CHEM 321 Quiz 3

what is Earths diameter?

12,756 km

what is the height of the atmosphere?

<100km

up to what height can humans live?

<8km

what are the regions of the atmosphere?

• troposphere

• stratosphere

• mesosphere

• tropopause

• stratopause

how does pressure change as we go up in space?

pressure continuously decreases as altitude increases

troposphere

• 0-12 km

• 15-18 km

• humans live in 8km of it

tropopause

• the boundary layer that separates the troposphere and stratosphere

• contains 85% of atmospheric gas

• -56 deg C

stratosphere

• 18-50 km

• temperature ranges from -56 - -2 deg C

• permanent temperature inversion

stratopause

• ~50 km in height

• where the ozone layer is

• -2 deg C

  • peak warmest (excluding atmosphere close to Earths surface)

mesosphere

• 50 - 80 km

• 50-100 deg C

• temperature decreases as we go up atmosphere

why is it so warm at the stratopause?

because theres absorption of radiation

composition of the atmosphere

why is water not included in the standard atmosphere?

its too variable!

the mixing ratio of CO2 was 314 ppm in 1976, what is it today (2026)?

426.07

• increase of 112 ppm in 50 years

• in picture: theres a constant increase

distribution of water in the atmosphere depends on …

height!

distribution of water in the atmosphere - troposphere

• up to several %

distribution of water in the atmosphere - tropopause

• water vapour is contained here and does not go up to stratosphere

distribution of water in the atmosphere - stratosphere

• no clouds here

  • because as warm, moist air from the troposphere rises (mostly via strong tropical convection), it cools down. By the time moisture reaches the top of the troposphere, the water vapor freezes into ice crystals and falls back down, leaving the air entering the stratosphere stripped of almost all moisture

• VERY LITTLE water in the stratosphere!!

distribution of ozone in the atmosphere

• almost no ozone in mesosphere (very minimal)

• LOTS of ozone in the stratosphere = stratospheric “ozone layer”

  • 25km above Earths surface

  • protects us from high solar radiations

electromagnetic radiation

as wavelength decrease…

energy increases

the major range has different effects on…

wavelengths

what effect does x-ray radiation have on molecules?

ionization (ejection of core electrons)

what effect does ultraviolet radiation have on molecules?

bonds break and molecules dissociate

(called ultraviolet because next to violet visible light!)

what effect does visible radiation have on molecules?

electronic transitions (movement of electrons between energy levels within a molecule)

rainbow!

what effect does infrared radiation have on molecules?

molecules vibrate = heat

(called infrared because next to red visible light!)

what effect does microwave radiation have on molecules?

molecules rotate

lights energy and wavelength - what was Albert Einsteins finding?

light can be considered not only a wave phenomenon but also to have particle-like properties in that it is absorbed (or emitted) by matter only in photons (aka finite packets)

lights energy and wavelength - the Einstein - Planck equation

E= hv = hc/λ

• since λv = c

• E = energy

• v = frequency = hc/λ

• h = Planck’s constant = 6.626 × 10^-34 J s

• c = speed of light = 2.99 × 10^8 m/s

• λ = wavelength

the shorter the wavelength of light, the …

greater the energy it may transfer to matter when absorbed

• inverse relation

what is absorbed light?

lights energy is absorbed in the volume of the material

what is transmitted light?

light emerges propagating in the same direction as the incident light

what is scattered light?

light emerges in a different direction from the incident light

light pic

absorption of radiation by gases: beer-lambert law

I/I₀ = e^{-σ x n x L}

• I = intensity transmitted

• I₀ = intensity of incident light

• σ = absorption cross section (cm²/molecule) = the molecule property (the molecule absorbing the light)

• n = number density of absorbing molecules (molecules/cm³) =. concentration

• L = optical path length (cm)

• e = euler’s number (=2.7182818)

what happens if you increase σ

• more molecules absorb light more effectively = less transmitted light

what happens if you increase n?

• more molecules are available to absorb light = less transmitted light

what happens if you increase L?

• light travels through more gas = more opportunities for absorption = less transmitted light

scattering of radiation by gases: beer-lambert law: Rayleigh scattering

I/I₀ = e^{-σ x n x L}

• I = intensity transmitted

• I₀ = intensity of incident light

• σ = scattering cross section (cm²/molecule)

• n = number density of molecules (molecules/cm³) =. concentration

• L = optical path length (cm)

• e = euler’s number (=2.7182818)

scattering of radiation by gases: at low wavelength…

light is scattered more efficiently than at high wavelengths

• the sky is blue because blue is short wavelengths, meaning it scatters stronger

absorption and scattering of radiation by gases: beer-lambert law

I/I₀ = e^{-(σ_abs x n + σ_raylaiegh x n ) x L}

• I = intensity transmitted

• I₀ = intensity of incident light

• σ_abs = absorption cross section (cm²/molecule)

• σ_rayleigh =scattering cross section (cm²/molecule)

• n = number density of absorbing molecules (molecules/cm³) =. concentration

• L = optical path length (cm)

• e = euler’s number (=2.7182818)

solar radiation

energy required to dissociate oxygen

O2 + hv → 2 O

what is the bond dissociation energy to convert O2 to 2 O

∆H rxn, 298K deg = 498.4 kJ/mol

minimum photon energy required to dissociate 1 molecule of O2 = 498,400 J/mol x (1 mol/6.023×10²³ molecules) = 8.27 × 10^-19 J/molecule

what is the minimum nm energy required to dissociate O2?

λmin = hc/E = [ (6.626×10^-34 J s) x (2.99 × 10⁸ m/s) ] / (8.27 × 10^-19 J/molecule) = 240nm

absorption of radiation by O2

• the minimum energy to dissociate O2 is 240nm

• schumann - runge continuum/band and herzberg continuum:

  • O2 + hv → 2O

• O + O2 (+M) → O3 (+M) + heat

  • M can be N2, H2O or O2 that carries away heat

absorption of radiation by O3

• N and O in the upper atmosphere absorb high energy solar radiation, and as it does this, atomic O reacts in the atmosphere making O3

• can effectively absorb between 200-290nm but not above 295!

  • resulting in a tail!

• O3 +hv → O2 + O

  • sink of O3 in the upper atmosphere!

electromagnetic radiation: dissociation of O3 - what is the longest wavelength of light that has enough energy to dissociate 1 molecule of O3?

Bond dissociation energy = Hrxn,298K = 105 kJ mol-1

∆Hrxn,298K = 105,000 J / mol (1 mol / 6.023 x 10²³ molecules) = 1.74 x 10^-19 J / molecule = minimum photon energy required to dissociate 1 molecule of O3

λmin = hc/E = [(6.626×10^-34 J s ) (2.99×10⁸ m /s )] / (1.74 10^-19 J molecule-1) = 1.136 × 10 ^-6 m = 1136 nm = the minimum energy required to dissociate 1 molecule of O3

• its in the infrared range! so not a lot of energy

the availability of light with sufficient photon energy is necessary, but…

not sufficient for a reaction to occur!

what are the requirements for O3 to dissociate?

1. sufficient energy required to bond dissociate

2. the target molecule must absorb light at the incident wavelength

• do not see O3 dissociate above 295nm because it can’t absorb the light at that range!

absorption

M + photon → M* → reaction or M + photon or M +heat

absorption 2

• atoms and molecules absorb light undergo a change in the organization of their electrons to an excited state M*

• however, M* is unstable and returns to their ground state

  • ground state = the lowest energy arrangement of electrons

• M* can undergo chemical reaction or release a photon or heat

absorption of radiation by N2, O2, and O3 in the upper atmosphere

what if there was no O3 in the stratosphere (excess UV radiation)?

1. dna mutations, skin cancer (melanoma), death

2. cataracts

3. crop failure

what if there was no O3 in the stratosphere (excess UV radiation) - dna mutations

• dna can absorb UV-B light

• a 1% decrease in overhead ozone results in 2% increase of UV-B intensity at ground level

• light skinned people maximum UV absorption from sunlight is at about 300nm

what if there was no O3 in the stratosphere (excess UV radiation) - skin cancer

• skin cancer due to overexposure to UV-B light

• highest incidence of malignant melanoma are US, Canada, South Africa, Australia, New Zealand, UK, western European countries and Scandinavia

• incidence of malignant melanoma is function of latitude

sunscreen

• full spectrum denotes UV-B and UV-A

• they contain organic substances or particles (or both) that absorb or scatter light

• zinc oxide and titanium dioxide are nanoparticles that appear transparent because of their small size

• however, if they absorb light they can produce by-products

  • ex. for OH radicals titanium dioxide particles have coats to prevent by-products

sunscreen - what does SPF 15 mean?

15 minutes of protection against UV-B

sunscreen - what is the new effectiveness scale

• rates effectiveness against UV-A

  • 4 stars = highest protection (80%)

  • 3 stars = high

  • 2 stars =. medium

  • 1 star = low

  • 0 stars = no protection

what if there was no O3 in the stratosphere (excess UV radiation) - cataracts

• front eye is exposed to uv light

• the cornea and lens filter 99% UV light

• absorption of UV can produce reactive molecules that attach to structural molecules in the eye and produce cataracts

  • can protect eyes with UV sunglasses (the goggles when you go skiing)

• looks like an opaque glass and can’t see very well

what if there was no O3 in the stratosphere (excess UV radiation) - crop failure

• UV-B exposure can affect photosynthesis to produce less leaf, seed, and fruit

what is a reaction mechanism?

a sequence of elementary reactions (or steps) by which overall chemical change occurs (and allows to deduce the rate law)

wat is a catalyst?

a molecule that lowers the reaction energy barrier (and speeds up the reaction) without being consumed (or created) in the net reaction

what is a catalytic cycle?

a reaction sequence in whihc a “chemical family” (catalyst) catalyzes a net reaction

the chapman mechanism

• mechanism to explain stratospheric O3 concentrations

• Ox ≡ O + O3 defines the “odd oxygen”

• rationale: O3 and O interconvert more rapidly than they are produced or destroyed

the chapman mechanism 2 - P (formation)

the rate of formation of O atoms = 2k1[O2]+k3[O3]

• look at the reactions where O is being produced (1 and 3)

the chapman mechanism 2 - L (destruction)

the rate of destruction of O atoms = k2[O2][M][O]+k4[O3][O]

• look at the reactions where O is being consumed (2 and 4)

the chapman mechanism 2 - steady state

• where [] stays constant over time

• at steady state: rate of formation (P) = rate of destruction (L)

the chapman mechanism 2 - another way of steady state

d[O]/dt = 0 = P – L

=> 2k1[O2]+k3[O3] = k2[O2][M][O]+k4[O3][O]

solve for [O]

=> [O] = 2k1[O2]+k3[O3] / k2[O2][M]+k4[O3]

the chapman mechanism 3

d[Ox]/dt = d[O]/dt + d[O3]/dt

= rate of formation of O - rate of destruction of O + rate of formation of O3 - rate of destruction of O3
= 2k1[O2] + k3[O3] - k2[O][O2][M] - k4[O][O3] + k2[O][O2][M] - k3[O3] - k4[O][O3]
= 2k1[O2] - 2k4[O][O3] = 0

• concentration of Ox = 0 and does not change over time

• input

  • [O] = 2k1[O2]+k3[O3] / k2[O2][M]+k4[O3] to 2k1[O2] - 2k4[O][O3]

• and solve for [O3]:

  • [O3]² = k1/k3 k2/k4 [M][O2]²

the chapman mechanism ___ predicts ____

qualitatively, altitude dependence of O3

• in pic: high at high altitude but goes to 0 because there is no light/ electromagnetic radiation as it approaches Earth

• 1 and 3 are dependent on light in the atmosphere

why does k1 = 0 s^-1 at the surface?

• in pic: high at high altitude but goes to 0 because there is no light/ electromagnetic radiation as it approaches Earth

• 1 and 3 are dependent on light in the atmosphere

why does the chapman mechanism predict more ozone than actually exists?

formation is overestimates

or

destruction is underestimates

and

there are additional reactions that destroy ozone (sinks) that the chapman doesn’t include

what are the 3 main catalytic cycles that cuase stratospheric O3 depletion

1. OH (hydroxyl radical)

2. NO (nitric oxide radical)

3. reactive halogen radicals (Cl, Br)

what are the catalyst formations (initiation) ?

1. OH: H2O + O → 2 OH

2. NO: N2O + O → 2 NO

2. halogen: CH3Cl → Cl + CH3

hv

the general mechanism for catalytic processes destroying stratospheric O3

hydroxyl radical mechanism

the termination reaction is what forms if the reaction O3 +O → 2O2 keeps happening and reacts with the intermediate HO2

nitric oxide radicals

the termination reaction is what forms if the reaction O3 +O → 2O2 keeps happening and reacts with the intermediate NO2 reacts with OH

halogen radicals mechanism (Cl or Br)

• inactive gases can be activated again to go to this reaction again:

  • Cl + O3 → ClO + O2
    ClO + O → Cl + O2

O and O2 in the atmosphere

• upper atmosphere: [O] » [O2]

  • high energy UV light reaches upper

  • oxygen molecules absorb this energy

  • energy breaks O2 bond

  • 2 separate oxygen atoms form

  • = photodissociation

• middle: [O] = [O2]

• lower atmosphere: [O] < [O2]

why does sunlight penetrate to different altitudes?

Type of radiation	Wavelength shown	How far it penetrates
High-energy UV	Below 100 nm	Mainly absorbed in the upper atmosphere
UV	200–330 nm	Absorbed at various altitudes, especially by O₂ and O₃
Longer-wavelength UV, visible and infrared	Above 330 nm	Much of it reaches Earth's surface

short wavelength UV is more energetic and gets abosorbed high in the atmosphere

• black line = solar radiation at the top of atmosphere

• green line = solar radiation that reaches Earths surface

  • radiation lost from the direct incoming sunlight through atmospheric process especially absorption

• atmosphere acts like a natural UV filter

  • lines get closer together in visible light region

what are reservoir species?

they are catalytically inactive molecules containing chlorine, present in the stratosphere

stratospheric chlorine is tied up in the form of ___

reservoir species such as chlorine nitrate (ClNO3) and hydrochloric acid (HCl)

reservoir species life spans

• relative long0lived

• they ‘tie up’ large amounts of active (short-lived) species but can slowly re-release them when activated

chlorofluorocarbons (CFCs)

• compounds containing only C, F, and Cl

• nontoxic

• nonflammable

• volatile

• non-bioaccumulative

• inert

• lifetime of 52-640+ years

• widely used since the 1940s in fridges and spray cans

warning about CFCs in 1974

• the solution to pollution was dilution

• CFCs did that and spread all over the globe

• but lack of tropospheric sinks (long lifetimes) meant that they eventually reached the stratosphere

• in the stratosphere CFCs would photo-dissociate because of the significant amounts of UV light

• CFCs are a source of reactive chlorine in the stratosphere and predicted catalytic ozone depletion

• CF2Cl →CF2Cl +Cl

first observation of polar stratospheric O3 loss (1985)

• large losses of total ozone in Antartica reveal seasonal ClOx/NOx interaction

first observation of polar stratospheric O3 loss (mid 1990s)

• O3 depletion extended form the poles to Australia and New Zealand in the South and Canada and Russia in the North

ozone depletion graph

left:

• during winter

  • temp is very cold

  • polar stratospheric clouds form

  • chemical reactions on these clouds convert inactive chlorine compounds into forms that can release reactive chlorine

• when spring arrives

  • sunlight retuns

  • sunlight break apart chlorine-containing molecules, releasing reactive chlorine

  • chlorine radicals catalytically destroy ozone

  • = ozone depleted region

right:

• ozone in 1993 < ozone in 1980-1982

• reductions in stratospheric ozone were also observed at lower latitudes (outside of Antartica)

what catalytic cycles contribute to stratospheric O3 loss?

all of them !

• HOx, Clx, NOx, and Ox are all involved

• thie relative contributions change with altiitude

HOx cycles

contribution is particularly important in the middle and upper stratosphere

Clx cycles