Air Pollution Outline

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Air Pollution Outline

• Meteorological Factors Affecting Air Pollution
• Acid Precipitation
• Air Pollution in Maryland

5/1/03
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REVIEW QUESTIONS
Describe how downdrafts in a severe thunderstorm
act to maintain updrafts. What is a gust
front? In a severe thunderstorm, downdrafts
spread out along the ground forcing warm, moist
surface air into the thunderstorm, thus
maintaining updrafts. A gust front is an outflow
boundary between cool the air of downdraft and
warmer surrounding air.
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• List three factors which contribute to the urban
  heat island
• 1.Urban building material such as concrete and
  asphalt
• absorb greater quantities of solar radiation than
  vegetation
• and soils do.
• City surfaces are impermeable, significantly
  reducing
• the evaporation rate
• At night, the building materials in cities
  release the
• additional heat they accumulated during the day
  and thus
• keep urban air warmer than that of outlying
  areas.

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List three steps in providing a weather
forecast. First data is collected and analyzed
on a global scale to provide a picture of
current state of the atmosphere. Next the NWS
employs a variety of techniques to establish the
future state of the atmosphere. Then the
forecast is disseminated to the public.
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Tornado formation roll cloud forms by wind shear
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Roll cloud is lifted by updrafts to form a
mesocyclone
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Mesocyclone tightens down into a tornado
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Sources and Types of Air Pollutants

• can be grouped into two categories primary and
  secondary.
• Primary pollutants are emitted directly from
  identifiable sources. They pollute the air
  immediately upon being emitted.
• Secondary pollutants are produced in the
  atmosphere when certain chemical reactions take
  place among primary pollutants.

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Meteorological factors
The solution to pollution is dilutionNOT TRUE!!!
The two most important atmospheric conditions
affecting the dispersal of pollution are (1)
the strength of the wind (2) the stability of
the air.
Boundary layer winds (winds from 1500 meters
down) mix pollutants horizontally while
convective mixing disperses pollutants away from
source regions.
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Effect of wind speed on dilution
The concentration of pollutants increases as wind
speed decreases.
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Role of atmospheric stability
Temperature profile for a surface inversion
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Role of Atmospheric Stability
Temperature-profile changes after the Sun has
heated the surface .
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Role of atmospheric stability
The vertical distance between the Earths surface
and the height to which convectional movements
extend is called the mixing depth. A deeper
mixing depth usually implies better air quality
since pollution is more dilute. Stable air
inhibits convective mixing. This leads to
shallow mixing depths, which results in
increased pollution levels. Convective mixing
is stimulated by the Sun, and therefore, mixing
depths are the deepest during the afternoon.
Likewise, mixing depths are deeper during the
summer than during the winter.
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Role of atmospheric stability
Temperature inversions produce very stable
atmospheric conditions in which mixing is
greatly reduced. There are two general types of
inversions surface inversions and inversions
aloft.
Surface inversions are the result of differential
radiative properties of the Earths surface and
the air above. The Earth is a much better
absorber and radiator of energy than air
thus, in the late morning and afternoon hours the
lower atmosphere is unstable. The opposite is
true in the evening a stable atmosphere with
little vertical mixing prevails.
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Role of Atmospheric Stability
Inversions aloft are associated with prolonged,
severe pollution episodes. These types of
inversions are caused by the sinking
air associated with the center of high pressure
systems (subsidence). As the air sinks it is
warmed adiabatically. Turbulence at the
very lowest part of the atmosphere prevents
subsidence from warming that portion of the
atmosphere. Los Angles pollution episodes as
well as those over the Mid-Atlantic region are
the result of inversions aloft associated with
strong high pressure systems.
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Role of Atmospheric Stability
Inversion Aloft
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Bermuda high
H
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Acid precipitation
The burning of fossil fuels (coal and petroleum
products), releases about 43 millions tons of
sulfur and nitrogen oxides into the atmosphere
over the United States every year.
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Acid Precipitation
Robert Angus Smith (1817-1884) was a 19th-century
Scottish chemist who investigated numerous
environmental issues. Smith did innovative
studies of air and water pollution and was one
of the few at the time to realize the importance
of finding solutions to the environmental
problems caused by urban growth. He is most
famous for his 1852 research on air pollution, in
the course of which he discovered acid rain.
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Acid Precipitation
Rain is naturally weakly acidic because CO2 from
the atmosphere dissolves in water. Unperturbed
rainwater has a pH of near 5. Precipitation
near urban areas has a much lower pH. This rain
or snow is called acid precipitation.
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Effects of Acid Precipitation
Scientific evidence is mounting that
acid-containing aerosols are harmful to human
health. It has been known for some time
that acid rain can lower the pH of lakes.
Ecosystems are very complex. Different lakes, or
different parts of a lake, can react
differently to acid precipitation. This
variation is due in large part to different types
of soil matrixes.
If the pH of a lake gets too low, the
ecosystem will no longer support much of the life
within it.
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Pollution in the Mid-Atlantic Region

• Maryland Department of the Environment
• University of Maryland
• Pennsylvania State University

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Most Unhealthy Air Quality Days Occur in the
Summer Season. Summer Weather in the
Mid-Atlantic Can be Characterized by the 4 Hs

• Hot
• Humid
• Hazy
• High Pressure
• These weather conditions occur frequently in
  mid-Atlantic summers and are often but not always
  associated with unhealthy air quality.

HAZY
HOT
Humid
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Pollutants of Concern During PoorAir Quality
Events

• Fine Particles
• Haze
• Ozone

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Ozone (O3)
O3 is a colorless gas made up of three oxygen
molecules. In the stratosphere, O3 is present in
large concentrations and protects the earth
by absorbing harmful UV radiation. Near the
surface, O3 is found in high concentrations in
industrialized areas and is harmful to human
respiratory systems and to plants and
materials. O3 is not emitted directly into the
atmosphere but is formed by a series of
reactions.
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Recipe for Ozone

• Emissions of O3
• Precursors
• A Sunny Day
• Hot Temperatures (typically in the 90s)
• Moderate or Light Surface Winds

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Fine Particles or Particulate Matter (PM)

• PM is made up of suspended particles of either
  solid or liquid pollutants.
• PM is grouped by size under 10 microns is called
  PM10, under 2.5 microns is called PM2.5.

• PM causes increased mortality and morbidity.
• Examples of PM include diesel soot, acids, dust,
  sulfates, nitrates, and organics.

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Haze

• ? Haze is a subset of PM and is primarily
  composed
• of sulfur and nitrogen compounds.
• Particles of a certain size can reflect or
  refract light,
• causing a reduction in visibility. This
  reduction
• in visibility is known as haze.
• Hazy conditions occur frequently in conjunction
  with
• severe O3 events.

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An Example of the Effects of Hazein the
Mid-Atlantic
The Great Smoky Mountains National Park
A Clear Day
A Hazy Day
Photos from www.epa.gov
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A Typical Day in a Pollution Episode
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A Day in a Pollution Episode

• The most severe episodes typically occur over
  multiple days, building up on the first day and
  tapering off on the last.
• As an introduction, a day in the middle of a
  pollution episode is discussed.
• While PM, O3 and haze events typically occur in
  conjunction with one another, we
• will focus here on an O3 event.

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Before Sunrise
In the late night and early morning hours during
a pollution episode certain effects are
commonly found O3 concentrations are at a
minimum, particularly near the urban
centers. Winds are light and variable. Haze
levels are at a maximum with visibility often
reduced to a few miles or less. These effects
are due to the development of the nocturnal
inversion.
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The Nocturnal Inversion

• On clear nights, a temperature inversion develops
  near the surface.
• - Air temperature usually decreases with height.
• An inversion is a layer of air where
  temperature
• increases with height.
• - Because the layer of air in the inversion is
  warmer than the air below it, the cooler air
  below the inversion cannot rise above it.
  Pollutants near the surface are therefore trapped
  below the inversion in the overnight hours.

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Temperature Inversion
Altitude
Temperature
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What causes the nocturnal inversion?

• While inversions can occur at various levels
• in the atmosphere (and we will see more
• examples later) and can be due to a variety
• of effects, the nocturnal inversion is caused
  by surface (or radiational) cooling.

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Nocturnal Inversion

• After sunset on clear nights, the ground surface
  cools rapidly. However, air is not a very good
  conductor of heat. As a result, only the layer
  of air in the first few hundred meters from the
  surface cools.
• The air further aloft remains warm creating what
  is called the "nocturnal inversion."

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Nocturnal Inversion
ABOVE

• O3 concentrations remains relatively high.
• Winds are moderate with some localized higher
  winds.

BELOW

• Ozone reacts with substances near to and deposits
  onto the earths surface its concentration
  virtually disappears.
• More pollution is released at the surface and is
  trapped under the inversion haze increases.

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O3 Times Series
This hourly O3 graph for a summer day near
Frederick, Maryland shows O3 concentrations
reaching a minimum in the early morning hours.
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Late Morning

• The ground heats up the air beneath the nocturnal
  inversion. This air becomes warmer than the air
  aloft, rises and mixes. The inversion layer
  disappears.
• Ozone and other pollutants above the inversion
  layer mix with the pollution under the layer
• This causes a dramatic increase in ground-level
  ozone, beginning around 10 AM

Altitude
Temperature
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Regional Scale O3
The air that is mixed downward during the late
morning and early afternoon hours is typically
high in O3 and other pollutants and
concentrations are often uniform over large
distances.
O3 concentrations along the western boundary of
the I-95 Corridor on August 17, 1999
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Regional Scale O3
In this case from July, 1999, the high elevation
monitor at Methodist Hill in southern PA is above
the nocturnal inversion. By late morning, mixing
has brought all monitors to the common regional
level.
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Afternoon

• By late morning, downward mixing of O3 leads to
  relatively uniform concentrations across the
  region.
• Local effects, related to emissions available,
  then dominate in the early afternoon hours.
• O3 is formed as UV radiation drive reactions of
  O3 precursors.

• Depending on a variety of factors, peak O3
  concentrations are reached during the mid to late
  afternoon hours.
• The highest concentrations occur downwind of the
  urban center.

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Ozone Map
On this day, winds were generally west or
southwest and highest O3 levels are found along
and east of the I-95 Corridor with lower
concentrations near the city centers.
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What modulates O3 concentrations?
The amount of O3 produced each day depends on a
variety of factors including Temperature Concent
rations of O3 and precursors mixed downward
during the late morning. Wind speeds. Local
emissions of O3 precursors Amount of available
sunlight Depth of vertical mixing
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Vertical Mixing Depth

• Just as surface-based inversions at night can
  trap pollutants near the surface, so higher level
  inversions can form in the afternoon hours and
  prevent pollution from mixing vertically.

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After Sunset

• If meteorological conditions remain the same, the
  temperature inversion forms again after dark as
  the ground cools faster than the air above.
• Ozone concentration above the inversion comes to
  equilibrium with other pollutants and then
  remains at a constant, relatively high level.
• Ozone trapped under the inversion reacts with
  other pollutants, particles and the surface the
  ozone concentration diminishes.

Ozone concentration remaining constant
Altitude
Temperature Inversion
Ozone concentration diminishing
Temperature
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A Multi-Day Pollution Episode
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Assume we start at noon on Day 1 with a
relatively clean air mass
Warmer gtgt
ltlt Cooler
Temperature
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As the sun sets, the surface begins to cool and a
transition takes place
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The surface cooling continues overnight
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On Day 2, the sun rises and the nocturnal
inversion begins to erode
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By noon, the nocturnal inversion is gone and any
air pollution that was aloft mixes down
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The process repeats now there is more O3
(Sunset Day 2)
Temperature
Warmer gtgt
ltlt Cooler
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Midnight Day 2
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Sunrise Day 3...
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By Noon on Day 3, Local Emissions and High
Background O3 Combine
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The End of a High Ozone Episode

• An ozone episode usually ends with the arrival of
  a clean air mass
• This can occur with a cold front or other
  low-pressure system like a tropical storm.
• An episode may also end prior to the passage of a
  cold front if widespread thunderstorms develop
  ahead of the front.

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Thunderstorms
This is an example of a strong squall line
bringing a high O3 event to a end.

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Longer Pollution Episodes
While in this example, the episode lasted three
days, it is not uncommon for high O3 events to
persist for longer periods. This episode in
July, 1997 lasted 7 days in the Baltimore
metropolitan area.
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Exceedances of the 1-Hour O3 Standard
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Color-Coded Air Quality Forecasts