Chapter 6 — Case Study 2: The Dinitrogen Pentoxide Puzzle and the Discovery of the Ozone Hole

How IR spectroscopy detected the chemistry of ozone depletion in Antarctic ice.

In the mid-1980s, atmospheric chemists were trying to understand a disturbing observation: the ozone layer over Antarctica was thinning rapidly each spring. By 1985, the spring "ozone hole" had become a major scientific and environmental emergency. Within a few years, the cause was identified: chlorofluorocarbons (CFCs) — industrial refrigerants — were breaking down in the stratosphere, releasing chlorine radicals that catalyzed ozone destruction. The Montreal Protocol (1987) phased out CFCs, and by 2020 the ozone layer is slowly recovering.

At the heart of the investigation was a question that Chapter 6 could answer: what chemistry is happening on the surfaces of polar stratospheric clouds?

The puzzle

Laboratory studies had shown that in the gas phase, chlorine radicals should be mostly sequestered into relatively unreactive forms like hydrogen chloride (HCl) and chlorine nitrate (ClONO₂). These compounds would not destroy ozone.

But something was freeing the chlorine. Atmospheric measurements showed rising levels of chlorine monoxide (ClO) and OClO — species that do destroy ozone. Where was the chlorine coming from?

The hypothesis (Solomon et al., 1986) was that polar stratospheric clouds provided surfaces for heterogeneous reactions:

$$HCl + ClONO_2 \xrightarrow{\text{ice surface}} Cl_2 + HNO_3$$

The $Cl_2$ produced would be photolyzed by spring sunlight into reactive chlorine radicals. The $HNO_3$ would stay on the ice, sequestering nitrogen oxides that normally moderate chlorine chemistry.

To test the hypothesis, atmospheric chemists needed to directly detect the species on Antarctic ice surfaces. IR spectroscopy was the tool of choice.

The experiment

In the early 1990s, laboratory experiments by researchers at NASA Goddard and elsewhere attempted to replicate polar-stratospheric conditions: ice surfaces at -80 °C, exposure to HCl and $ClONO_2$ vapor at stratospheric pressures. The gases were allowed to react on the ice, and products were monitored by FTIR spectroscopy (Fourier-transform infrared) — the modern, sensitive version of the IR instrument you learned about in this chapter.

The IR spectra of the ice surface showed characteristic bands:

  • The disappearance of $ClONO_2$ absorptions (including the characteristic $NO_2$ asymmetric stretch around 1715 cm⁻¹).
  • The growth of $HNO_3$ features (broad O-H around 3300, strong $NO_2$ stretches at 1680 and 1320 cm⁻¹ on nitric acid).
  • Transient intermediates including nitric acid hydrates.

These observations were consistent with the proposed heterogeneous chemistry. Chlorine nitrate was being consumed; nitric acid was being produced. The mechanism explained how cold, cloudy polar spring conditions could generate reactive chlorine.

Why IR?

Two reasons. First, IR is non-destructive — the ice sample is not consumed by the measurement, so the same sample can be probed repeatedly as the reaction progresses. Second, IR can detect species at very low concentrations on surfaces; the absorption coefficients are well characterized for the major atmospheric species.

Mass spectrometry could have been used (and was, for gas-phase products), but it requires vaporizing the sample, which is difficult for ices at low temperatures. IR lets you probe the surface as it reacts.

The Chapter 6 lesson

This case study shows Chapter 6 chemistry at global scale. The same instrument that identifies aspirin in a forensic lab can identify chlorine nitrate on an ice surface at 80 km altitude. The spectroscopic principles — $C=O$ absorbs here, $N-O$ absorbs there, hydrogen bonding broadens peaks — are universal.

The atmospheric chemistry is Chapter 40 territory in this book (green chemistry and environmental sustainability). But the detection tools are Chapter 6.

When you learn to read an IR spectrum, you are learning a tool used from bench chemistry to global atmospheric monitoring. The spectroscopic alphabet is small; the applications are vast.

Further reading. Solomon, S. (1999). Stratospheric ozone depletion: A review of concepts and history. Reviews of Geophysics, 37(3), 275–316. An accessible review by one of the principal scientific investigators. Abbatt, J. P. D. (2003). Interactions of atmospheric trace gases with ice surfaces. Chemical Reviews, 103, 4783–4800.