stratospheric chemistry and dynamics. For instance, the chemical processes controlling ozone formation and destruction in the lower stratosphere are more quantitatively understood as a result of AESA-sponsored research, which included in situ measurement of a wide variety of trace species, laboratory kinetics studies, and modeling of gas-phase and surface-catalyzed chemistry. Laboratory studies of heterogeneous processes have advanced understanding of the kinetics of reactions on aerosols, and aircraft-based in situ measurements have provided detailed knowledge of the number, size, and composition of stratospheric particles.

Important advances also have been made in the development of assessment models. AESA has used a combination of 2D and 3D global models for the assessment of HSCT impacts in the stratosphere, along with detailed box models for analyzing rapid photochemistry. AESA has performed extensive model vaCE°idation exercises by comparing predictions with measurements. This has highlighted some limitations that exist in accurately representing atmospheric transport. From these modeling studies, AESA has estimated that a fleet of 500 HSCTs (with a NO_x emission index of 5 g/kg) cruising between 17 and 20 km altitude could cause an Northern Hemispheric ozone column change in the range of -2.5 to +0.5 percent.¹ This range reflects current uncertainty in both kinetics and transport.

Despite the advances that have been made, some important uncertainties remain:

Emissions. Concern over the potential impact of emissions from HSCTs operating in the stratosphere has led to research into the development of ultra-low emission combustion systems for these aircraft. Because these engines have not yet been fully built and tested, considerable uncertainties remain about combustion processes and exhaust constituents. The mechanisms by which particulate emissions are formed are especially unclear.

Atmospheric Transport. Quantitative understanding of atmospheric transport is needed to fully assess the effects of aircraft emissions deposited in the upper troposphere and lower stratosphere. Current models do not simulate these processes accurately, as evidenced by discrepancies between modeled and measured "age-of-air" estimates.

Ozone Impacts. Several uncertainties remain in the models used to assess the impacts of HSCT emissions on ozone. This includes a discrepancy between

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Front Matter (R1-R10)

Executive Summary (1-4)

1 Introduction (5-7)

2 Scientific Discussion (8-34)

3 Recommendations (35-37)

4 Future Directions (38-41)

References (42-48)

Acronyms (49-50)