Definitions of Terms Used in Air Pollution meteorology
Solar flux
(also called insolation) = 1.34 x 103 w/m2
Average temperature = 15 °C -- Must be in balance
(equilibrium)
Conduction of Heat
Energy transfer through interaction of adjacent molecules -but no bulk movement of matter.
Convection
Movement of entire masses of air.
Sensible Heat
Energy in the form of Kinetic energy of molecules
Latent Heat
Heat of vaporization--stored in water vapor
Radiation
Electromagnetic radiation--the only way energy is transfered
through a vacuum.
Meteorology
The science of the atmosphere
Weather
Short term variation in atmosphere
Climate
Long term averages of weather
Humidity
Water content of air
Relative Humidity
The percent saturation of water in the atmosphere
Dew Point
The temperature at which water vapor condenses to liquid
water
Condensation Nuclei
Provide a surface area for water vapor to condense to liquid
water.
Interaction of Light with matter
Translational Energy
Rotational Energy
Vibrational Energy
Electronic Energy
Chemical bond is two electrons shared between atoms
When a photon of energy is absorbed by a molecule, one of
these shared electrons moves from a "ground-state"
molecular orbital to an empty "excited state" molecular
orbital. When this process occurs, the paired electrons (in the
unexcited molecule) become unpaired (in the excited state of
the molecule). There are two possible configurations for
unpaired electrons in the excited state that are called
"singlet" and "triplet." Singlet refers to a situation where both
electrons have different spin quantum numbers (+1/2 and –
1/2) and triplet refers to the situation where both electrons
have the same spin quantum number (either +1/2 or –1/2).
Chemists represent these electronic energy transformations
with energy level diagrams, and Figure 31.1 illustrates the
energy changes that would occur if a molecule of hydrogen
were to absorb one photon of energy. A requirement of the
quantum theory is that the energy absorbed be equal to the
difference in energy between the excited and ground states of
the molecule. Electrons in the exicted state are unpaired and
very reactive. Much of the chemistry that occurs in the
atmosphere is explained by the presence of these highly
reactive species. A stable, unpaired electron on a molecule is
called a "free-radical."
Nature of Light
Light can be thought of as waves, and to a certain extent they
are analogous to water and sound waves. Electric and
magnetic fields transmit energy in waves that are called
electromagnetic radiation. Ordinary light is a form of
electromagnetic radiation, as are x-rays, ultraviolet, infrared,
radar, and radio waves. All electromagnetic radiation travels
at approximately 300,000 km./sec. (186,000 mi./sec.). The
various forms of electromagnetic radiation differ from one
only in wavelength, and therefore in the energy they can
transmit. Figure 35.3 is a representation of the
electromagnetic spectrum with the visible portion shown in
color to emphasize the portion of the spectrum to which the
human eye is sensitive. The visible spectrum is white light
separated into its component wavelengths or colors. The
wavelength of light, typically measured in terms of millionths
of a meter (microns), extends from about 0.4 to 0.7 micron.
Figure 35.3 The electromagnetic spectrum, showing the
visible portion of the spectrum in color.
Waves of all kinds, including light waves, carry energy.
Electromagnetic energy is unique in that energy is carried in
small, discrete parcels called photons. Representations of a
blue, green, and red photon are shown in Figure 35.4. Blue,
green, and red photons have wavelengths of around 0.45, 0.55
and 0.65 micron respectively. The color properties of light
depend on its behavior both as waves and as particles.
Colors created from white light by passing it through a prism
are a result of the wave-like nature of light. A prism separates
the colors of light by bending (refracting) each color to a
different degree. Colors in a rainbow are the result of water
droplets, acting like small prisms, dispersed through the
atmosphere. Each water droplet refracts light into the
component colors of the visible spectrum. More commonly,
the colors of light are separated in other ways. When light
strikes an object certain color photons are captured by
molecules in that object. Different types of molecules capture
photons of different colors. The only colors we see are those
photons that the surface reflects. For instance, chlorophyll in
leaves capture photons of red and blue light and allow green
photons to bounce back, thus providing the green
Figure 35.4 Representation of blue, green and red photons,
demonstrating their relative wavelengths.
Example:
Nitrogen dioxide, a gas emitted into the atmosphere by
combustion sources, captures blue photons. Consequently,
nitrogen dioxide gas tends to look reddish brown
Figure 31.1 Stratification of the earth's atmosphere showing
changes in temperature and pressure with altitude.
Let's begin our discussion by looking at the Figure of
stratification of the atmosphere that we used in the last
lecture. As you can see, the atmosphere consists of four
distinct regions. The area closest to the earth's surface, the
troposphere, extends up about 10-16 km from the earth's
surface. The stratosphere is next, and reaches up to about 50
km. The mesosphere lies between 50 to 85 km from the
earth's surface, and the thermosphere goes from 85 km to 500
km away from the earth's surface.
The composition of the troposphere consists of mostly
nitrogen and oxygen gases. There are smaller amounts of
water vapor, argon, carbon dioxide, nitrogen oxides, sulfur
oxides, methane and additional trace gases. This region of the
atmosphere is where all life processes occur, and this region
of the atmosphere is the one most affected by anthropogenic
pollution. The reactions that take place in the troposphere
may be acid base reaction or photochemical reactions, and
substances in the troposphere usually have a shorter lifetime
than in other atmospheric regions. The ultimate fate of
chemical reactions in the troposphere is to be washed out
through precipitation events.
The stratosphere is not nearly as dense as the troposphere
and molecules in the stratosphere are therefore exposed to
much more intense radiation from the sun. This causes the
stable form of molecules to be smaller is size and have a
higher kinetic energy. Stratospheric ozone forms under these
conditions, absorbing much of the ultraviolet light coming in
from the sun. This absorption increases the average
molecular velocity. The composition of the stratosphere is
mainly nitrogen, oxygen, nitrogen oxides and ozone at this
point of the atmosphere.
The mesosphere contains mostly ions of the same molecules
that make up the stratosphere. Being closer to the sun, these
molecules are exposed to even more intense radiation that
has the ability to simply ionize small molecules into positive
ions and electrons.
The thermosphere consists of a mixture of ions and highly
charged atoms that are formed by the even more intense solar
radiation that occurs at the outer edge of the atmosphere. The
reason for these changes in atmospheric composition is the
different amount of solar radiation present at each level of the
atmosphere. Molecules act as very effective filters of light.
Each layer of the atmosphere absorbs some sunlight,
shielding the gases below from the radiation that it removes.
The reasons for these changes are based in Figure 31.2,
which shows the variation of atmospheric pressure vs.
altitude and temperature vs. altitude. The temperature of the
atmosphere at earth's surface is determined by radiation of
energy from the land back into the air, and by the density of
the gases in the air. Regions with higher ground temperatures
also have higher air temperatures. As you move away from
the earth's surface, convective heating has a smaller effect
and the air cools. Air temperature starts near 0° Celsius at
ground level, and drops to about -60° 18 km from the earth's
surface. The point where temperature begins to increase
defines the break between the troposphere and the
stratosphere. Temperature then increases to value of about
20° C at a distance of 50 km. This is the break between the
stratosphere and the mesosphere, and going high results in
temperature drops with increases in altitude, reaching a low
temperature of -100° C eighty-five km from the earth's surface.
This defines the break between the mesosphere and the
thermosphere, where temperature again increases with
increasing altitude. The earth's solar radiation budget is an
issue of major importance, and underlies the concerns
surrounding global warming. Figure 32.3 illustrates the
current accepted values for the solar budget.
Figure 32.2 Earth's radiation budget expressed on the basis
of portions of the 1,340 watts/m2 composing the solar flux.
On average, the earth receives 1,340 watts/m2 energy at the
top of the atmosphere, and many different things happen to
this radiation before it is ultimately returned back into space.
The average temperature of the earth is determined by the
ratio of energy received from the sun and the amount
returned back into space by convection and reflection. The
amount of light reflected from clouds has a definite effect on
the temperature of the earth's surface, and some scientists
argue that a short-term solution to global warming would be
to generate more clouds. The composition of the atmosphere
governs the rate at which infrared radiation is emitted back to
space. By adding molecules to the atmosphere that absorb
infrared radiation, we have effectively placed a "blanket" over
the atmosphere--resulting in warmer temperatures at ground
level.
Figure 31.1 Energy changes in a molecule of hydrogen when
a photon of energy is absorbed.
Figure 31.2 Molecules absorbing sufficient energy to break
chemical bonds and form molecular fragments containing
unpaired electrons (free radicals).
Nitrogen oxides are very important in atmospheric chemistry
because nitrogen contains 5 (an odd number) of valence
electrons and therefore is often found in the atmosphere as a
free radical.
Table 31.1 Composition of typical clean atmosphere,
residence times, and global cycles.
Gas
Conc. (ppm)
Residence
Time
Cycle
Ar
9340
---
No Cycle
Ne
18
---
No Cycle
Kr
1.1
---
No Cycle
Xe
0.09
---
No Cycle
N2
780,840
106 yr
Biological & Microbial
O2
209,460
10 yr
Biological & Microbial
CH4
1.65
7 yr
Biogenic & chemical
CO2
332
15 yr
Anthropogenic and
biogenic
CO
0.05-0.2
65 days
Anthropogenic &
chemical
H2
0.58
10 yr
Biogenic & chemical
N2O
0.33
10 yr
Biogenic & chemical
SO2
10-5 – 10-4
40 days
Anthropogenic &
chemical
NH3
10-4 – 10-3
20 days
Biogenic, chemical,
rainout
NO +
NO2
10-6 – 10-2
1 day
Anthropogenic,
chemical, lightning
O3
10-2 10-1
?
Chemical
HNO3
10-5 – 10-3
1 day
Chemical, rainout
H2O
Variable
10 days
Physio-chemical
He
5.2
10 yr
Physio-chemical
Table 31.2 Concentrations of trace substances in the
troposphere and in polluted urban air (concentrations
expressed in ppb).
Species
Clean Troposphere
Polluted Air
SO2
1 – 10
20 – 200
CO
120
1000 – 10,000
NO
0.01 – 0.05
50 – 750
NO2
0.1 – 0.5
50 – 250
O3
20 – 80
100 – 500
HNO3
0.02 – 0.3
3 – 50
NH3
1
10 – 25
HCHO
0.4
20 – 50
1 – 10
HCOOH
HNO2
0.001
1–8
CH3C(O)O2NO2
5 – 35
Non Methane
Hydrocarbons
500 - 1200
Figure 31.2 Vertical profiles of the earth’s atmosphere (not to
scale).
Figure 31.4 The atmospheric oxygen cycle.