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The photon flux incident from all propagation directions, resolved by wavelength when needed for photodissociation. It is distinct from the net radiative flux through a horizontal surface; scattered and diffuse photons can contribute even when their net vertical flux cancels.

A bond-dissociation energy specifies the energy needed for a stated dissociation into separated fragments. An energy from the lowest vibrational state, an energy from the bottom of a potential energy well, and a finite-temperature molar dissociation enthalpy require different conventions; do not silently mix them in an electronegativity formula.

Which fragments, in which quantum states, and at which temperature? For a neutral gaseous molecule with a specified two-electron covalent bond, the usual thermochemical convention is homolysis,Each fragment receives one of the two bonding electrons; the dots denote unpaired electrons. The final fragments are isolated from each other, with their own relaxed molecular geometry and specified electronic quantum states. The energy cost includes this relaxation, rather than merely stretching the initial chemical bond while freezing everything else. For a neutral parent, heterolysis instead produces ${\mathrm{A}}^{+}(g)+{\mathrm{B}}^{-}(g)$, or the opposite assignment of electric charges; those are different final states and different energies. A value for gaseous neutral fragments cannot be transferred to ions, a chemical solution, or a solid without accounting for those changed conditions.^[ref]

The distinction is useful beyond terminology. Pauling electronegativity compares consistently defined bond-dissociation energies; photodissociation can reveal a specified ground-state threshold; a thermal chemical reaction balance requires compatible enthalpies. A threshold is also different from an activation energy: it compares initial and final quantum states, whereas a barrier can obstruct the path between them.

The radical $\mathrm{BrO}$ participates in catalytic atmospheric ozone destruction. Its measured abundance is only one partition of the total bromine inventory; precursor delivery, photodissociation and reservoir reactions determine the relation between the two.

A low-temperature catalytic ozone destruction pathway involving ClO association to ClOOCl, photodissociation of the dimer and regeneration of chlorine atoms. Combined with two Cl + O3 reactions its net effect is $2{\mathrm{O}}_3\to 3{\mathrm{O}}_2$. Low temperature suppresses thermal dimer dissociation, while sunlight is needed for the photolytic step. Polar stratospheric clouds can activate reservoir chlorine before the sunlit cycle becomes rapid.

A compound composed of carbon, chlorine and fluorine, with no hydrogen. Many such fully halogenated molecules resist lower-atmospheric attack but release reactive chlorine after stratospheric photodissociation, contributing to catalytic ozone destruction.

The chlorine peroxide structure formed by association of two ClO radicals with collisional stabilization. Its low-temperature stability and photodissociation provide a route for catalytic ozone destruction. In a laboratory flow system, changing among three ClO precursors and examining infrared spectra helped distinguish the predominant ClOOCl product from an alternative ClOClO isomer. Product assignment and ultraviolet absorption support the proposed intermediate, but do not alone measure an entire atmospheric cycle's ozone-loss rate.^[ref]

Light initiates methane chlorination by photodissociation of $\mathrm{C}{\mathrm{l}}_2$ through homolysis. The elementary bookkeeping isHere $h\nu$ denotes the photon energy. The first propagation step replaces a chlorine radical with a methyl radical; the second regenerates a chlorine radical. Adding the two propagation equations cancels both free radicals and gives $\mathrm{C}{\mathrm{H}}_4+\mathrm{C}{\mathrm{l}}_2⟶\mathrm{C}{\mathrm{H}}_3\mathrm{Cl}+\mathrm{HCl}$: chloromethane and hydrogen chloride are made while the chain carrier is regenerated.^[ref]

For example, recombination $2{\mathrm{C}{\mathrm{H}}_3}^{\bullet }⟶{\mathrm{C}}_2{\mathrm{H}}_6$ of methyl radicals makes ethane and a covalent bond from two unpaired electrons, removing two carriers. Recombination of two ${\mathrm{Cl}}^{\bullet }$ also terminates carriers; in the gas phase, stabilization of the product requires disposal of released energy, for example to a third collision partner. The figure omits that partner. Termination competes with propagation, and further chlorination can consume chloromethane, so this cycle alone does not predict a pure product or a measured reaction rate.^[ref]

The atmospheric nitrogen oxide $\mathrm{N}{\mathrm{O}}_2$. Daylight photodissociation forms nitrogen monoxide and an oxygen atom; subsequent third-body recombination with oxygen forms ozone. Whether this makes net ozone depends on the pathway that regenerated nitrogen dioxide.

An electronically excited atomic oxygen state produced in part by ozone photodissociation. Reaction with water can yield two hydroxyl radicals, while collisional quenching returns the atom to its triplet ground term without the same radical production. Its short-lived steady balance is distinct from the lifetime of ozone or water.

In a direct-beam model with an effective ozone cross section $\sigma$, removal of an overlying column $\Delta N$ multiplies the transmitted photodissociation flux by $e^{\sigma \Delta N}$. A real photodissociation coefficient integrates wavelength-dependent cross section, quantum yield and actinic flux, including scattering and the local radiation field.

For one specified channel, the photolysis rate coefficient isIt depends on spectral actinic flux, absorption cross section, and channel quantum yield. Solar zenith angle, season, overhead absorbers, clouds and aerosol scattering affect the photon field; branching and collisional quenching affect the chemical yield. Absorption alone does not guarantee the desired dissociation.

Curve A is oxygen photodissociation; curve B is ozone photodissociation. Molecular oxygen needs relatively short-wavelength ultraviolet photons, which are strongly screened at lower altitudes, so its rate increases steeply with height. Ozone can dissociate using longer wavelengths that penetrate much further, producing the much larger, less height-sensitive rate shown by B. The plotted quantity is a first-order coefficient with units ${\mathrm{s}}^{-1}$, rather than a bimolecular rate constant.

Let $v_i$ denote the rate of reaction $i$. For odd oxygen defined as O plus O${}_3$, reactions 5 and 6 each consume one unit, while NO${}_2$ photodissociation in reaction 7 restores one. Thus the loss from reactions 4–8 is $L=v_5+v_6-v_7$. HO${}_2$ stationarity gives $v_5=v_6+v_8+v_4$, and NO${}_2$ stationarity within this restricted recycling loop gives $v_7=v_8$. ThereforeThe requested expression neglects the small termination correction $v_4$ in the odd-oxygen budget, while retaining it to determine the much slower total-radical balance. This is consistent when cycling greatly exceeds termination. The net main catalytic cycle is O plus O${}_3$ forming two O${}_2$ molecules; the NO-assisted route mainly interconverts O and O${}_3$ and cancels from this restricted net loss after NO${}_2$ recycling.

Low temperature favours association into the dimer and, especially, suppresses its thermally activated dissociation back to chlorine monoxide. More of the dimer can therefore follow photodissociation instead of the chemically unproductive thermal return. The supplied branching fraction is a local condition, not a temperature-independent universal yield.

Cold polar air also permits polar stratospheric clouds and efficient heterogeneous conversion of chlorine reservoirs into photolabile forms. The stratospheric polar vortex isolates and cools that air. Darkness permits activation and storage, but rapid catalytic ozone loss requires the return of sunlight. These chemical and meteorological conditions explain the springtime maximum; low temperature alone does not supply the photodissociation photons.

Write $D=[\mathrm{C}{\mathrm{l}}_2{\mathrm{O}}_2]$ and $c=[\mathrm{ClO}]$. In the steady-state approximation for the dimer, chlorine peroxide chlorine peroxide formation is balanced by its two competing removal channels:The association event rate is $k_1[M]c^2$; the associated disappearance rate of chlorine monoxide is twice that value. Each successful photodissociation event produces two chlorine atoms after the subsequent fast fragmentation, and their return to chlorine monoxide consumes two ozone molecules. Thermal dissociation instead returns the dimer to two chlorine monoxide radicals without destroying ozone. Thus the chemical branching fraction for an ozone-destroying event is $f=J_2/(J_2+k_3[M])=0.7$, givingThis is the sunlit chlorine monoxide dimer cycle, with net conversion $2{\mathrm{O}}_3\to 3{\mathrm{O}}_2$. It assumes that the dimer relaxes rapidly, downstream fragmentation and chlorine–ozone reactions are sufficiently fast, and competing chlorine sinks can be neglected. Counting the formation event twice and then multiplying by two again would overestimate loss by a factor of two.

The lost ozone-column photolysis shielding is $\Delta N=20(2.69\times 10^{16})=5.38\times 10^{17}\mathrm{molecules}\mathrm{c}{\mathrm{m}}^{-2}$, so its optical-depth decrement is $\sigma \Delta N=0.0538$. Under the stipulated effective-cross-section Beer-Lambert model, the transmitted photodissociation flux increases by $\exp (0.0538)$.

The short-lived oxygen singlet D atom satisfies $[\mathrm{O}({}^{1}D)]\simeq J_{{\mathrm{O}}_3}[{\mathrm{O}}_3]/(k_q[M])$ when quenching dominates. At fixed local ozone, water and air density, hydroxyl radical production is proportional to $J_{{\mathrm{O}}_3}$. ConsequentlyA linear optical-depth approximation would give $5.38\%$. A realistic calculation integrates the wavelength-dependent absorption cross section, quantum yield and actinic flux; clouds and multiple scattering modify the simple transmitted-beam model.

Ozone change can also alter the radiative temperature structure, water vapor and circulation, thereby changing the water reaction channel and the supply of ozone/precursors to the troposphere. If the held-fixed tropospheric ozone assumption is relaxed, altered ozone itself changes the photochemical source of oxygen singlet D atom. These are additional feedbacks, distinct from the isolated UV-transmission calculation above.

For the chemical sink, ${\tau }_{\mathrm{C}{\mathrm{H}}_4}\sim 1/(k[\mathrm{OH}])$. Warming increases the methane–hydroxyl radical rate constant and, with increased water vapor, often increases hydroxyl radical production, tending to shorten the chemical lifetime. Changes in ozone photodissociation, radical recycling, lightning and competing carbon monoxide/VOC/methane consumption can reinforce or oppose that trend. Increasing methane itself consumes hydroxyl radicals and lengthens its methane perturbation lifetime. The physical/chemical warming tendency should therefore be separated from scenarios with changed emissions; shorter chemical lifetime at fixed precursor conditions is a tendency, not an unconditional prediction for the future atmosphere.^[ref]

In a vertical-beam, effective-wavelength Beer-Lambert law approximation, the optical depth above altitude $z$ is $\tau (z)={\sigma }_{{\mathrm{O}}_2}{N}_{{\mathrm{O}}_2}(z)+{\sigma }_{{\mathrm{O}}_3}{N}_{{\mathrm{O}}_3}(z)$. Using centimetres consistently givesThe transmitted photon flux is proportional to $e^{-\tau }$. If the parent-gas absorption cross section and photodissociation yield are otherwise the same, they cancel in the ratio of photolysis rate coefficients:The corresponding fractions of the unattenuated beam are about $7.52\times 10^{-7}$ and $0.654$. Strong shielding by the overlying oxygen and especially ozone therefore makes destruction vastly faster higher in the stratosphere, even though the molecular absorption properties have not changed.

This numerical ratio uses the intended simplified radiation model. A real photolysis rate coefficient integrates wavelength-dependent actinic flux, absorption cross section and quantum yield. A nonvertical direct beam would use slant optical depth $\tau /\cos \theta$; diffuse scattering and the true solar spectrum would also matter. The supplied effective cross sections do not specify a unique broadband radiative-transfer calculation.

Put $X=[{\mathrm{O}}_3]$ and $u=T^{-1}$. The fast oxygen–ozone partition obeysDividing the production–loss balance by two and substituting givesUnder the specified restriction, all temperature dependence lies in the three rate coefficients: hold the photodissociation rates and the concentrations of ${\mathrm{O}}_2$, $\mathrm{M}$ and NO fixed. Then $d\ln A/du=-2100-510=-2610\mathrm{K}$ and $d\ln B/du=-1200\mathrm{K}$. Defining $Y=d\ln X/du$, differentiation yieldsThereforeThe factor two in the denominator multiplying $L_C$ comes from the additional dependence $[\mathrm{O}]\propto X/k_4$; ignoring that dependence would give the wrong sensitivity. This derivative has units of kelvin.

Lower winter temperatures favour formation and persistence of polar stratospheric clouds, providing particle surfaces on which reservoir chlorine can be activated, for exampleThese are heterogeneous surface processes; cooling changes particle abundance, adsorption, phase and reaction uptake, so a simple gas-phase Arrhenius argument is insufficient. Removal of nitric acid in sedimenting particles can also delay conversion of active chlorine back into chlorine nitrate. The relevant condensation thresholds depend on pressure and water/nitric-acid abundances, rather than a single universal temperature.

Once sunlight is present, $\mathrm{C}{\mathrm{l}}_2+\mathrm{h}\nu \to 2Cl$ provides active chlorine. In the chlorine monoxide dimer cycle,The net reaction is $2{\mathrm{O}}_3\to 3{\mathrm{O}}_2$, with chlorine regenerated. Low temperature favours formation of the ClOOCl dimer and suppresses its thermal dissociation, making photodissociation rather than thermal return of two ClO radicals more competitive. Laboratory flow experiments formed the dimer at 220–240 K and distinguished its infrared and ultraviolet signatures, supporting the chemical identity needed for this mechanism.^[ref]

Colder polar conditions can prepare and sustain greater springtime ozone loss, but darkness alone does not drive the photolytic cycle. In polar night reactive precursors can accumulate; sunlight is required for rapid dimer-cycle ozone destruction. This differs from the middle-stratospheric gas-phase cooling response derived above.^[ref]

Photodissociation of nitrogen dioxide forms an oxygen atom, followed by third-body recombination:If NO is converted back to nitrogen dioxide only by $\mathrm{NO}+{\mathrm{O}}_3\to \mathrm{N}{\mathrm{O}}_2+{\mathrm{O}}_2$, this is a null cycle with no net ozone production. Oxidation of a volatile organic compound by OH generates organic peroxy radicals, and often hydroperoxyl radicals, which instead oxidize NO without consuming ozone:The newly formed nitrogen dioxide can then photolyse, giving net ozone production. These chain reactions require sunlight, radical production and both precursor families.

In an NOx-limited ozone-production regime, NO is too scarce for efficient peroxy-radical conversion; competing peroxide-forming radical termination becomes important. Reducing nitrogen-oxide emissions then reduces ozone production. In a VOC-limited ozone-production regime, plentiful nitrogen oxides remove OH through $\mathrm{OH}+\mathrm{N}{\mathrm{O}}_2+\mathrm{M}\to \mathrm{HN}{\mathrm{O}}_3+\mathrm{M}$, so organic oxidation and radical supply are limiting. Reducing volatile-organic emissions is effective there, while a modest nitrogen-oxide reduction can initially increase ozone by reducing radical termination and NO titration. Sufficiently large nitrogen-oxide reductions can move the system into the limited regime and eventually lower ozone. The appropriate control therefore depends on the chemical regime and the regional transport history, not merely the largest local emission. The ozone isopleth diagram below uses an explicitly schematic response function to show derivative signs, not a fitted concentration or emissions-control prediction.^[ref]