environmental physics chapter 8: air pollution copyright © 2008 by dbs

Environmental Physics

Chapter 8:Air Pollution

Copyright © 2008 by DBS

Cartoon

Introduction

• Power production and energy use produces considerable adverse environmental effects

• Air pollution knows no boundaries, state or national

• Effects can be seen far from the source

Introduction

Mega Cities

Source: UNEP/WHO, 1992

http://www.who.int/peh/air/gemair.htm

Eastern Europe

• Eastern Europe after the fall of Communism

• Environmental concerns passed over in the name of progress

• Power plants burning high sulfur coal wcontaiing arsenic and cadmium

• Lack of air pollution controls on power plants

Air pollution in Copsa Mica, Romania, dubbed the most polluted city in the world.

Carbon black (soot) and metal smelting

Properties and Motion of the Atmosphere

• A

Permanent Variable


Air Pollutant:

Substances added by humans that are toxic or irritant to animals, vegetation, or property

Examples?


Anthropogenic vs. Natural Sources

A coal barge on the Monongahela River moves past a U. S. Steel Corporation coke plant at Clairton, Pennsylvania, 1973

Anthropogenic Natural

Wildfires across Oklahoma and TexasJan 2, 2006

Mt. Pinatubo, PhillipinesJune, 1991


Pressure

• Thin gaseous envelope – how thin?

• 99 % of the mass of the atmosphere is below 33 km (20 miles)

• We experience pressure,

P = f / A

Where P = pressure (psi, N/m2 = Pa), f = force (N) and A = area (m2)

6400 km

30 km

Question

Which exerts more pressure, a 4000 lb elephant standing on one leg with a foot size of 8" x 8" or a 120 lb woman standing on one leg in high-healed shoes with a heal size of 1"x1"?

P = f/A

4000/64 = 62.5 psi120/1 = 120 psi

so the woman exerts more pressure

Question

What is the pressure exerted by your finger if you hold back water leaking through a hole in a dike, as shown below?

Weight density of water Is 62 lb/ft2

P = 62 lb/ft3 x 1 ft = 62 lb = 0.43 psi ft2 x(144 in2/ft2 )


Pressure

Mean sea level pressure of air

= 15 lb/sq in

= 30 in. Hg (inches of mercury)

= 1000 mb (millibars)= 100,000 Pa (SI system)


Figure 8.1: Examples of atmospheric pressure. (a) The maximum weight you can lift with a 4-inch diameter suction cup is area × pressure = π (2)2 in2 × 14.7 lb/in2 = 184 lb. (b) Turning a glass of water (covered with a piece of paper) upside down forces some of the air out of the glass. The pressure of the remaining air plus the weight of the water is less than atmospheric pressure, so the water remains in the glass. (c) Sipping soda: atmospheric pressure forces the drink up the straw to the region of lower pressure in your mouth.

Demonstrations

Explain the following results

A: Hard-boiled egg sucked into a flask

B: Can Crushing


Mercury barometer

• ("bar" o "meter" - an instrument that measures "bars") – (invented by Toricelli, 1643)

• Height of mercury in column is proportional to air pressure

• Corrected for changes in temperature

1 bar = 1000 mb = 760 Torr = 76 cm Hg = 30” Hg


Buoyant Force and Air Temperature Profiles

A solid object will float if its density is equal or less than the density of the medium it is in

Figure 8.3: The buoyant force on a submerged object is equal to the weight of the displaced fluid. This is a result of the difference in pressure between the top and the bottom of the object. An object will float if the buoyant force is equal to or greater than the object’s weight.

Archimedes principle states that the buoyant force on an object is equal to the weight of the fluid displaced by the object



• Air warmed by the ground rises due to buoyancy

• Bouyant foce = upward force exerted on a parcel of air

As warm air rises it expands and cools



• Energy used for expansion of rising air is taken from the thermal energy of the air, air cools

• Air parcel will rise until its temperature equals that of the surrounding air

• If the warm-air parcel cools less rapidly than the surrounding air, its temperature will always be above the temperature of the surrounding air

• Mixing occurs

Figure 8.4: The temperature of ambient air normally decreases with increasing altitude. A parcel of air will rise until its temperature is equal to that of the surrounding air. An elevated inversion layer (a region in which the temperature of the ambient air increases with altitude) will put a lid on the rising air parcels.


Natural Dispersion of Air Pollutants

• Vertical motion (convection) and the winds are important dispersal mechanisms

• Mixing is restricted when there is an inversion

Troposphere usually well mixed (warm air rises, replaced by cooler air)

Cool air trapped below warm air prevents mixing

Inversion increases with calm winds


Natural Dispersion of Air Pollutants

• Vertical motion (convection) and the winds are important dispersal mechanisms

• Mixing is restricted when there is an inversion

Figure 8.5: View of Houston 15 miles from city center.

Photochemical Smog


• Temperature differences are also responsiblefor global air circulation

• Pollutants can travel long distances before settling out

• Emissions from one country affect another

e.g. US pollution from Midwest power plants goes to Canada

Figure 8.6: Earth’s wind engine. The rotation of the earth produces a complex wind pattern, as the cold polar air sinks toward the equator and the warm tropical air rises and moves toward the poles.

End

• Review

Air Pollutants and their Sources

• Gases, particulates (solids) and aerosols (liquid suspend in a gas)

CO, SO2, PM, HC’s, NOx

• Levels of pollutants in a particular area depend on type, amount emitted, method of release, meteorological conditions

• In general two sources: stationary or mobile

• We are affected by these primary pollutants and also by the products of chemical reactions that these pollutants undergo in the atmosphere


• US EPA reports air quality data for 5 principal pollutants

Figure 8.7: Emissions of major air pollutants in the United States by source: 2003. Particulates refer to those of size less than 10 microns (PM-10).

Every year more than 180 million tons emitted in US,

3.3 lb per person per day

(~amount of trash we generate)

1970-2003 US population increased by 39 %, vehicle miles increased by 155 %, total emissions of the 5 principal pollutants decreased by 50 %


Removal mechanisms:

• Sinks or scavenging mechanisms

– Absorption by vegetation, ground, and water

– Oxidation and conversion to precipitates

Measuring concentration

• No. molecules of a pollutant in one million molecules of air (N2 and O2)

• Expressed as a mixing ratio - Parts per million (v/v) (ppm)

e.g. 10 ppm = air containing one million molecules has 10 molecules of a pollutant

• Also mass per unit volume (μg / m3)

Conversion (at SATP 25 ºC and 1 atm):

concentration (ppm) = concentration (mg m-3) x 24.45 Molar mass

Mixing ratio is conserved if temperature or pressure changes


Carbon Monoxide (CO)

• Colorless, odorless, poisonous gas produced by incomplete combustion of carbon in gasoline:

C + ½ O2 → CO

• Binds with hemoglobin, prevents oxygenation of blood

• Average exposure 10-30 ppm, heavy traffic 50 – 100 ppm – causes dizziness, headaches and visual aberrations

• Levels

– Air quality standard (AQS) for CO is 9 ppm (10,000 μg/m3) for 8 hours

– No physiological effects are known to appear at outdoor AQS levels

– OSHA limits long-term workplace exposure to 50 ppm

• Total amount of CO emitted is more than all other pollutants combined, only a health hazard in heavy traffic


Sulfur Oxides (SO2/SO3)

• Colorless gas with a suffocating odor at concentrations > 3 ppm

• Produced by burning fossil fuels:

S + O2 → SO2

• Coal contains as much as 6% sulfur by weight, contributes most of the 16 million tons per year

• Also natural sources (sea spray) which add about 2x as much sulfur

• Damages respiratory tract, lung tissue

• Effects are pronounced in young, old and those with existing respiratory ailments

Air Pollutants and their SourcesLondon Smog 1952 / Donora Smog 1948

• Over 4,000 deaths were attributable to the Great London Smog of December 1952

• Donora Death Fog of October 1948 • Pollutants from Steel mill, zinc smelter,

sulfuric acid plant• Results: 7,000 inhabitants fell ill, 20 died

Smog = smoke + fog


Sulfur Oxides

• Further oxidizes to SO3 which combines with water to form sulfuric acid

2SO2 + O2 → 2SO3

SO3 + H2O → H2SO4

• Acid rain!

Figure 8.8: Effects of sulfur dioxide pollution on health. The figure shows ranges of concentrations and exposure times in which (a) the number of deaths reported was above expectation (light gray), (b) significant health effects have occurred (dark gray), and (c) health effects are suspected (middle gray).


Particulates

• Natural sources: soil, volcanic ash, pollen, sea salt spray

• Man-made sources: combustion products, fly ash

• Source determination:

– PM diameter > 1 μm associated with natural source

– PM diameter < 1 μm usually from combustion


Particulates

• Affect breathing, aggravate existing cardiovascular disease, possibly damage immune system

• Fine particles remain suspended in air

• PM-10 (diameter < 10 microns) and PM-2.5 reach lower respiratory tract

e.g NH4Cl, SO4

2- / NO3- salts

Natural: forest fires, volcanoes etc.

Man-made: fossil-fuel combustion, industry

Mineral dust from weathering of rocks and soils

Ultr

a-f

ine C

oarse

Fin

e


Particulates

• Absorb trace metals + toxic organics


Air Pollution and the 3rd world

• For most developing countries, industrial development is the priority (not environment!)

Nature of air pollutants in the developing world.

NOx concentrations are comparable

Total particulates (TSP) and SOx are much higher than industrialized nationsdue to fewer emission controls


Hydrocarbons and VOC’s

• Also known as volatile organic compounds (VOC’s)

• Natural - methane from wetlands, ruminants, rice paddies, landfills, emissions of isoprene and terpenes from tress account for 85 % of all HC’s in air

• Man-made sources – gasoline, natural gas etc. are far more reactive

• For this reason often divided into methane and non-methane hydrocarbons


Nitrogen Oxides, Photochemical Smog and Ozone

• Nitrogen oxides family: NO, NO2, N2O, N2O5

• Produced by oxidation of N2 in air

• Role in the formation of photochemical smog

• Health effects: eye irritation, reduced visibility and respiratory ailments

• Combination of many different gaseous and particulate pollutants, dominated by ozone

NO2 + Light energy → NO + O

O + O2 → O3

O3 + NO → NO2 + O2

• Equilibrium

• HC’s react with O, NO and NO2 to form reactive organic radicals e.g. PAN (peroxyacetyl nitrates)

• Produce more NO2 and prevent destruction of O3


• Ozone production is stimulated by sunlight and warm temperatures (summer problem)

• Many US urban areas do not meet 8-hour O3 standard of 0.080 ppm

• Pollution varies with time of day

Figure 8.9: Variation of NOx and ozone concentrations with time of day in the Los Angeles basin.


Acid Rain

• Rain falling through atmosphere polluted by SOx / NOx produces sulfuric + nitric acids

• pH scale is a measure of hydrogen ion concentration (H+)

• Each pH unit represents a factor of 10 change in solutions acidity

• Normal rainwater is weakly acidic (5.7) due to presence of dissolved CO2 as carbonic acid

Figure 8.10: Acidity is expressed by pH, which is a logarithmic number. A 1-unit change in pH represents a change in acidity by a factor of 10.

Acid Rain


• Primarily effects Eastern US, Canada and N. Europe

• Rainfall with pH 4.0-4.5 is common in NE and SE states

Figure 8.11: Change in annual average pH of precipitation in the eastern United States between 1955 and end of twentieth century.

NADP map


• Effect on crops

• Leaches nutrients e.g. Ca/Mg/Al from soil, bleaches chlorophyll from leaves

Effects of acid rain on a forest in Europe.


• Change in distribution of pH for Adriondack Lakes between 1930s and 1970s

• All fish die at pH < 5.0

• Lakes most at risk have hard insoluble bedrock, thin surface soils with low buffering capacity

• Naturally alkaline soils are resistant to acidification

Figure 8.12: Change in pH and fish population for 200 Adirondack lakes (in New York) above 600 meters altitude between the 1930s and 1970s.

No.

lake

s

In acidified lakes the number of fish and amphibians is declining due to reactions of aluminum ions with proteins in the gills of fish and the embryo's of frogs

High aluminum concentrations do not only cause effects upon fish, but also upon birds and other animals that consume contaminated fish

Acid Deposition

Results from technological fix of one problem

(local air pollution)

What was the fix?


• Taller smokestacks emit pollutants higher into the atmosphere

• Westerly winds transport pollutants to Canada and NE US from mid-west utilities

• Solutions: burning lower sulfur fuels, emissions control technology


• Toxic metals leaching into water supplies

• Damage to structures (limestone, marble)

• Damage to vehicles (rusting)

• Decreases visibility

• Decreases productivity of fisheries, forests and farms

Cartoon


Indoor air pollution usually is a greater threat to human health than outdoor air pollution.

Why?

Indoor environments often concentrate chemical and biological contaminantse.g. weather proofing / reducing home heating and cooling costs

We spend 90% of our time indoors


• Indoor Air Pollution


• According to the American Medical Association 50% of all illness is caused or aggravated by polluted indoor air


• According to the EPA, the most dangerous indoor air pollutants in developed countries are:

– Tobacco smoke and fine particulates

– Formaldehyde

– Radioactive radon-222 gas

• The only one of these that has a recommended indoor level is radon

Health EffectsRadon Gas

Radon decay products build up in confined space –are breathed in, stick to surface of airways and emit α-particles

Highly energetic α, β particles rip through tissue causing cellular and genetic damage


0

5,000

10,000

15,000

20,000

25,000

Dea

ths

per y

ear

Low

er e

stim

ate

Drunk Driving

Drownings Fires/Burns Air Transportation

Radon

Upp

er

estim

ate

Source: U.S. EPA’s Home Buyer’s and Seller’s Guide (Radon: National Academy of Sciences, Non-radon: National Safety Council)

End

• Review

Air Quality Standards

Signs of progress?


• London and Donora were turning points

• Death is no longer the sole factor to consider

• Changes in life expectancy and quality of life are now key factors


• Clean Air Act (CAA) – begun in 1955, followed by 1963 and 1967

• Amended in 1970

• National air quality standards (NAAQS) for six pollutants linked to public health effects

SOx, NOx, PM, O3, CO and lead

Nonattainment 2007

PPG 05-01-2008


• 1990 CAA amendments dealt with:

– Acid rain – proposals to cut sulfur by installing emission control tech (scrubbers)

– Smog – reduction using alternative fuels (ethanol, methanol, natural gas, electricity)

– air toxics

– Introduced evaporative emission controls

– Mandated new gas formulations for summer months

• California enacted its own plan for ultra-low emission vehicles (ULEV) and eventually zero emission vehicles (ZEV’s)


Primary – protect human health Secondary – protects welfare

Lead recently changed by factor of 10

TSP revised in 1987 and 1997


• Overall, emissions from all sources have been in decline

• 50 % decrease since 1970, despite 200 % increase in miles travelled

EPA: Latest Findings on National Air Quality – Status of Trends through 2006


• Megacities – total population > 10 million

Figure 8.13: Comparison of ambient levels of average annual total suspended particulate (TSP) matter, sulfur dioxide, and nitrogen dioxide concentrations among selected cities, expressed in μg/m3.

Automobile Emission Control

• Carbon monoxide control

– reduced by using excess air (O2 sensor) to complete combustion

• Nitrogen oxides control

– EGR – exhaust gas recirculation, lowers peak combustion temperature

• Hydrocarbon control

– from fuel evaporation and unburnt fuel)

– captured by charcoal filter, engine re-design to eliminate ‘blow-by’ gases

• Catalytic converter

– Further reduces pollutants

– Precious metals

– HC and CO oxidized, NOx reduced

Figure 8.14: Automobile emission controls.

Automobile Emission Control

• Emission standards required by CAA Amendments

• In additional to emission controls for gasoline, decreased emissions via:

– Switch to diesel

– Gas turbine

– Hybrid

– Electric

– Mass transit

Stationary Source Air Pollution Control Systems

• Fossil fuel plants produce majority SO2 and PM

General philosophies for meeting air quality:

1. use low sulfur fuel

2. remove sulfur prior to combustion

3. remove particulates and PM after combustion

4. shift fuels or power output in response to air quality

5. dilute and disperse gases through taller stacks


• Use of very tall chimney stacks

• Wind is higher at altitude

• Dilute and disperse pollutants

Figure 8.19: Chimneys are some of the tallest structures built by humans.


• After combustion control devices

• Particulates are removed via settling chambers, cyclone or inertial chambers, electrostatic precipitators, filters and scrubbers

Settling chambers - larger particles settle out

Cyclone collector – further settling of heavier particles, removes particles > 50 microns

Figure 8.16: Inertial or cyclone collector. As the gas undergoes circular motion, the heavier particles collide with the collector’s walls and fall to the bottom, where they are collected for disposal.


Electrostatic precipitator – metal wires and plates under large negative charge. Electric field ionizes the gas molecules which stick to the positive plate

Can remove ~99 % of particles, not good for < 1 micron

Bag filter – cloth or fiberglass, 99.9 % efficient down to 0.1 microns

Figure 8.17: A before-and-after sequence showing the effect of an electrostatic precipitator on stack gas emissions from a coal-fired power plant.


Since SO2 is a gas we need different technology for its removal…

Scrubbers – gases pass through a water spray

Flue-gas desulfurization – slurries of lime or limestone or dolomite react with SO2 to form calcium and magnesium sulfate

Lime: SO2 + CaO → CaSO3

2CaSO3 + O2 → 2CaSO4

Limestone: 2SO2 + 2CaCO3 + O2 → 2CaSO4 + 2CO2

Removes 98% of sulfur dioxide

Fig. 8-20, p. 278

Figure 8.20: Schematic of a typical fossil-fuel plant, showing equipment used to remove pollutants from the flue gas or boiler exhaust.


• Reduce amount of SO2 formed before combustion

– Use low sulfur coal

– Remove sulfur from fuel prior to combustion

– Convert coal to synthetic oil or gas

• Fluidized bed combustion (FBC) removes SO2 by burning crushed coal on a moving bed of air and sand to which limestone is added

Figure 8.21: A fluidized bed combustion (FBC) unit. When air is forced up from below, the bed of ash becomes fluidized (that is, the solids “flow” like a liquid). Crushed solid fuel burning in the bed heats the ash. Limestone added to the fluidized mixture reduces the amount of SOx emitted in the flue gases.

Summary

• Major air pollutants – SO2, PM, NOx, HC’s, O3 and CO

• Photochemical smog (primarily O3) is secondary in origin and formed by the photochemical reaction of NOx and HC’s

• Pollution control devices have reduced mobile and stationary pollution

• About half of currently operating coal-fired plants were built pre-1975 and have no sulfur controls

• Debate continues over the economic and human health benefits of air pollution control

environmental physics chapter 8: air pollution copyright © 2008 by dbs

Documents