Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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EXECUTIVE SUMMARY
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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.
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