A Review of NASA's 'Atmospheric Effects of Stratospheric Aircraft' Project (1999)

Scientists, policy-makers, and the aircraft industry are concerned that operation of a large fleet of high-speed civil transport (HSCT) aircraft could detrimentally affect the global atmosphere. HSCT emissions could have a direct effect on the chemistry of the stratosphere, resulting in changes in the distribution and total amount of ozone. These changes, in turn, may have indirect effects on ozone and on global climate through coupling with radiative and dynamical atmospheric processes. Hence, assessing the atmospheric impact of a fleet of HSCTs requires not only an understanding of the chemistry of the natural stratosphere and its possible perturbations by HSCT emissions, but also a quantitative understanding of the pathways for transport of HSCT emissions, and their resulting temporal and spatial distribution within the atmosphere.

The results of NASA's Atmospheric Effects of Stratospheric Aircraft (AESA) project have been summarized in a recent assessment report (Kawa et al., 1999). This NRC report evaluates the NASA assessment and also provides guidance for future research on atmospheric effects of stratospheric aircraft. Because this will be the final report issued by the Panel on Atmospheric Effects of Aviation (PAEAN), and because continued near-term funding of AESA and, in fact, the entire Atmospheric Effects of Aviation Program (AEAP) is highly uncertain as this report is being finalized, recommendations for the future are long-term in perspective and include some issues of importance for subsonic as well as supersonic aviation.

AESA has been a significant scientific effort. Research supported by AESA has improved the understanding of the atmospheric effects of stratospheric aircraft and has made important contributions to the fundamental understanding of

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

observations and model simulations of the NO_x/NO_y and the ClNO₃/HCl ratios, the kinetics of HNO₃ formation, and the reaction rate of ClO with HO₂. For heterogeneous reactions, issues that need further clarification include the processes controlling background aerosol sources and processing, the phase and composition of polar stratospheric cloud (PSC) particles, and the impact of the thermodynamic conditions of PSC formation on surface-catalyzed chemistry.

Climate Impacts. Supersonic aircraft emissions, particularly of water vapor, may cause significant radiative forcing in the stratosphere. 3-D climate models are an essential tool for quantifying these perturbations and understanding them within the context of the wide variety of natural and anthropogenic forcings of climate. Currently, however, these models have a very limited ability to represent many of the important atmospheric processes and feedbacks that exist within the climate system, and thus model estimates of aviation climate impacts are highly uncertain.

Addressing these uncertainties would require the following:

• Continued investigation of fundamental engine combustion chemistry and particle formation processes is needed, including laboratory, modeling, and field studies.
• Assessment studies should continue to explore a comprehensive range of potential emission indices particularly for NO_x and sulfur compounds.
• Emphasis should be placed on characterizing the global distribution and sources of aerosols in the lower stratosphere and upper troposphere, in order to properly gauge the relative impact of aircraft particle emissions.
• Further theoretical and measurement studies should be undertaken to quantify transport processes such as troposphere-stratosphere exchange and mid-latitude/low-latitude mixing.
• In situ field measurement campaigns, which advance our understanding of stratospheric chemistry and dynamics, should continue to be supported.
• There should be continued work on laboratory studies of the composition of PSCs and the fundamental kinetics and temperature dependences of the chemical processes associated with PSCs.
• Emphasis should be placed on quantifying the radiative impacts of aircraft emissions in the stratosphere, particularly of water vapor, and the consequent feedbacks that may exist within the climate system.
• The next generation of stratospheric assessment models should include chemical-dynamical feedbacks, higher vertical and horizontal resolution, accurate representation of relevant tropospheric processes, and be capable of including the effects of future changes in atmospheric composition and climate.

More generally, the panel is concerned that NASA's Atmospheric Effects of

Aviation program will soon be ending, and there are currently no plans for a new program to carry on with focused research on this topic. Although many parts of the AEAP's research program could feasibly be supported through other existing programs, there is concern that certain critical research topics would ''fall through the cracks'' between these other programs. Maintenance of an applied program such as AEAP adds considerable value to the overall research efforts by providing a focus for multiagency and multidisciplinary coordination, and by helping the research community provide a coherent source of input into national and international assessment activities.