Chemistry Labs

Environmental, green and energy chemistry

Photochemical reactions in the atmosphere

How sunlight initiates atmospheric reactions, creates and destroys ozone, and controls radical chemistry and pollutant lifetimes.

1 Lower secondary2 Upper secondary3 Undergraduate4 Advanced5 Research

IntuitionSunlight as a reagent

A molecule in air can absorb a photon and enter an excited state; that energy may break a bond or open a reaction pathway. The atmosphere is therefore a photochemical reactor whose behaviour depends on both sunlight and molecular absorption.

Only photons absorbed by a species can drive its primary photochemical step. Shorter wavelengths carry more energy, but the wavelength reaching a given altitude is filtered by gases, clouds and aerosols.

Comparez O₂, O₃ et NO₂, puis sélectionnez l’ozone ou NO₂. Repérez les bandes de l’ozone et le seuil approximatif de photolyse de NO₂ vers 400 nm.

SchoolSchool level: photons and photolysis

Definition: Photochemical laws

The Grotthuss–Draper law states that light must be absorbed for a photochemical change. The Stark–Einstein law (photochemical equivalence) says one absorbed photon activates at most one molecule in the primary event; subsequent chain chemistry can yield more than one product per photon.

Definition: Photon energy

Energy per photon follows E=hc/λE=hc/\lambda. A 300 nm photon carries about 6.6×10−196.6\times10^{-19} J, or 399 kJ mol⁻¹ of photons, sufficient for many atmospheric bond-breaking channels.

Emol=NAhc/λE_{\mathrm{mol}}=N_Ahc/\lambda

Example: A 300 nm photon

Use NAhc=119600N_Ahc=119600 kJ nm mol⁻¹ to estimate molar photon energy at wavelength 300 nm.

Solution

E=119600/300=399E=119600/300=399 kJ mol⁻¹. The energy per individual photon is 399000/NA≈6.6times10−19399000/ N_A≈6.6\\times10^{-19} J.

UndergraduateUniversity: photolysis rates

Definition: Actinic flux and J

Actinic flux counts photons arriving from all directions, unlike direct-beam irradiance. The photolysis frequency combines spectral absorption cross section σ\sigma, quantum yield ϕ\phi and actinic flux FF; its units are s⁻¹.

J=∫λσ(λ) ϕ(λ) F(λ) dλJ=\int_{\lambda}\sigma(\lambda)\,\phi(\lambda)\,F(\lambda)\,d\lambda

Ozone absorbs strongly in the Hartley band near 255 nm, with Huggins absorption extending into near-UV and weaker Chappuis absorption in visible light. NO₂ absorbs broadly into visible wavelengths; photolysis is possible mainly at wavelengths below about 400 nm in the lower atmosphere.

OX3+hν→OX2+O(1 D)(λ≲320 nm)\ce{O3 + h\nu -> O2 + O(1D)}\quad(\lambda\lesssim 320\,nm)

Excited oxygen O(X1X221D)\ce{O(^1D)} is rapidly quenched by NX2\ce{N2} and OX2\ce{O2}, but reaction with water vapour yields two hydroxyl radicals. The ground-state channel forms O(X3X223P)\ce{O(^3P)} instead.

O(X1X221D)+HX2O→2 OHNOX2+hν→NO+O(X3X223P)\ce{O(^1D) + H2O -> 2OH}\qquad \ce{NO2 + h\nu -> NO + O(^3P)}

Example: A noon photolysis estimate

For a clear-sky illustration take J(NOX2)=8.0×10−3J(\ce{NO2})=8.0\times10^{-3} s⁻¹. What fraction photolyses in 60 s if other loss processes are ignored?

Solution

First-order survival is e−Jt=e−0.48=0.619e^{-Jt}=e^{-0.48}=0.619; fraction photolysed is 1−0.619=0.3811-0.619=0.381, about 38%. Real JJ varies with solar zenith angle, clouds and altitude.

AdvancedAdvanced: ozone budgets and radicals

The Chapman cycle links oxygen photolysis to stratospheric ozone production and loss. Catalytic cycles involving NOx\ce{NOx}, ClOx\ce{ClOx}, BrOx\ce{BrOx} and HOx\ce{HOx} regenerate their radical catalyst, so a single radical can destroy many ozone molecules before termination.

Representative ozone-loss cycles
FamilyNet reactionTypical setting
NOxOX3+O→2 OX2\ce{O3 + O -> 2O2}Stratosphere; NO and NO₂ cycle
ClOx / BrOx2 OX3→3 OX2\ce{2O3 -> 3O2}Especially polar stratosphere
HOxO+OX3→2 OX2\ce{O + O3 -> 2O2}Upper stratosphere / mesosphere

In the troposphere, OH initiates oxidation of CO, methane and volatile organic compounds (VOCs). Peroxy radicals from VOC oxidation convert NO to NO₂ without consuming ozone; subsequent NO₂ photolysis can produce net ozone. High NOx with scarce VOC chemistry can instead make ozone production VOC-limited, so the effective control strategy is regional and nonlinear.

[OX3]ss=JNOX2[NOX2]kNOX+OX3[NO][\ce{O3}]_{ss}=\dfrac{J_{\ce{NO2}}[\ce{NO2}]}{k_{\ce{NO+O3}}[\ce{NO}]}

This Leighton relationship follows only when NO₂ photolysis and NO+OX3\ce{NO + O3} are the dominant paired reactions. HO₂ and RO₂ provide additional NO-to-NO₂ conversion, so observed ozone often exceeds the simple photostationary prediction. OH acts as the atmospheric “detergent”: its small abundance belies rapid oxidation and strong control on pollutant lifetimes.

Example: Applying Leighton’s relation

At noon let JNO2=8.0×10−3J_{NO2}=8.0\times10^{-3} s⁻¹, kNO+O3=1.9×10−14k_{NO+O3}=1.9\times10^{-14} cm³ molecule⁻¹ s⁻¹ at 298 K and [NO2]/[NO]=1[NO2]/[NO]=1. Estimate ozone.

Solution

The ratio gives [O3]=J/k=4.2×1011[O3]=J/k=4.2\times10^{11} molecules cm⁻³. At 1 atm and 298 K, air density is about 2.46×10192.46\times10^{19} cm⁻³, so this is roughly 17 ppb. The estimate is idealized; peroxy chemistry breaks the two-reaction assumption.

For a trace gas removed mainly by reaction with OH, the approximate lifetime is τ=1/(k[OH])\tau=1/(k[OH]). The methane OH-loss lifetime is roughly 9–12 years under present conditions; this is a global effective value, not the lifetime at every location.

τX≃1kX+OH[OH]\tau_X\simeq\dfrac{1}{k_{X+OH}[OH]}

Example: Methane lifetime estimate

Use k=6.3×10−15k=6.3×10^{-15} cm³ molecule⁻¹ s⁻¹ and [OH]=1.0×106[OH]=1.0×10^6 molecules cm⁻³ as an illustrative mean.

Solution

k[OH]=6.3×10−9k[OH]=6.3×10^{-9} s⁻¹, so τ=1.59×108τ=1.59×10^8 s, about 5.0 years. This single-mean estimate is shorter than the evaluated total methane lifetime, about 9–12 years, because real OH varies in space and time and methane has other sinks and feedbacks.

ResearchResearch frontier: ozone and photochemistry

References