Aviation and the Global Atmosphere

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Current aircraft engines emit aerosol particles and gaseous aerosol precursors into the upper troposphere and lower stratosphere that may affect air chemistry and climate. Aircraft engines also directly emit soot and metal particles. Liquid aerosol precursors include water vapor, oxidized sulfur in various forms, chemi-ions (charged molecules), nitrogen oxides, and unburned hydrocarbons.

Large numbers (about 1017/kg fuel) of small (radius 1 to 10 nm) volatile particles are formed in the exhaust plumes of cruising aircraft, as shown by in situ observations and model calculations. These new particles initially form from sulfuric acid, chemi-ions, and water vapor; they grow in size by coagulation and uptake of water vapor and other condensable gases. The conversion fraction of fuel sulfur to sulfuric acid in the young plume is inferred to be likely in the range of 0.4 to about 20%.

Subsonic aircraft emissions near the tropopause at northern mid-latitudes are a significant source of soot mass and sulfate aerosol surface area density and number concentration, according to existing measurements and model results. Aircraft generate far less aerosol than that emitted and produced at the Earth's surface or by strong volcanic eruptions. Aircraft emissions injected directly at 9- to 12-km altitudes are more important than similar surface emissions because of longer atmospheric residence times in the upper troposphere. The impact of present aircraft emissions on the formation of polar stratospheric clouds is much smaller than what is expected for a projected fleet of supersonic aircraft.

Regional enhancements in concentrations of aircraft-produced aerosol have been observed near air traffic corridors. Global changes in sulfate aerosol properties at subsonic air traffic altitudes were small over the past few decades. The contribution of aircraft emissions to changes or possible trends in these regions is difficult to determine because of the large variability of natural sources.

Contrails are visible line clouds that form behind aircraft flying in sufficiently cold air as a result of water vapor emissions. Contrail formation can be accurately predicted for given atmospheric temperature and humidity conditions. In the exhaust, water droplets form on soot and sulfuric acid particles, then freeze to form contrail particles. Models suggest that contrails would also form without soot and sulfur emissions by activation and freezing of background particles. Increasing fuel sulfur content results in more and smaller ice particles. In the future, aircraft with more fuel-efficient engines will produce lower exhaust temperatures for the same concentration of emitted water vapor, hence will tend to cause contrails at higher ambient temperatures and over a larger altitude range.

Persistent contrails often develop into more extensive contrail cirrus in ice-supersaturated air masses. Ice particles in such persistent contrails grow by uptake of water vapor from the surrounding air. The area of the Earth covered by persistent contrails is controlled by the global extent of ice-supersaturated air masses and the number of aircraft flights in those air masses. Present contrail cover will increase further as air traffic increases. The properties of persistent contrails depend on the aerosol formed in exhaust plumes. Regions of ice-supersaturation vary with time and location and are estimated to cover an average of 10 to 20% of the Earth's surface at mid-latitudes. Ice-supersaturation in these regions is often too small to allow cirrus to form naturally, so aircraft act as a trigger to form cirrus clouds.

The mean coverage of line-shaped contrails is currently greatest over the United States of America, Europe, and the North Atlantic; it amounts to 0.5% on average over central Europe during the daytime. The mean global linear contrail coverage represents the minimum change in cirrus cloud coverage from air traffic; its present value is estimated to be 0.1% (possibly 0.02 to 0.2%).

Aviation-induced aerosol present in exhaust plumes and accumulated in the background atmosphere may indirectly affect cirrus cloud cover or other cloud properties throughout the atmosphere. Observations and models are not yet sufficient to quantify the aerosol impact on cirrus cloud properties.

Satellite and surface-based observations of seasonal and decadal changes in cirrus cover and frequency in main air traffic regions suggest a possible relationship between air traffic and cirrus formation. Cirrus changes in main air traffic regions suggest global cirrus cover increases of up to 0.2% of the Earth's surface since the beginning of jet aviation, in addition to the 0.1% cover by line-shaped contrails. Observed cirrus cover changes have not been conclusively attributed to aircraft emissions or to other causes.

Contrails cause a positive mean radiative forcing at the top of the atmosphere. They reduce both the solar radiation reaching the surface and the amount of longwave radiation leaving the Earth to space. Contrails reduce the daily temperature range at the surface and cause a heating of the troposphere, especially over warm and bright surfaces. The radiative effects of contrails depend mainly on their coverage and optical depth.

For an estimated mean global linear-contrail cover of 0.1% of the Earth's surface and contrail optical depth of 0.3, radiative forcing is computed to be 0.02 W m-2, with a maximum value of 0.7 W m-2 over parts of Europe and the United States of America. Radiative forcing by linear contrails is uncertain by a factor of about 3 to 4 (range from 0.005 to 0.06 W m-2), reflecting uncertainties in contrail cover (x 2) and contrail mean optical depth (x 3).

In the current atmosphere, the direct radiative forcing of accumulated aircraft-induced aerosol is smaller than that of contrails. The optical depth of aircraft-induced aerosol is less than 0.0004 in the zonal mean and is much smaller than that of stratospheric aerosol from large volcanic eruptions or mean tropospheric aerosol abundances.

Indirect radiative forcing is caused by aviation-induced cirrus that is produced in addition to line-shaped contrail cirrus. This forcing is likely positive and may be larger than that from line-shaped contrails. Radiative forcing from additional cirrus may be as large as 0.04 W m-2 in 1992. Indirect forcing from other cloud effects has not yet been determined and may be either positive or negative.

In the future, contrail cloudiness and radiative forcing are expected to increase more strongly than global aviation fuel consumption because air traffic is expected to increase mainly in the upper troposphere, where contrails form preferentially, and because aircraft will be equipped with more fuel-efficient engines. More efficient engines will cause contrails to occur more frequently and over a larger altitude range for the same amount of air traffic. For the threefold increase in fuel consumption calculated for a 2050 scenario (Fa1), a fivefold increase in contrail cover and a sixfold increase in radiative forcing are expected. The contrail cover would increase even more strongly if the number of cruising aircraft increases more than their fuel consumption. For other 2050 scenarios (Fc1 and Fe1), the expected cirrus cover increases by factors of 3 and 9, respectively, compared to 1992. Higher cruise altitudes will increase contrail cover in the subtropics; lower cruise altitudes will increase contrail cover in polar regions. Future climate changes may cause further changes in expected aircraft-induced cirrus cover.

The future aerosol impact of aviation will increase with fuel consumption. The trends depend on future fuel-sulfur content, engine soot emissions, and the efficiency with which fuel sulfur is transformed into aerosol behind the aircraft.

Aerosol microphysical and chemical processes are similar in subsonic and supersonic aircraft plumes. Aerosol properties will differ because soot emission levels, aerosol formation potential, and plume dilution properties vary with engine type and atmospheric conditions at cruise altitudes. Significant increases in stratospheric aerosol are expected for the operation of a large fleet of supersonic aircraft, at least for non-volcanic periods.

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