Advances in the Chemistry of Atmosphere

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Advances in the Chemistry of Atmosphere
Welcome to

• CHEM-ATOC 419/619

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Instructor

• Prof. P. Ariya
• Burnside Hall 809, Otto Maas 421
• Tel [phone], -3615 E-mail parisa.ariya_at_mcgill.
  ca
• Office hours Drop by or call for an appointment

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Grading

• For undergraduate students
• Midterm 30
• Research Project 35
• Final examination 35
• For Graduate students
• Research project 65
• Final exam 35

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Course Objectives

• Serves as an introductory but comprehensive
  course to the field of atmospheric Chemistry
• Purpose is to
• Introduce (photo)chemical, atmospheric kinetic,
  field studies, and modeling concepts in a
  visual and practical manner using updated
  scientific research
• Provide an opportunity for students to apply
  selected atmospheric concepts in practical
  manner through a research project.

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COURSE OUTLINE

• Introduction Earths atmosphere, chemical
  composition and its vertical structure
• Radiation balance of atmosphere green house
  gases, absorption and photochemistry
• Oxidation potential of the atmosphere
  atmospheric oxidants and homogeneous chemistry
• Aerosols and heterogeneous chemistry
• Selected topics Chemistry of ozone hole and
  air pollution
• Formation process of cloud chemical reactions
  in and on cloud particles
• State-of-the-art field measurement techniques in
  atmospheric chemistry
• Atmospheric modeling 0, 1-D, 2-D and 3-D
  modeling
• Chemistry of the climate change
• Your research topics!

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Textbooks

• Chemistry of the Upper and Lower Atmosphere B.J.
  Finlayson -Pitts and J.N. Pitts, Jr., Academic
  Press, 2000.
• Atmospheric Chemistry and Physics From Air
  Pollution to Climate Change J.H. Seinfeld, S.N.
  Pandis, 1998.
• Introduction to Atmospheric Chemistry, Daniel J.
  Jacob, Princeton University Press, 1999.
• Available in the McGill bookstore

You do not need to purchase Any books!
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Useful References

• Climate Change, The IPCC Scientific Assessment
  J.T. Houghton et al., eds., Cambridge Univ.
  Press, Cambridge, UK, 1990.
• Climate Change 1995 The Science of Climate
  Change J.T. Houghton et al., eds., Cambridge
  Univ. Press, Cambridge, UK, 1996.
• Climate Change 2001 The Science of Climate
  Change J.T. Houghton et al., eds., Cambridge
  Univ. Press, Cambridge, UK, 2000.
• Climate Change 2004 The Science of Climate
  Change J.T. Houghton et al., eds., Cambridge
  Univ. Press, Cambridge, UK, 2003.
• Scientific Assessment of Ozone Depletion 1994
  World Meteorological Organization, Global Ozone
  Research and Monitoring Project - Report No. 37.
• Scientific Assessment of Ozone Depletion 1998
  World Meteorological Organization, Global Ozone
  Research and Monitoring Project - Report No. 44.
• Biogeochemistry, An Analysis of Global Change
  W.H. Schlesinger, 2nd ed. 1997.
• Composition, Chemistry, and Climate of the
  Atmosphere H.B. Singh, ed., 1995.
• Atmospheric Change An Earth System Perspective
  T. E. Graedel and P. J. Crutzen, 1993.
• Chemistry of Atmospheres (2nd Edition), R. P.
  Wayne, 1991.
• The Physics of Atmospheres J.T. Houghton, 2nd
  edition, 1986.
• Chemistry of the Natural Atmosphere (2nd Edition)
  P. Warneck, 2000.
• Atmospheric Chemistry and Global Change Edited by
  G.P. Brasseur, J.J. Orlando, and G.S. Tndall,
  Oxford University Press, 1999.
• Earth Under Siege From Air Pollution to Global
  Change R.P. Turco, Oxford University Press, 1997.
  

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Importance of our Atmosphere

• We are always affected by the atmosphere
• Many natural disasters are linked with the
  atmosphere
• There is concern that our climate is changing and
  atmosphere is a pivotal player
• Chemistry can play a key role in the physical
  processes
• Atmospheric chemists have been the leaders in
  fundamental photochemistry, kinetics, molecular
  dynamics, and high-resolution spectroscopy!!!
• .

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climate and the atmosphere

• Climate is the weather conditions at a certain
  location averaged over a specified time period
  (e.g., winter climate over Quebec), and their
  departure from long-term averages
• Climatology is the study of climate, its controls
  and variability

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Atmosphere

• Atmosphere is a thin envelope of gases and tiny
  particles that surround Earth
• 99 of atmosphere's mass is confined to a layer
  of thickness ¼ of the earth's diameter
  atmosphere around Earth is thus thin, like the
  peel of an apple
• atmosphere is essential for life contains oxygen
  and carbon dioxide for life sustaining processes,
  supplies water and shields life from harmful
  ultraviolet radiation from Sun

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• Discoveries of Air Composition
• Greek Philosophers Earth, water, fire and air -
  Aristotle (384-322 BC) classified water and air
  spheres as one.
• da Vinci (1452-1519) and Mayow (1641-1679) Fire
  air and foul air. "Fire air" (oxygen) was
  isolated by Swedish chemist, Scheele (1742-1786).
  
• Laplace (1749-1827) showed carbon dioxide as a
  component of animal respiration and Black
  identified it in atmosphere.
• Ramsey (1852-1916) identified argon and a few
  other nobel gases in the atmosphere (Nobel
  prize!)
• Ozone discovery by Schonbein (1799-1868) and
  Houzeau made the first measurement in 1858.
  Methane was identified in 1862 by Boussingault
  and 50 years later was detected in the atmosphere
  by IR solar spectrum.
• During the 20th century CFCs, Nitrogenated
  species, hydrogen, non-methane hydrocarbon and
  etc.

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Evolution of the Atmosphere Early (Primeval)
Phase

• Earth's birth was about 4.6 billion years ago
• lava, ashes, gases from volcanoes ("outgassing")
  form the Earth's primeval atmosphere, hugging the
  planet due to the gravitational field of the
  Earth the atmosphere consisted of mostly CO2
  (carbon dioxide), N2 (nitrogen) and water vapor
  (H2O

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Early Phase

• surface temperature might have been as high as 85
  to 110 oC (compared to 15oC today) planet cooled,
  water vapor condensed to form clouds and rain,
  hence oceans a lot of the CO2 in atmosphere
  dissolved in rainwater life formed about 2
  billion years ago, and photosynthesis produced
  oxygen (O2)

• ozone (O3) shield formed

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Evolution of the Atmosphere Modern Phase

• main atmospheric components are N2 (78.08 by
  volume) and O2 (20.95) in the layer below 80 km
  other constituents are water vapor, trace gases
  and aerosols
• water vapor concentration is highly variable,
  ranging from 0 to 4

• trace amounts of CO2, O3 and other gases
• aerosols are tiny liquid and solid particles
  originating from forest fires, wind erosion of
  soil, crystals from ocean spray, volcanic
  emission, and meteoric dust

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ATMOSPHERIC COMPOSITION
(volume)
N2 78.08
O2 20.95
CO2 0.033
Ar 0.934
Ne 1.82E-03
He 5.24E-04
Kr 1.14E-04
Xe 8.7E-06 H2 5.0E-05 CH4 2.0E-04
N2O 5.0E-05
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MEASURES OF ATMOSPHERIC COMPOSITION The
objective of atmospheric chemistry is to
understand the factors that control the
concentrations of chemical species in the
atmosphere. MIXING RATIO The mixing ratio CX
of a gas X (equivalently called the mole
fraction) is defined as the number of moles of X
per mole of air. Units are mol/mol or v/v since
the volume occupied by an ideal gas is
proportional to the number of molecules.
Pressures in the atmosphere are sufficiently low
that the ideal gas law is always obeyed to within
1.
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• The mixing ratio of a gas has the virtue of
  remaining constant when the air density changes
• Consider a balloon filled with room air and
  allowed to rise in the atmosphere. As the balloon
  rises it expands, so that the number of molecules
  per unit volume inside the balloon decreases
  however, the mixing ratios of the different gases
  in the balloon remain constant.
• The mixing ratio is therefore a robust measure of
  atmospheric composition.

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Recent Changes in Gas Concentrations
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• NUMBER DENSITY
• The number density nX of a gas X is defined as
  the number of molecules of X per unit volume of
  air.
• It is expressed commonly in units of molecules
  cm-3 (number of molecules of X per cm3 of air).
  Consider the bimolecular gas-phase reaction

• The loss rate of X by this reaction is equal to
  the frequency of collisions between molecules of
  X and Y multiplied by the probability that a
  collision will result in chemical reaction.
• The collision frequency is proportional to the
  product of number densities nXnY. When we write
  the standard reaction rate expression (1.1)

where k is a rate constant, the concentrations
in brackets must be expressed as number
densities.
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• Concentrations of short-lived radicals and other
  gases which are of interest primarily because of
  their reactivity are usually expressed as number
  densities.
• Another important application of number densities
  is to measure the absorption or scattering of a
  light beam by an optically active gas.
• The degree of absorption or scattering depends on
  the number of molecules of gas along the path of
  the beam and therefore on the number density of
  the gas.
• Consider in this atmosphere an optically active
  gas X. A slab of unit horizontal surface area and
  vertical thickness dz contains nXdz molecules of
  X. The integral over the depth of the atmosphere
  defines the atmospheric column of X as (1.2)

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Absorption of radiation by an atmospheric column
of gas.
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• The number density and the mixing ratio of a gas
  are related by the number density of air na
  (molecules of air per cm3 of air)

(1.3) The ideal gas law gives
(1.4) where R 8.31 J mol-1 K-1 is the gas
constant. The number density of air is related to
N and V by
(1.5) where Av 6.023x1023 molecules mol-1 is
Avogadro's number. Substituting equation (1.5)
into (1.4) we obtain
(1.6) and hence
(1.7) We see from (1.7) that nX is not conserved
when P or T changes.
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• A related measure of concentration is the mass
  concentration ?X, representing the mass of X per
  unit volume of air (we will also use ?X to denote
  the mass density of a body, i.e., its mass per
  unit volume the proper definition should be
  clear from the context). ?X and nX are related by
  the molecular weight MX (kg mol-1) of the gas

(1.8) The mean molecular weight of air Ma is
obtained by averaging the contributions from all
its constituents i
(1.9) and can be approximated (for dry air) from
the molecular weights of N2, O2, and Ar
(1.10)
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Effect of humidity on air density
Question In surface air over the tropical oceans
the mixing ratio of water vapor can be as high as
0.03 mol/mol. What is the molecular weight of
this moist air?
Answer. The molecular weight Ma of moist air is
given by where Ma,dry 28.96x10-3 kg mol-1 is
the molecular weight of dry air derived in (1.10)
, and MH2O 18x10-3 kg mol-1. For CH2O 0.03
mol/mol, we obtain Ma 28.63x10-3 kg mol-1.
A mole of moist air is lighter than a mole of dry
air.
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Air Pressure and Air Density

• Pressure decreases as we ascend in the vertical
  direction

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Vertical Pressure Profile
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A reminder about Pressure

• Surface air pressure is proportional to weight of
  column of air over a unit area at Earth's surface
• Units of pressure are force/unit area
  imperial units pounds/in2 metric units
  Pascals (mass in kg and area in m2)
• Pressure is often measured in millibars (mb) 1
  Pa 0.01 mb
• an inflated automobile tire has a pressure of
  about 30 pounds/in2
• surface air pressure is about 14.7 pounds/in2, or
  about 1013 mb
• Conversion 1 pound/in2 68.9 mb

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Vertical Structure of the Atmosphere

• the radiosonde, developed in the 1920's,
• is a small instrument package carried aloft by
  helium balloons equipped with a radio transmitter
  
• measures temperature, air pressure and humidity,
  giving soundings (variation in the vertical) of
  these quantities

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Vertical Structure of the Atmosphere

• troposphere temperature decreases with height at
  about 6.5 oC per km of ascent, weather occurs in
  this layer, which extends from the surface to
  about 12 km
• stratosphere temperature is initially isothermal
  (i.e. constant) with height, then increases with
  height to about 50 km this layer is ideal for
  jet travel as it is above weather disturbances
• mesosphere temperature decreases with height, to
  about 80 km thermosphere temperature increases
  once again with height above 80 km

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Vertical Structure of the Atmosphere

• the boundaries between these layers are the
  tropopause, stratopause and mesopause there are
  thus 3 regions of relative warmth Earth's
  surface, stratopause and above 80 km

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Atmospheric Chemistry elsewhere
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What about Mars?
Artists Picture of Mars and its Atmosphere
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• Mars has a very thin atmosphere made mostly of
  carbon dioxide. The surface pressure on Mars is
  only about 0.7 of the average surface pressure
  at sea level on Earth.
• Also, the atmospheric pressure on Mars changes
  seasonally because the temperature is cold enough
  that some of the carbon dioxide freezes during
  the winter and "snows" onto the polar cap. This
  greatly reduces the amount of carbon dioxide left
  in the atmosphere.
• During the summer, when the polar cap warms up
  again, the carbon dioxide goes back into the
  atmosphere. Mars also has a lot of dust in its
  atmosphere, and winds occasionally create large
  dust storms.

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Mars Atmosphere
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• Aerosols
• "Aerosol" is a general term describing a
  dispersed condensed phase suspended in a gas.
• Atmospheric aerosol particles are typically
  between 0.01 and 10 mm in diameter (smaller
  particles grow rapidly by condensation while
  larger particles fall out rapidly under their own
  weight).
• General measures of aerosol abundances are the
  number concentration (number of particles per
  unit volume of air) and the mass concentration
  (mass of particles per unit volume of air).

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SATELLITE VIEW
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Why aerosols are important in atmosphere?

• Air Pollution local, regional and global
• e.g., reduction of visibility
• Climate change Global
• (direct and indirect effect)
• Health hazard

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Impact on Earths Climate
h?

• Influence the amount of sunlight that impinges
  on the surface.
• Alter the cloud albedo
• Offset a significant fraction of the warming
  due to accumulation of greenhouse gases in the
  atmosphere.
• Chemical reactions at the surface as well as
  inside particles

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This cloud has more small droplets, hence
reflects more sunlight (Brighter Cloud)
This cloud has only few droplets, hence reflects
less sunlight (Darker Cloud)
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Aerosol and Particle Formation
Bulk-to-particle-conversion(Formation from
Solids)
Gas-to-particle-conversion (Formation from the
Gas Phase)
Drop-to-particle-conversion (Formation from the
Cloud Droplets)
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Aerosols
Chemical reactions (e.g., h?, HO, O3, )
Secondary aerosols
Primary Aerosols
Daniel Jacob, 1999
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• Main Types of Aerosols
• Continental/ Desert Aerosols
• Marine Aerosols
• Industrial Aerosols
• Volcanic Aerosols
• Organic Forest Hazes
• Smoke/ Biomass
• Burning Aerosols

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An Example Ionosphere

• Ionosphere is not actually a layer, but an
  electrified region where ions and free electrons
  exist
• Plays a major role on radio communications
• D region reflects AM signals, thus enabling
  transmission over large distances on Earth

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Radio Wave Reflections
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• Summary