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Air Pollution- Particles in the Air
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Air quality and particlesEpidemiological () evidence associates
atmospheric particles with diseases and mortality.Particles
penetrate the lungs, blocking and irritating air passages.Particles
themselves could exert toxic effects. (toxic substances present in
the particles) [Polycyclic Aromatic Hydrocarbons are examples of
toxic substances in aerosols.]
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Particle deposition as a function of particle diameters in
various regions of the lung.The larger a particle is, the more
likely it is deposited into the upper part of the respiratory
system.The smaller a particle is, the deeper it can get into the
respiratory system.
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TermsTSP: Total suspended particulate matterRSP: Respirable
suspended particulate matterPM10: Particulate matter of less than
10 um; equivalent to RSP.PM2.5: Particulate matter of less than 2.5
um
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Air quality standards for particlesUS15 ug/m3 (annual average)
for PM 2.5 um65 ug/m3 (daily average) for PM 2.5 umHong Kong RSP:
55 ug/m3 (annual), 180 ug/m3 (daily) TSP: 80 ug/m3 (annual), 260
ug/m3 (daily)
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Visibility and AerosolsIn a clear sky without any particles, one
can see as far as 100-300 km.
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Physics of visibilityVisibility depends on the transmission of
light through the atmosphere and the ability of the eye to
distinguish an object because it contrasts with the background.A
change in contrast with viewing distance occurs for both dark,
light and bright objects. With dark objects, the atmosphere
introduces light, called "air light", to the sight path, and the
dark object appears lighter at increasing distance until it bleeds
into the background at the horizon.With light objects, light is
lost from the sight path with increasing viewing distance.
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In both cases, the result is the sameThe contrast between the
object and the background approaches zero when the light coming
along the line of sight from the object approaches the intensity of
the light from the background at the horizon. When the eye can no
longer distinguish the difference, the contrast, between the object
and the background, the object can not be seen, and it is said to
be beyond the limit of visibility.
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The mathematic expression for the extinction of contrast:I =Io
exp[-bx]b: scattering coefficient or absorption coefficient, x:
distance I: light intensity or contrast. b = bsg + bsp + bag +
bapbsg and bsp are extinction coeff. caused by light scattering due
to gas and particles respectively.bsp = bsdp + bsw are broken into
contributions due to dry particles and water associated with
particles.bag and bap are extinction coeff. caused by light
absorption due to gas and particles respectively.
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Visibility reduction due to particles is size-dependent Particle
Scattering and absorption cross-section per unit volumeParticles of
0.1-1.0 um size are efficient light-scattering centers.
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Fine aerosol particles are a major contributor to visibility
impairment (Blue Ridge Mountain area in the U.S.)
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Measurement of visibilityHuman observationsOptical
measurementsMeasured by a integrating nephelometerVisual range
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Study questionsHow do particles cause adverse effect on human
health? How do particles affect visibility?
