Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Box 1-3. Time Scales in Aviation and the Atmosphere
Understanding the time scales of the processes involved is important in assessing the impact aviation can have on the atmosphere now and in the future. It
takes many years for a new aircraft design to progress from the drawing board into service. Once aircraft are operative, their emissions remain in the
atmosphere for periods ranging from days to centuries, with some climatic effects felt on even longer time scales. Furthermore, although new technologies
would have an immediate effect on emissions from new aircraft, any impact on the global abundance of short-lived atmospheric constituents would be limited
by the rate of introduction of the new technology into the global fleet. A rough idea of the various time scales involved is provided. The processes that remove
trace species from the atmosphere can be chemical (e.g., the oxidation of methane), physical (e.g., in rain or by dry deposition onto land or sea), or biological
(e.g., the uptake of CO 2 by plants). The rate of each process typically varies with season and location in the atmosphere. These rates can be combined to
produce a rough estimate of how long each constituent remains in the atmosphere. A constituent with a short lifetime responds quickly to any change in
emissions. A trace species with a long lifetime responds slowly to a change in emissions. The atmospheric effects of H 2 O, NO x , SO x O, and soot are all
relatively short-lived. Broadly speaking, the tropospheric lifetimes of these constituents are a couple of weeks or less; that of any ozone produced by NO x is a
month or so. The stratospheric time scales involved are longer but are all well under a decade. By contrast, emissions of carbon dioxide affect the
atmosphere for a long time (about 100 years), with little difference for emissions into the stratosphere or troposphere. The main factors affecting how quickly
new aircraft are introduced are technological feasibility, certification, and commercial viability. Typically, new technology is likely to be a decade in its
gestation, although this time scale may be reduced if there are significant market opportunities. The project launch of a new aircraft type by an airframe
manufacturer is normally concurrent with the launch of new engines supplied by competing manufacturers. Development of the engine culminates in
airworthiness and emissions certification, usually 3-5 years later-but the time scale for entry into service is dictated by the airframe manufacturer and its
customer airlines. Once the engine has achieved airworthiness certification, it is installed on the airframe, and the aircraft typically then takes another year to
complete the airworthiness and noise certification process before initial deliveries are made to customers. With commercial airlines, individual aircraft will
operate for 25 years or more in revenue service. A good product, including its derivatives, will have a substantial production period (possibly 25 years or
longer); therefore, the overall time scale between introduction into service of an aircraft type and withdrawal from service may exceed 50 years. Development
of the infrastructure for air transportation (airports, air traffic control, etc.) can take years or even decades. This development is driven by overall increased
demand for air transportation, both for passengers and freight. It is limited by the availability of financial resources and local environmental concerns about
noise and increased ground traffic around new or expanded airports.
The future growth of aviation will depend heavily on factors such as economic growth (at global and regional levels), the demand for travel (in an age of rapid advances
in information technology), the development of infrastructure to support air travel and available flight technology, and the availability and cost of fuel. Increases in
demand will not translate directly into increases in emissions. Changes in engine efficiency, airplane design (size and shape), and operational practice are all expected
to lead to more efficient use of fuel because there are strong commercial reasons for airlines and other operators to keep fuel costs down.
The scenarios used in this report have been developed using models of passenger demand on a regional and global basis that assume future economic growth rates
as found in the IS92 scenarios (particularly IS92a). In all cases, it is assumed that infrastructure (e.g., airports) and technology will be developed so that this growth is
not constrained. Different aircraft types and fuel use are included, so CO 2 and H 2 O emissions-which depend solely on the amount of fuel burned-can be calculated
directly. SO 2 emissions are estimated simply by assuming what the sulfur composition in the fuel will be. Emissions of NO x , CO, and hydrocarbons depend strongly on
combustor technology-particularly the mixing of fuel and air in the combustion chamber, as well as temperature and pressure. These emissions are estimated using
semi-empirical relationships between in-flight fuel flow and emissions of NO x , CO, and hydrocarbons derived from ICAO engine certifications together with determined
flight patterns. Emissions of all of these compounds for global air traffic are produced on a 3-D grid (latitude, longitude, altitude) that can be used in global atmospheric
models. Little equivalent information regarding emissions exists for particulates other than total sulfur emissions.
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