SPADE Mission Statement

STRATOSPHERIC PHOTOCHEMISTRY, AEROSOLS AND DYNAMICS EXPEDITION

S. C. Wofsy, Project Scientist, SPADE
R. S. Stolarski, Program Scientist, AESA

 

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

  1. To study chemical processes potentially affecting ozone at altitudes most strongly influenced by stratospheric aviation by making comprehensive measurements of radicals and reservoir species, including HOx radicals (OH and HO2) and NO2, for the first time from the ER-2, along with NO, NOy, ClO, O3, HCl, sulfate aerosols, and UV and visible irradiances. The mission included night, sunrise, day and sunset (i.e., diurnal) observations over California and northern and southern survey flights to provide a rigorous diagnosis of factors regulating concentrations of radicals.
  2. To examine distributions of tracers whose concentrations in the lower stratosphere vary on time scales ranging from months to years. These studies provide essential data for analyzing dispersal and eventual removal of aircraft exhaust emitted into the lower stratosphere.
  3. To determine the effects of heterogeneous chemistry on concentrations of radicals and reservoir species by obtaining data for various stages of decay of the Mt. Pinatubo aerosol and by examining morning-evening differences.
  4. To lay the groundwork for 1994 field missions of the HSRP and UARP.

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

    1. The new HOx instrument performed reliably with excellent signal-to-noise ratio, apparently free of artifacts. A pulse of HOx radicals was observed at visible sunrise, not predicted by photochemical models, possibly indicating production of a readily photolyzed species at night (such as HONO) by heterogeneous chemistry. Daytime OH and HO2 concentrations were generally consistent with model simulations; definitive comparisons must await follow final calibrations and careful study of the full data set including NOx, albedo, particle surface areas, overhead and in situ O3, etc.

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.

    1. Plots of CO2 versus N2O tracers revealed very low scatter, both in spring and fall, however, the relationships changed markedly over the five month period reflecting seasonal changes and long-term trends in tropospheric CO2 concentrations. The findings indicate that atmospheric motions smoothed out latitudinal variations in less than 5 months. Near the tropopause at subtropical latitudes the signature of recent input of CO2 and H2O from the troposphere into the stratosphere was clearly observed.

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.

    1. Diurnal variations of NO, NO2, HOx, and ClO were observed in experiments that successfully followed radical concentrations through sunrise and sunset in air parcels with equivalent tracer concentrations. Measurements (including NO2 for the first time by the ER-2) provide a nearly complete set of chemical data to assess critically current understanding of gas-phase and heterogeneous reaction rates in the stratosphere. The results demonstrate the importance of short-wave albedo in regulating radical concentrations, and they suggest that improvements are needed in model treatments of solar ultraviolet irradiance for low sun angles (i.e., solar zenith angles > 85 deg). Several flights showed conclusively that light reflected by underlying cloud fields can strongly perturb key constituent concentrations, especially NO and NO2.

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.

    1. The anomaly in measured HCl concentrations first observed during the earlier Airborne Arctic Stratospheric Expedition II (AASE-II) (i.e., low values of HCl relative to expectations from photochemical models) was confirmed. The cause remains obscure, with possibilities including incorrect specification of total inorganic chlorine (Cly), missing or incorrect chemistry, or unknown instrument artifacts.

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.

    1. Several sharply delineated air parcels were observed with markedly low sulfate aerosol surface area, high NOy and ClO, and low N2O and CO2 concentrations (indicating old air - Steve/Rich, please clarify the term "old"). Preliminary analysis suggested possible denitrification (i.e., irreversible removal of NOy) and dehydration. These air parcels appear to represent virtually unmodified air from the end stage of the polar vortex, giving evidence of large-scale latitudinal descent, processing by polar stratospheric clouds, and possibly substantial ozone loss during the winter.

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.

    1. The volcanic aerosol from Mt. Pinatubo was observed to have aged and settled over the winter season, providing new information needed to understand the evolution of stratospheric aerosols after a major eruption. Measurements of NO, ClO, O3, NO2, NOy, and aerosol surface area confirm the influence of heterogeneous hydrolysis of N2O5 on sulfate aerosols in the subtropics. Concentrations of ClO, and the ClO/HCl ratio, declined as the aerosol surface area declined, while NOx, and the NOx/NOy ratio, increased.

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.

    1. Dynamical and chemical signatures of the ER-2 wake were observed several times during SPADE. The emission index (EI) at cruise was 3-5 g NO2/kg fuel, consistent with Pratt & Whitney estimates for the J-75 engine for cruise conditions. Coincidentally this is the range currently being sought for advanced engines intended to power future HSCTs.

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:

  • Chemical cycles in the lower stratosphere
    • Radical families and ozone recombination rates
    • Diurnal studies and reservoir species
  • Aerosols in the lower stratosphere
  • Photochemistry in the lower stratosphere
  • Chlorine chemical cycles and budget
  • Polar air chemistry
  • Stratospheric transport and tracer fields
  • Aircraft exhaust
    • Locating at cruise altitude
    • Measurement of plume structures and emission indices

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.