1.3. Emissions and the Environment

The global environmental issues addressed in this report are climate changes and depletion of stratospheric ozone (see Boxes 1-1 and 1-2). Major international assessments of these issues are made periodically, and no attempt is made here to revisit such reports (for more information, see IPCC, 1996a,b; WMO, 1999, and references therein). These reports are used for contextual information and as a background to the possible impact of aviation on the atmosphere. There have also been several recent assessments of the impacts of aviation in Europe and the United States of America (Stolarski et al., 1995; Friedl et al., 1997; Schumann et al., 1997; Brasseur et al., 1998).

Aviation represents only one of many perturbations associated with future scenarios. There are many sources of climate-active substances. For example, greenhouse gases are emitted from a wide range of industrial, domestic, and agricultural activities, and there are numerous sources of aerosols (e.g., sulfate from fossil fuel combustion and volcanoes and carbonaceous aerosols from fossil fuel and biomass burning). Increases in tropospheric ozone are expected to result from increasing methane, nitrogen oxides, carbon moNOxide, and hydrocarbon emissions. Similarly, perturbations to stratospheric ozone result from chlorofluorocarbons (CFCs) and CFC-substitutes and from halon and methyl bromide emissions in addition to any potential perturbations resulting from aircraft emissions.

Some of the processes related to aviation emissions and their location in the atmosphere are shown in Figure 1-2. Subsonic aircraft fly in the troposphere and lower stratosphere, whereas supersonic aircraft fly in the stratosphere 80-85% of the time, with cruise altitudes several kilometers above those of subsonic aircraft. The differing chemical and physical processes in the two regions must be taken into account.

The troposphere is principally heated near the Earth's surface, and the temperature decreases with altitude. Warm, moist air tends to underlie cool, dry air, leading to frequent vertical turbulent motions that exchange air throughout this region. In contrast, the temperature in the stratosphere generally increases with altitude; the result, as reflected in its name, is that this region is stratified. Vertical motions are much slower here than in the troposphere. The stratosphere is also much drier than the troposphere, and clouds rarely form at this level.

Table 1-1 lists aircraft emissions that are important from an atmospheric perspective, with summaries of the roles that they play. These emissions can be usefully divided into two categories, depending on how they affect climate: Direct, as with CO2 (where the emitted compound is the species that can modify climate), and indirect, where the climate species is not the same as the emitted species-as with modified cirrus cloud coverage resulting from particles and particle precursors.

1.3.1. Carbon Dioxide

The behavior of CO2 within the atmosphere is simple and well understood. There are no important formation or destruction processes that take place in the atmosphere itself. Atmospheric sources and sinks occur principally at the Earth's surface and involve exchanges with the biosphere and the oceans (e.g., Schimel et al., 1995, 1996). The effect of CO2 on climate change is direct and depends simply on its atmospheric concentration. CO2 molecules absorb outgoing infrared radiation emitted by the Earth's surface and lower atmosphere. The observed 25-30% increase in atmospheric CO2 concentrations over the past 200 years has caused a warming of the troposphere and a cooling of the stratosphere.

There has been much discussion about how stabilization of CO2 concentrations might be achieved in the future (e.g., IPCC, 1996a, 1997a,b). One of the most important factors is the accumulated emission between now and the time at which stabilization is reached. The way in which annual emissions vary over time is less important. Two findings from these IPCC reports are worth noting:

If global anthropogenic CO2 emissions were maintained near 1994 levels, the atmospheric concentration would continue to rise for more than 200 years; by the end of the 21st century, it would have reached about 500 ppmv (compared to its pre-industrial value of 280 ppmv).
A range of carbon cycle models indicates that stabilization of atmospheric CO2 concentrations at 450, 650, or 1000 ppmv could be achieved only if global anthropogenic CO2 emissions drop to 1990 levels by approximately 40, 140, or 240 years from now, respectively, and substantially below 1990 levels subsequently.

Table 1-1: Species contributing to climate and ozone change.

Emitted Species Role and Major Effect at Earth's Surface CO2 Troposphere and Stratosphere Direct radiative forcing warming H2O Troposphere Direct radiative forcing warming Increased contrail formation radiative forcing warming Stratosphere Direct radiative forcing warming Enhanced PSC formation O3 depletion enhanced UV-B Modifies O3 chemistry O3 depletion enhanced UV-B NOx Troposphere O3 formation in upper troposphere      radiative forcing warming      reduced UV-B Decrease in CH4 less radiative forcing cooling Stratosphere O3 formation below 18-20 km reduced UV-B O3 formation above 18-20 km enhanced UV-B Enhanced PSC formation O3 depletion enhanced UV-B SOxO and H2SO4 Troposphere Enhanced sulfate aerosol concentrations Direct radiative forcing cooling Contrail formation radiative forcing warming Increased cirrus cloud cover radiative forcing warming Modifies O3 chemistry Stratosphere Modifies O3 chemistry Soot Troposphere Direct radiative forcing warming Contrail formation radiative forcing warming Increased cirrus cloud cover radiative forcing warming Modifies O3 chemistry Stratosphere Modifies O3 chemistry

Notes: 1) Positive radiative forcing will tend to warm the Earth's surface; negative forcing will tend to cool the Earth's surface. 2) Any warming at the Earth's surface caused by increases in atmospheric CO2 will be accompanied by a cooling of the stratosphere. 3) Any decrease in ozone, whether in the troposphere or stratosphere, will tend to enhance UV-B radiation and cool the Earth's surface. Conversely, any increase in ozone will tend to reduce UV-B and heat the Earth's surface. This dual effect is shown in this table only for tropospheric ozone. 4) Chemi-ions and metal particles are also emitted from aircraft. They may play an important role in contrail and enhanced cirrus formation (see Chapter 3), in which case their emission would lead to a tendency to warm the Earth's surface. 5) Hydrocarbons and CO are also emitted and contribute in a minor way to tropospheric ozone formation.

The amount of CO2 formed from the combustion of aircraft fuel is determined by the total amount of carbon in the fuel because CO2 is an unavoidable end product of the combustion process (as is water). The subsequent transport and processing of this CO2 in the atmosphere follows the same pathways as those of other CO2 molecules emitted into the atmosphere from whatever source. Thus, CO2 emitted from aircraft becomes well mixed and indistinguishable from CO2 from other fossil fuel sources, and the effects on climate are the same. The rate of growth in aviation CO2 emission is faster than the underlying global rate of economic growth, so aviation's contribution, along with those of other forms of transportation, to total emissions resulting from human activities is likely to grow in coming years. The radiative and climate implications are addressed in Chapter 6.

1.3.2. Water

The natural cycle of water in the atmosphere is also complex, involving a suite of closely coupled physical processes. This is particularly true in the troposphere, where there is continual cycling between water vapor, clouds, precipitation, and ground water. Water vapor and clouds have large radiative effects on climate and directly influence tropospheric chemistry. The stratosphere is much drier than the troposphere. Nevertheless, water vapor is important in determining radiative balance and chemical composition, most dramatically in polar ozone loss through the formation of polar stratospheric clouds (PSCs).

Emissions of water vapor by the global aircraft fleet into the troposphere are small compared with fluxes within the natural hydrological cycle; however, the effects of contrails and enhanced cirrus formation must be considered (Chapter 3). Water vapor resides in the troposphere for about 9 days. In the stratosphere, the time scale for removal of any aircraft water emissions is longer (months to years) than in the troposphere, and there is a greater chance for aircraft emissions to increase the ambient concentration. Any such increase could have two effects: A direct radiative effect with a consequent influence on climate, and a chemical perturbation of stratospheric ozone both directly and through the potentially increased occurrence of polar stratospheric clouds at high latitudes. The implication of releases in the stratosphere are discussed in Chapters 4 and 6.

1.3.3. Nitrogen Oxides

Nitrogen oxides (NO and NO2 are jointly referred to as NOx) are present throughout the atmosphere. They are very influential in the chemistry of the troposphere and the stratosphere, and they are important in ozone production and destruction processes. There are a number of sources (oxidation of N2O, lightning, fossil fuel combustion) whose contribution to NOx concentrations in the upper troposphere are not well quantified.

In all regions, the chemistry of the atmosphere is complex; aircraft NOx emissions are best viewed as perturbing a web of chemical reactions with a resultant impact on ozone concentrations that differs with location, season, and so forth. In the upper troposphere and lower stratosphere, aircraft NOx emissions tend to cause increased ozone amounts, so increased ozone and its greenhouse effects are the main issues for NOx emissions from subsonic aircraft. The pathways of other atmospheric constituents are also affected. Principal among these effects for NOx emissions is the reduction in the atmospheric lifetime and concentration of methane, another greenhouse gas. On the other hand, NOx emissions at the higher altitudes (18 km or above) of supersonic aircraft tend to deplete ozone. These and other issues related to the atmospheric chemistry of aviation emissions are discussed in Chapters 2 (past and present) and 4 (future). The effects on ultraviolet fluxes at the Earth's surface are discussed in Chapter 5.

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 NOx budget there; any long-term changes in the surface area of particles would affect stratospheric NOx, 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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