Laughing gas no laughing matter

Time for controls? ...

A few years ago I pooh-poohed a paper by my colleague Ravi Shankara at NOAA. He made the point that nitrous oxide - otherwise known by dentists of yore as ‘laughing gas’, or chemists as N2O - will become the dominant ozone-depleting substance emitted in this century.

I reckoned it was a bit of a beat-up. Nitrous oxide is a stable naturally-occurring gas that’s emitted from soils. But high in the stratosphere it can be broken down by UV rays in sunlight to form nitric oxide (NO). That nitric oxide destroys ozone in a catalytic cycle of reactions similar to the one involving chlorine from CFCs. My point was that the cycle involving nitric oxide has always been the most important ozone loss cycle.

The increasing concentration of chlorine (and bromine) from man-made CFCs just provided new pathways for ozone depletion that further reduced its equilibrium concentration.

But it’s the extra man-made increases in nitrous oxide that Ravi was concerned about. The future role of nitrous oxide on ozone depletion (as well as climate change) will be even more important in the decades ahead. It was reported last month that nitrous oxide - like the other major greenhouse gases - is currently increasing at the rate of about 3 percent per decade as shown below- due mainly to over-application of nitrogenous fertilisers in agriculture. And long after atmospheric chlorine has reverted to background levels (due to the success of the Montreal Protocol), it will still be increasing. At the moment the concentration of N2O is about 335 parts per billion (ppb), but if the current rate of increase persists, by the end of the century its concentration would have compounded to be about 430 ppb. That’s about 50 percent more than in 1950 before those increases in atmospheric chlorine became significant.

So its contribution to ozone depletion will also increase. Maybe Ravi’s concerns weren’t such a beat-up after all?

It’s just a quirk of fate that the importance of the NO-cycle hasn’t already increased in recent years despite the increase in its major source gas. That’s because, while chlorine levels are high, reactions that occur between the nitrogen and chlorine oxides have the effect of locking both up in more stable forms that don’t affect ozone. But that won’t be the case in the future when the amount of chlorine in the atmosphere reverts to its natural level.

The calculated effect on ozone levels from these increases nitrous oxide aren’t huge. By year 2100, the expected reduction in ozone is less than 10 percent.

Despite that, there’s been discussion of whether nitrous oxide should be brought under the control of the Montreal Protocol, partly because of the attendant benefits to climate change. The case for its inclusion is stronger than that for the new man-made HFCs (i.e., hydrofluorocarbons, molecules that are used as replacement chemicals in refrigerators) that are now controlled under the recently ratified Kigali Amendment. These HFCs have no effect on ozone, but are powerful greenhouse gases. Nitrous oxide, on the other hand, is both an ozone-depleting substance and a greenhouse gas.

But - to control excess emissions, you’d need to have the agricultural sector on board. That’s a big ask. They’re already being asked to carry an unfair burden of the climate change problem. The problem’s not of their making. Its main cause is the build-up of carbon dioxide in the atmosphere from the burning of fossil fuels. But unluckily for farmers (and luckily for the fossil fuel barons), our day of reckoning can be delayed by curbing agricultural emissions of methane and nitrous oxide.

Further control on laughing gas will be N(2)O joke for them.