Contrails Formation
Aircraft emit a number of emissions into the atmosphere, such as CO2 and NOx, that lead to changes in the atmospheric composition. Analysis of emissions inventories show that aviation emissions are concentrated at altitudes between 10 and 12 km (Lee and Raper, 2003). Aircraft can give rise to condensation trails or contrails.
A contrail is a visible line cloud, consisting of tiny ice particles that form behind an aircraft if the ambient air is cold enough. The Schmidt-Appleman criteria describes the thermodynamics of contrail formation (Appleman, 1953), Schumann (1996). Contrail formation is dependent on physical parameters such as the atmospheric temperature and pressure, fuel energy content, aircraft-induced emission of water vapour and the aircraft propulsion efficiency. Contrails form below a threshold temperature, due to the increase in relative humidity occurring in an engine plume when the engine plume mixes with the surrounding atmosphere (Schumann, 2005), (Schumann, 1996).
In dry air, contrails dissolve immediately, thus having little to no impact on radiative forcing (RF). In moist air, contrails grow due to the intake of ambient water vapour. They may also persist in ice-supersaturated regions of the upper troposphere. Persistent linear contrails can spread in the atmosphere due to wind shear, mixing and uplift to form contrail-cirrus (Lee et al, 2010). There is also evidence that particles emitted in the engine exhaust plume influence cirrus formation. This is often referred to as soot-cirrus (Lee et al, 2010). Contrail-cirrus and soot-cirrus are collectively referred to as aviation-induced cirrus. Over congested airspace of areas such as central Europe and parts of the US, the cirrus coverage due to aircraft can reach up to 6-10% coverage (Burkhardt and Kärcher, 2011)
Contrails act in a similar manner to cirrus cloud in reducing the amount of short-wave (SW) solar radiation reaching the Earth’s surface and reducing the long-wave (LW) radiation emitted into space. The net positive contrail RF is a result of the opposing LW and SW radiation scattering. Uncertainty can arise in RF calculations as errors in the treatment of either LW or SW effects can result in overall uncertainty in the net RF (Myhre and Stordal, 2001), (Rädel and Shine, 2008). The RF of contrails and contrail-cirrus depends on:
- Geographic location
- Altitude
- Time of the day and year
- Background state of the atmosphere (e.g. whether there are lower clouds or not)
The IPCC Aviation Report stated that contrails may have a climate impact comparable to that of CO2 (Prather et al, 1999). More recently studies such as Frömming et al (2010) and Lee et al (2009) estimate that the RF due to linear contrails is about one fifth of the total aviation RF. The RF from contrail-cirrus has been calculated as nine times as great as the RF from linear contrails alone. Contrail-cirrus may be the greatest contributor to the aviation-related RF (Burkhardt and Kärcher, 2011).
Prather et al (1999) predicted that the climate effects of contrails would increase more rapidly in the future than other climate perturbations such as CO2 emissions from aviation. This is due to a number of factors, including predicted increases in propulsion efficiency. There is a focus in the aviation industry on increasing the efficiency of propulsion technology to reduce fuel consumption (Lee, 2010). However, the increase in aircraft propulsion efficiency can result in an increase of linear contrails coverage. This suggests a significant future increase in coverage from linear contrails, since the demand for air transport is expected to increase by 5% per annum over the next 20 years.
Contrail Mitigation
Operational changes have been proposed to avoid or reduce the formation of contrails. Several studies have been carried out on operational changes in aircraft flight altitudes (Sausen et al, 1998), (Fichter et al, 2005). The results showed that contrails formed in mid-latitude areas are suppressed by flying higher in extra tropical regions, while contrails formed in tropical areas are avoided by flying in lower altitudes.
Technical mitigation options have also been proposed including changes in aircraft engine design (Haglind, 2008), use of alternative fuels (Ponater et al, 2006) and the use of fuel additives (Gierens, 2007). Using alternative fuels such as liquid hydrogen would allow contrails to form at a higher threshold temperature than kerosene-based fuel. However, the climate impacts from contrails formed may be different, as the physical properties of these contrails differ from those formed by kerosene combustion (Ponater et al, 2006).
The use of fuel additives has also been investigated (Montgomery et al, 2005). Early research showed that in theory, additives could decrease the potential for condensation to the extent that higher super-saturation would be necessary for the formation of contrails. However, more recent research found this method to be not effective. This is due to a number of factors including the inability of current technology to produce a sufficiently effective fuel additive (Gierens, 2007).
Contrail Research at CATE
There are currently several on-going research programmes at CATE that are looking into the climate impacts of contrails. These studies used CATE’s suite of models, such as FAST [link to FAST model], COMA [link to COMA model], Edwards-Slingo Radiative Transfer Model (RTM) [link to E-S model] and LinClim [link to LinClim model].
FAST is used to generate 4D inventories of distance travelled from global aircraft movement data. The COMA model uses these distance inventories, fleet propulsion efficiency, meteorological data and satellite observations of contrail coverage to calculate global contrail coverage. The coverage data are then transferred to the Edwards-Slingo Radiative Transfer Model (RTM), to produce RF estimates due to these coverages. LinClim is then applied to produce the temproral evolution of contrails RF and their related contribution to temperature increase.
The following figures show an example results from these models. This study investigated the effects of increases in propulsion efficiency, η, and the growth of air travel on contrail coverage (Lee, 2010).
In another application, RF (left panel) and temperature response (right panel) from linear contrails were estimated for years between 2000 and 2100 (Lee, 2010).
