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Title: Advances in the Chemistry of Atmosphere

Advances in the Chemistry of Atmosphere
Welcome to
  • CHEM-ATOC 419/619

  • Prof. P. Ariya
  • Burnside Hall 809, Otto Maas 421
  • Tel 398-6931, -3615 E-mail parisa.ariya_at_mcgill.
  • Office hours Drop by or call for an appointment

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

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.

  • 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
  • Chemistry of the climate change
  • Your research topics!

  • 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!
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.

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

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

  • 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

  • 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
  • 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

Evolution of the Atmosphere Early (Primeval)
  • 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

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

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

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
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
  • 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.

Recent Changes in Gas Concentrations
  • The number density nX of a gas X is defined as
    the number of molecules of X per unit volume of
  • 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
  • Concentrations of short-lived radicals and other
    gases which are of interest primarily because of
    their reactivity are usually expressed as number
  • 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)

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.
  • 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
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 Pressure and Air Density
  • Pressure decreases as we ascend in the vertical

Vertical Pressure Profile
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

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

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

Atmospheric Chemistry elsewhere
What about Mars?
Artists Picture of Mars and its Atmosphere
  • 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.


Mars Atmosphere
  • 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
  • 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).

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

Impact on Earths Climate
  • 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
  • Chemical reactions at the surface as well as
    inside particles

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

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

Radio Wave Reflections
  • Summary