Published on EGEE 101: Energy and the Environment (https://www.e-education.psu.edu/egee101)

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Lesson 9: Look Out - Smog!

Overview:

Young "Bo" Mathews and "his" General Lee ("Dukes of Hazzard" reference!)
Click for the transcript.

[Dr. Mathews drives onto camera by sliding his car on an unpaved parking lot. The tune from "The Dukes of Hazard" is playing the background.] [Dr. Mathews climbs out of his window as race car drivers do.] Dr. Mathews: You know I love "The Dukes of Hazard" as a kid growing up in England watching my three television channels. Fortunately part of the attraction is all the dust I've just generated. Unfortunately, these very small particles from these unpaved roads are going to stay in the atmosphere for quite a long time. They can cover quite large distances along with other emissions like sulfate aerosols, particulates coming out of my exhaust, and other fumes. [The camera zooms into the car's exhaust pipe and then back to Dr. Mathews.] They are going to cause problems. Part of these problems are regional haze and also lower air quality. This is what we are going to look at and how we can reduce them. One way is, don't drive like me. [Dr. Mathews struggles back into his car as the theme song begins to play again. The sound of a car peeling out plays as Dr. Mathews drives away.] [Video ends]

Lesson Objectives

This lesson is about the consequences of all the moving about that we do in our vehicles. You will be working towards the following learning objectives in this lesson:

  • Discuss pollution formation and control strategies
  • Relate vehicle changes to pollution reduction
  • Discuss fuel changes to pollution reduction
  • Articulate pollution impacts upon health

Wake Up Your Brain

What is Smog?

What we typically call smog is primarily made up of ground-level ozone (photochemical smog).  While ozone in the stratosphere, high above the Earth protects human health and the environment, ground-level ozone is the main harmful ingredient in smog.

Ozone is a secondary pollutant - it is not released into the atmosphere directly, but is formed by other released pollutant interactions. The equation that is important is as follows:

NOx + unburned hydrocarbons or any volatile organic compounds (VOCs) → sunlight over the arrow O3 

Smog over LA
Here the layer of warm air helps contain the world-famous LA Smog. The mountains help to contain the pollution and the cold air layer also.
Credit: CA ARB

Watch the following video that talks about the science of smog. It is almost 6 minutes long.

Click here for a transcript of The science of smog video.

On July 26, 1943, Los Angeles was blanketed by a thick gas that stung people's eyes and blocked out the Sun. Panicked residents believed their city had been attacked using chemical warfare. But the cloud wasn't an act of war. It was smog.

A portmanteau of smoke and fog, the word "smog" was coined at the beginning of the 20th century to describe the thick gray haze that covered cities such as London, Glasgow, and Edinburgh. This industrial smog was known to form when smoke from coal-burning home stoves and factories combined with moisture in the air. But the smog behind the LA panic was different. It was yellowish with a chemical odor. Since the city didn't burn much coal, its cause would remain a mystery until a chemist named Arie Haagen-Smit identified two culprits: volatile organic compounds, or VOCs, and nitrogen oxides.

VOCs are compounds that easily become vapors and may contain elements, such as carbon, oxygen, hydrogen, chlorine, and sulfur. Some are naturally produced by plants and animals, but others come from manmade sources, like solvents, paints, glues, and petroleum.

Meanwhile, the incomplete combustion of gas in motor vehicles releases nitrogen oxide. That's what gives this type of smog its yellowish color. VOCs and nitrogen oxide react with sunlight to produce secondary pollutants called PANs and tropospheric, or ground-level, ozone.

PANs and ozone cause eye irritation and damage lung tissue. Both are key ingredients in photochemical smog, which is what had been plaguing LA.

So why does smog affect some cities but not others? Both industrial and photochemical smog combine manmade pollution with local weather and geography. London's high humidity made it a prime location for industrial smog. Photochemical smog is strongest in urban areas with calm winds and dry, warm, sunny weather. The ultraviolet radiation from sunlight provides the energy necessary to breakdown molecules that contribute to smog formation. Cities surrounded by mountains, like LA, or lying in a basin, like Beijing, are also especially vulnerable to smog since there's nowhere for it to dissipate. That's also partially due to a phenomenon known as temperature inversion, where instead of warm air continuously rising upward, a pollution-filled layer of air remains trapped near the Earth's surface by a slightly warmer layer above.

Smog isn't just an aesthetic eyesore. Both forms of smog irritate the eyes, nose, and throat, exacerbate conditions like asthma and emphysema, and increase the risk of respiratory infections like bronchitis. Smog can be especially harmful to young children and older people and exposure in pregnant women has been linked to low birth weight and potential birth defects. Secondary pollutants found in photochemical smog can damage and weaken crops and decrease yield, making them more susceptible to insects. Yet for decades, smog was seen as the inevitable price of civilization. Londoners had become accustomed to the notorious pea soup fog swirling over their streets until 1952 when the Great Smog of London shut down all transportation in the city for days and caused more than 4,000 respiratory deaths.

As a result, the Clean Air Act of 1956 banned burning coal in certain areas of the city, leading to a massive reduction in smog. Similarly, regulations on vehicle emissions and gas content in the US reduced the volatile compounds in the air and smog levels along with them.

Smog remains a major problem around the world. Countries like China and Poland that depend on coal for energy experience high levels of industrial smog. Photochemical smog and airborne particles from vehicle emissions affect many rapidly developing cities, from Mexico City and Santiago to New Delhi and Tehran. Governments have tried many methods to tackle it, such as banning cars from driving for days at a time. As more than half of the world's population crowds into cities, considering a shift to mass transit and away from fossil fuels may allow us to breathe easier.

Credit: TEDEd, Kim Preshoff

We also normally think of smog as something that occurs in cities, due to excessive traffic and emissions outputs.

What Does Smog Require?

Smog requires:

  • Warm temperatures (thus smog is a summer issue in the US).
  • Sunlight (thus it does not form at night).
  • Some form of volatile organic compound, such as gasoline but also natural VOCs emissions from forests, etc., are a significant source
  • NOx – there are both natural and anthropologic emissions.
Photograph of smog in London, the city is only partially visible in the haze.
Smog is a city problem (mostly) because lots of cars generate the pollutants that form smog. It appears in the morning hours and can last late into the evening. Not only does it reduce visibility — it is damaging to human health and can even kill! This example is from LA where there is pollution from vehicles, electricity generation, and factories combining.
Credit: Artinun/adobe.stock.com

Other Components of Smog

Ozone (O3) is the main component of photochemical smog. Particulates from combustion can also be present. The most dangerous smog is when both ozone and sulfate aerosols are present. The famous Killer Fog of London during the mid 1900s and the late 1800s was comprised of sulfate aerosols and smoke (particulates) from the coal fires used to heat homes and businesses. Over a 4-day period in 1952, the London Smog was probably responsible for about 3,000 or so "extra deaths" that year. The English expression would be "kick the bucket." And it would have been the old, ill, and infirm who died, not healthy Londoners. In New York City during the late 1800s, improving the air quality took the passing of a law that required anthracite coal (little smoke and low sulfur, so no sulfate aerosols) as the fuel of choice. This law, ultimately, was responsible for the rapid growth of anthracite mining in NE Pennsylvania at the turn of the century (1900's).

Temperature Inversion

Smog is a problem because it is concentrated pollution. In the normal events of the average day, our pollutants are released to rise above our cities to be blown elsewhere (this upsets the Swedish when British pollution kills their forests (acid deposition related)). However, we can also have a temperature inversion. This is when a layer of warm air acts like a lid containing pollutants sitting over a layer of cold air. The situation is more common in valleys as the cold air can collect in the bottom of the valley (this is how fog forms, stagnant cold air sitting over river valleys).

Temperature inversion.  pollution from traffic is stuck under a warm inversion layer which is stuck under cooler air.
Temperature Inversion
Credit: The Amazing World of Science [1] is licensed under a CC BY-SA 4.0 [2] License.

At night, the ground level temperature cools faster than the air above it. Pollutants become trapped under the layer of warm air. As the sun rises in the morning, the ground level temperature warms up faster than the air above it, pushing the air upwards, which breaks up the warm air layer, allowing the pollutants to escape. However, if there is no wind, the air can become stagnant.

-Marconia County Web Site.

Ozone (Holes)

No matter what subject I am discussing, there are always students who think that the ozone hole is the reason for everything. Hint: It is not. Ozone holes are not something I would normally cover, but due to overwhelming interest in the subject (from your input), some explanation of the issue is in order here. But note it is not the major cause of climate change.

Globe covered with a thin blue layer.  The layer represents the atmosphere.
The atmosphere of the earth does not extend very far up in comparison to the size of the planet. The common analogy is the skin of an orange represents the atmosphere while the orange represents the planet. But the atmosphere is an important protective barrier for harmful rays, in particular, UV (UV-A & UV-B) or ultraviolet rays.
Credit: NASA

The sun emits all kinds of wavelengths that impinge on the outer surface of the planet (the atmosphere). Fortunately, the ozone that is high in the atmosphere screens much of the UV rays out. The ozone layer is in the stratosphere between 10 and 30 miles up.

Picture of a tube of SPF 15 sunblock.
Think of the ozone layer as sunscreen for the planet.
Credit: Sunblocks.com

When I was in Australia (one of the skin cancer leading nations) most of the kids on the beach were running around wearing full coverage bathing suits that look like a thin wetsuit. The material provided protection against the UV rays. In the center of town was the cancer treatment center. It is not uncommon for some of the older residents to go once a year to have the skin cancers removed. In the US skin cancer is more common on the left arm than on the right, why? It is due to how we drive and expose the left arm to more sun exposure.

Ad discouraging people from exposing their skin to the sun, "You Know What To Do" accompanies images of ways to protect your skin.
Be smart this year, don't burn. 
Credit: Australian Sun Smart Program

What causes ozone holes?

The chemicals that we use to provide air conditioning, etc. are very stable, so stable that in the atmosphere they can rise to the outer layer of the stratosphere where the ozone resides. Once there the chlorine molecules can destroy 10,000 ozone molecules. The chemicals of concern come from commercial and industrial uses. The chemicals (refrigerants) are also used in heat-pumps and in refridgerators. 

Harmful sources of ozone damaging the protective layer
Click here to see graph tabular data
Harmful sources of ozone damaging the protective layer
Source of Ozone Percent
Solvent Cleaning Products 36.1%
Sterilization 3.0%
Refrigeration & Air Conditioning 29.6%
Foam Products 14.3%
Aerosols 5.0%
Other Products Including Halons 12.0%

Solution

The solution is relatively simple; STOP using ozone-destroying chemicals (Chlorofluorocarbons - CFC's, and others)! The Montreal Protocol became effective in 1989 with 160 countries eliminating or phasing out ozone-depleting chemicals. You can help by having your vehicle air conditioning fixed at locations where they do not release the chemicals into the atmosphere, and have your refrigerator refrigerant disposed of properly before the fridge goes into the landfill. As ozone is formed naturally, the ozone hole(s) will be repaired naturally. The problem is time - this is not going to occur until you are all much older (about 40 years). The process of formation is slow. Those ozone-destroying chemicals that were released in the 80's are still on their way to the ozone layer. So you and your family will need to use sunscreen and protect your skin and eyes against the UV-B rays in particular (to prevent cataracts). Unfortunately, penguins have no such protection. Take extra care if you are close to the equator (more exposure due to the Earth's angle towards the sun) and close to the poles (but don't worry about the Czechoslovakians!)

Watch the following 1:51 minute video about the destruction of the Ozone Holes.

Click here for a transcript of NASA's Big Ozone Holes Headed For Extinction By 2040 video.

Say goodbye to large ozone holes. A new study from NASA scientists suggests by the year 2040, the Antarctic ozone hole will be permanently smaller than the giant holes of today.

Since the early 90s, observed hole sizes have been larger than 12 million square miles with exact sizes changing each year. The ozone hole is a seasonal thinning of the ozone layer over Antarctica. The ozone hole size varies in part due to levels of ozone-depleting chemicals in the atmosphere. Man-made chemicals that destroy ozone are transported from the equator to the poles. In the Southern Hemisphere, they are trapped by the winds of the polar vortex, a ring of fast-moving air that circles the South Pole.

Although levels of these chemicals have been declining since the late 1990s due to the Montreal Protocol they will remain in the atmosphere for years, affecting ozone levels well into the century.

High in the atmosphere, the chemicals react with sheets of iridescent ice clouds which trigger the destruction of ozone. In years with warm temperatures, fewer ice clouds form, resulting in holes that are smaller. In years with cold temperatures, more ice clouds form, resulting in holes that are larger. But in order to understand how hole sizes will change in the future, scientists needed a more accurate picture of levels of man-made chemicals in the atmosphere.

Using NASA’s AURA satellite, scientists determined how chemical levels In the ozone hole, varied each year. With this new information, we can look into the future and say with confidence that ozone holes will be consistently smaller than 8 million square miles by 2040. And that will really be a milestone that we’re finally past the era of big ozone holes.

Credit: NASA Goddard [3]

Misconceptions

Smog is caused by the ozone hole! Nope! Ozone at ground level contains ozone that was created at ground level from a combination of pollutants in the summer months in polluted areas. Smog has nothing to do with the ozone hole(s).

Climate change is enhanced by the ozone holes as it lets in more energy. Nope! The energy wavelength of interest for climate change is IR not UV. Climate change and the hole are however related, the gases that deplete the ozone are also greenhouse gases, and climate change can affect the ozone layer.

There is no ozone over the pole. Nope! We are discussing a reduction in the concentration of ozone - not the complete removal - so a "hole" is a bit misleading.

It is only a problem in the Southern Pole. Hmmmmm. Okay, it is a problem in the South Pole, mostly because during certain times of the year the "hole" goes walkabout and loses the circular shape and can cross populated areas such as the coastline of Australia. The North Pole "hole" has further to travel before crossing highly populated Europe. UV-B does impact phytoplankton and so it is a problem in both poles as phytoplankton is vital for the food chain. No phytoplankton, no krill, no krill no krill-eating whales, etc.

EPA Smog Publications

Click on the image below to open the EPA report on Smog.

While this is an old report, the information has remained the same. Please read pages 1-3, and page 8 for more details about the health impacts of smog and how it can be avoided.

 link to EPA's pdf entitled Smog-Who Does It Hurt? What You Need to Know About Ozone and Your Health [4]
Click on the image above to open EPA's PDF report.
Credit: EPA

Change the Fuel, Vehicle, Driving Habits

Reviewing SMOG Prevention Measures

We have seen that areas with smog problems tend to be major cities as that is where there are lots of (dirty) vehicles. Smog is also an issue in areas close to major refineries such as the Texas basin where much of the refining capacity for that region is located (because of Texas oil and offshore oil from the Gulf of Mexico). So if there are 4 things needed to form smog, then we can reduce smog by reducing the precursors. There is not much we can do to block out the sun, or to reduce the outside temperature, so we are limited to reducing the precursor emissions of NOx and VOCs. In particular, we need to reduce or eliminate the anthropologic emissions of NOx and VOCs.

Reducing VOCs

Photograph of a gasoline pump with a black plastic vapor catching device to seal vapors in the tank (and capture them).
The black plastic concertina-like device makes a seal with the gas cap tube to prevent gasoline vapors from escaping. This is called gasoline vapor recovery. If you see one of these you are in a non-attainment area for air quality (smog). They are no longer needed due to better storage tank designs in vehicles.
Credit: AQMD

We have already discussed reducing the volatility of the gasoline fuel as one method of reducing the VOC emissions. This takes place in the blending stage of the refinery operation. By limiting the quantity of the more volatile compounds, the gasoline will evaporate less. This results in the refinery having larger quantities of light compounds to dispose of (in less profitable ways) which raises the cost of the gasoline just as do all the approaches where something needs to be changed. This reduced volatility fuel is a "summer blend." The lower temperatures of Fall, Spring, and Winter reduce the gasoline emissions, and thus reduce smog formation because smog requires warm (mostly summer) temperatures.

So the summer blend is only used in the summer and only in those locations with poor air quality (non-attainment area). You might ask how do gasoline vapors escape? When you "fill her up" the first thing you do is open the cap to the gasoline tank. What happens next on a summer day? Hsssss.... Gasoline vapors escape! And it is not only from the open "gas" tank, but also other locations: from the engine, the exhaust, fuel lines, and the gasoline tank. One approach is to capture these vapors from the tank with special nozzle attachments on the metal part of the gasoline hose. While it won't capture all of the volatile gasses escaping from the moment you unscrew the cap, it will capture the vapors that will be displaced by filling the tank up with gasoline. These vapors are cooled back into a liquid and sent back to the refinery.

Other Approaches...

  • Not overfilling the tank also prevents gasoline spills (and fires — how may people have you seen smoking while filling up?)
  • Another approach is to fill up at times when the smog formation opportunities are lowest, such as in cooler evening hours (not enough sunlight and lower temperatures). This includes our use of the many gasoline-powered tools we tend to have, such as lawnmowers, weed whackers, etc. Filling up your lawnmower in the evening would also reduce VOC emissions.
  • Oxygenated fuels also reduce VOC emissions because the oxygen within the fuel enhances the mixing process of fuel with oxygen, and so less fuel (VOCs) will escape the combustion process. The catalytic converter will also oxidize some of the escaping VOCs to water and carbon dioxide.

Reducing NOx From Vehicles

This table provides a quick summary of the methods used to reduce NOx emissions in vehicles. For those of you keeping score at home, the information here combines NOx emissions issues from this lesson, earlier lessons, namely Transportation (L05), and issues in upcoming lessons: Climate Change (L11 & 12), and Acid Deposition (L10). 

Methods used to reduce NOx emissions in vehicles
Method Result
Oxygenated Fuels The addition of MTBE (or ethanol) in the summer reduces NOx by lowering the temperature of the combustion process.
Alternative Fuels Methane (compressed natural gas), ethanol, etc. will have lower levels of NOx emissions.
Catalytic Converter The NOx is reduced to N2 in the catalytic converter. It does not eliminate emissions but it will drastically reduce the emissions. However, all our gasoline vehicles have them, so this is not a solution to our current problems as it is already in place.
Improved Catalytic Converters One of the reasons to reduce sulfur from gasoline to very low levels is because the S (as a catalyst poison) would reduce the effectiveness of the catalytic converter. The new lower S standards (<30 ppm S in gasoline) allow for better catalytic converter operation and also newer more effective catalysts.
Hybrid Cars Hybrid cars manage to reduce pollutant emissions via a combination of higher efficiencies and perhaps by utilizing the electric engine in those already polluted areas such as the centers. In Europe, one proposal would require that gasoline-fueled vehicles be banned from the inner city parking areas thus encouraging the use of cleaner cars. Thus, a hybrid might function as a gasoline engine for the commute to work but as an electric engine when within the city. 
Emission Free Vehicles Electric or fuel cell cars have no NOx emissions from the vehicle itself. Encouraging their use (fleet vehicles, tax credits, parking availability-fuel cell car only, and other approaches such as allowing these cars in high occupancy vehicle lanes with 1 driver and no passengers) would aid electric vehicle adoption.
Getting Old Cars Off the Road A few vehicles are the cause of much of the pollution. If we have a buy-back of your old Junker program (then squish the car and recycle it) we can remove them from the road. Emission checking (emission inspection along with the traditional vehicle inspection) is also a key component of ensuring the pollution control equipment within the car is functioning but also for failing super and high polluting vehicles. Emission Inspections are often required in vehicles in non-attainment areas. 
Efficiency and Conservation AGAIN! Always a right answer! All the methods of increasing vehicle efficiencies, such as increases in MPG and passenger occupancy, and the uses and development of more efficient methods of public transportation, are again applicable here and are all discussed in detail in Lesson 5 for those of you who would benefit from a review.

NOx Reduction Approaches (Utilities)

 Graphic of air showing about half nitrogen and half oxygen.
"Air is mostly nitrogen molecules (green in the above diagram) and oxygen molecules (purple). When heated hot enough (around 3000 °F), the molecules break apart and oxygen atoms link with the nitrogen atoms to form NOx, an air pollutant. "
Credit: DOE

Nitrogen is 78% of the volume of air. Normally, nitrogen atoms float around joined to each other like chemical couples. But when air is heated - in a high-temperature boiler's flame, for example - these nitrogen atoms break apart and join with oxygen. This forms "nitrogen oxides" - or, as it is sometimes called, "NOx" (rhymes with "socks"). NOx can also be formed from the atoms of nitrogen that are trapped inside coal (fuel NOx).

Part of the Clean Air Act is a reduction in NOx emissions from power plants using Low NOx burners. Three others are worth mentioning here:

  • Fluidized Bed Combustion: This technique utilizes low combustion temperatures, which produce much less NOx.
  • Addition of a Catalyst Bed: This approach is also used to remove NOx concentrations - however, as these catalytic reactors are very expensive to operate, this technology is often run only during those summer months when smog pollution is most significant (called a selective catalytic reduction (SCR) reactor in the figure below).
  • Urea or ammonia injection: Another approach is to add nitrogen-containing compounds that will be reactive and bind with the nitrogen atoms (single) to form nitrogen molecules (N2)
Diagram of the Selective catalytic reduction system
Here the selective catalytic reduction system is shown along with an ESP particulate collector. Urea or ammonia is also injected into the flue gas to react with the NOx in the presence of the catalyst beds.
Credit: LANL

Related Topics for NOx Reduction

Many of the approaches for NOx reduction are similar to those we discussed in the previous and upcoming lessons (Acid Deposition), so for the details on these techniques, I'd suggest you keep this in mind when we research those materials.

What is Regional Haze? Regional Haze Revisited

London shrouded in haze.
A view of London
Credit: © Chris M/adobe.stock.com [5]
Map of the UK with the area around Liverpool circled.
My home town of Chester is close
to Liverpool in a key industrial 
region of the North West of England

When I was a young boy the family moved to the plains of Cheshire, a region in the northwest of England that is very flat. From my bedroom window, I could see a couple of castles (I would have to lean out of the window precariously). When I go back home, my memories of the great view are rarely repeated because the view is now often hazy. The NW of England is an industrial area: Manchester, Liverpool, etc. My bedroom window view is also of Stanlow, one of the largest European refineries (Shell).

Tankers offload oil in the Mersey estuary and it is piped to the refinery. All the industry, the utilities, and transportation (lots of cars in a small place) contribute to the pollution. It is nothing like the peace and tranquility of State College. The cause of the haze is very small particles that can stay suspended in the air for days, and can thus travel large distances on the wind. There are a number of different types of particles depending on where you are, and the local events. Haze can be a natural event when the air is humid and contains many tiny suspended water particles (think Florida on a hot and humid day - recall this is one of the reasons that solar cells are not as efficient in Florida).

Visibility often is measured as the farthest distance from which a person can see a landscape feature. Haze currently reduces natural visibility from 90 miles to between 14 and 24 miles in the eastern United States, and from 140 miles to between 33 and 90 miles in the western United States. Visibility generally is worse in the eastern United States due to higher average humidity levels and higher levels of particulate matter from manmade and natural sources.

- EPA

See It For Yourself

Natural Causes

Some of the natural particular matter (PM) emissions are from forest fires. The smoke contains fine ash particles and fine soot particles (if you have ever owned a fireplace, you know that the routine of having a chimney sweep removes soot periodically!) All combustion can produce small particles, and it is the small particles that contribute to visibility reductions and health-related problems.

Aerial view of the 2002 forest fires in Arizona.
An image captured by NASA of forest fires in Arizona.
Credit: NASA
Cotopaxi volcano eruption with huge ash cloud
Ash cloud from a volcanic eruption.
Credit: © ecuadorquerido/adobe.stock.com [6]

Volcanic eruptions are also natural sources of particulates in the atmosphere. In addition to their potentially catastrophic consequences on the immediate localities and regions, they can also send particles high (many miles) into the atmosphere, and the fine particles can be very dangerous to aircraft (particularly those with jets), often forcing airline travel delays and the creation of alternative routes around the volcanic activity.

Manmade Causes

cars on highway with exhaust
Car exhaust
Credit: © Nady/adobe.stock.com [7]

Dust from unpaved roads, the use of leaf blowers (often gasoline-powered, which means that VOCs are also released, contributing to more smog), and mowing the lawn, etc., all can add particles into the air and increase the smog problem, so those activities are best avoided on days where air quality is already low. Would people rather make sure the yard looks good, or do their part to preserve higher air quality?

Deisel soot
This is what diesel soot looks like, viewed by a scanning electron microscope. Just to give you a sense of the scale here, the average human hair is about 80 microns thick. If we can reduce the particulate emissions from diesel engines, that will help regional haze abatement too.
Credit: Jonathan Mathews

The Clean Air Act

lungs of nonsmoker.  They are mostly black as a result of air pollution.
You might think these are the
lungs of a smoker but they are
not. This is a result of air
pollution. There are many
nations currently struggling
with massive air pollution in
very overcrowded cities.
Credit: AQMD

The Clean Air Act classifies the size of these fine particles by two terms:

  • PM10: Particulate Matter below 10 microns
  • PM2.5: Particulate Matter below 2.5 microns

Dust is anything below 100 microns, and so we are focused on very small particles. The old standard was a limitation of the number of PM10 particulates in the air. Now, that law remains and there is another standard focusing on the very smallest particles below 2.5 microns. This is because the health impact of these smallest particles is greater than the slightly larger particles. Small particles enter into the lungs and can stay there (recall the discussion of Black Lung). Particles smaller than 1 micron not only enter the lungs but can also enter the bloodstream. This is a great concern because the small particles have a very large surface area (recall this is why we pulverize coal). Heavy metals and carcinogens from the combustion process coat these small particles. In the blood these chemicals and metals cause disease. Similar to the smog influences, these particles impact those who already suffer from limited lung capacity the most. So if you smoke, this is what you are adding to your lungs.

The cigarette is an example of biomass combustion. It produces much of the same carcinogens as does any combustion. In fact, the only way of producing these very small particles of < 1 micron is via the combustion process

Cigarette burning in an ashtray.
Credit: © Vesna/adobe.stock.com [8]

The Environmental Protection Agency (EPA) classifies environmental tobacco smoke (ETS) as a class A carcinogen, the most hazardous classification for cancer causing agents. ETS kills as many as 53,000 nonsmokers in the United States annually; 3,000 from cancer deaths and 50,000 from cardiovascular disease and other tobacco-related illnesses.

- Smoking Cessation Program

Take a good look at the chart below. You'll see that, when the air is unhealthy, you are better off staying indoors and avoiding strenuous exercise (if only I could use this excuse more often to watch football and drink beer while eating chicken wings!)

Air Quality Index (AQI): Particulate Matter (PM)
Index Values Levels of Health Concerns PM 2.5
Cautionary Statements*
PM10
Cautionary Statements*
0-50 Good None None
51-100** Moderate None None
101-150 Unhealthy for Sensitive Groups People with respiratory or heart disease, the elderly, and children should limit prolonged exertion. People with respiratory diseases such as asthma should limit outdoor exertion.
151-200 Unhealthy People with respiratory or heart disease, the elderly, and children should avoid prolonged exertion; everyone else should limit prolonged exertion. People with respiratory diseases such as asthma should avoid outdoor exertion; everyone else should limit prolonged exertion.
201-300 Very Unhealthy People with respiratory or heart disease, the elderly, and children should avoid any outdoor activity; everyone else should avoid prolonged exertion. People with respiratory diseases such as asthma should avoid any outdoor activity; everyone else, especially the elderly and children, should limit outdoor exertion.
301-500 Hazardous Everyone should avoid any outdoor exertion; people with respiratory or heart disease, the elderly, and children should remain indoors. Everyone should avoid any outdoor exertion; people with respiratory diseases such as asthma should remain indoors.

* PM has two sets of cautionary statements which correspond to the two sizes of PM that are measured:

  • Particles up to 2.5 micrometers in size (PM2.5)
  • Particles up to 10 micrometers in size (PM10)

Progress

As you can see we are getting better air quality (but this is still problematic for those at-risk groups).

Comparison map showing improvements in air visibility between ~2000 and 2015.  The east side of the nation has reduced visibility in comparison to the west (generally).
The regional haze is finally getting better after very limited progress during the 1990s and onwards. There has been an improvement but the east of the nation is still significantly impacted. 
Credit: EPA

EPA site on Regional Haze

Click on the image below to visit the EPA's (Environmental Protection Agency) great site about Regional Haze:

link to the EPA website about Regional haze [9]

Baghouse, ESP, or a cyclone?

black smoke coming out of a smoke stack
smokestack
Credit: AQMD

The emissions from this smokestack are atrocious! Fortunately, we don't see this often in this country. Now, in other countries, that's a different story. Before there was regulation on particulate emissions this is what the stack of a coal power plant would look like. Even when the stack emissions are not dark we are getting pollution in the form of small particles that we need to clean before they reach the atmosphere. Not only is it a health issue, but it is also a waste of coal. Home fireplaces might produce black smoke because of the inefficient method of combustion but many modern fireplaces might have a catalyst to reduce particulate emissions.

What we need is a method of filtering, or pulling the particles out of the hot flue gas before they reach the stack. For coal-fired utilities, there are 3 approaches: a) Electrostatic Precipitators, b) Baghouses for pulverized coal combustion; for a Fluidized bed, c) a cyclone is commonly used.

Watch the following 3:56 minute video about Particulate Matter.

Click here for a transcript of the video about particulate matter.

Dr. Mathews: So when we discuss particulate matter, we are talking about small particles that are smaller than 10 microns, and there's a sub-classification there of small particles smaller than 2.5. So PM 2.5. These very small particles have a number of contributions to environmental challenges.

And so anything that is on the smaller side, less than these 10 microns, can certainly go into the lungs. They can go into the bloodstream. Certainly, the smaller particles have very large surface areas. This is where carcinogenic material can go on as well. And that's why it's the smaller, smaller than 2 and 1/2 microns that are particularly troublesome for environmental issues, for human health, particularly.

Some of these small particles are going to be aerosols. They contribute to climate change. Actually, in a good way. They reflect more of the energy back, sunlight back, and so actually cause the planets. When we have volcanic emissions, you can sometimes see a cooling event on a couple-year period of time, if not longer in some cases, due to all that fine ash being up in the atmosphere.

And so we need to be able to handle these particulates and prevent them to go into the atmosphere. And we can do that by a couple of approaches. So if we'd like to remove them from a gas stream, we can do it through a cyclone. And so with a cyclone, the material comes in and gets rotated. And through that very, very high centrifugal forces, the particles get thrown out to the sides and can be extracted, whereas the clean air comes out.

In some cases, we will have these particles flowing through filters. And so, again, maybe some will go through. Some will stopped and get absorbed. And so your vacuum cleaner, for example-- we can have some filters that will prevent these going through.

We also have examples where we use electrostatic precipitators, for example. And so what we're going to do there is put an electric charge. That electric charge is going to give the particle a charged surface. And if I have a positive plate, then it'll get attracted to the plate, and we can remove the particulates that way.

And so PM 10, PM 2.5 Again, variety of approaches to clean these emissions, depending on what the size is and what the material is. They contribute, certainly, to regional haze. Obviously, some things like sulfur and aerosols make things like smoke worse. And of course, we've got other issues with very small particles going and affecting human health. And so that's PM 10, PM 2.5.

And just to give you an idea of how small that is, a human hair would be ~80 microns. And so we're talking about very small particles that we can't see very well. And because they're small, they can have a long time in the atmosphere. They're not going to fall and be brought down to the ground very easily. They can be blown around.

Credit: JPM
Bag house at Penn State.
The Baghouse at Penn State.
Credit: J. Mathews

This Baghouse used to be at University Park, it gives you some idea of the scale of these systems. It is about a 6 story building, the flue gas passing through before exiting out of the flue. Within the building, there were 1000's of very large bags much like a vacuum cleaner bag (that can stand >110 °C). This building (and the stack) were demolished when we moved away from coal to natural gas for the steam plant. An Office of Physical Plant (OPP) building sits there now.

Baghouse and Cyclone Systems

We have discussed fluidized beds already in the course (Lesson 02). The one shown below has both a baghouse (labeled as a fabric filter) and a cyclone system to return particles back to the fluidized bed if large enough, or if small enough they go through the baghouse (fabric filter) before the other products of combustion exit the stack. The cyclone uses centrifugal forces to separate particles from a fluid.

Watch the following 1 minute video: Baghouse Basics

Click here for a transcript of the Baghouse Basics video.

There are many different sizes and designs, but baghouses all operate in the same basic way. One. The dust-laden or particle laden air or gas stream enters the bag house, travels along the surfaces of multiple fabric tubes and then passes outward or inward through the fabric. Two. The larger particles fall down into a hopper while the smaller particles accumulate on the fabric surfaces. Three. A cleaning mechanism occasionally removes the particles from the fabric tubes and they fall down into the hopper from which they are discharged. Four. the clean air or gas stream exits from the top of the baghouse.

Credit: Convergence Training by Vector Solutions [10]
diagram identifiying the baghouse and cyclone systems of a fluidized bed boiler. The process is described in text and video
A baghouse and a cyclone system. 
Credit: DOE

Here is my effort at showing the particle trajectory. The separation due to centrifugal forces occurs because of the mass difference (size and density differences) between air and the particles (char, ash). The smallest particles have the least amount of centrifugal forces and thus are the hardest to separate (larger particles are much easier) to separate from the air. Obviously, there is a cost to adding these components (power-plants are large-scale operations) to reduce the emissions.

Cyclones: air flows into the cyclone, spirals down, particles come out the bottom and the air flows back up and out the top.
An image of the airflow that occurs within the cyclones.
Credit: ARROW Corp.

You can see a much better particle pathway in this video. Watch this 1 minute of the following cyclone animation. You can skip the first 30 seconds of company blurb.

Click here for a transcript of the Hurricane System - High Efficiency Cyclones video.

Hurricanes are customized cyclones to serve different needs and which results from optimization functions such as maximize efficiency, minimize cost, or minimize size. Combining stochastic numerical optimization by changing the eight independent cycling dimensions. With a knowledge of particle agglomeration ACS is able to design the perfect geometry for each case. The colored dots indicate the particulates to be collected and the blue arrow represents the flow of gas that leaves the equipment.

For dusts with tendency to agglomerate such as biomass or coal fly ash among others. Efficiency is maximized thanks to the clustering effects of particles. In fact, the cyclone doesn't see as many small particles but clustered particles with bigger dimensions and thus easier to collect. Agglomeration increases with wide particle size distributions beyond residence times in the cyclone and large inlet particle concentrations. Agglomeration is maximized with the new developed hurricane MK cyclones. Emissions can be under 15% of those of other cyclones being as low as 30 milligrams a normal cubic metre for many industrial processes.

Credit: Advanced Cyclone Systems [11]

Electrostatic Precipitators

The schematic below shows a boiler with Electrostatic Precipitators that capture the fly ash (ash leaving with the flue) and the small particulates. Also shown is a low-NOx burner to reduce NOx emissions (less smog and what else???)

Diagram of a fluidized bed boiler with the  electrostatic precipitators called out. Described in text.
Electrostatic Precipitators
Credit: LANL

Read

ExplainThatStuff! has a great article, Electrostatic smoke precipitators [12]. Pay particular attention to the section on how it works.

Lesson 9 Coverage Map

This map represents a visual summary of the topics and ideas relating to smog and problems with air quality. It is interactive so move your mouse over the text below for more details.

Accessible Version (word document) [13]

Deliverable

After looking at this map, please take the L09 quiz.


Source URL: https://www.e-education.psu.edu/egee101/node/655

Links
[1] https://www.mrgscience.com/ess-topic-63-photochemical-smog.html
[2] https://creativecommons.org/licenses/by-sa/4.0/
[3] https://www.youtube.com/channel/UCAY-SMFNfynqz1bdoaV8BeQ?pbjreload=102
[4] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson08/L08_smog_EPA.pdf
[5] https://stock.adobe.com/images/los-angeles-skyline-with-smog-and-smoke/334253951
[6] https://stock.adobe.com/images/cotopaxi-volcano-eruption/92030899?prev_url=detail
[7] https://stock.adobe.com/images/pollution-from-the-exhaust-of-cars-in-the-city-in-the-winter-smoke-from-cars-on-a-cold-winter-day/247258123
[8] https://stock.adobe.com/images/closeup-of-cigarette-on-ashtray-with-a-beautiful-wisp-of-smoke/271850288
[9] https://epa.maps.arcgis.com/apps/Cascade/index.html?appid=e4dbe2263e1f49fb849af1c73a04e2f2
[10] https://www.youtube.com/channel/UCHXdDvAkuK5BjhrYHY4hOzQ
[11] https://www.youtube.com/channel/UC4e6MKRiaG47SfHGjtUejyQ
[12] https://www.explainthatstuff.com/electrostaticsmokeprecipitators.html
[13] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson09/Lesson%209%20Coverage%20Map.docx