Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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2.1. Relating Aircraft Emissions to Atmospheric Ozone
2.1.1. Introduction
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The chemical products of aircraft jet fuel combustion are emitted at the engine nozzle exit plane as part of a high-velocity plume. This gaseous and particulate stream
is subject to chemical and dynamical processes that influence downstream composition. Eventually, plume constituents irreversibly mix with, and are diluted by,
ambient air. Subsequently, some of the emitted species act in concert with other natural and anthropogenic chemicals to change ozone abundances in the Earth's
atmosphere. The ultimate fates of these aircraft-derived species are determined by larger-scale chemical and transport processes.
Concerns about NO and NO2 (i.e., NOx ) emissions from present-generation subsonic and supersonic aircraft operating in the upper troposphere (UT) and lower
stratosphere (LS) were raised more than 20 years ago by Hidalgo and Crutzen (1977) because these emissions could change ozone levels locally by several percent
or so. Despite extensive research and evaluation during the intervening years, WMO-UNEP (1995) concluded that assessments of ozone changes related to aviation
remained uncertain and depended critically on NOx chemistry and its representation in complex models. Because of large uncertainties in present knowledge of the
tropospheric NO x budget, little confidence has been placed in previous assessments of quantitative model results of subsonic aircraft effects on atmospheric ozone.
Assessment tools and their input data continue to improve, however, and reconsideration is appropriate in the light of the extensive research results published since
the WMO-UNEP (1995) assessment.
The research results published since the WMO-UNEP (1995) assessment have addressed a number of issues relevant to the assessment of ozone impacts of present
aviation. These issues have included the development of improved aircraft NO x emission inventories, updating of evaluated chemical kinetic and photochemical
databases, studies of aircraft plume chemistry, and the development of three-dimensional (3-D) modeling tools. Reviews have also been published of U.S. (Friedl,
1997) and European (Schumann et al., 1997; Brasseur et al., 1998) research programs addressing ozone and other environmental impacts of present aviation.
In this chapter we evaluate, from a qualitative and quantitative standpoint, the impact on atmospheric ozone of aircraft exhaust species, emitted either directly from
engines or produced as secondary products of processes occurring in aircraft plumes. Our evaluation is based primarily on global model calculations rather than ozone
trends because expected changes are not easily discerned from observations, as discussed below. We use intermodel comparisons and atmospheric observations of
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