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
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
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.