Introduction
Conclusions
Nitrogen Oxides
Nitrogen oxides in the atmosphere can have many harmful effects on the environment. N2O, a greenhouse gas, comes from both natural and anthropogenic sources, including bacteria in rice paddies. NO2 contributes to smog in the troposphere. An estimated 24 Tg Nitrogen/year are released from fossil-fuel combustion. In the United States, 40-45% of the NOx emissions come from transportation [1], [2]. In a hydrogen economy, if hydrogen is produced without using fossil fuels and its energy is utilized with fuel cells, it is possible that NOx emissions could be reduced.
Benefits of Reducing NOx EmissionsNitrogen oxides can have harmful effects on ozone in the troposphere and the stratosphere. N2O is unreactive enough in the troposphere that it can reach the stratosphere and undergo photolysis:
N2O + hv(UV) → N2 + O*
N2O + O* → 2NO
In the stratosphere, NO helps contribute to ozone depletion. This is a problem because ozone absorbs much of the UV light that would otherwise reach the Earth’s surface and have harmful effects like cancer. NO in the stratosphere can destroy ozone through the following reactions:
NO + O3 → NO2 + O2
NO2 + O → NO + O2
In the troposphere, NO2 is also one of the main components of ozone formation, otherwise known as smog. NO2 itself gives smog a brown color, because it absorbs blue light. NO2 creates ozone through the following reactions:
NO2 + hv → NO + O
O + O2 → O3
Credit: NOAA
Peroxyl radicals are formed from hydrocarbons and then react with NO to make NO2. Because of this, NO2 can be used repeatedly to form more ozone [1]. If we moved to a hydrogen economy, then there could be less NOx produced because there might be less fossil fuel combustion. NOx would not be released from tailpipes, and hydrogen gas could be produced without the use of fossil fuel energy. This could cause less ozone depletion in the stratosphere and less ozone produced in the troposphere. One model suggests that ozone in the troposphere might decrease by 1-8 ppb [3]. Also, N2O is a potent greenhouse gas. Since the various forms of nitrogen are interconnected through the nitrogen cycle, if NOx emissions are reduced, then the amount of N2O would decrease.
Smog over Asia.
Drawbacks of Reducing NOx Emissions
There actually are some drawbacks to having less NOx emissions. NOx concentrations affect the concentration of OH radicals [3]. NO can convert HO2 to a hydroxyl radical through the reaction:
HO2 (radical) + NO → NO2 + OH (radical)
If there is less NOx, then there will be less OH radicals. OH radicals catalyze the destruction of methane and other gases:
OH (radical) + CH4 → CH3 (radical) + H2O
Through a series of reactions, the methyl radical turns into CH2O. This can react with hydroxyl radicals again, until eventually carbon dioxide is formed [1]. If there are less hydroxyl radicals, then these molecules will have longer lifetimes in the atmosphere.
One model by Schultz et al. predicts an increase in the lifetime of methane of up to 26%, which would increase the amount of methane in the atmosphere [3]. Since methane is a greenhouse gas, a longer methane lifetime in the atmosphere would cause an increase in the greenhouse effect.
The extent of the effects of these pros and cons depend on how the hydrogen is produced. If hydrogen is produced from the combustion of fossil fuels, then NOx will still be emitted, and the positive and negative effects described above would be diminished. Also, if the energy from hydrogen is obtained from hydrogen combustion instead of fuel cells, there is the potential for NOx to be produced. However, there are some hydrogen combustion methods that could theoretically not produce NOx. The various effects of NOx in the atmosphere illustrate the problems with the assumption that hydrogen fuel will be a catch-all solution for environmental problems. A hydrogen economy could just make our problems different than they are today.
Sources:
1. Spiro, T.G. and W.M. Stigliani, Chemistry of the Environment. Second ed. 2003, Upper Saddle River, NJ: Prentice Hall.
2. Seinfeld, J.H. and S.N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. 1998, New York: John Wiley & Sons, Inc.
3. Schultz, M.G., et al., Air pollution and climate-forcing impacts of a global hydrogen economy. Science, 2003. 302: p. 624-627.
