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CH1
X-ray tube
An X-ray tube is a vacuum tube that produces X-rays. They are used in Xray machines. X-rays are part of the electromagnetic spectrum, an
ionizing radiation with wavelengths shorter than ultraviolet light. X-ray
tubes evolved from experimental Crookes tubes with which X-rays were
first discovered in the late 1800s, and the availability of this controllable
source of X-rays created the field of radiography, the imaging of opaque
objects with penetrating radiation. X-ray tubes are also used in CAT
scanners, airport luggage scanners, X-ray crystallography, and for
industrial inspection.
Coolidge X-ray tube, from around 1917. The heated cathode is on the left,
and the anode is right. The X-rays are emitted downwards.
As with any vacuum tube, there is a cathode, which emits electrons into
the vacuum and an anode to collect the electrons, thus establishing a flow
of electrical current, known as the beam, through the tube. A high voltage
power source, for example 30 to 150 kilovolts (kV), is connected across
cathode and anode to accelerate the electrons. The X-ray spectrum
depends on the anode material and the accelerating voltage [1].
In many applications, the current flow (typically in the range 1mA to 1A)
is able to be pulsed on for between about 1ms to 1s. This enables consistent
doses of x-rays, and taking snapshots of motion. Until the late 1980s, X-ray
generators were merely high-voltage, AC to DC variable power supplies.
In the late 1980s a different method of control was emerging, called high
speed switching. This followed the electronics technology of switching
power supplies (aka switch mode power supply), and allowed for more
accurate control of the X-ray unit, higher quality results, and reduced Xray exposures.
Electrons from the cathode collide with the anode material, usually
tungsten, molybdenum or copper, and accelerate other electrons, ions and
nuclei within the anode material. About 1% of the energy generated is
emitted/radiated, usually perpendicular to the path of the electron beam,
as X-rays. The rest of the energy is released as heat. Over time, tungsten
will be deposited from the target onto the interior surface of the tube,
including the glass surface. This will slowly darken the tube and was
thought to degrade the quality of the X-ray beam, but research has
suggested there is no effect .
Eventually, the tungsten deposit may become sufficiently conductive that
at high enough voltages, arcing occurs. The arc will jump from the
cathode to the tungsten deposit, and then to the anode. This arcing causes
an effect called "crazing" on the interior glass of the X-ray window. As
time goes on, the tube becomes unstable even at lower voltages, and must
be replaced. At this point, the tube assembly (also called the "tube head")
is removed from the X-ray system, and replaced with a new tube assembly.
The old tube assembly is shipped to a company that reloads it with a new
X-ray tube.
The X-Ray photon-generating effect is generally called the
Bremsstrahlung effect, a contraction of the German brems for
braking, and strahlung for radiation.
The range of photonic energies emitted by the system can be adjusted by
changing the applied voltage, and installing aluminum filters of varying
thicknesses. Aluminum filters are installed in the path of the X-ray beam
to remove "soft" (non-penetrating) radiation. The number of emitted Xray photons, or dose, are adjusted by controlling the current flow and
exposure time.
Simply put, the high voltage controls X-ray penetration, and thus the
contrast of the image. The tube current and exposure time affect the dose
and therefore the darkness of the image.
Crookes tube:-
Crookes X-ray tube from early 1900s. The cathode is on the right, the
anode is in the center with attached heat sink at left. The electrode at the
10 o'clock position is the anticathode. The device at top is a 'softener' used
to regulate the gas pressure.
Historically, x-rays were discovered radiating from experimental
discharge tubes called Crookes tubes invented by British physicist William
Crookes and others. As the medical and other uses of x-rays became
apparent, workshops began to manufacture specialized Crookes tubes to
produce x-rays. These were the first x-ray tubes. These first generation
cold cathode or Crookes x-ray tubes were used until the 1920s.
Crookes tubes generated the electrons needed to create x-rays by
ionization of the residual air in the tube, instead of a heated filament, so
they were partially but not completely evacuated. They consisted of a glass
bulb with around 10-6 to 5×10-8 atmospheric pressure of air (0.1 to 0.005
Pa). An aluminum cathode plate at one end of the tube created a beam of
electrons, which struck a platinum anode target at the center generating xrays. The anode surface was angled so that the x-rays would radiate
through the side of the tube. The cathode was concave so that the electrons
were focused on a small (~1 mm) spot on the anode, approximating a point
source of x-rays, which resulted in sharper images. The tube had a third
electrode, an anticathode connected to the anode. It improved the X-ray
output, but the method by which it achieved this is not understood. A
more common arrangement used a cupped plate anticathode (similar in
constriction to the cathode) in line with the anode such that the anode was
between the cathode anf the anticathode.
To operate, a DC voltage of a few kilovolts to as much as 100 kV was
applied between the anodes and the cathode, usually generated by an
induction coil, or for larger tubes, an electrostatic machine. This created
and then accelerated a small number of ions from the low pressure gas in
the tube. These struck further gas atoms, knocking electrons off them,
creating more positive ions in a chain reaction. All the positive ions were
attracted to the cathode. When they struck it, they knocked electrons out
of the metal, which were accelerated along with the electrons knocked
from the gas atoms toward the anode target. When these high speed
electrons struck the atoms of the anode, they created x-rays by one of two
processes, either Bremsstrahlung or x-ray fluorescence.
Crookes tubes were unreliable. As time passed, the residual air would be
absorbed by the walls of the tube, reducing the pressure. This increased
the voltage across the tube, generating 'harder' x-rays, until eventually the
tube stopped working. To prevent this, 'softener' devices were used (see
picture). A small tube attached to the side of the main tube contained a
mica sleeve or chemical that released a small amount of gas when heated,
restoring the correct pressure.
The glass envelope of the tube would blacken is use due to the X-rays
affecting its structure.
Coolidge tube
Coolidge side-window tube (scheme)
(1)K: filament
(2)A: anode
(3)Win and Wout: water inlet and outlet of the cooling device (C)
The Crookes tube was improved by William Coolidge in 1913. The
Coolidge tube, also called hot cathode tube, is the most widely used. It
works with a very good quality vacuum (about 10-4 Pa, or 10-6 Torr).
In the Coolidge tube, the electrons are produced by thermionic effect from
a tungsten filament heated by an electric current. The filament is the
cathode of the tube. The high voltage potential is between the cathode and
the anode, the electrons are thus accelerated, and then hit the anode.
There are two designs: end-window tubes and side-window tubes.
In the end-window tubes, the filament is around the anode, the electrons
have a curved path.
What is special about side-window tubes is:
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An Electrostatic Lens to focus the beam onto a very small spot on the
anode
The anode is specially designed to dissipate the heat and wear
resulting from this intense focused barrage of electrons:
o Mechanically spun to increase the area heated by the beam.
o Cooled by circulating coolant.
The anode is precisely angled at 1-20 degrees off perpendicular to the
electron current so as to allow escape of some of the X-ray photons
which are emitted essentially perpendicular to the direction of the
electron current.
The anode is usually made out of tungsten or molybdenum.
The tube has a window designed for escape of the generated X-ray
photons.
The power of a Coolidge tube usually ranges from 1 to 4 kW.
Rotating anode tube
Simplified rotating anode tube schematic
A: Anode
C: cathode
T: Anode target
W: X-ray window
Modern rotating anode X-ray tube
The rotating anode tube is an improvement of the Coolidge tube. X-ray
production using this method is very inefficient (99% of incident energy is
converted to heat) the dissipation of heat at the focal spot is one of the
main limitations on the power which can be applied. By sweeping the
anode past the focal spot the heat load can be spread over a larger area,
greatly increasing the power rating. With the exception of dental X-ray
tubes, almost all medical X-ray tubes are of this type.
The anode consists of a disc with an annular target close to the edge. The
anode disc is supported on a long stem which is supported by bearings
within the tube. The anode can then be rotated by electromagnetic
induction from a series of stator windings outside the evacuated tube.
Because the entire anode assembly has to be contained within the
evacuated tube, heat removal is a serious problem, further exacerbated by
the higher power rating available. Direct cooling by conduction or
convection, as in the Coolidge tube, is difficult. In most tubes, the anode is
suspended on ball bearings with silver powder lubrication which provide
almost negligible cooling by conduction.
A recent development has been liquid gallium
lubricated fluid dynamic bearings which can withstand
very high temperatures without contaminating the tube
vacuum. The large bearing contact surface and metal
lubricant provide an effective method for conduction of
heat from the anode .
The anode must be constructed of high temperature
materials. The focal spot temperature can reach 2500°C
during an exposure, and the anode assembly can reach
1000°C following a series of large exposures. Typical
materials are a tungsten-rhenium target on a
molybdenum core, backed with graphite. The rhenium
makes the tungsten more ductile and resistant to wear
from impact of the electron beams. The molybdenum
conducts heat from the target. The graphite provides
thermal storage for the anode, and minimizes the
rotating mass of the anode.
Increasing demand for high-performance CT scanning
and angiography systems has driven development of
very high performance medical X-ray tubes.
Contemporary CT tubes have power ratings of up to 100
kW and anode heat capacity of 6 MJ, yet retain an
effective focal spot area of less than 1 mm2.
Hazards of x-ray production from vacuum
tubes
2 High Voltage rectifier tubes capable of producing X-rays.
Any vacuum tube operating at several thousand volts or more can produce
x-rays as an unwanted byproduct, raising safety issues.
The higher the voltage, the more penetrating the resulting radiation and
the more the hazard. Color televisions and computer CRT displays
operate at 30-40 kilovolts, making them the main concern among
household appliances.
Historically, concern has focused less on the cathode ray tube, since its
thick glass envelope is impregnated with several pounds of lead for
shielding, than on high voltage (HV) rectifier and voltage regulator tubes
inside. In the 1970s it was found that a failure in the HV supply circuit of
some GE TVs could leave excessive voltages on the regulator tube, causing
it to emit X-rays. The models were recalled and the ensuing scandal
caused the US agency responsible for regulating this hazard, the Center
for Devices and Radiological Health of the Food and Drug Administration
(FDA), to require that all TVs include circuits to prevent excessive
voltages in the event of failure. This hazard was eliminated with the
advent of all solid state TVs, which have no tubes beside the CRT. Since
1969 the FDA has limited TV X-ray emission to 0.5 mR (milliroentgen) per
hour. The tubes in the image to the right have a brown discoloration or
"sunburn". This is caused by the condensation of vaporised anode
material on the glass enclosure.
More recently, concern has been expressed about the magnetron tubes in
microwave ovens, although with operating voltages of only 4-5 kilovolts it
is doubtful whether any x-rays produced could penetrate the tube
envelope, typically constructed of ceramic and metal. No verified reports
of oven magnetrons producing measurable X-rays appear to have been
published.
Description
A vacuum tube consists of electrodes in a vacuum in an insulating heatresistant envelope which is usually tubular. Many tubes have glass
envelopes, though some types such as power tubes may have ceramic or
metal envelopes. The electrodes are attached to leads which pass through
the envelope via an airtight seal. On most tubes, the leads are designed to
plug into a tube socket for easy replacement.
The simplest vacuum tubes resemble incandescent light bulbs in that they
have a filament sealed in a glass envelope which has been evacuated of all
air. When hot, the filament releases electrons into the vacuum: a process
called thermionic emission. The resulting negatively charged cloud of
electrons is called a space charge. These electrons will be drawn to a metal
plate inside the envelope, if the plate (also called the anode) is positively
charged relative to the filament (or cathode). The result is a flow of
electrons from filament to plate. This cannot work in the reverse direction
because the plate is not heated and does not emit electrons. This very
simple example described can thus be seen to operate as a diode: a device
that conducts current only in one direction. The vacuum tube diode
conducts conventional current from plate (anode) to the filament
(cathode); this is the opposite direction to the flow of electrons (called
electron current).
Vacuum tubes require a large temperature difference between the hot
cathode and the cold anode. Because of this, vacuum tubes are inherently
power-inefficient; enclosing the tube within a heat-retaining envelope of
insulation would allow the entire tube to reach the same temperature,
resulting in electron emission from the anode that would counter the
normal one-way current. Because the tube requires a vacuum to operate,
convection cooling of the anode is typically not possible. Instead anode
cooling occurs primarily through black-body radiation and conduction of
heat to the outer glass envelope via the anode mounting frame. Cold
cathode tubes do not rely on thermionic emission at the cathode and
usually have some form of gas discharge as the operating principle; such
tubes are used for lighting (neon lamps) or as voltage regulators.
The vacuum tube is a voltage-controlled device, with the relationship
between the input and output circuits determined by a transconductance
function. The voltage between the control grid and the cathode controls
the amount of current in the tube that goes from cathode to anode.
Control grid current is practically negligible in most circuits. The solidstate device most closely analogous to the vacuum tube is the JFET,
although the vacuum tube typically operates at far higher voltage (and
power) levels than the JFET.
Bremsstrahlung produced by a high-energy electron deflected in the
electric field of an atomic nucleus
Bremsstrahlung cross section for the emission of a photon with energy 30
keV by an electron impacting on a proton.
Bremsstrahlung (German pronunciation: ( listen), from bremsen "to
brake" and Strahlung "radiation", i.e. "braking radiation" or
"deceleration radiation"), is electromagnetic radiation produced by the
acceleration of a charged particle, such as an electron, when deflected by
another charged particle, such as an atomic nucleus. The term is also used
to refer to the process of producing the radiation. Bremsstrahlung has a
continuous spectrum. The phenomenon was discovered by Nikola
Tesla[citation needed] during high frequency research he conducted between
1888 and 1897.
Bremsstrahlung may also be referred to as free-free radiation. This
refers to the radiation that arises as a result of a charged particle that is
free both before and after the deflection (acceleration) that causes the
emission. Strictly speaking, bremsstrahlung refers to any radiation due to
the acceleration of a charged particle, which includes synchrotron
radiation; however, it is frequently used (even when not speaking
German) in the more narrow sense of radiation from electrons stopping in
matter.
The word Bremsstrahlung is retained from the original German to
describe the radiation which is emitted when electrons are decelerated or
"braked" when they are fired at a metal target. Accelerated charges give
off electromagnetic radiation, and when the energy of the bombarding
electrons is high enough, that radiation is in the X-ray region of the
electromagnetic spectrum. It is characterized by a continuous distribution
of radiation which becomes more intense and shifts toward higher
frequencies when the energy of the bombarding electrons is increased.
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Outer Bremsstrahlung
"Outer bremsstrahlung" is the term applied in cases where the energy loss
by radiation greatly exceeds that by ionization as a stopping mechanism in
matter. This is seen clearly for electrons with energies above 50 keV.
Inner Bremsstrahlung
"Inner bremsstrahlung" is the term applied to the less frequent case of
radiation emission during beta decay, resulting in the emission of a photon
of energy less than or equal to the maximum energy available in the
nuclear transition. Inner bremsstrahlung is caused by the abrupt change in
the electric field in the region of the nucleus of the atom undergoing decay,
in a manner similar to that which causes outer bremsstrahlung. In
electron and positron emission the photon's energy comes from the
electron/nucleon pair, with the spectrum of the bremsstrahlung decreasing
continuously with increasing energy of the beta particle. In electron
capture the energy comes at the expense of the neutrino, and the spectrum
is greatest at about one third of the normal neutrino energy, reaching zero
at zero energy and at normal neutrino energy.
Beta particle-emitting substances sometimes exhibit a weak radiation with
continuous spectrum that is due to both outer and inner bremsstrahlung,
or to one of them alone.
Secondary radiation
Bremsstrahlung is a type of "secondary radiation", in that it is produced
as a result of stopping (or slowing) the primary radiation (beta particles).
In some cases, e.g. 32P, the Bremsstrahlung produced by shielding this
radiation with the normally used dense materials (e.g. lead) is itself
dangerous; in such cases, shielding must be accomplished with low density
materials, e.g. Plexiglass (lucite), plastic, wood, or water; because the rate
of deceleration of the electron is slower, the radiation given off has a
longer wavelength and is therefore less penetrating.
CH2
albedo
The albedo of an object is a measure of how strongly it reflects light from light
sources such as the Sun. It is therefore a more specific form of the term reflectivity.
Albedo is defined as: the ratio of total-reflected to incident electromagnetic
radiation. It is a unitless measure indicative of a surface's or body's diffuse
reflectivity. The word is derived from Latin albedo "whiteness", in turn from albus
"white", and was introduced into optics by Johann Heinrich Lambert in his 1760
work Photometria. The range of possible values is from 0 (dark) to 1 (bright).
general, the albedo depends on the direction and directional distribution
of incoming radiation. Exceptions are Lambertian surfaces, which scatter
radiation in all directions in a cosine function, so their albedo does not
depend on the incoming distribution. In realistic cases, a bidirectional
reflectance distribution function (BRDF) is required to characterize the
scattering properties of a surface accurately, although albedos are a very
useful first approximation.
Albedos of typical materials in visible light range from up to 0.9 for fresh
snow, to about 0.04 for charcoal, one of the darkest substances. Deeply
shadowed cavities can achieve an effective albedo approaching the zero of
a blackbody. When seen from a distance, the ocean surface has a low
albedo, as do most forests, while desert areas have some of the highest
albedos among landforms. Most land areas are in an albedo range of 0.1 to
0.4.[6] The average albedo of the Earth is about 0.3.[7] This is far higher
than for the ocean primarily because of the contribution of clouds.
Human activities have changed the albedo (via forest clearance and
farming, for example) of various areas around the globe. However,
quantification of this effect on the global scale is difficult.
The classic example of albedo effect is the snow-temperature feedback. If a
snow-covered area warms and the snow melts, the albedo decreases, more
sunlight is absorbed, and the temperature tends to increase. The converse
is true: if snow forms, a cooling cycle happens. The intensity of the albedo
effect depends on the size of the change in albedo and the amount of
insolation; for this reason it can be potentially very large in the tropics.
The Earth's surface albedo is regularly estimated via Earth observation
satellite sensors such as NASA's MODIS instruments onboard the Terra
and Aqua satellites. As the total amount of reflected radiation cannot be
directly measured by satellite, a mathematical model of the BRDF is used
to translate a sample set of satellite reflectance measurements into
estimates of directional-hemispherical reflectance and bi-hemispherical
reflectance.
The Earth's average surface temperature due to its albedo and the
greenhouse effect is currently about 15°C. For the frozen (more reflective)
planet the average temperature is below -40°C (If only all continents being
completely covered by glaciers - the mean temperature is about 0°C). The
simulation for (more absorptive) aquaplanet shows the average
temperature close to 27°C.
Albedo works on a smaller scale, too. People who wear dark clothes
in the summertime put themselves at a greater risk of heatstroke than
those who wear lighter color clothes.
What is the definition of albedo?
The albedo is the relative reflectivity of light from a surface, which for
solar radiation indicates the percentage of solar energy that is not
absorbed.
White-sky and black-sky albedo
It has been shown that for many applications involving terrestrial albedo,
the albedo at a particular solar zenith angle θi can reasonably be
approximated by the proportionate sum of two terms: the directionalhemispherical reflectance at that solar zenith angle, , and the bihemispherical reflectance, the proportion concerned being defined as the
proportion of diffuse illumination D.
Albedo α can then be given as:
Directional-hemispherical reflectance is sometimes referred to as blacksky albedo and bi-hemispherical reflectance as white sky albedo. These
terms are important because they allow the albedo to be calculated for any
given illumination conditions from a knowledge of the intrinsic properties
of the surface.
Astronomical albedo
The albedos of planets, satellites and asteroids can be used to infer much
about their properties. The study of albedos, their dependence on
wavelength, lighting angle ("phase angle"), and variation in time comprises
a major part of the astronomical field of photometry. For small and far
objects that cannot be resolved by telescopes, much of what we know
comes from the study of their albedos. For example, the absolute albedo
can indicate the surface ice content of outer solar system objects, the
variation of albedo with phase angle gives information about regolith
properties, while unusually high radar albedo is indicative of high metallic
content in asteroids.
Enceladus, a moon of Saturn, has one of the highest known albedos of any
body in the Solar system, with 99% of EM radiation reflected. Another
notable high albedo body is Eris, with an albedo of 0.86. Many small
objects in the outer solar system[12] and asteroid belt have low albedos
down to about 0.05.[13] A typical comet nucleus has an albedo of 0.04.[14]
Such a dark surface is thought to be indicative of a primitive and heavily
space weathered surface containing some organic compounds.
The overall albedo of the Moon is around 0.072, but it is strongly
directional and non-Lambertian, displaying also a strong opposition
effect.[15] While such reflectance properties are different from those of any
terrestrial terrains, they are typical of the regolith surfaces of airless solar
system bodies.
Two common albedos that are used in astronomy are the (V-band)
geometric albedo (measuring brightness when illumination comes from
directly behind the observer) and the Bond albedo (measuring total
proportion of electromagnetic energy reflected). Their values can differ
significantly, which is a common source of confusion.
In detailed studies, the directional reflectance properties of astronomical
bodies are often expressed in terms of the five Hapke parameters which
semi-empirically describe the variation of albedo with phase angle,
including a characterization of the opposition effect of regolith surfaces.
The correlation between astronomical (geometric) albedo, absolute
magnitude and diameter is:[16] ,
where A is the astronomical albedo, D is the diameter in kilometres, and H
is the absolute magnitude.
Other types of albedo
Single scattering albedo is used to define scattering of electromagnetic
waves on small particles. It depends on properties of the material
(refractive index); the size of the particle or particles; and the wavelength
of the incoming radiation.
Albedo also refers to the white, spongy inner lining of a citrus fruit rind.[17]
According to Dr. Renee M. Goodrich, associate professor of food science
and human nutrition at the University of Florida, the albedo is rich in the
soluble fiber pectin and contains vitamin C.
Some examples of terrestrial albedo effects
lighter color clothes.
Trees
Because trees tend to have a low albedo, removing forests would tend to
increase albedo and thereby could produce localized climate cooling.
Cloud feedbacks further complicate the issue. In seasonally snow-covered
zones, winter albedos of treeless areas are 10% to 50% higher than nearby
forested areas because snow does not cover the trees as readily. Deciduous
trees have an albedo value of about 0.15 to 0.18 while coniferous trees have
a value of about 0.09 to 0.15.[3] The difference between deciduous and
coniferous is because coniferous trees are darker in general and have
cone-shaped crowns. The shape of these crowns trap radiant energy more
effectively than deciduous trees.
Studies by the Hadley Centre have investigated the relative (generally
warming) effect of albedo change and (cooling) effect of carbon
sequestration on planting forests. They found that new forests in tropical
and midlatitude areas tended to cool; new forests in high latitudes (e.g.
Siberia) were neutral or perhaps warming.[21]
[edit] Snow
Snow albedos can be as high as 0.9; this, however, is for the ideal example:
fresh deep snow over a featureless landscape. Over Antarctica they
average a little more than 0.8. If a marginally snow-covered area warms,
snow tends to melt, lowering the albedo, and hence leading to more
snowmelt (the ice-albedo positive feedback).
Water
Water reflects light very differently from typical terrestrial materials. The
reflectivity of a water surface is calculated using the Fresnel equations (see
graph).
Reflectivity of smooth water at 20 C (refractive index=1.333)
At the scale of the wavelength of light even wavy water is always smooth so
the light is reflected in a locally specular manner (not diffusely). The glint
of light off water is a commonplace effect of this. At small angles of
incident light, waviness results in reduced reflectivity because of the
steepness of the reflectivity-vs.-incident-angle curve and a locally
increased average incident angle.[22]
Although the reflectivity of water is very low at low and medium
angles of incident light, it increases tremendously at high angles of
incident light such as occur on the illuminated side of the Earth
near the terminator (early morning, late afternoon and near the
poles). However, as mentioned above, waviness causes an
appreciable reduction. Since the light specularly reflected from
water does not usually reach the viewer, water is usually considered
to have a very low albedo in spite of its high reflectivity at high
angles of incident light.
Note that white caps on waves look white (and have high albedo) because
the water is foamed up (not smooth at the scale of the wavelength of light)
so the Fresnel equations do not apply. Fresh ‘black’ ice exhibits Fresnel
reflection.
Clouds
Clouds are another source of albedo that play into the global
warming equation. Different types of clouds have different
albedo values, theoretically ranging from a minimum of near
0 to a maximum approaching 0.8. "On any given day, about
half of Earth is covered by clouds, which reflect more
sunlight than land and water. Clouds keep Earth cool by
reflecting sunlight, but they can also serve as blankets to trap
warmth."[23]
Albedo and climate in some areas are already affected by artificial clouds,
such as those created by the contrails of heavy commercial airliner traffic.
A study following the burning of the Kuwaiti oil
fields by Saddam Hussein showed that
temperatures under the burning oil fires were as
much as 10oC colder than temperatures several
miles away under clear skies.
Aerosol effects
Aerosol (very fine particles/droplets in the atmosphere) has two effects,
direct and indirect. The direct (albedo) effect is generally to cool the
planet; the indirect effect (the particles act as CCNs and thereby change
cloud properties) is less certain.
Aerosols can modify the Earth’s radiative balance through the aerosol
direct and indirect effects.
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direct effect. Aerosols directly scatter and absorb radiation.
The scattering of radiation causes atmospheric cooling, whereas
absorption can cause atmospheric warming.
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Aerosol indirect effect
Aerosols modify the properties of clouds through a subset of the
aerosol population called cloud condensation nuclei (CCN).
Increased CCN concentrations lead to increased cloud droplet
number concentrations (CDNC). A greater number of cloud droplets
leads to increased cloud albedo, increased light scattering and
radiative cooling (first indirect effect). Increased CDNC also leads to
reduced precipitation efficiency and increased lifetime of the cloud
(second indirect effect).