Stratospheric Chemistry and the Ozone Layer – Comprehensive Study Notes

Page 1 – Introductory Information

• Course: CHEM 202 – Introduction to Environmental Chemistry, subsection Stratospheric Chemistry & the Ozone Layer.
• Lecturer: Dr. Fatima Haydous, Department of Chemistry, American University of Beirut.

Page 2 – Stratosphere & Its Peculiarities

• Stratosphere lies above the troposphere and below the mesosphere and is defined by a temperature inversion (temperature rises with altitude).
• Cooler–heavier air underneath minimizes convection → very stable, weak vertical mixing.
• Bulk composition ≈ that of the whole atmosphere: \mathrm{N2} and \mathrm{O2} dominate.
• Exposure to higher-energy solar photons triggers unique chemistry, most notably the formation and destruction of ozone.

Page 3 – Where Do We Find Ozone?

• Ozone ((\mathrm{O_3})) occurs at all altitudes, but the maximum mixing ratio (peak) is between 15 km and 30 km → the ozone layer.
• Stratospheric ozone ≈ “good ozone.”
• Elevated ozone in the boundary-layer/troposphere ≈ “bad ozone.”

Page 4 – Good vs. Bad Ozone

Good (stratospheric) ozone:
• Mixing ratio: parts-per-million level, yet vital.
• Absorbs UV radiation between 200\ \mathrm{nm} \le \lambda \le 315\ \mathrm{nm} → biological shield.
Bad (tropospheric) ozone:
• Powerful oxidant → respiratory irritation, crop damage.
• Acts as a greenhouse gas.

Page 5 – Solar Radiation Basics

• Solar output spans <100\ \mathrm{nm} (far-UV) to 1000\ \mu\mathrm{m} (far-IR).
• Intensity maximum ≈ 550\ \mathrm{nm} (yellow).
• Visible & IR photons → heat & light; UV photons (shorter λ) constitute a small fraction of total flux but drive photochemistry and can be lethal biologically.

Page 6 – UV Sub-Bands & Biological Weighting

Page 7 – What Actually Reaches the Surface?

• Green curve = solar UV output.
• Dashed O(2) curve: molecular oxygen absorbs \lambda<240\ \mathrm{nm}.
• Dotted O(3) curve: ozone absorbs 200–315 nm.
• Result: Nearly all UV-C, most UV-B absorbed aloft; ≈90 % of ground-level UV is UV-A.

Page 8 – Biological Impacts of UV

• Reduced stratospheric ozone → more UV-B reaches the troposphere → higher skin-cancer incidence, cataracts, plant stress.
Positive body responses to UV-B:
(i) Melanin synthesis (tanning) – partial protection.
(ii) Vitamin D synthesis peaks at \lambda \approx 296\ \mathrm{nm} (in 270–300 nm window).
• UV-A penetrates deeper, accelerates skin ageing.

Page 9 – Sunscreens & Their Chemistry

• Sunscreens: organic UV absorbers (e.g.
benzyl salicylate, cinnamates) ∴ strong absorption 250–325 nm; often include radical scavengers.
• Sunblocks: physical scatterers (ZnO, TiO(_2)) → reflect broad spectrum including UV-A.

Page 10 – Quantifying Sunscreen Efficiency

• Beer’s law: A = \varepsilon b c.
• Sun Protection Factor (SPF): if SPF = 15 → 93\% of UV-B blocked. Stay-time multiplier = \frac{100}{100-93}=15.

Page 11 – Limitations & Broad-Spectrum Protection

• Most chemical absorbers poor for UV-A; physical blocks (ZnO, TiO(_2)) attenuate the entire UV range.

Page 12 – Global Solar UV Index (UVI)

• Joint WMO–WHO metric.
• Measures 250–400 nm surface irradiance weighted by human skin response → public-health guidance scale.

Page 13 – Dobson Unit (DU) & Ozone Column

• Definition: Thickness (in hundredths of a cm) of pure ozone at 0^\circ!\mathrm{C} & P^\circ that equals total ozone in a 1-m² air column.
• Example: 300 DU ⇒ 3\ \mathrm{mm} pure O(3). • From PV = nRT, n = \frac{P V}{R T}; inserting V = 0.003\ \mathrm{m^3},\ T=273\ \mathrm{K} yields moles O(3) in the column.

Page 14 – Measuring Ozone Aloft

• Ozone absorbs 200–315 nm, (\lambda_{\max}≈255\ \mathrm{nm}.)
• Instruments: ground-based Dobson/Brewer spectrophotometers, balloon sondes, aircraft, satellites (e.g.
TOMS).
• Ozone vertical maximum ≈25 km altitude.

Page 15 – Temporal & Spatial Variability

• Global mean ≈300 DU; tropics ≈250 DU; high-latitudes ≈450 DU.
• Daily fluctuations ±20–30 DU; higher values in winter/spring, lower in summer/autumn.

Page 16 – Chapman Mechanism (Four Reactions)

Synthesis:

1. \mathrm{O_2 + h\nu\ (\lambda<240\ nm) \rightarrow O(^3P)+O(^3P)} \Delta H^{\circ}=+498.4\ \mathrm{kJ\ mol^{-1}} (slow).

2. \mathrm{O + O2 + M \rightarrow O3 + M} \Delta H^{\circ}=-106.5\ \mathrm{kJ\ mol^{-1}} (fast).
   Decomposition:

3. \mathrm{O3 + h\nu\ (200–315\ nm) \rightarrow O2^{}+O^{}} \Delta H^{\circ}=+386.5\ \mathrm{kJ\ mol^{-1}} (fast).

4. \mathrm{O + O3 \rightarrow 2O2} \Delta H^{\circ}=-391.9\ \mathrm{kJ\ mol^{-1}} (slow; Ea≈18\ \mathrm{kJ\ mol^{-1}}). (M = third body, often N(2)/O(_2)).

Page 17 – Photolysis Thresholds

• \lambda{\text{diss}}(\mathrm{O2})<240\ \mathrm{nm}. Longer λ pass through to lower layers.
• Thus, O(_2) photolysis confined to upper stratosphere.

Page 18 – Odd Oxygen Concept

• Reactions 2 & 3 interconvert \mathrm{O},\ \mathrm{O_3} rapidly → individual lifetimes short, but odd-oxygen family persists months–years.
• Reaction 3 not reverse of 2; theoretical reverse requires \lambda<1123\ \mathrm{nm} (not absorbed).

Page 19 – Key Wavelength for O(_3) Photolysis

Using E=\frac{hc}{\lambda}, \Delta H^{\circ}=386.5\ \mathrm{kJ\ mol^{-1}} ⇒ \lambda_{max}=309.5\ \mathrm{nm}.
Ozone absorbs strongly 200–315 nm → critical UV shield.

Page 20 – Why an Ozone Layer Exists

• Slow reaction 4 + balance of synthesis/decomposition -> maximum ozone ≈23 km.
• Ozone synthesis/destruction governed solely by O-species plus sunlight in Chapman view.

Page 21 – Vertical Distribution Logic

Upper stratosphere (≈50 km): plentiful photons, little O(2) → limited O(3).
Lower stratosphere (≈20 km): lots of O(2), few energetic photons. Middle (≈23 km): both ingredients optimum → O(3) peak.
Actual altitude varies with season & latitude.

Page 22 – Catalytic Ozone Destruction: Generic Scheme

\begin{aligned}
X + O3 &\rightarrow XO + O2\
XO + O &\rightarrow X + O2\ \hline O + O3 &\rightarrow 2O_2 \quad (\text{net})
\end{aligned}
X = HO·, NO·, Cl·, Br·, etc.
X regenerated → catalyst.

Page 23 – Relative Importance by Altitude

• 50 km: \mathrm{HOx} ≈ 70 % of O(3) loss.
• ~30 km: \mathrm{NOx} dominates. • Near tropopause: \mathrm{NOx} ≈ 70 % of removal.

Page 24 – HOx Sources: Water & Methane

Water:
• Troposphere humid (several %), but tropopause cold (≈–50 °C) → water freezes, stratosphere dry.
Methane:
• Does not freeze; diffuses upward; photochemistry + O-atoms → water + HOx radicals.
Reactions produce OH either by photolysis of H(2)O or CH(4) + O.

Page 25 & 26 – HOx Catalytic Cycle Details

Key steps:
\begin{aligned}
\mathrm{OH + O3} &\rightarrow \mathrm{HO2 + O2}\ \mathrm{HO2 + O} &\rightarrow \mathrm{OH + O2}\ \hline O + O3 &\rightarrow 2O2 \end{aligned} Hydrogen atom can also enter: \mathrm{H + O3 \rightarrow OH + O_2}.
Methane abundance (anthropogenic/agricultural) modulates HOx-driven destruction.

Page 27 – NOx Sources

Anthropogenic: combustion (heating, power, vehicles).
Natural: reduction/oxidation in soils & water → N(2)O (from nitrification/denitrification). N(2)O rises, survives troposphere.

Page 28 – Fate of N(_2)O in Stratosphere

90 % destroyed:
\begin{aligned}
\mathrm{N2O + h\nu} &\rightarrow \mathrm{N2 + O}\
\mathrm{N2O + O} &\rightarrow \mathrm{N2 + O2} \end{aligned} 10 % forms NO: \mathrm{N2O + O^{*} \rightarrow 2NO}.
N(_2)O = most significant anthropogenic ozone-depleting species.

Page 29 – NO Production & Catalytic Cycle

High (>30 km) photolysis of N(2): \lambda<126\ \mathrm{nm}. Catalytic sequence: \mathrm{NO + O3 \rightarrow NO2 + O2}
\mathrm{NO2 + O \rightarrow NO + O2}
Reservoir path: \mathrm{NO + OH + M \rightarrow HNO_2 + M}.

Page 30 – ClOx Sources

Naturals: CH(3)Cl (oceanic biology, biomass burning), HCl (volcanoes), sea-salt aerosols (Cl⁻). Anthropogenic: CFCs. CH(3)Cl + OH (troposphere) or h\nu (stratosphere) releases Cl·.

Page 31 – ClOx & Mixed Halogen Cycles

Classical cycle:
\mathrm{Cl + O3 \rightarrow ClO + O2}
\mathrm{ClO + O \rightarrow Cl + O2} Net: O + O3 \rightarrow 2O_2.
Other coupled cycles (e.g.
BrO + ClO, HOCl photolysis) account for ≈60 % of halogen-driven loss.

Page 32 – CFC Properties & Uses

• Chemically/biologically inert, low viscosity, low surface tension, low boiling points → refrigerants, aerosol propellants, foam-blowing agents, electronics cleaners.
• Stable liquids under modest pressure; vaporization absorbs heat.

Page 33 – CFC Numbering Example

Rule: subtract 90 from code ⇒ C : H : F counts; remaining sites = Cl.
CFC-115 → 115 – 90 = 25 ⇒ 2 C, 0 H, 5 F → residual 1 Cl ⇒ \mathrm{CF3CF2Cl}.

Page 34 – Photolysis of CFCs

In troposphere: inert.
In stratosphere: \mathrm{CFCl3 + h\nu\ (\lambda<290\ nm) \rightarrow \cdot CFCl2 + Cl\cdot} → subsequent releases yield multiple Cl· per molecule.

Page 35 – Ozone Depletion Potential (ODP) & GWP

• \text{ODP} = \frac{\text{O3 loss per mass of substance}}{\text{O3 loss per mass of CFC-11}}.
Range 0.1–1 for most CFCs; HCFCs 0.01–0.1; HFCs 0.
• Must also weigh Global Warming Potential (100 yr) – many replacements are potent greenhouse gases.

Page 36 – Montreal Protocol

International treaty (1987 → amendments) phased out CFC production/consumption; curbs stratospheric Cl loading.

Page 37 – Continuing Research

Search for alternatives retaining desirable CFC qualities yet benign toward ozone/GHG budgets.

Page 38 – HCFCs as Transitional Substitutes

• Addition of H increases reactivity; OH abstraction yields shorter tropospheric lifetimes → smaller ODP.
• Example: HCFC-123 \mathrm{CF3CHCl2}, ODP ≈0.02 of CFC-11.
• Downsides: flammability, limited use in some equipment.

Page 39 – HFCs (No Chlorine)

• Replace all Cl with F or H. C–F bond strong → hard to photolyse; ODP = 0.
• Example refrigerant: \mathrm{CF3CH2F} (HFC-134a).
• But high IR absorption → high GWP; costly synthesis.

Page 40 – Bromine-Containing Halons

• Derivatives where Br substitutes for H/Cl/F. Naming example Halon 1211 (CBrClF formula counts).
• Exceptional fire-suppression (density + radical quenching).
• Br more efficient than Cl at destroying O(_3) → very high ODP; production restricted.

Page 41 – Halon Behaviour & Regulation

• ODP for Br-halons extremely high; releases occur directly during fire-fighting.
• Existing supplies (“banked reserves”) allowed only under strict control.

Page 42 – Halon Flame-Quenching Chemistry

Effectiveness order: F < Cl < Br < I (C–F too strong, C–I too weak).
HX formed reacts with flame radicals (H·, OH·) lowering combustion energy.

Page 43 – Methyl Bromide & Overall Summary

• CH(3)Br natural & synthetic (soil fumigation). Potent O(3)-destroyer; phased-out uses except critical exemptions.
• Summary: CFCs stable → stratospheric Cl; HCFC/HFC transitional replacements; Br compounds highly destructive; all have GWP considerations.

Page 44 – Other Radical Chemistry Classes

1. Catalytic cycles – net O(_3) destruction (already covered).

2. Null cycles – convert X ↔ XO with no net odd-oxygen change.

3. Holding cycles / reservoir formation – temporarily sequester active radicals (e.g.
   N(2)O(5), HCl, ClONO(_2)).
   These interactions complicate predictive modeling.

Page 45 – Example Null Cycle (NOx)

\begin{aligned}
NO2 + h\nu &\rightarrow NO + O\ O + O2 + M &\rightarrow O3 + M\ NO + O3 &\rightarrow NO2 + O2
\end{aligned}
Net: photolysis of O(_3) but rapid re-formation → no change in odd-oxygen inventory (daytime only).

Page 46 – Holding Cycle: N(2)O(5)

NO3 + NO2 + M \rightleftharpoons N2O5 + M
N(2)O(5) stable, contains 5–10 % of total NOx → reservoir. Reversible; leaks back active NOx over time or transports species to troposphere.

Page 47 – Polar (Antarctic/Arctic) Ozone Holes

• Ozone monitored globally; severe spring depletion seen yearly over Antarctica (peak Sep–Oct), area ≈28 million km².
• Requires: cold dark winter, isolation (polar vortex), accumulation of reservoir species → rapid loss upon first sunlight.

Page 48 – Seasonal Context

• Antarctic summer: Oct–Mar (continuous daylight).
• Antarctic winter: Mar–Oct (continuous darkness) → sets stage for vortex chemistry.

Page 49 – Polar Vortex & Polar Stratospheric Clouds (PSCs)

• Winter darkness + rotation → stable vortex; T ≈ −80 °C; formation of PSCs.
• Type I PSC: ≈1 µm particles, HNO(3)·3H(2)O.
• Type II PSC: up to 10 µm, nearly pure H(_2)O ice.

Page 50 – Heterogeneous Activation of Chlorine

Reservoirs on PSC surfaces:
\mathrm{ClONO2 + HCl \rightarrow Cl2 + HNO3} \mathrm{ClONO2 + H2O \rightarrow HOCl + HNO3}
Upon spring sunlight: \mathrm{Cl_2 + h\nu \rightarrow 2Cl\cdot},\ \mathrm{HOCl + h\nu \rightarrow Cl\cdot + OH\cdot}.

Page 51 – Rapid Ozone Loss Sequence

Dimer cycle independent of O·:
\begin{aligned}
ClO + ClO + M &\rightarrow Cl2O2 + M\
Cl2O2 + h\nu &\rightarrow Cl + ClOO\
ClOO + M &\rightarrow Cl + O2 + M\ 2(Cl + O3 &\rightarrow ClO + O2)\ \hline \text{Net: } 2O3 &\rightarrow 3O_2
\end{aligned}
Complete column losses >50 % within days; recovery starts when vortex breaks and reservoirs re-form.

Page 52 – Observational Concerns

• Low-O(_3) air can drift to populated southern latitudes → elevated UV-B.
• At Arctic, events similar but less severe (weaker vortex, warmer temperatures).

Page 53 – Strategies for Recovery & Current Trends

• Montreal Protocol parties now targeting HFC phase-down (Kigali Amendment).
• Stratospheric aerosol injection (SAI) proposals may affect ozone recovery & circulation.
• Energy-efficient refrigeration, controls on equipment trade, mitigation of short-lived substances (<6 months lifetime).
• COVID-19 led to temporary dip in HFC consumption (2020–22 vs 2018–19).

Pages 54–55 – 2024 Ozone-Hole Update (AUB Slide Placeholder)

• Placeholder slides noting 2024 update & university branding – no additional technical content supplied.