Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
CO Energy
Biomass burning
NO x
4.2.1.6. Tropospheric Transport
Source 2015 2050
Energy
Biomass burning
+15%
+9%
+45%
+7%
+66%
+21%
+107%
+22%
VOCs Energy-related sources
(not isoprene) +23% +66%
In addition to transport by resolved-scale winds, all models considered here include parameterizations of vertical transport by sub-grid-scale processes such as
convection and turbulent mixing in the boundary layer. Again, the manner in which these processes are parameterized differs from model to model. In this context, it is
worth noting that four of the models (or their close counterparts) used in this exercise (ECHAm 3 /CHEM, HARVARD, UKMO, and Tm 3 /KNMI) were also involved in a
model intercomparison exercise sponsored by the World Climate Research Program (WCRP) in 1993 (Jacob et al., 1997). As part of this exercise, each model
simulated a scenario in which a fictitious tracer with a 5.5-day e-folding lifetime was emitted in the Northern Hemisphere mid-latitude UT. The vertical gradient in the
simulated fields was similar in several of the participating models. However, there were significant inter-model differences in the simulated rates of meridional tracer
transport in the UT.
4.2.2. Definition of Scenarios
This section describes the scenarios for aircraft emissions evaluated for this assessment. The premises for current (circa 1992) and future (roughly 2015 and 2050)
aircraft fleet emissions, along with descriptions of actual emissions databases, are given in Chapter 9. The assumptions used for the background atmosphere in model
calculations of the effects of aircraft emissions on O 3 are important and influence the results. In the following sections, we discuss the basis for background
atmospheres used in model calculations and the aircraft scenarios evaluated.
4.2.2.1. Background Atmospheres
Boundary conditions for CH 4 in the background atmosphere are 1714, 2052, and 2793 ppbv for the years 1992, 2015, and 2050, respectively. These amounts are
based on the IPCC IS92a scenario (IPCC, 1992, 1995). Updated projections for future CH 4 concentrations (WMO, 1999) are smaller than those assumed here. Recent
observations by Dlugokencky et al. (1998) show that CH 4 levels currently are leveling off. If this trend continues during the next century, with little or no increase in the
CH 4 concentration, the increase in background O 3 will also be substantially less than that calculated in these studies.
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