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
- 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.
- 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.
- 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.
- 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
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
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