Nitrogen oxides (NOx) act as indirect greenhouse gases by producing the tropospheric greenhouse gas 'ozone' via photochemical reactions in the atmosphere. The impact of NOx gases on global warming is not all bad though. Like tropospheric ozone, NOx gases also affect the global greenhouse gas budget through their effect on the atmospheric abundance of hydroxyl (OH) radicals.

The breakdown of NOx gases gives rise to increased OH abundance and so helps to reduce the lifetimes of greenhouse gases like methane. Sources of NOx include fossil fuel burning, biomass burning and emission from soils.

NOx is also emitted directly into the troposphere by lightning, ammonia oxidation and aircraft. The main sink for NOx gases is oxidation in the atmosphere, however significant amounts of NOx arising from soils can be used up in the tree canopy before it escapes to the troposphere. Another pathway for NOx in the atmosphere is that of dry deposition back on land, such deposition can then lead to increased emissions of the direct greenhouse gas nitrous oxide (N2O).

Human Impact

The largest global source of NOx emission is that of fossil fuel burning, with a recent estimate of 33 Tg per year. Emissions are rapidly increasing in areas such as East Asia, where increases in fossil fuel use by transport are helping to push emissions up. Rapid increases in global air travel are also a concern, with great potential for increased emission of NOx directly to the troposphere.

Potential for control

As fossil fuel combustion is the largest source of NOx gases, then reducing global fossil fuel usage is the most direct route to reducing NOx emissions. Though emissions from the developed world have remained relatively constant over the last few years, increased use of fossil fuels in the developing world could mean I large increase in global emissions over the next few decades. Reducing transport emissions, in particular aviation, probably provides the most direct way to tackle NOx gases.