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 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.

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 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.

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.

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

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.

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.

Effects of Acid Precipitation

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 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.

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 90s) 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

A Hazy Day

Photos from www.epa.gov

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.

Temperature Inversion

Altitude

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
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.

BELOW

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.

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

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
140 120 100 So. Carroll Frederick Long Park Ashburn Little Buffalo Methodist Hill

Ozone (ppbv)

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.

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.

Altitude

Ozone concentration remaining constant Temperature Inversion

Ozone concentration diminishing

Temperature

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
<< Cooler

2,000 1,000

Temperature

Warmer >>

As the sun sets, the surface begins to cool and a transition takes place

Wind

Elev (ft)

6,500

4,000

O3 Profile
<< Cooler Temperature Warmer >>

2,000 1,000

The surface cooling continues overnight

Wind

Elev (ft)

6,500

4,000

O3 Profile
<< Cooler Temperature Warmer >>

2,000 1,000

On Day 2, the sun rises and the nocturnal inversion begins to erode

Wind

Elev (ft)

6,500

4,000

O3 Profile
<< Cooler Temperature Warmer >>

2,000 1,000

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
<< Cooler Temperature Warmer >>

2,000 1,000

The process repeats now there is more O3 (Sunset Day 2)

Wind
Elev (ft)

6,500

4,000

O3 Profile
<< Cooler Temperature Warmer >>

2,000 1,000

Midnight Day 2

Wind

Elev (ft)

6,500

4,000

O3 Profile
<< Cooler Temperature Warmer >>

2,000 1,000

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
<< Cooler Temperature Warmer >>

2,000 1,000

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

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.
200 180 160
Ozone (ppbv)

140 120 100 80 60 40 970710 970713 Date Peak Ozone 1-Hour 8-Hour 970716 970719

Exceedances of the 1-Hour O3 Standard

Baltimore Forecast Area - 1-Hour NAAQS Exceedances 18 16 14

Number of Days

12 10 8 6 4 2 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 Year

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