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Air Pollution Outline
•Meteorological Factors Affecting Air Pollution
•Acid Precipitation
•Air Pollution in Maryland
5/1/03
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.
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.
2. City surfaces are impermeable, significantly reducing
the evaporation rate
3 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.
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.
Tornado formation: roll cloud
forms by wind shear
Roll cloud is lifted by updrafts to
form a mesocyclone
Mesocyclone tightens down into
a tornado
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.
Meteorological factors
The solution to pollution is dilution…NOT 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.
Effect of wind speed on dilution
The concentration of pollutants increases as wind speed
decreases.
Role of atmospheric stability
Temperature profile for a surface inversion
Role of Atmospheric Stability
Temperature-profile changes after the Sun has
heated the surface .
Role of atmospheric stability
The vertical distance between the Earth’s 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.
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 Earth’s 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.
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.
Role of Atmospheric Stability
Inversion Aloft
Bermuda high
H
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.
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.
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.
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.
Pollution in the Mid-Atlantic
Region
Maryland Department of the Environment
University of Maryland
Pennsylvania State University
Most Unhealthy Air Quality Days Occur in the
Summer Season. Summer Weather in the MidAtlantic Can be Characterized by the “4 H’s”
•
•
•
•
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.
Pollutants of Concern During Poor
Air Quality Events
• Fine Particles
• Haze
• Ozone
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.
Recipe for Ozone
• Emissions of O3
“Precursors”
• A Sunny Day
• Hot Temperatures
(typically in the 90’s)
• Moderate or Light
Surface Winds
Fine Particles
or Particulate Matter (PM)
• PM is made up of
• PM causes increased
suspended particles of
mortality and
either solid or liquid
morbidity.
pollutants.
• Examples of PM
• PM is grouped by size:
include diesel soot,
under 10 microns is
acids, dust, sulfates,
called PM10, under 2.5
nitrates, and organics.
microns is called
PM2.5.
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.
An Example of the Effects of Haze
in the Mid-Atlantic
The Great Smoky Mountains National Park
A Clear Day
Photos from www.epa.gov
A Hazy Day
A Typical Day in a Pollution
Episode
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.
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.
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.
Altitude
Temperature Inversion
Temperature
Inversion
Pollution trapped
below inversion
Temperature
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.
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."
ABOVE
• O3 concentrations remains relatively high.
• Winds are moderate with some localized higher
winds.
Nocturnal Inversion
BELOW
•Ozone reacts with substances near to and deposits onto
the earth’s surface – its concentration virtually
disappears.
•More pollution is released at the surface and is trapped
under the inversion – haze increases.
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.
• 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
Late Morning
Temperature
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
Regional Scale O3
140
120
Ozone (ppbv)
100
So. Carroll
Frederick
Long Park
Ashburn
Little Buffalo
Methodist Hill
80
60
40
20
0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Time (EST)
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.
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.
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.
What modulates O3 concentrations?
The amount of O3 produced each day depends
on a variety of factors including:
Temperature
Concentrations 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
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.
• 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.
Altitude
After Sunset
Ozone concentration
remaining constant
Ozone concentration
diminishing
Temperature
Temperature
Inversion
A Multi-Day Pollution Episode
Assume we start at noon on “Day 1”
with a relatively clean air mass
Wind
Elev (ft)
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
As the sun sets, the surface begins to
cool and a transition takes place
Elev (ft)
Wind
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
The surface cooling continues overnight
Elev (ft)
Wind
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
On Day 2, the sun rises and the
nocturnal inversion begins to erode
Elev (ft)
Wind
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
By noon, the nocturnal inversion is
gone and any air pollution that was
aloft mixes down
Elev (ft)
Wind
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
The process repeats – now there is
more O3 (Sunset Day 2)
Elev (ft)
Wind
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
Midnight Day 2
Elev (ft)
Wind
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
Sunrise Day 3...
Wind
Elev (ft)
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
By Noon on Day 3, Local Emissions and
High “Background” O3 Combine
Wind
Elev (ft)
6,500
4,000
O3 Profile
2,000
1,000
<< Cooler
Temperature
Warmer >>
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 lowpressure 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.
Thunderstorms
This is an example of a strong
squall line bringing a high O3
event to a end.
Longer Pollution Episodes
This episode in July, 1997
lasted 7 days in the
Baltimore metropolitan
area.
200
180
160
Ozone (ppbv)
While in this example, the
episode lasted three days, it
is not uncommon for high
O3 events to persist for
longer periods.
140
120
100
Peak Ozone
80
1-Hour
8-Hour
60
40
970710
970713
970716
Date
970719
Exceedances of the 1-Hour O3 Standard
Baltimore Forecast Area - 1-Hour NAAQS Exceedances
18
16
Number of Days
14
12
10
8
6
4
2
0
1990
1991
1992
1993
1994
1995
1996
Year
1997
1998
1999
2000
2001
Color-Coded Air Quality Forecasts
Purple – 1-hr Avg. of over 150 ppb (Rare)
Red – 1-hr Avg. of between 125 and 150 ppb
Orange – 1-hr Avg. of between 105 and 124 ppb
Yellow – 1-hr Avg. of between 80 and 104 ppb
Green – 1-hr Avg. of 79 ppb or lower