Atmospheric chemistry

The atmosphere today is about 78% nitrogen, 21% oxygen and just under 1% argon, with small amounts of carbon dioxide, water vapour and other gases. Small does not mean unimportant.

Greenhouse gases such as carbon dioxide, methane and water vapour let sunlight through but absorb some of the infrared radiation the warm Earth gives off, then send it back out in all directions, including downwards. This keeps the surface warmer than it would otherwise be. Burning fossil fuels and clearing forests have raised carbon dioxide levels, and farming cattle and growing rice release methane, which adds to the effect and causes climate change.

Pollutants come mainly from burning fuels:

• carbon monoxide from incomplete combustion, a toxic gas with no smell;
• sulfur dioxide from sulfur impurities in fuels, which causes acid rain;
• oxides of nitrogen, formed when nitrogen and oxygen react in hot engines, which also cause acid rain and breathing problems;
• soot particles, which damage lungs and darken clouds.

High above us, the ozone layer absorbs harmful ultraviolet light. Chlorine from CFCs, once used in fridges and aerosol sprays, destroyed some of it. Many countries agreed to ban CFCs, and the ozone layer is now recovering.

Ozone in the stratosphere

Ozone is constantly made and destroyed. Short-wavelength UV breaks the O=O bond homolytically, giving oxygen atoms that join O_2 to make ozone; ozone absorbs longer-wavelength UV and splits again:

The balance between these sets a steady ozone concentration, and the UV absorbed in the process is what shields the surface.

Chlorine radicals

CFCs such as CCl_2F_2 are unreactive near the ground, so they survive long enough to drift up to the stratosphere. There, UV breaks the weakest bond, C–Cl, releasing chlorine radicals. These destroy ozone in a chain reaction:

Add the two steps and the chlorine cancels: overall \mathrm{O_3 + O \rightarrow 2O_2}. The chlorine radical is a catalyst, regenerated each cycle, so one atom can remove many thousands of ozone molecules before being locked away. Nitrogen monoxide from aircraft can do the same job. HFCs, which contain no chlorine, replaced CFCs because C–F bonds are too strong to break this way, though HFCs are themselves potent greenhouse gases.

Why some gases are greenhouse gases

A molecule absorbs infrared only if a vibration changes its dipole moment. CO_2 has no permanent dipole, but its asymmetric stretch and bend do change the dipole, so it absorbs. N_2 and O_2 have no such vibration and are transparent to infrared.

Smog

In sunlight, NO_2 from exhaust splits into NO and an oxygen atom, which forms ozone at ground level. With unburnt hydrocarbons present, this builds up into photochemical smog. Catalytic converters reduce the problem by converting NO and CO into N_2 and CO_2.

The Chapman mechanism and why it fails

Ozone is a bent molecule (O–O 127.2 pm, angle 116.8°) whose delocalised π system absorbs strongly between 200 and 310 nm. The Chapman scheme is

where M is a third body (N_2 or O_2) that carries away the excess energy of the new bond. Steps 2 and 3 interconvert O and O_3 rapidly, so it is useful to track odd oxygen, \mathrm{O_x = O + O_3}, which is made only in step 1 and destroyed only in step 4. Setting production equal to loss and using the fast-step equilibrium [\mathrm{O}]/[\mathrm{O_3}] = j_3/(k_2[\mathrm{O_2}][\mathrm{M}]) gives

With measured rate constants this overestimates real ozone by roughly a factor of two. The missing loss comes from catalytic cycles \mathrm{X + O_3 \rightarrow XO + O_2}, \mathrm{XO + O \rightarrow X + O_2}, with X = OH, NO, Cl or Br. Each speeds up the slow step 4 without being consumed.

Reservoirs and the ozone hole

Most stratospheric chlorine is held in inactive reservoirs, HCl and ClONO_2. Over Antarctica in winter, temperatures fall below about 195 K and polar stratospheric clouds form. On their surfaces the reservoirs react:

The nitric acid stays in the particles, removing the nitrogen oxides that would otherwise re-capture ClO. When sunlight returns in spring, Cl_2 photolyses and the ClO dimer cycle (\mathrm{2ClO + M \rightarrow Cl_2O_2}, then photolysis back to 2Cl + O_2) destroys ozone without needing atomic oxygen, which is scarce in the lower stratosphere.

The troposphere: OH and NO_x

Photolysis of ozone below about 320 nm gives excited O(^1D), which reacts with water to form two hydroxyl radicals. OH starts the oxidation of almost every trace gas: methane, CO, SO_2 and volatile organic compounds. In sunlit air NO_2 photolysis and the reaction \mathrm{NO + O_3 \rightarrow NO_2 + O_2} set up a photostationary state, [\mathrm{O_3}] = j_{\mathrm{NO_2}}[\mathrm{NO_2}]/k[\mathrm{NO}]. Organic peroxy radicals from VOC oxidation convert NO to NO_2 without using ozone, upsetting that balance, and ozone accumulates: this is photochemical smog.

Atmospheric chemistry now combines field measurements, laboratory kinetics and global models, and the gaps between them drive research.

OH reactivity measurements, which give the total first-order loss rate of OH directly, often exceed the sum of known reactions with measured species, especially in forests. Isoprene oxidation turned out to recycle OH through peroxy-radical isomerisation, overturning older mechanisms. Criegee intermediates, carbonyl oxides formed in alkene ozonolysis, were first detected directly in 2012 and react with SO_2 fast enough to matter for sulfate formation.

Aerosols are the largest uncertainty in climate forcing. Secondary organic aerosol forms from low-volatility oxidation products, including highly oxygenated molecules formed by autoxidation. New particle formation from sulfuric acid, ammonia, amines and iodine oxoacids has been characterised in chamber experiments; how these particles grow to cloud-forming sizes is still being worked out.

On ozone, Antarctic recovery is visible but slow, with return to 1980 levels projected around the 2060s. Atmospheric monitoring found an unexpected rise in CFC-11 emissions in the 2010s, later traced to unreported production and since reduced, a demonstration of how measurement networks enforce treaties. Nitrous oxide is now the largest ozone-depleting emission and is not controlled by the Montreal Protocol. The 2016 Kigali Amendment phases down HFCs for their climate impact, while their replacements, hydrofluoroolefins, break down to trifluoroacetic acid, a persistent compound accumulating in water.

Methods include chemical ionisation mass spectrometry, laser-induced fluorescence for OH and HO_2, satellite retrievals of NO_2 and HCHO, and inverse modelling to locate emission sources from concentration data.