1. Air Pollution Outline Meteorological Factors Affecting Air Pollution
Acid Precipitation
Air Pollution in Maryland
5/1/03

2. 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.

3. 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.

4. 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.

5. Tornado formation: roll cloud forms by wind shear

6. Roll cloud is lifted by updrafts to form a mesocyclone

7. Mesocyclone tightens down into a tornado

8. 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.

9. 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.

10. Effect of wind speed on dilution
The concentration of pollutants increases as wind speed
decreases.

11. Role of atmospheric stability
Temperature profile for a surface inversion

12. Role of Atmospheric Stability
Temperature-profile changes after the Sun has
heated the surface .

13. 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.

14. 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.

15. 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.

16. Role of Atmospheric Stability
Inversion Aloft

17. Bermuda high H

18. 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.

19. Acid Precipitation
Robert Angus Smith ( ) 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.

20. 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.

21. 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.

22. Pollution in the Mid-Atlantic Region
Maryland Department of the Environment
University of Maryland
Pennsylvania State University

23. Most Unhealthy Air Quality Days Occur in the Summer Season
Most Unhealthy Air Quality Days Occur in the Summer Season. Summer Weather in the Mid-Atlantic 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.
HAZY
HOT
Humid

24. Pollutants of Concern During Poor Air Quality Events
Fine Particles
Haze
Ozone

25. 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.

26. Recipe for Ozone Emissions of O3 “Precursors” A Sunny Day
Hot Temperatures (typically in the 90’s)
Moderate or Light Surface Winds
Note here that O3 is a secondary pollutant. It is not emitted directely into
The atmosphere but is formed by a series of reactions involving primary
Emissions of NOx and VOC, there are called “precursors”
More on thiws latere.

27. 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.
In accompanying bibliography should cite recent volume of Joournal of Air and Waste ;which reviews health effects of PM>

28. Haze of sulfur and nitrogen compounds.
 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.

29. An Example of the Effects of Haze in the Mid-Atlantic
The Great Smoky Mountains National Park
A Clear Day
A Hazy Day
Photos from

30. A Typical Day in a Pollution Episode

31. 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.

32. 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.

33. 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.

34. Temperature Inversion
Altitude
The trapping of emissions beneath the noccturnal inversion is the reason why haze concentrations are highest in the late night and early morning hours.
Temperature Inversion
Pollution trapped below inversion
Temperature

35. 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.

36. 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."

37. Nocturnal Inversion ABOVE BELOW
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.

38. 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.

39. 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

40. O3 concentrations along the western boundary of the I-95 Corridor on
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

41. 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.

42. 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.

43. 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.

44. 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

45. 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.

46. Ozone concentration diminishing
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

47. A Multi-Day Pollution Episode

48. Assume we start at noon on “Day 1” with a relatively clean air mass
<< Cooler
Temperature
Warmer >>

49. As the sun sets, the surface begins to cool and a transition takes place
Warmer >>
<< Cooler
Temperature

50. The surface cooling continues overnight
O3 Profile
Elev
(ft)
1,000
2,000
4,000
6,500
Wind
Warmer >>
<< Cooler
Temperature

51. On Day 2, the sun rises and the nocturnal inversion begins to erode
Warmer >>
<< Cooler
Temperature

52. By noon, the nocturnal inversion is gone and any air pollution that was aloft mixes down
Warmer >>
<< Cooler
Temperature

53. The process repeats – now there is more O3 (Sunset Day 2)
Temperature
<< Cooler
Warmer >>

54. Midnight Day 2 O3 Profile Warmer >> << Cooler Temperature
Elev
(ft)
1,000
2,000
4,000
6,500
Wind
Warmer >>
<< Cooler
Temperature

55. Sunrise Day 3...
Warmer >>
<< Cooler
Temperature

56. By Noon on Day 3, Local Emissions and High “Background” O3 Combine
Warmer >>
<< Cooler
Temperature

57. 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.

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

59. 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.

60. Exceedances of the 1-Hour O3 Standard

61. Color-Coded Air Quality Forecasts
Purple – 1-hr Avg. of over 150 ppb (Rare)
Yellow – 1-hr Avg. of between 80 and 104 ppb
Orange – 1-hr Avg. of between 105 and 124 ppb
Red – 1-hr Avg. of between 125 and 150 ppb
Green – 1-hr Avg. of 79 ppb or lower