Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
1.3.4. Particles and Particle Precursors
A similarly complex system of atmospheric processes and effects exists for particles. There are many types of particles, each with its own complex physics and
chemistry. Natural types of particles include salt particles from sea spray, wind-blown soil, and sulfate aerosols produced from naturally emitted sulfur-containing
gases. Aerosols resulting from human activities include sulfate aerosols and soot from fossil fuel burning. Carbonaceous aerosols are produced from biomass burning
and fossil fuel burning.
Particles related to aviation (principally sulfate aerosols and soot particles) are discussed in Chapter 3 together with contrail and cloud formation. Aircraft engines
actually emit a mixture of particles (including metal particles and chemi-ions) and gases (e.g., SO2 ). These emissions evolve in the engine exhaust and the atmosphere
to form a variety of particles mainly composed of soot from incomplete combustion and sulfuric acid (H2SO4 ) from sulfur in the aviation fuel. These particles are
capable of seeding contrails and cirrus clouds, thus potentially changing the total cloud cover in the upper troposphere. The climate impact of clouds is a balance of
their capabilities to reflect sunlight back to space and to trap outgoing infrared radiation from the Earth's surface. For high clouds, the latter effect is larger, and
increased cirrus coverage would result in a warming tendency. (This effect is opposite in sign to that of surface emissions of SO2 , which mainly affect low-altitude
clouds and produce a cooling effect.)
Particles are also involved in the chemical balance of the atmosphere. It is well established that the sulfate aerosol layer in the stratosphere is critically important in
determining the NO x budget there; any long-term changes in the surface area of particles would affect stratospheric NO x , hence ozone. The chemical issues related to
particles are discussed in Chapters 2 and 4.
1.3.5. Atmospheric Models
Atmospheric models attempt to describe the workings of the atmosphere; the detail of the description depends on factors such as the scientific understanding of the
processes involved, the time scale of interest, and the available computer resources. Different models include different facets of the atmosphere system. For instance,
state-of-the-art climate models are similar to weather prediction models, but additionally may include descriptions of the ocean and the biosphere so that the exchange
of heat and carbon dioxide can be modeled. The next generation of models is likely to include chemical processes and be derived from the current generation of
models that contain detailed descriptions of the chemistry.
A wide range of different types of atmospheric models are used within this report, depending on the problem of interest. Chemical transport models are used to
calculate changes in chemical composition resulting from aviation emissions (Chapters 2 and 4); microphysical models are used to calculate changes in particle
composition (Chapter 3), and radiative transfer and climate models are used to assess the possible impact on UV-B radiation (Chapter 5) and climate (Chapter 6).
Model uncertainties arise from one of two main sources: Incorrect or poorly quantified descriptions of the processes involved, and missing processes. These
uncertainties are typically reduced over time as the state of the underlying scientific knowledge evolves. Although it is difficult to quantify these uncertainties, one of the
major aims of this report is to give a clear idea of the uncertainties associated with model calculations.
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