By Bjorn Fehrm

April 18, 2025, ©. Leeham News: We do a Corner series about the state of developments to improve the emission situation for Air Transport. We try to understand why development has been slow.

We examine the non-CO2 effects of Air Transport that contribute to global warming. Over the last weeks, we have looked at contrails, which have the largest impact on global warming, larger than CO2, Figure 1.

NOx is a smaller contributor, but it contributes about 20% of the total to Global Warming.

Figure 1. A summary of the CO2 and non-CO2 Effective Radiative Forcing (ERF) contributions from Air Transport. Source: The report “The contribution of global aviation to anthropogenic climate forcing for 2000-2018” by Lee et al. (2021).

NOx and Global Warming

NOx, or Nitrogen Oxides, are created during combustion in the zone where sufficiently high temperatures exist, above 1,300°C, during a minimum reaction time called the residence time.

As the air contains both necessary components for generating NOx, the way the high temperature is created, with Jet-A1 or Hydrogen, is not important. What’s needed is a high-temperature burn process and the presence of Nitrogen and Oxygen for the creation of the NOx ( certain fuels that contain nitrogen molecules can increase the NOx emissions, though).

NOx are created in thermal car engines, power station boilers, and gas turbine engines. The car engine and boiler NOx generate high enough concentrations close to living species to cause harmful smog and acid rain.

From Wikipedia about the NOxes that are harmful to human beings:

In atmospheric chemistry, NOx is shorthand for nitric oxide (NO) and nitrogen dioxide (NO2), the nitrogen oxides that are most relevant for (groung based, my comment) air pollution. These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone.

The regulations for ground transport regarding emissions, including limits for NOx emissions, started in the 1960s, with California’s first tailpipe emission standard in 1966.

Nitrous oxide (N2O), which is less harmful, is also included in air transport NOx emissions. Although it contributes less severely to ground air pollution, it is involved in ozone depletion and global warming.

Gas Turbine NOx emission

Gas turbine manufacturers have worked for over 50 years with their combustor technology to lower NOx emissions. I described the problem and technology in my series “New engine development,” Parts 17 and 18.

It’s about dividing the combustion into different areas, where a richer pilot part maintains steady combustion, and a lean burn part with low NOx emissions dominates when a higher output is commanded.

Figure 2. The TAPS combustor’s Pilot and Main areas. Source: GE Aviation.

The ICAO Aircraft Engine Emissions Databank contains test stand data from full throttle runs of all commercial Turbofan engines. It’s hosted by EASA on their website. Here the official introduction:

The ICAO Aircraft Engine Emissions Databank contains information on exhaust emissions of production aircraft engines, measured according to the procedures in ICAO Annex 16, Volume II, and where noted, certified by the States of Design of the engines according to their national regulations. The databank covers engine types which emissions are regulated, namely turbojet and turbofan engines with a static thrust greater than 26.7 kilonewtons. The information is provided by the engine manufacturers, who are solely responsible for its accuracy. The European Union Aviation Safety Agency (EASA) is hosting the databank on behalf of ICAO and is not responsible for the contents.

This comprehensive list of all Turbofans (I don’t think it has any turbojets now that Concorde is not flying) has official emission data, including CO2, NOx, and Soot. Based on it, a few airports (less than 2% of the world’s airports, London Heathrow being one) have started to levy emission charges for visiting airliners, which include NOx emissions.

To my knowledge, there are no underway emission fees based on NOx; so far, there have only been CO2 emission fees, which are levied on flights over the EU in accordance with the EU ETS emission regulation.