Atmospheric Effects of Aviation: A Review of NASA's Subsonic Assessment Project (1999)

Over the course of the last few years, SASS-sponsored research has unquestionably made important progress towards its goal of quantifying the atmospheric impacts of aviation, and has also contributed a great deal to our fundamental understanding of atmospheric chemistry and dynamics. Some notable accomplishments include:

• development and testing of several core components of the GMI
• successful completion of the major field campaigns SUCCESS and SONEX
• compiling climatology data on atmospheric reactive nitrogen
• laboratory studies of some key gas-phase and heterogeneous reactions
• development of aircraft fuel-use and emission databases
• compiling a historical database for contrail effects
• improving our understanding of emissions through bench and field tests and plume/wake models

The panel commends SASS on the work done thus far. The panel also commends SASS for responding to both program management recommendationsmade in the panel's interim review report (i.e., drawing up a detailed strategic plan for the program and putting strong program leadership in place). The results can be seen in the last year's progress.

Throughout the discussions of the previous chapter, many suggestions are made for exploring new issues, or for continuing research activities currently under way. The panel realizes, however, that in the near term SASS may not

have the resources to carry out all the desired research activities. Given below then, is a list of issues that the panel would like to highlight as its highest priority. Some of these issues are already alluded to in the SASS strategic plan, but it is not clear that they are given sufficient priority or resources. In addition, suggestions are made about the general strategy and management of SASS. Some of these science and management recommendations echo those made in the panel's interim review of SASS (NRC, 1997a).

The impact of aviation on homogeneous ozone chemistry in the troposphere appears to be understood well enough to satisfy SASS's program objectives. However, heterogeneous reactions couple strongly with these gas-phase processes, and may significantly affect ozone concentrations. Thus assessments of the impacts of aviation on ozone levels must include the uncertainties associated with the climatology of aerosols and the heterogeneous processes occurring on these aerosols.

The Freidl (1997) report states that "additional characterization of engine exhaust, particularly of SO_x oxidation products and of particle emissions, is required to define the initial state of aircraft exhaust emissions." The panel agrees that more emissions characterization studies for the species listed above, as well as for a wider range of NMHCs, would be worthwhile. To accurately interpret field measurements of these exhaust species, methods need to be developed to distinguish in situ between emissions from aircraft and those from other sources.

The chemical and radiative effects of aerosols, contrails, and cirrus clouds remain an unresolved issue, though the panel notes SASS's increased attention to this matter. Aircraft particulate emissions must be assessed in the context of the ambient particle field, however, and it is not clear how SASS expects to accomplish this. The panel recommends that SASS assemble baseline data on the ambient particle fields in the upper troposphere and lower stratosphere, so that the relative effects of aircraft-emitted particles can be properly gauged. SASS should begin by undertaking the collection of currently available aerosol datasets.

The upper troposphere and lower stratosphere are highly interactive regions, and cross-tropopause transport may significantly affect aviation's impacts in both regions, since the species emitted by subsonic aircraft are deposited primarily in the tropopause region. Cross-tropopause transport processes are not well enough known to permit appropriate representation in models, and thus need to be an important focus of AEAP analyses. It is important that the SASS and AESA components of AEAP sort out this composite assessment with whatever type of collaboration will be most effective.

Assessments of the atmospheric effects of aircraft emissions come largely from model predictions. Thus, the dependence of the model results upon the parameterizations of sub-grid processes, stratosphere-troposphere exchanges, and the chemical schemes selected needs to be carefully tested. SASS should place a high priority on carrying out detailed sensitivity studies, and the tropospheric components of the GMI should participate in international model intercomparison studies.

A large portion of SASS's current budget is devoted to the Global Modeling Initiative. This level of support will be justified only if the GMI can incorporate aerosol/contrail/cirrus processes into its model framework. Given the complexity of the GMI, in the near term it might be more cost effective to invest in alternative, simpler modeling tools that could include parameterizations of these processes.

Finally, the GMI efforts should be more closely coordinated with the SASS-sponsored climate modeling studies being done at NASA-GISS.

SASS-sponsored field campaigns like SONEX and SUCCESS have proven extremely valuable in addressing questions related to aircraft emissions, ambient aerosol concentration, and the properties of contrails and cirrus clouds. Given the infrequency with which such complex and costly studies can be undertaken, it is important that they be carefully planned to provide answers to the most pressing uncertainties related to the impacts of aviation. The results of SUCCESS and SONEX should be used to guide the planning and conduct of SASS's next campaign, helping to pinpoint the most important scientific questions and the most effective measurement methodologies.

Much of SASS's work thus far has focused on quantifying the absolute magnitude of aircraft emissions and the resulting impacts upon the concentration of atmospheric species such as ozone. We agree that SASS experimental research should focus as much as possible directly on the effects of aircraft emissions. However, the importance of these perturbations can be evaluated only in the context of the natural distribution and variability of the relevant chemical species, as well as the magnitude of other anthropogenic perturbations. Obviously SASS does not have the resources to study the global climatologies and sources for all relevant gases and aerosols (as well as clouds) on its own. SASS should give much higher priority then to exploring how data accumulated by other programs can be used to provide some context in which aircraft effects can be evaluated. We recommend that SASS devote more of its time and money to establishing cooperative efforts with other research programs, both within and beyond NASA, and to evaluating the possible applicability of datasets and information gathered by others. It might even be appropriate to designate an investigator to focus on that task alone.

In particular, some of the research sponsored by NASA's Office of Earth Science may be quite useful in reducing uncertainties, as would some of the climatology database work being coordinated by IGAC. Likewise, SASS should be preparing to use the existing and anticipated data from satellite platforms, identifying the specific data products that will be relevant to evaluating the impacts of aircraft emissions. Methods, infrastructure, and techniques to simulate data flows should be created now, so that when remote-sensing platforms come on line, SASS will be prepared to use the data effectively.