Disclaimer: This material is being kept online for historical purposes. Though accurate at the time of publication, it is no longer being updated. The page may contain broken links or outdated information, and parts may not function in current web browsers. Visit https://espo.nasa.gov for information about our current projects.
This statement has been prepared on behalf of investigators of the Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE) which was based at the NASA Ames Research Center, Moffett Field, CA in late-1992 and early-1993. An ER-2 aircraft was used as the instrument platform for primarily stratospheric observations (see enclosed configuration drawing). SPADE was the first expedition dedicated specifically to the objectives of the High-Speed Research Program (HSRP). It plays a role in the continuing development of the ER-2 instrument suite for stratospheric observations, hitherto principally sponsored by NASA's Upper Atmosphere Research Program (UARP).
OBJECTIVES
OPERATIONAL HISTORY
The SPADE instrument payload was among the heaviest and most complex flown by NASA's ER-2. Owing to the difficulty of integrating this payload, the mission was divided into two phases: an extended test phase in October and November 1992, and a brief test phase plus operational flights in April and May 1993.
Test flights in the fall of 1992 showed that the payload exceeded ER-2 constraints on both total fuselage weight and center of gravity, requiring significant weight reductions by fuselage instruments prior to operational missions in the spring. Tests of concepts for diurnal flights demonstrated the need for simplified flight plans. Valuable data sets were collected for comparison with previous and subsequent observations.
By the time of the spring mission, weight reductions made to O3, MMS, H2O, and HOx instruments with considerable effort by the instrument teams, brought the payload below the total fuselage weight constraint. But although the center of gravity met design limits, aircraft performance was unacceptable in turbulent conditions, forcing deletion of the Microwave Temperature Profiler (MTP, a piggyback investigation) and the telemetry package from the payload.
The spring operational phase achieved all mission objectives. In addition, the ER-2 observed, on several occasions, the composition of polar air at the end stage of winter chemistry, and its own wake (allowing measurement of the NOx emission index for the ER-2 engine at cruise conditions).
The SPADE mission tested a number of the concepts that underlie the stratospheric models used for assessment of high-speed civil transport (HSCT) effects on the stratosphere. In some cases these concepts appear to be sound, some appear doubtful (i.e., further analysis required to determine the significance of observed discrepancies), and some clearly need to be revised.
PRELIMINARY RESULTS
Significance for the HSRP: The HOx radicals, OH and HO2, are key constituents in determining the rate of photochemical transformations amongst the various forms of nitrogen and chlorine compounds. In addition, catalytic cycles of the HOx radicals participate in the destruction of lower stratospheric ozone. Prior to SPADE, no measurements of OH and HO2 had been made in the lower stratosphere, and the concentrations calculated in models could not be verified by measurement. SPADE measurements demonstrated that the fundamentals of the HOx simulations in models are correct, providing the first experimental confirmation that HOx catalytic cycles currently dominate ozone recombination below 20 km altitude. But the measurements uncovered discrepancies which, when understood, may modify model simulations of future HSCT perturbations. Preliminary estimates of uncertainty in the measurements are +/- 30%. While these may impact the quantitative calculation of the impact of an HSCT fleet, the result is not expected to be qualitatively different.
Significance for the HSRP: One of the most difficult aspects of assessing aircraft perturbations is to determine the validity of the transport properties of the models. The addition of the CO2 instrument to the ER-2 payload provides a unique tracer for transport for time scales as short as a season. The results from SPADE appear to show that air is mixed rapidly between middle latitudes and the tropics, and that air is exchanged between the troposphere and stratosphere at subtropical latitudes (where models typically don't have much exchange). These results should help to better define the dispersal and the effective lifetime of HSCT exhaust products in the stratosphere. Transport uncertainties are difficult to quantitatively estimate. The calculation of HSCT impact on ozone should scale approximately linearly with the estimated transported lifetimes of pollutants, but these lifetimes may still be uncertain by a factor of 2 or more.
Significance for the HSRP: One of the best tests of present understanding of the chemistry of ozone-destroying radicals in the stratosphere is to observe concentration changes when the sun rises or sets. The SPADE measurements provided the most complete radical measurement set to date. Because of the completeness of this set of measurements, along with observations of associated ultraviolet light, aerosols and tracers, the possible explanations for deviations from model predictions are severely constrained. When analysis is completed, the data should increase confidence (i.e., decrease uncertainty) in predicted ozone perturbations resulting from aircraft emissions.
Significance for the HSRP: Several suggestions have been put forward to explain the HCl results. These include a possible pressure dependence in the chlorine partitioning which manifests itself as a dependence on altitude. Recent laboratory results suggesting such a dependence in the photolysis rate of chlorine nitrate (ClONO2) leads to changes in the partitioning in the correct direction when included in models, but cause other problems of understanding. One model calculation indicated that a worst case impact may be that the aircraft perturbation calculation yields results similar to those obtained for low atmospheric chlorine scenarios (i.e., an approximate doubling of ozone depletion predictions). Until the cause of this anomaly is understood, it is difficult to make any more definitive statements concerning its impact.
Significance for the HSRP: When the wintertime polar vortex breaks up, fragments with perturbed polar chemistry are dispersed to lower latitudes. SPADE measurements were made in fragments of the polar vortex which were over California in early May. The relative importance of these fragments is uncertain. Particular interest focuses on the evidence these data provide for denitrification or dehydration during Arctic winter, since HSCT inputs of H2O and NOx directly into this region may affect the potential for these processes.
Significance for the HSRP: Heterogeneous reactions on sulfate aerosols are the key to present model calculations which predict relatively modest impacts of HSCTs on stratospheric ozone at low altitudes. The aerosols from Mt. Pinatubo provided a large enhancement in the surface area available for reaction. The SPADE measurements were made at a time when the Mt. Pinatubo aerosol surface area had decreased significantly from its maximum of 30 times background during AASE-II. The surface area during SPADE was still approximately five times background. These data will help us to map out the dependence of radical concentrations and ozone chemistry on the aerosol surface area.
Significance for the HSRP: A potential uncertainty in the evaluation of HSCT effects is whether the emission indices measured in ground tests are those which will actually occur at cruise in the stratosphere. During SPADE, several observations were made of the ER-2's wake. The measured EI for NOx appears to confirm that data from ground tests can be directly utilized in modeling calculations. Flight measurements of the ER-2 exhaust also indicate the possibility of such observations contributing to the understanding of near field interaction (i.e., between the aircraft wake and exhaust) effects on global chemistry, by demonstrating successful strategies for measuring wake chemistry of operational aircraft.
REPORTING
A workshop was conducted in September 1993 for discussion of results from SPADE and AASEII, and to form teams for preparation of scientific publications. As a result, a set of linked publications will be developed to interpret and communicate results in a coherent and concise manner.
Major topics to be addressed include:
The scientific publications will be summarized in "The Atmospheric Effects of Stratospheric Aircraft (AESA): A Fourth Program Report," which is to be prepared in coordination with the next AESA annual meeting in June 1994.