Introduction
Conclusions
Ozone Destruction by Hydrogen
Ozone (O3) is a critical component of our atmosphere because it absorbs in the UV region, which protects life from the harmful effects of UV radiation.
Ozone Formation and DestructionOzone is in the stratosphere is formed from oxygen molecules reacting with high energy photons:
O2 + hv(<242 nm) → 2O
O + O2 + M → O3 + M
Ozone can be destroyed by absorbing radiation:
O3 + hv (200-320nm) → O + O2
O2 + O → 2O2
It can also be destroyed by NO, hydroxyl radicals, chlorine (including CFC’s) and bromine [1].
The Hole in the Ozone Layer
Image of the ozone hole above Antarctica.
Ice Clouds over Scandinavia.
The ozone layer becomes thinner near the South Pole during the spring. During the winter, the air gets very cold and stratospheric ice clouds form. On the ice particles, HNO3, HCl and ClONO2 accumulate and the following reactions occur:
HCl + ClONO2 → Cl2 + HNO3
H2O + ClONO2 → HOCl + HNO3
HNO3 remains on the ice particles and the chlorine gas separates and moves above the ice clouds. Normally, nitrogen compounds would make chlorine unable to destroy ozone because ClONO2 would form. However, because of this physical separation of HNO3 and Cl2, ClONO2 cannot form. In the spring when there is more daylight, the photons will cause Cl2 to break into two Cl radicals and HOCl to break into a hydroxyl radical and a chlorine radical. These products will destroy ozone through the following reaction:
Cl (radical) + O3 → ClO (radical) + O2
The ClO radicals form dimers, which can separate into O2 and two Cl radicals. The Cl radicals can be used again to destroy ozone molecules [1].
How a Hydrogen Economy Could Affect Ozone in the StratosphereThere could be a general decrease in ozone depletion from a decrease in NOx emissions. However, having more H2 in the atmosphere could make ozone thinning at the poles worse. More H2 in the atmosphere could increase the amount of water vapor, which could decrease the temperature of the lower stratosphere.
The size of the ozone hole is increasing over time.
One model shows that if the lower stratosphere cools by 0.5 degrees C (a result of a increase in atmospheric H2O of 0.5 ppmv), the northern polar vortex will have a size increase of 7%, and the southern polar vortex will have a size increase of 4% [2]. Also, if the lower stratosphere is 0.5 degrees C colder, the polar ice clouds will be present for 5-8 days longer. This would allow the Cl2 and HOCl levels in the stratosphere to increase. Then, in the spring when daylight produces Cl radicals, more Cl radicals will be produced, which will react with ozone. The predicted increase in ozone depletion is 5-8% around the North Pole and 3-7% around the South Pole. The effect will be greater in the north because there are already so many Cl radicals in the south that the amount of ozone depletion is near maximum. This model suggests that a hydrogen economy could end up worsening ozone depletion around the poles, especially the North Pole. However, the extent of ozone depletion will depend on how long it takes for CFC’s in the atmosphere to decrease, as the reduction of CFC levels will decrease the ozone holes [2].
Another model predicts a much lower increase in stratospheric water vapor, and therefore a smaller change in temperature: a 0.1 to 0.3 degrees C decrease [3]. This model predicts that the changes in ozone concentrations due to a hydrogen economy will be negligible. These studies show the difficulty in predicting the effects of a hydrogen economy. They also illustrate the need to proceed cautiously because the effects are so uncertain.
Sources:
1. Spiro, T.G. and W.M. Stigliani, Chemistry of the Environment. Second ed. 2003, Upper Saddle River, NJ: Prentice Hall.
2. Tromp, T.K., et al., Potential environmental impact of a hydrogen economy on the stratosphere. Science, 2003. 300: p. 1740-1742.
3. Warwick, N.J., et al., Impact of a hydrogen economy on the stratosphere and troposphere studied in a 2-D model. Geophysical Research Letters, 2004. 31: p. L05107; 1-4.
