the six principal air pollutants.

• The vast majority of areas that experienced unhealthy air did so because of one or both of two pollutants—ozone and particulate matter (PM).

Vehicle Sources of Pollution

• Main problem: Gasoline and diesel fuels are mixtures of hydrocarbon compounds which contain hydrogen and carbon atoms. These atoms combine with oxygen during combustion.

• Also contain additives such as lead.• Nitrogen atoms are also present and are

combined with oxygen to produce gases.

Mobile Sources of Pollution

• Primary Pollutants from Engine Combustion– Carbon Monoxide (CO) – Lead (Pb)– Hydrocarbons (VOCs)– Nitrogen Oxides (NOX)

• Secondary Pollutants from Engine Combustion– Nitrogen Dioxide (NO2)– Ozone (O3)

Carbon Monoxide (CO)

• Colorless and odorless gas• Most abundant air pollutant• Major source (~ 77%) is motor vehicle

exhaust• Poorly running automobiles emit large

amounts by incomplete combustion (low temperatures an/or not enough O2 in engine).

Carbon Monoxide (CO)

• Extremely toxic – causes heart disease long-term, or death within an hour if concentration is 1600 ppm or more

• Hemoglobin in the blood is 200 times more attracted to carbon monoxide than oxygen (leading to oxygen starvation of the body)

• Most hazardous to human health of all criteria pollutants

Lead• Lead is a metal found naturally in the

environment as well as in manufactured products. 

• The major sources of lead emissions have historically been motor vehicles (such as cars and trucks) and industrial sources. 

• Due to the phase out of leaded gasoline, metals processing is the major source of lead emissions to the air today.

• Exposure results in brain damage and memory loss, as well as low IQ in children

Volatile Organic Compounds (VOCs)

• The term VOC stands for volatile organic compounds.

• Also called hydrocarbons, entirely made of carbon and hydrogen atoms

• VOCs are carbon-containing chemicals that evaporate easily at room temperature.

• Usually gasoline vapors and uncombusted fuel.• No direct adverse health effects, but they react

with other substances to form smog

Nitrogen Oxides (NOx)

• Nitrogen oxides, or NOx, is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts.

• The primary manmade sources of NOx are motor vehicles.

• Due to the high temperatures and oxygen in a vehicle engine, N separates from N2 and combines with O2 to form NOx

Nitrogen Dioxide (NO2)

• Can be emitted directly as a primary pollutant, but most is created by the oxidation of Nitric Oxide (NO)

• Toxic gas that has a pungent odor and gives the air a yellow to reddish-brown color

• Corrosive and can react with other substances to produce other secondary pollutants

• Irritates the lungs and can reduce immunity to infection at high levels

• Breaks down readily, so at its highest levels when traffic levels are the highest

Ozone• At the surface, ozone (O3) is a pollutant - it

irritates the respiratory system and damages plants

• Component of Photochemical Smog• Created differently than it is produced in the

stratosphere• VOC + NOx + Heat + Sunlight = Ozone

• Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC that help to form ozone. 

SmogA combination of the words “smoke” and

“fog”Two different types of smog occur 1. Photochemical smog; dry air smog

with ozone, peroxyacyl nitrate (PAN) and formaldehyde (see previous lecture)

2. Classic (London) Smog: original meaning of smog; mixture of smoke, sulfurous particles and soot with a fog

Photochemical Smog

• Occurs on sunny days due to intense traffic.

• Oxides of nitrogen and hydrocarbons react in the presence of sunlight to produce a mixture of aerosols and gases.

• Photochemical smog contains ozone, ketones, formaldehyde, and PAN (peroxyacetyl nitrates).

• The next slides illustrates the chemical reactions

Photochemical Smog

Peak Smog Hours

The Summertime Pollutant

• Peak ozone levels typically occur during hot, dry, stagnant summertime conditions. 

• The length of the ozone season varies from one area of the United States to another. 

• Southern and Southwestern states may have an ozone season that lasts nearly the entire year.

Photochemical Smog: Health Effects

• Increased incidents of respiratory distress.• Repeated exposures to ozone:

– Increased susceptibility to respiratory infection

– Lung inflammation– Aggravation of pre-existing respiratory

diseases such as asthma. – Decreases in lung function and increased

respiratory symptoms such as chest pain and cough.

Automobile Emission Control Strategies

Automobile Emission Control Strategies

• CO Reduction– Increase O2 to the combustion chamber from the

carburetor.– Higher air-to-fuel ratio– CO2 instead of CO from exhaust.

• VOC Reduction– Capture unburned vapors from fuel tank and

carburetor using Charcoal Filters– Recycle VOC back into the combustion chamber via

the ERG valve.– Increase combustion temperatures

Automobile Emission Control Strategies

• Problem: Higher air to fuel ratios (CO reduction) and and higher temperatures (VOC reduction) increase the amount of NOx emitted via engine exhaust.

• Solution: catalytic converter

What is a catalytic converter (CC)?

• A device that promotes (catalyzes) reactions in order to convert HC, CO, and NOx into less harmful compounds before they exit the vehicle.– HC + O2 CO2

+ H2O (oxidation)

– 2CO + O2 2CO2 (oxidation)

– 2NO N2 + O2 or 2NO2 N2 + 2O2 (reduction)

• The catalyst in most CCs is Platinum which is why CCs are so expensive.

Are Catalytic Converters the Answer?

• Until recently, it was thought that catalytic converters offered the greatest potential for decreasing mobile source pollution.

• The newest-model cars can remove up to 95% of the emissions.

• However, mobile source pollution still a problem because of the ever increasing number of vehicles.

• Real improvements in vehicle emissions will come from a switch from gasoline as a fuel and from reductions in the number of vehicles and miles driven.