Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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3.1. Introduction
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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
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