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3.1. Introduction
Recent advances in our understanding of heterogeneous chemistry in the lower
stratosphere and the role of aerosols and clouds in climate forcing have increased
the need to understand the influence of these aircraft emissions on atmospheric
composition. Aerosol particles from aviation-comprising soot, metals, sulfuric
acid, water vapor, and possibly nitric acid and unburned hydrocarbons-may influence
the state of the atmosphere in many ways. These particles may provide surfaces
for heterogeneous chemical reactions, both in the exhaust plume and on regional
and global scales; represent a sink for condensable atmospheric gases; absorb
or scatter radiation directly; and change cloud properties that may affect radiation
indirectly. Persistent contrails can directly cause additional cirrus clouds
to form. In addition, aerosol particles may enhance sedimentation and precipitation
of atmospheric water vapor, hence affecting the hydrological cycle and the budget
of other gases and particles. Changes in cloud formation properties and cloud
cover may also affect actinic fluxes in the atmosphere and ultraviolet-B (UV-B)
radiation at the surface.
This chapter addresses the following questions related to aviation-induced
aerosol particles:
What are the processes that produce aerosols and contrails in the plume of
a jet aircraft engine?
What is the relationship of the aircraft aerosol source to background aerosol
abundances and trends in the atmosphere?
Why do persistent contrails form, and what are their properties?
What is the relationship of aircraft-induced aerosol and contrails to cirrus
cloudiness and trends?
What are the radiative properties of aviation-induced aerosol, contrails,
and cirrus clouds, and how do they affect the Earth-atmosphere system?
What changes in the effects of aviation-induced aerosol might occur in response
to future changes in climatological conditions or aircraft operating parameters?
Section 3.2 describes the theoretical and experimental
basis of the emission and formation of aerosol in aircraft plumes. Section
3.3 describes findings and calculations of regional and global aerosol distributions
and their trends, quantifies the change in aerosol mass and surface density
from present aviation emissions at global scales, and compares aircraft sources
with volcanic and other natural or anthropogenic sources. Section
3.4 reviews recent results on contrails and cirrus clouds, provides estimates
for regional and global contrail coverage, and describes measured contrail particle
properties. Section 3.5 presents recent evidence that
may relate changes in cirrus cloudiness and related climate parameters to aircraft
emissions. Section 3.6 describes changes in radiative
fluxes from contrails as a function of various cloud parameters using three
one-dimensional radiation transport models, and presents a computation of global
radiative forcing from contrails. Finally, Section 3.7
identifies climatological and aircraft operational parameters that may influence
the future importance of aviation-induced aerosol and cloudiness and estimates
future contrail cover for a fixed climate.
The material in this chapter relies on Chapters 7 and
9 to describe emissions present at the engine exit plane
and total emissions from global air traffic; Chapters 2
and 4 to discuss the chemical implications of changes
in aerosol properties; and Chapters 5 and 6
to describe changes in UV-B radiation and climate from aerosols.
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