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Chemistry · Ch 14 — Environmental Chemistry

Stratospheric Pollution

14.2.2

Stratospheric Pollution

Formation and Breakdown of Ozone

The stratosphere holds a natural reservoir of ozone (O₃) that shields the earth from harmful UV radiation (around 255 nm), which otherwise causes skin cancer (melanoma). Ozone forms when UV light splits molecular oxygen into free oxygen atoms, which then combine with more O₂:

O2(g)→hνO(g)+O(g)O_2(g) \xrightarrow{h\nu} O(g) + O(g)

O(g)+O2(g)⇌O3(g)O(g) + O_2(g) \rightleftharpoons O_3(g)

Because ozone is thermodynamically unstable and continually decomposes back to O₂, a dynamic equilibrium normally exists between ozone formation and breakdown.

CFCs and the Chlorine Radical Chain

The main culprit behind ozone-layer depletion is the class of chemicals called chlorofluorocarbons (CFCs or freons) — unreactive, non-flammable, non-toxic compounds once widely used in refrigerators, air conditioners, foam manufacture and electronics cleaning. Precisely because they are so unreactive, CFCs survive long enough to drift up into the stratosphere, where intense UV radiation breaks them apart, releasing chlorine free radicals:

CF2Cl2(g)→hνCl∙(g)+CF2Cl∙(g)(i)CF_2Cl_2(g) \xrightarrow{h\nu} Cl^{\bullet}(g) + CF_2Cl^{\bullet}(g) \quad (i)

The chlorine radical attacks ozone:

Cl∙(g)+O3(g)→ClO∙(g)+O2(g)(ii)Cl^{\bullet}(g) + O_3(g) \rightarrow ClO^{\bullet}(g) + O_2(g) \quad (ii)

and the chlorine monoxide radical regenerates a fresh chlorine radical by reacting with atomic oxygen:

ClO∙(g)+O(g)→Cl∙(g)+O2(g)(iii)ClO^{\bullet}(g) + O(g) \rightarrow Cl^{\bullet}(g) + O_2(g) \quad (iii)

Because the chlorine radical is regenerated at the end of the cycle, a single CFC molecule can destroy a very large number of ozone molecules — CFCs act as continuous carriers of chlorine radicals into the stratosphere.

The Antarctic Ozone Hole

In the 1980s, scientists working in Antarctica discovered a pronounced seasonal depletion of stratospheric ozone over the South Pole, known as the ozone hole. The mechanism involves a special seasonal cycle:

  • In summer, NO₂ and methane react with chlorine monoxide and chlorine atoms, forming stable "chlorine sinks" (chlorine nitrate and HCl) that limit further ozone destruction:

ClO∙(g)+NO2(g)→ClONO2(g)(iv)ClO^{\bullet}(g) + NO_2(g) \rightarrow ClONO_2(g) \quad (iv)

Cl∙(g)+CH4(g)→CH3∙(g)+HCl(g)(v)Cl^{\bullet}(g) + CH_4(g) \rightarrow CH_3^{\bullet}(g) + HCl(g) \quad (v)

  • In winter, extremely cold polar stratospheric clouds form over Antarctica. Their surfaces let chlorine nitrate hydrolyse to hypochlorous acid, and let it react with HCl to release molecular chlorine:

ClONO2(g)+H2O(g)→HOCl(g)+HNO3(g)(vi)ClONO_2(g) + H_2O(g) \rightarrow HOCl(g) + HNO_3(g) \quad (vi)

ClONO2(g)+HCl(g)→Cl2(g)+HNO3(g)(vii)ClONO_2(g) + HCl(g) \rightarrow Cl_2(g) + HNO_3(g) \quad (vii)

  • When spring sunlight returns, it breaks up the polar clouds and photolyses HOCl and Cl₂ back into chlorine radicals:

HOCl(g)→hνOH∙(g)+Cl∙(g)(viii)HOCl(g) \xrightarrow{h\nu} OH^{\bullet}(g) + Cl^{\bullet}(g) \quad (viii)

Cl2(g)→hν2Cl∙(g)(ix)Cl_2(g) \xrightarrow{h\nu} 2Cl^{\bullet}(g) \quad (ix) …

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