Non-CO2 emissions and contrails, explained

Aviation emits more than just carbon dioxide. Airbus and the wider industry are taking a proactive role to address these non-CO₂ emissions. What are they, and how can they be mitigated?

Burning fuel creates both carbon dioxide (CO₂) and non-CO₂ emissions. Primary non-CO₂ emissions are water vapour and nitrogen oxides, collectively known as NOx. They also include sulphur oxides, carbon monoxide, soot, unburned hydrocarbons and aerosols.

In certain conditions, water vapour and some non-CO₂ emissions can cause condensation trails – contrails – which are the white streaks you see behind aircraft at altitude. They are essentially ice cirrus clouds. Although contrails  both reflect incoming solar radiation and trap outgoing heat, they have a net warming effect. There is still uncertainty on the magnitude of the effect, but all climate models agree.

Difficult to measure

Contrails form when the water vapour in the exhaust of an aircraft transforms into ice crystals, encapsulating non-CO₂ emissions also present in the exhaust.

Their formation and size are influenced by a multitude of factors, making their presence difficult to predict with certainty. For example, some ice crystals are bigger than others. This is a good thing as they are heavier and dissipate faster, making the contrail more short-lived.

Although cooling contrails exist, the majority are considered to have a warming effect, including all contrails that are emitted at night as there is no sunlight to reflect back. 

While only 10%-15% of aircraft contrails are persistent and most of these last only a few hours, they can exacerbate climate change by trapping heat radiating from the Earth’s surface. Their length can be significant: the average contrail is 150 kilometres long, which equates to about ten minutes of flying.

Balancing CO₂ and non-CO₂ impacts

The effect of non-CO2 emissions on climate can be mitigated by accurately forecasting the conditions in which contrails will form and persist. This allows flights to navigate around, below or above the locations most conducive to contrail formation, if air traffic control allows. 

However occasionally these alternative trajectories are longer, increasing both CO₂ emissions and the cost of operating the flight. The trade-off is delicate, but it concerns only a limited number of flights. It’s estimated that just 10% of flights, mostly long haul, create 80% of warming contrails.

To reduce or mitigate the impact of these flights, the aviation industry is accelerating research into the most effective solution, with many collaborations to combine research with the testing of solutions.

Indeed, in the context of the EU-funded SESAR Joint Undertaking, Airbus is leading CICONIA, a project that aims to introduce more accurate meteorological forecasts to help determine optimised trajectories and flight planning.

Additionally, Airbus is engaged in several projects looking at different fuel types and compositions to determine the optimal mixture for mitigating non-CO₂ emissions. This includes studying all alternative aviation fuels compositions for both CO₂ and non-CO₂ emissions, and the size of ice crystals formed from the water vapour they emit. 

Airbus is actively working with the research ecosystem to understand the impact of real flights, with different fuel types and trajectories, on contrail formation. This will improve contrail impact modelling in the longer term.