Aerosols and Particulate Matter

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Aerosols and Particulates
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Aerosols and Particulate Matter

• Atmospheric particulate matter
• Ranges in size from ½ mm down to molecular
  dimensions
• Either solid or liquid droplets
• Visible part of air pollution
• Aerosol
• A system of small (lt100 µm diameter) colloidal
  solid or liquid particles suspended in a gaseous
  phase

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The Origin of Aerosols

• (i) Primary particulate material (dispersion
  aerosols) directly derived from dispersal of
  solids and liquids physical origin
• (ii) Secondary particulate material result of
  chemical reactions in the atmosphere - chemical
  origin

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Primary Particulates

• sources can be biological
• viruses, bacteria, spores, fungi, pollen,
  vegetation ( forest fires)
• terrestrial
• dust, sand, soils, pulverized rocks, volcanoes
• aquatic (mists)
• raindrops, fog, ocean spray
• extraterrestrial
• meteoric debris

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Aerosols are small particles in the atmosphere.

• clouds and aerosols are both particles.
• Aerosols are composed of many different
  substances, and their composition is important to
  what they do in the atmosphere.
• Aerosols include soil dust from deserts, and sea
  salt, and soot. Some of these influence clouds,
  especially the formation and size of cloud
  droplets.
• The sulfate aerosols that form after volcanic
  eruptions, and from industrial emissions, reflect
  solar energy as well as absorb solar energy in
  the atmosphere.
• Both of these processes keep energy away from the
  Earths surface.
• Hence, sulfate aerosols are a source of global
  cooling.

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Aerosol particles larger than about 1 micrometer
in size are produced by windblown dust and sea
salt from sea spray and bursting bubbles.
Aerosols smaller than 1 micrometer are mostly
formed by condensation processes such as
conversion of sulfur dioxide (SO2) gas (released
from volcanic eruptions) to sulfate particles and
by formation of soot and smoke during burning
processes. After formation, the aerosols are
mixed and transported by atmospheric motions and
are primarily removed by cloud and precipitation
processes.
http//www.nasa.gov/images/content/113650main_aero
sol_pie_chart.jpg
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Anthropogenic aerosol includes dust from land use
changes, sulfate from industrial emissions, and
soot and particulate organic matter from fossil
fuel combustion and biomass burning. This aerosol
perturbs the Earth's radiation balance directly
by scattering and absorbing incoming solar
radiation and indirectly by altering cloud
properties such as cloud extent, lifetime, and
reflectivity.
http//www.pmel.noaa.gov/home/images/forcing.gif
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Secondary Particulates

• atmospheric chemical reactions
• ? both organic and inorganic particulates
• e.g. Particulate iron oxide is formed during the
  combustion of pyrite-containing coal
• 3FeS2 8O2 ? Fe3O4 6SO2
• oxidation of SO2 containing aerosols to
  sulphuric acid
• 2SO2 O2 2H2O ? 2H2SO4
• in the presence of pollutants
• H2SO4 (droplet) 2NH3 (g) ? (NH4)2SO4 (droplet)
• H2SO4 (droplet) CaO (particle) ? CaSO4
  (droplet) H2O
• in low humidity, water is lost by aerosol to give
  solid aerosol

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Secondary Particulates

• CaO particulates are produced via
• CaCO3 heat ? CaO CO2
• component of coal ash
• oxidation of hydrocarbons to give
  partially-oxidized intermediates results in hazes
• oxidation of terpenes in the Great Smokey
  Mountains
• oxidation of nitrogen oxides to give
  photochemical smog
• Composition of aerosols may be used to trace its
  source
• generally, the proportions of elements in
  particulate matter reflect the relative
  abundances in parent material

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Size Spectra and Sedimentation

• Most important physical characteristic is size
• determines sedimentation rate (settling)
• optical properties
• health effects
• condensation of particles

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Size Spectra and Sedimentation

• Sedimentation described by Stokes Law

Large particles (dense) settle faster than small
(light) ones
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Size Spectra and Sedimentation

• Aitken particles (condensation nuclei) can
  diffuse quickly and then coagulate at a rate
• Smaller particles ( molecular scale) undergo
  random walk Brownian motion
• e.g. At a density of 2 g/cm3, particles lt 1 µm in
  diameter can remain in the atmosphere for several
  weeks

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The different processes that influence the
composition and the size-distribution of
aerosols.
http//cerea.enpc.fr/en/mphase.html
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Absorption and Chemistry on Particle Surfaces

• Small particles suspended in the atmosphere have
  large surface areas per unit mass
• ( 106 m2 / g )
• and can interact with gas phase molecules.
• ? Many volatilized species (including metals)
  end up bound to atmospheric aerosols

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Volatile species distribute themselves partly
between the gas and particle phases
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Absorption and Chemistry on Particle Surfaces

• Partitioning depends upon species volatility and
  is determined by the temperature dependence of
  the partition equilibrium constant
• log KP mP/T bP
• mP, bP constants
• mP enthalpy of desorption of the gas from the
  solid
• KP is small at high temperature and gases will
  desorb.
• Less volatile compounds have larger KPs (and
  mPs)

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Absorption and Chemistry on Particle Surfaces

• Large surface area-to-volume ratio of aerosols
  increases the likelihood of surface reactions.
• Common aerosol surfaces include
• hydrocarbon (PAHs)
• sulphuric acid
• water

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Absorption and Chemistry on Particle Surfaces

• These tiny particles can provide a surface where
  the ozone destruction reactions take place very
  rapidly. Aerosols only have an effect because of
  the currently high levels of stratospheric
  chlorine released from ozone destroying
  substances. They improve a chlorine atom's
  effectiveness at destroying ozone molecules,
  producing a short-lived spurt in ozone depletion.
• One disturbing point to note is that it appears
  to take much longer for aerosols to be removed
  from polar regions than from tropical regions.
  The polar regions, particularly Antarctica, are
  particularly susceptible to major drops in
  stratospheric ozone.

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Optical Effects

• When aerosol diameters are of the same order as
  the wavelengths of visible light, optical effects
  (light scattering) can occur
• Rayleigh Law (molecules)

(strictly true for very small particles, need
more complicated theories (Mie scattering) to
include all sizes
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Optical Effects

• But note
• Sky is blue (viewing scattered light) blue light
  is more effectively scattered
• Sunsets are red (viewing transmitted light) blue
  component is depleted
• eg. Blue haze of Great Smokey Mountains
• Climatic Effects !!!

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Condensation

• Clouds and liquid aerosols
• water condenses to form droplets when its vapour
  pressure exceeds the equilibrium vapour pressure
  of liquid water Po
• Consider the vapour/liquid equilibrium

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Condensation

• As the ratio P/Po exceeds 1 (100), ?G becomes
  negative droplets form spontaneously
• Small drops have high curvature and therefore
  high surface tension
• (thermodynamically unstable) ? evaporation

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Surface tension must be included!
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Condensation/Evaporation Condensation/evaporation
is a key process for the aerosol composition and
distribution. Gaseous species may condense onto
existing aerosols, or species in the aerosol
phase may evaporate. This mass transfer between
the aerosol and the gaseous phases depends on the
difference of concentrations of gaseous species
far from aerosols and the concentrations of
gaseous species at the surface of aerosols (which
are assumed to be at local thermodynamic
equilibrium with the aerosol internal
composition).
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Aerosols can warm and cool

• The aerosol layer reflects solar energy back to
  space, which contributes to cooling.
• But the aerosol might absorb and re-emit solar
  radiation, warming the atmosphere.
• Time of day and whether the aerosols are high or
  low in the atmosphere impacts how effectively
  they add to cooling or warming.
• The mechanism of cooling is related to sulfate
  aerosols which form after the eruption. If these
  aerosols are injected into the stratosphere, then
  they can stay there for over a year.

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Clouds

• A cloud reflects solar energy back to space,
  which contributes to cooling.
• A cloud also absorbs the Earths radiation and
  re-emits it back to the surface.
• It therefore has a greenhouse effect.
• A cloud also emits infrared radiation upwards.
• If the clouds are low in the atmosphere, then
  they contribute to surface warming.
• This is why it does not get as cold at night even
  if there are very thin clouds.
• If the clouds are very high in the atmosphere,
  like those associated with thunderstorms, they
  are very cold at the top, the atmosphere above
  them is thin,
• they emit efficiently to space and contribute to
  cooling.

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Clouds

• In a warmer environment the atmosphere will hold
  more water. (Remember, water is a greenhouse gas
  hence, will add to warming! A positive feedback.)
  If there is more water, then it is reasonable to
  expect more clouds. This will contribute to
  cooling because of reflection of solar energy,
  but there is also a warming effect, especially at
  night. Without calculations it is not easy to
  argue if clouds cool or if they warm. However,
  clouds do have the possibility of helping to
  regulate, cool, the Earth in a warming
  environment hence, they can be a negative
  feedback. As you think and rethink all that we
  have seen, the ice age cycles, the role of
  greenhouse gases, the increase of greenhouse
  gases, you develop an intuition. That intuition
  is that a stable climate on Earth is strongly
  reliant on water being able to exist in all three
  phases. As a gas it is supreme absorber. As ice
  it is supreme reflector. As clouds it?s a
  balance. And as a liquid, water is life and
  energy. It?s ultimately about water, water,
  water.

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http//climateknowledge.org/figures/Earth_System_R
eflection.JPG
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http//www.wunderground.com/blog/RickyRood/archive
.html?tstamp200704
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http//climateknowledge.org/figures/Cloud_Feedback
.JPG
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http//climateknowledge.org/figures/Earth_System_A
bsorption.JPG
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Arctic Warming
Scientists believe that the warming of the Arctic
climate and decreases in the area and thickness
of sea ice are caused by greenhouse gas warming.
The Arctic region also experiences large periodic
influxes of aerosols originating from the
industrial regions to the south. Using data from
the DOE ARM Climate Research Facility in Barrow,
Alaska, Vogelmann and Lubin determined that
enhanced aerosol amounts can make clouds emit
more thermal energy to the surface. In an
aerosol-cloud process, increased aerosol
concentrations cause the cloud droplets to become
smaller and, within clouds of fixed water
amounts, more abundant. Vogelmann and Lubin
discovered that this process makes many clouds
more opaque and emit more thermal energy to the
surface, by an average of 3.4 watts per square
meter, which is comparable to that by increased
greenhouse gases.
Because sunlight is generally weak in the Arctic,
the clouds, via their emission of thermal energy,
normally exert a net warming on the Arctic
climate system throughout most of the year,
except briefly during the summer.
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In a process known as the first aerosol indirect
effect, enhanced aerosol concentrations cause the
droplets in a cloud to be smaller and more
numerous within a cloud of fixed water amount.
This study found that this process can make the
clouds more opaque and emit more thermal energy
to the surface.
"We have concluded that the aerosol-cloud process
- called the first aerosol indirect effect -
operates in the clouds we studied, and results in
a greater downward thermal emission from the
cloud," Vogelmann said. "Its contribution to the
surface warming is comparable to that by the
so-called greenhouse effect."
http//www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?
prID06-09
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Aerosols and Global Warming

• Compared with the greenhouse gases aerosols are
  usually short lived - a few days in the
  troposphere.
• Because of their short lifetime and their
  localized sources, they are not evenly
  distributed in the atmosphere.
• Often aerosols are associated with polluted air.
• Two regions of particular interest are South Asia
  and China.

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http//earthobservatory.nasa.gov/
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Volcanic Contributions

• Most volcanoes do not penetrate the stratosphere.
  
• In fact, only a small number of eruptions have
  produced a significant amount of aerosols in this
  century.
• One example is Mt. Pinatubo, which injected 30
  million tons of aerosols into the stratosphere
  during its 1991 eruption in the Philippines. That
  amount is represented by the peak in the
  following graph.

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The topmost graph shows measurements from Barrow,
Alaska the lower graph represents measurements
taken at Mauna Loa, Hawaii.
http//images.google.ca/imgres?imgurlhttp//www.e
pa.gov/ozone/science/images/aerosol2.gifimgrefurl
http//www.epa.gov/ozone/science/myths/aerosol.ht
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This image shows a magnified view of aerosol
particles collected in the industrial city of
Port Talbot, England. Many of the particles
measure roughly 2.5 microns across, small enough
to easily enter and damage human lungs. This
Micrograph adapted from Sixth Annual UK Review
Meeting on Outdoor and Indoor Air Pollution
Research 15th16th April 2002 Web Report W12,
Leicester, UK. Credit MRC Institute for
Environment and Health
http//www.nasa.gov/lb/vision/earth/environment/ny
_air.html
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We affect our weather!

• According to a recent NASA study, the millions of
  people who work in New York City not only alter
  the population density each work day, they add to
  the density of tiny particles in the air called
  aerosols. Researchers have discovered for the
  first time in an American city a workweek pattern
  of aerosols clouding the air, believed to be
  created by the comings and goings of people
  working in the city. The study also finds these
  urban aerosols, thickest on Wednesdays and
  lightest on weekends, can affect air temperatures
  and clouds in big cities.

http//www.nasa.gov/lb/vision/earth/environment/ny
_air.html
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We affect our weather!

• The study, conducted using New York City as its
  test case, finds that aerosol cycles are affected
  by city structures, geography, and human
  activities like vehicle use and construction. The
  researchers gathered data over four summers from
  2000 to 2004 to complete the study. Specifically,
  they used aerosol information from the Moderate
  Resolution Imaging Spectroradiometer aboard
  NASA's Terra satellite, and daily and hourly
  measurements from NASA's AErosol RObotic NETwork
  (AERONET), a ground-based network of sensors that
  detects aerosols in the air.

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We affect our weather!

• Aerosols are known to serve as nuclei for water
  and ice droplets to form around. Previous
  research indicates that greater concentrations of
  aerosols disperse water among more nuclei,
  leading to higher concentrations of cloud drops
  that are smaller in size. These smaller drops may
  fail to grow big enough to fall as rain,
  therefore, reducing rainfall in urban areas.
  Opposite to this, however, heated urban surfaces
  such as roads, parking lots and buildings cause
  stronger convection, or the rising of hot air,
  which can cause more rainfall. The overall effect
  on rainfall over New York City is actually a
  result of these two competing processes. Thus
  aerosols do indeed play a role in big city
  weather.

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