Atmospheric Chemistry

Lecture 1: Chemical Principles and

Stratospheric Ozone Chemistry

Jim Smith
Atmospheric Chemistry Division / NCAR
jimsmith@ucar.edu
Basic Chemical Concepts

Chemical composition and units of measurement

Reactants: molecules, radicals, ions

Reactions: primary, binary, tertiary, chain reaction,
catalysis

Equilibrium

Reaction Rates and Lifetimes

Photochemistry

Dry and wet deposition
Basic Chemical Concepts
Chemical composition
of the atmosphere

The bulk composition of

the air (99.997% by vol.)
consists of mainly N2, O2,
Ar, CO2. These are stable
species with little or no
interesting chemistry!

About 99% of the mass of
the atmosphere is located
below 50 km, in the
stratosphere and the
troposphere.
Basic Chemical Concepts
Units of measurement

Because we deal with such trace amounts of material, we have adopted

special units of measure to describe the concentration of gas phase
chemical compounds in the atmosphere.
• Number Density (molecules/cm3)
• Mass Density (kg/m3 or g/cm3).
• Frequently we use dimensionless volume mixing ratio:
ni
µi =
na
where ni is the number density of species i and na is the air number
density.
• and mass mixing ratio:
ρi
µ~i =
ρa
where each now refers to the mass density of species i and of air.
 Basic Chemical Concepts
Units of measurement, continued …

Mixing ratios are usually volume mixing ratios unless indicated, and
usually presented as follows:
µ term
10-6 parts per million (ppmv)
10-9 parts per billion (ppbv)
10-12 parts per trillion (pptv)

The “v” in the units refers to volume mixing ratios, which are used so
much more frequently than mass mixing ratios that we often get lazy
and leave the “v” off the units.

Going from one unit to the next is accomplished by approximating the
atmosphere as an ideal gas and then using the ideal gas law:

pV = nRT , where p is pressure, V is volume,

n is number of moles, R is the gas constant (0.0821 L-atm/(mol-K)).

You will also need to know Avogadro’s constant, which defines the
mole as 6.022x1023 molecules.
 Basic Chemical Concepts
Reactants

Reactants are individual chemical compounds that participate in reactions. Several
types are important in atmospheric processes:

Free radicals: neutral compounds containing an odd number of electrons. These are
highly reactive. The most important free radical in the atmosphere is hydroxyl, which is
the combination of oxygen and hydrogen atoms:
O H

to form a complex with one unpaired electron that REALLY wants to find another
electron:
O H

Ions: an atom or molecule that carries a positive or negative electric charge as a result
of having lost or gained one or more electrons.
Ions are important
• in the upper reaches of the atmosphere (above 6km, from cosmic radiation),
• in clouds (lightning from thunderstorms), and
• in the liquid droplets themselves.
• E.g., the dissolution of nitric acid in clouds or fog, and is one of the primary
contributors (along with H2SO4) to “acid rain.”
HNO3 + H2O H3O+ + NO3-
 Basic Chemical Concepts
Reactions

unimolecular reaction (reaction involving one species), e.g., photodissociation of
ozone: O + hν (290 nm < λ <350 nm) O + O (1)
3 2

The oxygen atom reacts quickly with water to form 2 molecules of hydroxyl in a
bimolecular reaction (reaction involving two reactants):
O + H2O 2 OH (2)
The net result of this sequence is the formation of OH from ozone photolysis:
O3 + hν + H2O 2 OH + O2

Reactions 1 and 2 are elementary reactions, that is, they cannot be subdivided into
two or more simpler reactions. Together they form a chain reaction. Reaction 1 is the
one that started it up, and this is called the initiation step. Subsequent steps, such as
reaction 2 and others that may follow are called propagation steps in the reaction.

A chain reaction will end with the formation of stable products (called the termination
step):
OH + NO2 + M HNO3 + M
In this reaction, nitric acid is formed in a termolecular reaction (a reaction involving
three reactants). M is commonly N2 or O2 and stabilizes the product through collisions.
Without such collisions, the nitric will have so much internal energy that is will fall apart
as soon as it is formed.
 Basic Chemical Concepts
Reactions, continued …
A catalytic cycle is one in which a molecule significantly changes or enables a
reaction cycle without being altered by the cycle itself. The example that we
will see later today is the catalytic destruction of stratospheric ozone, which is
carried out by the following mechanism:
X + O3 XO + O2

O3 + hν O + O2

O + XO X + O2

Net: 2 O3 + hν 3 O2
Here, the radical X (which can be Cl, NO, Br, etc.) catalyzes the destruction of
ozone and is regenerated in the process, thus allowing for the cycle to repeat
over and over … “a little goes a long way”!
 Basic Chemical Concepts
Equilibrium

Many chemical reactions occur in both directions such that the products are
able to re-form the reactants. The result is that equilibrium will be established
between products and reactants. In the example:
HNO3 H+ + NO3-
Equilibrium can be described by:
[H + ][NO 3− ]
K= = 15.4M
[HNO3 ]

Note that all reactions have an equilibrium constant associated with them. In
the gas phase reaction:
CH4 + 2O2 CO2 + 2H2O
The equilibrium constant is about 10140. Most gas phase reactions are like this,
that is, they are irreversible.
 Basic Chemical Concepts
Reaction Rate Laws

The speed of a chemical reaction is called the reaction rate. The science that deals
with reaction rates is called chemical kinetics. A basic understanding of chemical
kinetics is crucial to atmospheric chemistry.

In chemical kinetics-speak, unimolecular reactions are part of a class of reactions called
first-order reactions:
k
A products

We define k as the reaction rate constant, which is a constant of proportionality
between the rate at which A is depleted and the concentration of A, defined as:
d [A]
− = k ⋅ [A].
dt

If we look at the rate at which A is depleted, the solution to the above differential
equation is:
[A] = [A]o exp(− kt )

Where [A]o is the initial concentration of A. In this process, A will have a chemical
lifetime that can be characterized by:
1
τ=
k
 Basic Chemical Concepts
Reaction Rate Laws, continued …

Most reactions in the atmosphere are bimolecular, and we call these reactions
second-order reactions. In the reaction:
k
aA + bB ... + cC + ...
the rate coefficient is defined as:
1 d [A] a b 1 d [C]
− = k ⋅ [A] ⋅ [B] = .
a dt c dt

Analogous to the case of first order reactions, the chemical lifetime of A is
defined as: 1
τA =
k[B]b

What if you have numerous reactions that compete in depleting A? How do we
determine the lifetime of A? We sum up the individual contributions as follows:

τ −1 = τ a−1 + τ b−1 + ...

 Basic Chemical Concepts
Chemical lifetimes
A plot of the lifetime of
chemical compounds in
the atmosphere shows a
wide variety, from
seconds (in the case of
free radicals), to centuries
(in the case of stable
molecules). Also note that
the chemical lifetime
determines the spatial and
temporal variability of the
species. Long-lived gases
such as CFCs are well
mixed in the troposphere,
whereas radicals exhibit
much more temporal and
spatial variability.
 Basic Chemical Concepts
Reaction Rate Laws, continued …

Lastly, many reactions are termolecular, or third-order, reactions. The
example given above demonstrates one such reaction:
OH + NO2 + M HNO3 + M

How do we deal with calculating the rates of such reactions? In most
termolecular reactions, one compound is present at equal concentrations
before, during, and after the reaction. In the example above, the species M is
one of these compounds. Since M remains constant during the reaction, we
can simplify the kinetics by representing it as a pseudo-second-order
reaction:
d [NO2]
− = k ′ ⋅ [OH] where k ′ = k [M ]
dt

All the rules of second order reactions thus apply!

 Basic Chemical Concepts
Reaction Rate Laws, cleaning up …

Don’t forget, that in the case of a chain reaction:
k1
A B + C
k2
B + D products

you need to consider both the production and loss of B when you apply rate
equations:
d [B]
= k1 ⋅ [A] - k 2 ⋅ [B][D]
dt

Another important concept is that, often, compounds are in steady state. If B
were in steady state in the above example, then
d [B]
= k1 ⋅ [A] - k 2 ⋅ [B][D] = 0
dt
 Basic Chemical Concepts
Photochemistry

One of the unique features of the “atmospheric chemical reactor” is that many reactions
are initiated by the absorption of photons of solar radiation. We have already seen and
example of this:
O3 + hν (290 nm < λ <350 nm) O + O2

Just as we defined a rate constant for a chemical reaction, so too can we define a
photodissociation rate constant Ji (sometimes in the vernacular you hear these referred
to as “J-values,” given by:
d [O3 ]
− = J O3 ⋅ [O3 ].
dt

Note the important difference between J values and regular rate constants. In the case
of photochemistry, J is not constant but is a function of must be computed for each
compound considering the time of day, and altitude. The expression for this is:
λmin
Ji = ∫ σ i (λ ) ⋅ φi (λ ) ⋅ I (λ ) ⋅ dλ
λmax

where σi(λ) is the absorption cross-section of compound i (often given in cm2/molecule),

I(λ) is the photon flux (units of photons/cm2), and Φi(λ) is the quantum yield of the
reaction, which states the probability that the absorption of a photon will result in a
reaction.
 Basic Chemical Concepts
Deposition to Surfaces

For many gases and for particles as well, deposition to surfaces competes with
chemical reaction for the depletion of those species. We will consider dry and wet
deposition.

Dry deposition refers to the flux into surfaces in the absence of precipitation (but the
surface can be wet), whereas wet deposition applies to deposition into fogs, clouds,
rain, or snow where the gas is incorporated in the bulk solution.

In both cases, the relationship between the concentration of species i and the vertical
flux, Φi, to a surface is given by:
Φ i = v d [Ci ]
where vd is deposition velocity of species i with units of, most commonly, cm/s.
The deposition velocity is difficult to define by first principles, depending on:
• the type of surface (tree, grass, asphalt, etc)
• atmospheric conditions (temperature, pressure, wind)
• the reference height at which the concentration of species i is measured
 Basic Chemical Concepts
Gas – particle partitioning

Both can be a function of the solubility of the gas, so an important parameter
that dictates the deposition velocity is the Henry’s law constant, Hc, for the
gas. Henry’s law is the expression that defines the partitioning of a species
between gas and solution phase:
[Cso ln ] = H c [C gas ]

For solutions of water, molecules that dissolve readily are termed hydrophilic,
whereas those that are sparingly soluable are termed hydrophobic.
• Examples of hydrophilic compounds, for which wet
deposition or dry deposition onto wetted surfaces is a
significant loss mechanism include the agents of acid
rain, H2SO4 and HNO3, and ammonia.
• Examples of hydrophobic compounds are hydrocarbons
such as the alkanes, e.g., methane.
 Stratospheric Ozone Chemistry: Origins
Chapman Mechanism

Ozone
: is produced by the photodissociation of O2 by solar UV radiation:
O2 + hν (λ < 242 nm) O + O
2 [ O + O2 + M O3 + M ]
Net: 3O2 + hν 2O3

The following reaction sequence then recycles ozone back into O2:
O3 + hν O + O2
O + O3 2O2
Net: 2O3 + hν 3O2

Ozone in the stratosphere is maintained as a result of a dynamic balance
between these formation and destruction processes. From 1930 to 1975, this
was the basis for our understanding of the ozone layer. We now know that the
loss mechanism from the Chapman scheme accounts for only 20% of ozone
loss.
Stratospheric Ozone Chemistry
Catalytic Ozone Destruction Cycle: Chlorine
(Rowland and Molina, 1975)
Stratospheric Ozone Chemistry
Catalytic Ozone Destruction Cycle: Chlorine
(Rowland and Molina, 1975)

ClO + ClO + M Cl2O2 + M

Cl2O2 + hv Cl + ClO2
ClO2 + M Cl + O2 + M
then: 2 x (Cl + O3) 2 x (ClO + O2)
net: 2 O3 3 O2

Termination Steps

ClO + NO2 + M ClONO2 + M

Cl + CH4 HCl + CH3
Stratospheric Ozone Chemistry
Catalytic Ozone Destruction Cycle: Chlorine

Measured ozone loss (DU) over

Predicted ozone loss (%) Antarctica … showing a
vs. year calculation was made nosedive in the 1970s.
Stratospheric Ozone Chemistry

View this movie at:

http://www.atm.ch.cam.ac.uk/tour/anim_toms.html
Stratospheric Ozone Chemistry
O3 and ClO in the lower stratosphere over Antarctica, as
recorded by J. Anderson et al. (Sept. 1987)

[ClO] (ppb)

[O3] (ppm)
The good news (sort of): Proved chlorine chemistry is causing ozone
destruction!
The bad news: ClO levels were 100 times higher than predicted by gas-
phase models … something was missing in the models…
Stratospheric Ozone Chemistry
Polar Stratospheric Clouds
 Stratospheric Ozone Chemistry
Polar Stratospheric Clouds

The polar winter leads to
the formation of the polar
vortex which isolates the
air within it.

Cold temperatures form
inside the vortex; cold
enough for the formation
of Polar Stratospheric
Clouds (PSCs). As the
vortex air is isolated, the
cold temperatures and
the PSCs persist.
Stratospheric Ozone Chemistry
Polar Stratospheric Clouds, continued …

ClONO2

HCl

ClONO2

HCl
Stratospheric Ozone Chemistry
Polar Stratospheric Clouds, the rest of the story

Recall the termination steps for the Chlorine catalytic cycle:
ClO + NO2 + M ClONO2 + M
Cl + CH4 HCl + CH3

Once the PSCs form, heterogeneous reactions take place and convert
the inactive chlorine reservoirs to more active forms of chlorine.
HCl + ClONO2 HNO3 + Cl2
ClONO2 + H2O HNO3 + HOCl
HCl + HOCl H2O + Cl2
N2O5 + HCl HNO3 + ClONO
N2O5 + H2O 2 HNO3

No ozone loss occurs until sunlight returns to the air inside the polar
vortex and allows the production of active chlorine and initiates the
catalytic ozone destruction cycles. Ozone loss is rapid. The ozone hole
currently covers a geographic region a little bigger than Antarctica and
extends nearly 10km in altitude in the lower stratosphere.
Stratospheric Ozone Chemistry
Proof of PSC role: Mt. Pinatubo Eruption (June 1991)

The largest eruption

of the 20th Century

20 million tons
of SO2 emitted into
the stratosphere,
which rapidly forms
H2SO4 aerosols.
Stratospheric Ozone Chemistry
Stratospheric aerosols following Pinatubo eruption as measured by
SAGEII (Aerosol optical depth indicated)
Before During

0.5-1 µm2/cm Core after 5 mos.

18 mos. after 35 cm2/cm

Avg: 20 µm2/cm
 Stratospheric Ozone Chemistry
Following the Pinatubo eruption, 1992-93 levels of ozone
were at record lows.

Observations from various
stations recorded a reduction in
stratospheric Ozone of between
6-30%.

Pinatubo aerosols may have
been responsible for the a loss
of 10% of Antarctic ozone
"before" the Antarctic ozone hole
formed in the winter of 1992.

Over the Antarctic ozone was
50% lower than normal between
13-16 km altitude and was totally
absent between 16-18 km.

Ozone hole appeared over
Europe for the 1st time.
Stratospheric Ozone Chemistry
Equitorial Ozone following Pinatubo eruption as measured by TOMS
(from NASA/Goddard Space Flight Center Scientific Visualization Studio)

View this movie at:

http://svs.gsfc.nasa.gov/vis/a000000/a002100/a002183/