Chemistry Labs

Grade 12

The ozone layer, the greenhouse effect

Stratospheric ozone shields life from ultraviolet radiation, while greenhouse gases absorb outgoing infrared radiation and warm the lower atmosphere. These distinct processes are explained through photochemistry, molecular vibrations and Earth’s radiative balance.

1 Lower secondary2 Upper secondary3 Undergraduate4 Advanced5 Research

IntuitionTwo different atmospheric roles

High above the surface, ozone absorbs much harmful ultraviolet radiation. Near the surface, ozone is itself an air pollutant. Greenhouse gases are a separate issue: they absorb selected wavelengths of Earth’s infrared emission, slowing heat loss to space.

Switch between the Chapman ozone cycle with catalytic chlorine destruction and the greenhouse-layer diagram. Trace ultraviolet absorption, incoming sunlight, outgoing infrared radiation and atmospheric back-radiation.

SchoolSchool level: ozone in the stratosphere

The Chapman cycle describes how sunlight continually forms and destroys ozone. Short-wave UV splits oxygen molecules; oxygen atoms then combine with O₂, while ozone absorbs UV and can split again.

OX2→hν2 OO+OX2+M→OX3+MOX3→hνOX2+OO+OX3→2 OX2\ce{O2 ->[h\nu] 2 O}\qquad\ce{O + O2 + M -> O3 + M}\\\ce{O3 ->[h\nu] O2 + O}\qquad\ce{O + O3 -> 2 O2}

Definition: Ozone-depleting substances

Chlorofluorocarbons (CFCs) are stable in the lower atmosphere. In the stratosphere, energetic UV can break a C–Cl bond and release a chlorine radical, Cl·, which participates in catalytic ozone destruction.

CFClX3→hνCFClX2X ∙ +ClX ∙ \ce{CFCl3 ->[h\nu] CFCl2^. + Cl^.}
ClX ∙ +OX3→ClOX ∙ +OX2ClOX ∙ +O→ClX ∙ +OX2net: OX3+O→2 OX2\ce{Cl^. + O3 -> ClO^. + O2}\qquad\ce{ClO^. + O -> Cl^. + O2}\\\text{net:}\ \ce{O3 + O -> 2 O2}

Because Cl· is regenerated, one radical can destroy many ozone molecules before it is removed into a reservoir compound. On polar stratospheric clouds, reactions convert relatively inactive chlorine reservoirs into forms that release reactive chlorine when sunlight returns, contributing to the seasonal Antarctic ozone hole.

The 1987 Montreal Protocol controls production and consumption of many ozone-depleting substances. Its global phase-down has allowed the ozone layer to begin recovering, although long-lived compounds mean recovery takes decades.

SchoolSchool level: the greenhouse effect

Earth absorbs sunlight, then emits infrared radiation. Greenhouse gases absorb and re-emit some infrared wavelengths, including downward toward the surface. This natural effect keeps the surface warmer than it would be without an atmosphere; increasing greenhouse-gas concentrations strengthens the effect.

Te=(S(1−A)4σ)1/4≈255 K;Ts≈288 KT_e=\left(\frac{S(1-A)}{4\sigma}\right)^{1/4}\approx255\ \mathrm{K};\qquad T_s\approx288\ \mathrm{K}

Example: Earth’s effective temperature

Estimate the effective radiating temperature using solar constant S=1361 W m⁻², planetary albedo A=0.30 and Stefan–Boltzmann constant σ=5.67×10⁻⁸ W m⁻² K⁻⁴.

Solution

At equilibrium, absorbed solar flux averaged over the sphere is S(1−A)/4≈238 W m⁻². Setting it equal to σT⁴ gives T≈255 K (about −18 °C). Earth’s actual mean surface temperature is about 288 K (15 °C); the ≈33 K difference reflects the natural greenhouse effect, not a temperature of one uniform atmospheric layer.

Selected greenhouse gases
GasImportant feature
CO₂Combustion, land-use change; long-lived
CH₄Wetlands, agriculture, fossil fuels; stronger warming per mass than CO₂ over 100 years
N₂OAgriculture and industry; long-lived
H₂O vapourMost abundant natural greenhouse gas; concentration responds rapidly to temperature

Infrared absorption occurs when molecular vibration or rotation changes a molecule’s dipole moment. CO₂’s bending and asymmetric-stretch vibrations absorb infrared; its symmetric stretch is infrared-inactive. N₂ and O₂ do not absorb strongly in this way, while H₂O, CH₄ and N₂O have infrared-active modes.

Global warming potential over 100 years (illustrative IPCC AR6 values)
GasGWP₁₀₀ relative to CO₂=1
CO₂1
CH₄ (non-fossil)≈27
N₂O≈273

Example: Methane in CO₂-equivalent

A source emits 2.0 kg of non-fossil methane. Use GWP₁₀₀≈27 to estimate its carbon-dioxide equivalent.

Solution

CO₂e = mass × GWP₁₀₀ = 2.0 × 27 ≈ 54 kg CO₂e. This comparison integrates climate influence over 100 years; it is not a claim that methane and CO₂ have identical atmospheric lifetimes or effects at every time scale.

UndergraduateUniversity glimpse: absorption and radiative forcing

For a narrow spectral band, Beer–Lambert attenuation is I/I₀ = exp(−σN), where σ is an absorption cross-section and N is the absorbing-column number density. Across a real atmospheric column, temperature, pressure, clouds, overlapping bands and emission also matter; the simple equation is an instructive starting point, not a full climate model.

ΔF≈5.35ln⁡(C/C0) W m−2\Delta F\approx5.35\ln(C/C_0)\ \mathrm{W\,m^{-2}}

Example: Forcing from 280 to 420 ppm CO₂

Use the approximate relation ΔF≈5.35 ln(C/C₀) W m⁻² to estimate the forcing for C₀=280 ppm and C=420 ppm.

Solution

ΔF≈5.35 ln(1.5)=5.35×0.405≈2.17 W m⁻². This is a radiative imbalance estimate; feedbacks and the ocean’s heat uptake determine the eventual temperature response.

References