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Unit: Aviation Environment - Air
Introduction to Environmental Science & Technology – 3rd
Edition
By
Gilbert M. Masters & Wendel P. Ela
Contents
Chapter 7: Air Pollution .................................................................................................. 2
7.1 – 7.2: Introduction & Overview of Emissions ........................................................ 2
7.3: The Clean Air Act ................................................................................................ 3
7.5: Criteria Pollutants ................................................................................................. 7
Airport Systems: Planning, Design, and Management.................................................... 13
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Unit: Aviation Environment - Air
Chapter 7: Air Pollution
7.1 – 7.2: Introduction & Overview of Emissions
Substances that are emitted directly into the atmosphere are called primary
pollutants. Other substances that are created by various physical processes and chemical
reactions that take place in the atmosphere are called secondary pollutants. For example,
nitrogen oxides and hydrocarbons emitted when fuels are burned are primary pollutants,
but the ozone that is created when those chemicals react with each other in the
atmosphere is a secondary pollutant.
Most primary pollutants enter the atmosphere as a result of combustion,
evaporation, or grinding and abrasion. Automobile exhaust emissions and power plant
stack gases are created during combustion; volatile substances such as gasoline, paints,
and cleaning fluids enter the atmosphere by evaporation; whereas dust kicked up when
land is plowed and asbestos fibers that flake off of pipe insulation are examples of
grinding and abrasion.
CH4 +2 O2 → CO2 + 2 H2O
(7.1)
Equation 7.1 is a complete combustion of a pure hydrocarbon fuel; methane
(CH4). The products of combustion are simple carbon dioxide and water. If the
temperature of combustion isn’t high enough, or there isn’t enough oxygen available, or
if the fuel isn’t given enough time to burn completely, then the fuel will not be
completely oxidized, and some of the carbon will be released as carbon monoxide instead
of CO2. Additionally, some of the fuel will not be completely burned, so there will be
emissions of various partially combusted hydrocarbon. When these are factored into the
combustion equation, the following results:
CH4 + O2 → mostly (CO2 + 2 H2O) + traces of [CO + (HC)]
(7.2)
When the temperature of combustion is high enough, some of the nitrogen in the
air (air is 78% N2 and 21% O2) reacts with the oxygen in the air to form various nitrogen
oxides (NOx). Since NOx is formed when combustion temperatures are high, it is referred
to as thermal NOx.
Air (N2 + O2) + Heat → Thermal NOx
(7.3)
In reality, most fuels are not pure, and have a number of other elements in them
such as nitrogen, sulfur, lead, mercury, and other unburnable materials called ash.
Burning fuel with these “impurities” in them releases additional NOx (called fuel
NOx), oxides of sulfur (SOx), lead (Pb), mercury (Hg), more particulate matter, and ash.
Combining the effects of imcomplete combustion, combustion in air, and combustion of
fuels that are not pure hydrocarbons yields the following qualitative description of
combustion:
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Fuel (H, C, S, N, Pb, Hg, ash) + Air (N2 + O2) → Emissions (CO2, H2O, CO, NOx,
SOx, Pb, particulates) + ash
(7.4)
7.3: The Clean Air Act
Efforts to address the nation’s air pollution problem began with the passage of the
Air Pollution Control Act of 1955. It provided funding only for research and not control,
but it was an important milestone because it opened the door to federal participation in
efforts to deal with air pollution. This was then followed by a series of legislative actions
by Congress that included the Clean Air Act Amendments of 1963, 1966, 1970, 1977,
and 1990, all of which are often grouped together and called The Clean Air Act (CAA).
The majority of the CAA was established in the 1970 amendments. In those
amendments, the US EPA was required to establish National Ambient Air Quality
Standards (NAAQS) and states were required to submit State Implementation Plans
(SIPs) that would show how they would meet those standards. Additionally, the Act
required New Source Performance Standards (NSPS) to be established that would limit
emissions from certain specific types of industrial plants and from motor vehicles.
Air Quality & Emission Standards
NAAQS have been established by the EPA at two levels: primary and secondary.
Primary standards are required to be set at levels that will protect public health and
include an “adequate margin of safety,” regardless of whether the standards are
economically or technologically achievable. Secondary air quality standards are meant to
be even more stringent than primary standards. Secondary standards are established to
protect public welfare.
NAAQS now exist for the six criteria pollutants (to be discussed later). The CAA
requires that the list of criteria pollutants be reviewed periodically and that standards be
adjusted according to the latest scientific information. Originally, the particulate standard
did not refer to the size of particulates, but as of 1987, it was modified to include only
particles with an aerodynamic diameter of less than or equal to 10 micrometers (PM10). In
1997, an additional fine particles category for particulates smaller than 2.5 microns was
added (PM2.5).
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EXAMPLE 7.1 – Air Quality Standards as Mass per Unit Volume
Table 7.1 shows that California’s air quality one-hour standard for
NO2 is 0.25 ppm. Express this as a concentration by mass at 25°C and
1 atm pressure.
Solution The Ideal Gas Law is used to show that 1 mol of an ideal gas
at 1 atm and 25°C occupies a volume of 24.465 L (24.465  10-3 m3).
g
MW  1  14  2  16  46
mol
therefore
g
0.25  10 6 m 3  NO2  / m 3 air   46
mol
NO2  
3
m
24.465  10 3
mol
g
g
 470  10 6 3  470 3
m
m
Notice that parts per million by volume (ppm) is a dimensionless
volume fraction, independent of temperature and pressure, so it is the
preferred measure
In addition to establishing NAAQS, the CAA also requires the EPA to establish
emission standards for mobile sources such as cars and trucks. The 1970 amendments to
the CAA gave the auto industry a five-year deadline to achieve a 90 percent reduction in
emission from new cars. The EPA is also required to establish emissions standards for
certain large stationary sources such as fossil-fuel-fired power plants, incinerators,
Portland cement plants, nitric acid plants, petroleum refineries, sewage treatment plants,
and smelters of various sorts.
The Clean Air Act Amendment of 1977
The 1996 Amendments had to deal with two important questions. First, what
measures should be taken in nonattainment areas that were not meeting the standards?
Second, should air quality in regions where the air is cleaner than the standards be
allowed to degrade toward the standards, and if so, by how much?
For nonattainment areas, the 1970 Act appeared to prohibit any increase in
emissions whatsoever, which would have eliminated industrial expansion and severely
curtailed local economic growth. To counter this, the EPA adopted a policy of emission
offsets. To receive a construction permit, a major new source of pollution in a
nonattainment area must first find ways to reduce emissions from existing sources. The
reductions, or offsets, must exceed the anticipated emissions from the new source. The
net effect of this offset policy is that progress is made toward meeting air quality
standards in spite of new emissions sources being added to the airshed.
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Offsets can be obtained in a number of ways. For example, emissions from
existing sources in the area might be reduced by installing better emission controls on
equipment that may or may not be owned by the permit seeker. In some cases, a permit
seeker may simply buy out existing emission sources and shut them down. Emission
offsets can be “banked” for future use, or they can be sold or traded to other companies
for whatever the market will bear. In addition to offsets, new sources in nonattainment
areas must use emission controls that yield the Lowest Achievable Emission Rate
(LAER) for the particular process. LAER technology is based on the most stringent
emission rate achieved in practice by similar sources, regardless of the economic cost or
energy impacts.
The 1970 Amendments were not specific about regions that were cleaner than
ambient standards required and in fact appeared to allow air quality to deteriorate to those
standards. The 1977 Amendments settled the issue of whether or not this would be
allowed by establishing the concept of prevention of significant deterioration (PSD) in
attainment areas. Attainment areas are put into one of three classes and the amount of
deterioration allowed is determined by the class. In PSD areas, Best Available Control
Technology (BACT) is required on major new sources. BACT is less stringent than
LAER, as it does allow consideration of economic, energy, and environmental impacts of
the technology, but it can be more strict than allowed by NSPS. In all PSD areas, the
allowable increments of air quality degradation are constrained by the NAAQS. That is,
in no circumstance would air quality be allowed to deteriorate to the point where the area
is no longer in compliance with ambient air quality standards.
The Clean Air Act Amendment of 1990
The CAA Amendment of 1990 significantly strengthened the EPA’s efforts to assure
healthful air in the US and broadened its scope to include control of pollutants that affect
a global problem – stratospheric ozone depletion. Principle changes int eh Act included
the following:
 Creation of a new Acid Rain Program (Title IV)
 New requirements for nonattainment areas (Title I)
 Tightened automobile emission standards and new fuel requirements (Title II)
 New toxic air pollution controls (Title I)
 Phase-out schedule for ozone-depleting substances (Title VI)
The Acid Rain Program
One of the most important shortcomings of the Clean Air Act before the 1990
Amendments was its inability to deal effectively with acid rain (or, more correctly, acid
deposition). The 1990 Amendments approached this problem in a new and creative way
by creating a market-based “cap-and-trade” system to control emissions.
The cap-and-trade system is based on a more flexible allowance system, where
one allowance authorizes the owner to emit 1 ton of SO2. Large coal-fired power plants
are not allowed to emit any more tons of SO2 than the number of allowances they own. If
insufficient allowances are owned to cover emissions, the owners are subject to an
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excess-emissions penalty. The intent is for these allowances to be bought and sold or
banked in the same way that other commodities are traded. New sources that have no
allowances have to purchase them from existing sources or from annual EPA auctions.
The idea, of course, is that major sources will find the least-expensive ways to cut their
emissions and then sell some of their allowances to other who can’t reduce their
emissions as cheaply. The goal is at least-cost emission limitations that allows sources the
flexibility they need to make the most cost-effective choices. By ratcheting down the
number of allowances that the EPA issues reduction in SO2 emissions from 1980 levels
by 2010.
The cap-and-trade system went into effect in 1995, so we now have more than a
decade of results to use to evaluate how well it is working. Between 1993 and 2002, the
EPA’s Acid Rain Program resulted in an SO2 emission reduction of over 30%. And it is
well on its way to meeting its 2010 goal of cutting emissions in half compared to 1980
emissions.
Success with the SO2 cap-and-trade program led to a similar effort to control NOx
emissions called the NOx Budget Trading Program (NBP). The program became
operational in 2003, and early results are promising. In 2005, out of the 2,570 NO x
emitters affected under NBP, all but 3 sources were in compliance, and emissions were
11% lower than 2004 and 57% lower than in 2000 before implementation of the NBP.
By switching to lower-sulfur coal (which also contains less mercury) and
installing scrubbers, not only have SOx and NOx been reduced, as Title IV intended, but
also highly toxic mercury emissions have been cut as well as ambient levels of PM2.5 and
ozone. The combined annual health and welfare benefits in 2010 of the over $120 billion
(over 90% of which is attributable to protected reductions in PM2.5), whereas the annual
costs of SOx and NOx controls is estimated at only $3 billion.
National Emissions Standards for Hazardous Air Pollutants (NESHAP)
Earlier versions of the Clean Air Act established NESHAP, and though it, a small list of
pollutants (asbestos, benzene, beryllium, coke oven emissions, inorganic, arsenic,
mercury, radionuclides, and vinyl chloride) were controlled. The 1990 Amendments
extend that ist to include 189 pollutants listed in the legislation. The list can be changed
by the EPA, and if there are objections to any additions, the burden of proof is on the
petitioner, who must show that the chemical may not reasonably be anticipated to cause
adverse human health or environmental effects. Emission standards for chemicals on the
list are based on the Maximum Achievable Control Technology (MACT).
Ozone Depleting Substances
Finally, Title VI of the 1990 Amendments was written to protect the stratospheric
ozone layer by phasing out ozone-depleting substances such as chlorofluorocarbons
(CFCc). The phase-out was mandated to be at least as strict as that required by the
international treaty known as the Montreal Protocol.
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Unit: Aviation Environment - Air
7.5: Criteria Pollutants
Most of the monitoring of emissions, concentrations, and effects of air pollution
has been directed toward the six criteria pollutants: ground-level ozone (O3), carbon
monoxide (CO), sulfur dioxide (SO2), small particulates (PM10 and now PM2.5), nitrogen
dioxide (NO2), and lead (Pb).
Carbon Monoxide
Over two-thirds of the mass of all the pollution emissions in the US is the
colorless, odorless, tasteless, poisonous gas, carbon monoxide. It is produced when
carbonaceous fuels are burned under less than ideal conditions. Incomplete combustion,
yielding CO instead of CO2, results when any of the following variables are not kept
sufficiently high: (1) oxygen supply, (2) combustion temperature, (3) gas residence time
at high temperature, and (4) combustion chamber turbulence. These parameters are
generally under much tighter control in stationary sources such as power plants than in
motor vehicles, and CO emissions are correspondingly less.
Over 80% of total CO emissions are from the transportation sector, and almost all
of the CO in urban areas comes from motor vehicles. Hourly atmospheric concentrations
of CO over our cities often reflect city driving patterns: peaks occur on weekdays during
the morning and late afternoon rush hours, while on weekends there is typically but one
lower peak in the late afternoon. Personal exposure to CO is very much determined by
the proximity of motor vehicle traffic, with some occupational groups such as cab
drivers, police, and parking lot attendants receiving far higher tan average doses.
The usual way to express the amount of carboxlhemoglobin in the blood is as a
percentage of the saturation level, %COHb. The amount of COHb formed in the blood is
related to the CO concentration and the length of time exposed, as suggested in the
following:
%COHb = β(1 - eγt)[CO]
7.6)
Where
%COHb = carboxyhemoglobin as a percent of saturation
[CO] = carbon monoxide concentration in ppm
γ = 0.402/hr
β = 0.15% / ppm CO
t = exposure time in hours
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EXAMPLE 7.2 Federal Standard for CO
Estimate the %COHb expected for a one-hour exposure to 35 ppm (the federal
standard)
Solution From (7.6):
%COHb = 0.15%/ppm[1-exp(0.402/hr  1 hr)][35 ppm] = 1.7%
Oxides of Nitrogen
Although 7 oxides of nitrogen are known to occur, NO, NO2, NO3, N2O, N2O3,
N2O4, and N2O5 – then only two that are important air pollutants are nitric oxide (NO)
and nitrogen dioxide (NO2). There are two sources of nitrogen oxides when fossil fuels
are burned. Thermal NOx is created when nitrogen and oxygen in the combustion air are
heated to a high enough temperature about (1000K) to oxidize the nitrogen. Fuel NOx
results from the oxidation of nitrogen compounds that are chemically bound in the fuel
molecules themselves.
About 95% of anthropogenic emissions of NOx are in the form of NO, which is a
colorless gas that has no known adverse health effects at concentrations found in the
atmosphere. However it does oxidize to NO2 which can irritate the lungs, cause
bronchitis and pneumonia, and lower resistance to respiratory infections. In addition to
adverse human effects, it has other environmental effects. It reacts with the hydroxyl
radical (OH) in the atmosphere to form nitric acid (HNO3) that corrodes metal surfaces
and contributes to the acid rain problem. It also can cause damage to terrestrial plants.
Volatile Organic Compounds (VOCs)
This class of compounds consists of volatile compounds that enter the atmosphere
when solvents, fuels, and other organics evaporate, along with unburned and partially
burned hydrocarbons that are emitted from tailpipes and smoke stacks when fossil fuels
are not completely combusted.
The transportation sector is responsible for almost half of anthropogenic
(manmade, excluding wildfires and prescribed burnings) VOC emissions, and despite
rapidly rising miles driven in motor vehicles, emissions decreased by 39% from 1993 to
2003. Industrial sources account for almost all of the other half of VOC emissions, with
much of that again being caused by vaporization of hydrocarbons. Less than 2% of VOCs
result from fossil-fuel combustion in power plants and industrial boilers.
Particulate Matter
Atmospheric particulate matter consists of any dispersed matter, solid or liquid, in
which the individual aggregates range from molecular clusters of 0.005 micrometers
diameter to coarse particles up to about 100 micrometers (roughly the size of a human
hair). As a category of criteria pollutant, particulate matter is extremely diverse and
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complex since size and chemical composition, as well as atmospheric concentration, are
important characteristics.
A number of terms are used to categorize particulates, depending on their size and
phase (liquid or solid). The most general term is aerosol, which applies to any tiny
particles, dispersed in the atmosphere. Solid particles are called dusts if they are caused
by grinding or crushing operations. Solid particles are called fumes if they are formed
when vapors condense. Liquid particles may be called mist, or more loosely, fog. Smoke
and soot are terms used to describe particles composed primarily of carbon that result
from incomplete combustion. Smog is a term that was derived from smoke and fog,
originally referring to particulate matter but now describing air pollution in general.
Particulate matter (PM) can be emitted directly as carbonaceous soot particles
from incomplete combustion, or it can be formed in the atmosphere, as, for example,
when gaseous SO2 and NOx emissions are transformed into liquid droplets of sulfates
(sulfuric acid) and nitrates (nitric acid). In the eastern part of the US, with its greater
reliance on coal-fired power plants, sulfates (and associated ammonium) are the most
important components of PM, whereas in California and other areas of the west, carbon
(soot) and nitrates dominate.
Size and Chemical Composition
The original NAAQS for particulates did not take size into account. Larger
particles could dominate the mass per unit volume measure but be unimportant in terms
of human health risk. In 1987, however, the PM10 standard was introduced, and in 1997,
the PM2.5 standard was added. Particles smaller than 2.5 µm (i.e., PM2.5) are referred to as
fine particles, whereas those between 2.5 µm and 10 µm are the coarse fraction of PM10.
Coarse particles tend to settle quickly, so their spatial impact is limited to areas near their
source, but fine particles have longer atmospheric lifetimes and are capable of traveling
vast distances.
Although particles may have very irregular shapes, their size can be described by
an equivalent aerodynamic diameter determined by comparing them with perfect spheres
having the same settling velocity. The particles of most interest have aerodynamic
diameters in the range of 0.1 µm to 10 µm (roughly the size of bacteria). Particles smaller
than these undergo random (Brownian) motion and, through coagulation, generally grow
to sizes larger than 0.1 µm. Particles larger than 10 µm settle quickly, a 10 µm particle,
for example, has a settling velocity of approximately 20 centimeters per minute.
We can use a fairly simple analysis to calculate the settling velocity of a spherical
particle. When such a particle reaches its terminal velocity, the gravitational force pulling
it down is balanced by a drag force that we can estimate. For particle that are larger than
about 1 µm, with density much greater than air, we can use a simplified version of
Stoke’s Law to approximate the drag force:
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Gravitational force = drag force

mg  d 3 g  3vd
6
2
d g
v
18
Where:
m = mass of the particle (g)
g = gravitational acceleration = 9.80 m/s2
d = particle diameter (m)
ρ = particle density (g/m3)
η = viscosity of air = 0.0172 g/m  s
v = settling velocity (m/sec)
EXAMPLE 7.4 Settling Velocity of a Spherical Particle
Find the settling velocity of a spherical droplet of water with diameter 2 µm.
Using (7.24) with the density of water equal to 106g/m3 gives
2
g 
m
2  10  6 m 10 6 3  9.8 2 
2
d g
m 
s 

v

g 
18

18 0.0172 s 
m 

m
v  1.27  10  4
s
Which is about 0.5 m/hr.
Solution


Oxides of Sulfur
About 86% of the 15 million tons per year of anthropogenic sulfur oxide
emissions are the result of fossil fuel combustion in stationary sources, and most of that is
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emitted by coal-fired power plants. Only about 5% comes from highway vehicles. The
only significant noncombustion sources of sulfur emissions are associated with petroleum
refining, copper smelting, and cement manufacture.
All fossil fuels as they are extracted from the ground contain some sulfur. Coal, which
has the most, typically contains from 1 to 6% sulfur. About half of that is organic sulfur
that is chemically bound to the coal. The other half is simply physically trapped in the
noncarbon portion of coal, and much of that half can be removed by pulverizing and
washing the coal before combustion. The amount of sulfur in petroleum tends to be less
than a few percent, and if refined, almost all of that sulfur is removed during processing.
Gasoline, for example, has much less than 1 ppm sulfur. Natural gas as it leaves the
wellhead contains a considerable amount of sulfur in the form of highly toxic hydrogen
sulfide (H2S) that must be removed before the gas can be used. After natural gas is
cleaned, however, negligible amounts of sulfur remain, which makes it a highly desirable
replacement fuel for coal.
When these fuels are burned, the sulfur is released mostly as sulfur dioxide (SO2)
but also with small amounts of sulfur trioxide (SO3). Sulfur dioxide, once released, can
convert to SO3 in a series of reactions, which, once again, involve a free radical such as
OH˙:
SO2 + OH˙→HOSO2˙
(7.25)
HOSO2˙ + O2 →SO3 +HO2˙
(7.26)
The HO2˙ radical can the react with NO to return the initial OH2˙ . Sulfur trioxide reacts
very quickly with H2O to form sulfuric acid, which is the principle cause of acid rain.
Lead
Most lead emissions in the past were from motor vehicles burning gasoline
containing the antiknock additive, tetraethyllead Pb(C2H5)4.
In the US (and just a handful of other developed countries), almost all lead
emissions from gasoline have been eliminated so that by 1993, per capita emissions of
lead in the US were only 3% of the world average. The decrease in the US was originally
motivated by the Clean Air Act Amendments of 1970 that dictated a 90% drop in CO,
NOx, and hydrocarbon emissions from automobiles. The auto industry chose to use
catalytic converters as their principal emission control system for those three pollutants.
Catalytic converters, it turns out, are quickly rendered ineffective when exposed to lead,
so beginning in the late 1970s, many cars were designed to only burn unleaded fuels. By
the mid 1990s, after virtually all cars in the US had catalytic converters, leaded gasoline
was completely phased out. As a result, total US lead emissions decreased 93% between
1982 and 2002 with the major remaining sources being industrial processes (particularly
metal-processing plants) and leaded fuels used for aviation and nonroad vehicles.
Unfortunately, in most of the rest of the world, leaded gasoline is still the predominant
fuel and lead emissions remain very high.
Lead is emitted into the atmosphere primarily in the form of inorganic
particulates. Much of this is removed from the atmosphere by settling in the immediate
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vicinity of the source. Airborne lead may affect human populations by direct inhalation,
in which case people living nearest to major sources such as highways or metalsprocessing plants are at greatest risk. Despite the elimination of most lead emissions from
motor vehicles, the soil around highways is still heavily contaminated, and it can become
airborne when disturbed. It also can also be tracked into homes where it may end up
embedded in carpeting ready to become airborne once again. Wiping your feet on a
doormat is one easy and effective way to help reduce that hazard.
Inhalation of lead can occur indoors as well as outdoors. A major indoor source of
lead is chipped and flaking particles of lead-based paints that were commonly used in the
past. Paint containing Pb3(CO3)2(OH)2 was widely used in white faces. Those substances
are no longer allowed for paints used indoors, but they can be used for exterior surfaces.
When those paints chip, peel, or are sanded, lead particles become dusts that are easily
ingested or inhaled. Chips of leaded paint are somewhat sweet and are all too often eate
by children living in older homes.
Although most human exposure to lead is from inhalation, it can also be ingested
after airborne lead is deposited onto soil, water, and food crops such as leafy vegetables
and fruits. It also can be ingested when water systems are contaminated. Lead once was
used to make water pipes, but those uses have been banned in new systems in the US.
Lead can still leach out of those older water systems, especially if the water is acidic or
particularly soft. For such systems, running the water from a tap for a minute before
drinking it is highly recommended.
Lead poisoning can cause aggressive, hostile, and destructive behavioral changes,
as well as learning disabilities, seizures, and severe and permanent brain damage, and
even death. Measurements in actual communities suggest that an increase in airborne lead
concentration of 1 µg/m3 (the NAAQS is 1.5 µg/m3) results in an increase of about 1 to 2
µg per deciliter (µg/dL) in human blood levels. Blood levels of 10 to 20 µg/dL are
associated with reduced intelligence and detrimental effects on growth and development
of children, leading the US Public Health Service to label lead as the greatest
environmental health threat to our children. As lead was phased out of gasoline in the US,
the average concentration of lead in blood dropped from 16 to 3 µg/dL, which makes it
one of the most successful of all environmental achievements.
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Unit: Aviation Environment - Air
Airport Systems: Planning, Design, and Management
By Richard de Neufville & Amedeo Odoni
Chapter 6: Environmental Impacts
6.4: Air Quality and Mitigation of Air Pollution
Aircraft engines burn fuel, thus producing emissions with potential effects on
global climate change and on local air pollution. This section focuses on the latter effects,
since contributions to local air pollution are another important source of friction between
airport operators and neighboring communities. Concerns about the relationship between
air quality and health have been growing over the years, thus increasingly making air
pollution the focus of environmental controversies involving airports. In public hearings
in 2001 about the disputed proposed Runway 14/32 at Boston/Logan, more comments
and complaints were filed about potential effects on air quality than about noise. This was
a sharp reversal from what had been the case in earlier years. This may be a problematic
development for airport/community relations, as the long-term health effects of air
pollutants emanating from aircraft are certainly difficult to measure and so far poorly
understood by medical science.
Major airports can indeed be significant source of air pollutants in metropolitan
areas because of emissions from landing, departing, and taxiing aircraft and from airportrelated concentrations of automobiles and other ground traffic. About 0.5-1% of the
exhaust emitted from jet engines consists of some of the air pollutants that are of primary
concern in determining local air quality, a combination of nitrogen oxides (NOx),
hydrocarbons (HC), volatile organic chemicals VOCs), carbon monoxide (CO), sulfur
oxides (SOx), and trace chemicals species, and carbon-based soot particles. The
concentrations of nitrogen oxides, hydrocarbons, and carbon monoxide may be
particularly high near busy commercial airports, with the ground operations of aircraft
and other vehicles being major contributors. In addition, soot emissions may add to
ambient particulate levels, while sulfur oxide emissions from aircraft may be low-level
local contributors to acid rain. As an indication of the magnitudes involved, it is
estimated that New York/Kennedy and New York/LaGuardia generate about 1900 and
1500 tons per year respectively, of nitrogen oxide emissions (which are heavy
contributors to smog) and VOCs. They thus ranked as the first and fourth sources of thee
pollutants in the Metropolitan New York Area – the others in the top five being two large
power plants and the Hempstead incinerator. Figure 6-6 shows a comparison made by the
US EPA between 1990 airport contributions to nitrogen oxide levels in 10 metropolitan
areas and those projected for 2010. In every case, the EPA expected the contribution of
airports to more than double by 2010.
International and national efforts concerning aviations’s role in global climate
issues also have important implications for local air quality. The ICAO’s Committee on
Aviation Environmental Protection (CAEP), which has been charged with studying
technology, operational, and market-based opportunities for reducing both the global and
local impacts of aircraft engine emissions, has considerable international influence.
13
SYST460/560 – Fall 2009
Instructor: Dr. Lance Sherry, lsherry@gmu.edu
George Mason University
Center for Air Transportation Systems Research
http://catsr.ite.gmu.edu
Unit: Aviation Environment - Air
Several national civil aviation organizations are working closely with CAEP on these
objectives.
Fig. 6-6. Airport contributions to nitrogen oxide levels
The ICAO and national standards for emissions certification of aircraft engines
currently focus on impacts near airports. They establish limits, expresses in terms of mass
of emissions per unit of engine thrust, on nitric oxide, carbon monoxide, unburned
hydrocarbons, and smoke for a reference landing and takeoff (LTO) cycle below 914 m
(3000 ft) of altitude above the airport. Experts currently believe that aircraft above this
altitude do not have any discernible effect at ground level near the airport.
Legislation in several developed countries require environmental impact
statements on proposed airport projects to deal in depth with air quality impacts.
Estimation of these impacts typically uses computer databases and models. For example,
in the US, a two-step estimation process is used to compute:
1. Total emissions from all aircraft operations, ground service equipment, motor
vehicles, and fuel storage and transfer facilities from appropriate local databases.
Estimates must include CO, NOx, HC, VOC, and particulate total emissions and local
concentrations, including odor-causing hydrocarbon emissions.
2. Ambient pollutant concentrations at various locations inside and near the airport, with
a computer-based model. In the US, since 1998 the FAA mandates the use of its
Emissions and Dispersion Modeling System (EDMS) The Industrial Source Complex
(ISC) model used by the US EPA is also sometimes applied internationally.
National or local authorities are adopting various mitigation measures in efforts to
preserve or improve air quality near airports. As in the case of noise, these measures
range from mild to severe interventions. The most important among them include the
following:
SYST460/560 – Fall 2009
Instructor: Dr. Lance Sherry, lsherry@gmu.edu
14
George Mason University
Center for Air Transportation Systems Research
http://catsr.ite.gmu.edu
Unit: Aviation Environment - Air
1. Extensive air quality monitoring. For example, Aeroports de Paris (ADP)
measures air pollution at 80 locations in the Paris region, analyzes the levels of
CO, NO, NO2, SO2, O3, and HC and reports results and trends on a quarterly
basis. Similarly, Manchester (UK) monitors air quality at and around the airport
and publishes the results
2. Aircraft operations. Possibilities include towing of aircraft between gates and
maintenance areas, as is done at San Francisco and at Zurich; limiting the use of
auxiliary power units (APU) to certain times of the day (Copenhagen, Zurich);
provision of central power via passenger buildings, to reduce need for APUs
(many airports in the US, including most renovated passengers buildings); placing
restrictions on the times when engines can be run up (Copenhagen prohibits runups between 23:00 and 05:00 local time); and airfield design – or airfield design
change – aimed at reducing taxiing distances (as in the case of many midfield
passenger terminals).
3. Reducing delays while taxiing and idling. Airport delays contribute to emission, if
these delays are suffered with engines running. Extensive observations have
shown that the maximum departure rate at Boston/Logan is reached when nine
aircraft are taxiing out for departure. Having more than nine aircraft on the
outbound taxiway system does not increase that departure rate. Thus, emissions
(and noise impacts) can be reduced, without any adverse effects on runway
capacity, through the simple expedient of not allowing additional aircraft to begin
taxi-out, as long as nine or more aircraft are already on the outbound taxiway
system. The critical threshold value (nine in the case of Boston/Logan) will vary,
of course, from airport to airport.
4. Emissions-based landing fees. A few airports impose an emissions-based
surcharge to encourage use of “cleaner” engines by airlines. The first use of this
approach has been Zurich. In 1997 it reduced all landing fees by 5% and started
recovering the lost amount (5% of the total landing fees collected previously)
through surcharges based on the emissions characteristics of the different types of
aircraft. Specifically, Zurich classified aircraft into five categories, with Category
1 being the worst emissions performers, which pay a 40% surcharge on their
(reduced) landing fee, and Category 5 being the best. Geneva also adopted the
same approach in response to Swiss federal air quality legislation. Several other
airports reportedly were considering emissions-based charges at the beginning of
the 21st century.
SYST460/560 – Fall 2009
Instructor: Dr. Lance Sherry, lsherry@gmu.edu
15