Table 1. Platforms for Airborne Atmospheric Chemistry Measurements
The effect of human activity on the composition of the atmosphere is an issue at the heart of global change because of its strong implications for climate, the biosphere, and public welfare. Major chemical perturbations are expected over the next century due in particular to rising human population coupled to rising fossil fuel consumption, changing patterns of agricultural production and rapid land use change, the phase-out of chlorofluorocarbons coupled to the phase-in of replacement products, and the rise in aircraft emissions including possibly a supersonic fleet in the stratosphere. This report identifies a set of critical problems in atmospheric chemistry for the year 2000 and beyond, and assesses the role of space-based measurements of the EOS program in addressing these problems.
The main driving force of atmospheric chemistry research is the need to develop sound environmental policy related to the following questions:
Answers to these questions require substantial improvement of our current knowledge of the physical, chemical, and biological processes affecting atmospheric chemistry. Major scientific issues needing to be resolved are:
We begin with a brief review of measurement platforms (ground-, aircraft-, and space-based) expected to be operational for atmospheric chemistry observations in the year 2000 and beyond. We then discuss a strategy for effectively using these platforms to address these issues.
2. ATMOSPHERIC CHEMISTRY MEASUREMENT PLATFORMS FOR THE YEAR 2000 AND BEYOND.
2.1 Ground- and aircraft-based platforms
A. Ground-based sensors
A wide range of atmospheric chemistry measurements are made from the ground. These include: ambient concentrations of stable gases, radicals, and aerosols; wet and dry deposition fluxes; vertical profiles of atmospheric composition and structure by active sensors such as LIDAR; and atmospheric structure and composition measured by microwave sounders. As instrumentation evolves and the scientific questions are refined, strategies for deploying these instruments have demanded more rigorous experimental designs. Complex arrays of instruments are common, allowing simultaneous observations of a wide variety of species in order to characterize the oxidizing power of the atmosphere.
Ground-based observations are increasingly made for long periods to observe seasonal and interannual changes and long-term trends. The ALE/GAGE network for CFCs and NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL) network for greenhouse gases are excellent examples of long-term monitoring programs. The Network for the Detection of Stratospheric Change (NDSC), is another example of a coordinated, long-term, international ground-based stratospheric monitoring program. Multiple instruments measuring a variety of stratospheric species (profile and total column O3, H2O, NO2, aerosols, ...) are located at five sites spread from the Arctic to the Antarctic. In addition to monitoring programs like NDSC, short-term, intensive, ground-based programs are required to provide a more comprehensive set of measurements needed to elucidate the processes responsible for the long-term changes. These integrated experiments, involving closely coordinated measurements by aircraft and ground stations, have evolved rapidly (for example, the ABLE missions, BOREAS, MLOPEX) to provide sets of measurements spanning a wide range of spatial and temporal scales.
The increasing sophistication of deployed ground-based sensors will provide important opportunities for linking these observations to observations from space. Satellite measurements extend local measurements to the global domain, thereby making it possible to address global-scale atmospheric chemistry problems. However, the satellites measure radiances, while in situ techniques usually measure the actual quantities of interest (e.g., concentrations) by techniques other than radiance measurements. In order to provide a quantitatively reliable set of observations in the year 2000 and beyond, it will be important to continue to develop and deploy integrated experimental designs that enhance and exploit complementarity between ground-based and satellite sensors and validate the satellite measurements. Even ground-based sensors that measure radiance, such as passive microwave, provide valuable checks and enhancements of satellite measurements derived from radiances because of complementarity of point of view and measurement scale.
B. Airborne sensors
Platforms for airborne atmospheric chemistry measurements include NASA's ER-2, DC-8, and P-3; NCAR's WB-57F; and unmanned airborne vehicles (UAVs). Each aircraft has unique capabilities and limitations, summarized in Table 1. A wide range of instruments has been developed for airborne in situ measurements, including short-lived free radicals (ClO, OH) which are very difficult to measure at ground level. Also, a unique airborne ozone-aerosol lidar has been developed. These instruments potentially provide data for concentrations of radicals from all of the major families important in the atmosphere, for short- and long-lived tracers, and for aerosol size distributions and composition.
Table 1. Platforms for Airborne Atmospheric Chemistry
Measurements
Aircraft measurements offer superb capabilities for accurate measurements over a large range at fine spatial scales; hence, airborne observations should provide a primary source of "ground truth" for satellite sensors and a keystone for integrated experiments linking ground-based measurements, small-scale process studies, in situ (aircraft) observations, and satellite measurements. However, aircraft operate over limited regions and time periods, and they are further constrained by operational limitations (weather, proximate airfields). The best approach to obtain truly global data sets of high quality usually requires a combination of ground-based, airborne, and satellite observations.
2.2 Space-based platforms.
The current EOS program includes six satellite instruments
dedicated to atmospheric chemistry measurements: HIRDLS,
MLS, MOPITT, SAGE III, TES and ODUS (an instrument to be
provided by Japan for flight on CHEM-1). The capabilities
of each of these instruments are summarized in Table 2.
MOPITT (to be launched in June 1998 on the EOS AM-1
platform) will provide 3-D mapping of CO (a key gas
regulating the oxidizing power of the troposphere) and
horizontal mapping of the atmospheric column of methane.
HIRDLS, MLS, TES, and ODUS (to be launched together on the
CHEM-1 platform in December 2002) will map an extensive
ensemble of trace species (HIRDLS and MLS in the
stratosphere and upper troposphere and TES in the lower
stratosphere and troposphere, with ODUS providing horizontal
mapping of the atmospheric column of ozone). SAGE III (to
be launched in August 1998 on the Russian Meteor 3M-1
satellite) will provide 3-D mapping of ozone, water vapor,
aerosols, NO2,
and some other species. EOSP, on the EOS AM-2
platform, will measure the optical depth and polarization of
the tropospheric and stratospheric aerosol. Prior to EOSP,
two other EOS instruments will be monitoring atmospheric
aerosol burdens. The MODIS (AM-1,-2, PM-1) instruments will
retrieve aerosol optical depth and particle sizes in a
similar fashion to the current NOAA/AVHRR aerosol
measurements. More wavelengths have been added to improve
the aerosol retrieval over land. The MISR (AM-1,-2)
instruments will measure aerosol optical depth and particle
sizes using simultaneous multiple wavelengths and multiple
zenith angles.
3. ADVANTAGES AND LIMITATIONS OF SPACE-BASED
MEASUREMENTS.
The obvious merit of space-based measurements is their
unique capability for continuous global mapping of the
concentrations of trace species. This mapping is critical
for understanding sources, sinks, and chemical and dynamical
processes controlling species with short atmospheric
lifetimes
(a few months or less) and, hence, large spatial and
temporal variability. In addition, space-based measurements
can measure atmospheric composition at higher altitudes than
can be conveniently attained by conventional sampling means.
The drawbacks of space-based measurements are high cost,
lower spatial resolution, limitations in instrument
sensitivity, and limitations in the number of species that
can be measured. In situ measurements can achieve
better
accuracy and spatial resolution, at lower cost.
Consequently, in situ measurements from the surface and from
aircraft will remain the approach of choice for many studies
that focus on detailed processes, especially in the
troposphere. For long-lived gases with comparatively uniform
concentrations in the atmosphere, ground-based sampling at a
limited network of sites provides a cheaper alternative to
space-based or aircraft observations.
For these reasons judicious synergism between space-,
ground-, and aircraft-based measurements holds the key for a
successful atmospheric chemistry research program over the
next decades. While results of process studies based
primarily on aircraft and/or ground-based measurements
require satellite measurements in order to extend them to
the global domain, space-based measurements can also play an
extremely valuable role in the design of process studies by
identifying a specific problem. A good example is the
tropospheric ozone maximum over the south Atlantic in
spring, which was first identified by analysis of TOMS and
SAGE II satellite measurements and was later confirmed and
interpreted with aircraft observations in the
GTE/NASA/TRACE-A expedition.
Complementarity between in situ and space-based
measurements
is a key to solving atmospheric chemistry problems of global
or regional scale. Neither measurement model provides
complete coverage in terms of time and space scales,
resolutions, or species type, but well-coordinated use of
all types of measurements is capable of addressing the
atmospheric chemistry questions raised in this document.
4. MAJOR STRATOSPHERIC CHEMISTRY PROBLEMS IN THE YEAR
2000
AND BEYOND.
In this section, we anticipate what the major scientific
questions are likely to be, show how satellite, in situ, and
ground-based data may be jointly used to address them, and
identify some possible gaps in existing plans.
4.1 What controls the concentration of ozone in the lower
stratosphere?
Both the total column ozone abundance and the net radiation
at the surface are sensitive to changes in ozone
concentration in the lowest part of the stratosphere.
Moreover, interannual and interdecadal trends attributed to
chlorofluorocarbon (CFC) interactions with polar
stratospheric clouds (PSCs) and sulfate aerosols are large
in this layer, and the interactive chemical and dynamical
processes which control trace species concentrations are
complex. Exchanges of air between polar regions and
mid-latitudes, between tropics and mid-latitudes, and
between the troposphere and tropical/mid-latitude lower
stratosphere are all important factors influencing changes
in this region. Fundamental gaps in understanding remain,
and there will remain a need for improved quantitative
understanding of both the chemical and dynamical processes
in this layer in the year 2000 and beyond.
The data sets will serve two central purposes: (1) Monitor
change in the ozone layer with enough resolution and
specificity to precisely locate changes and their
relationship to the position of the tropopause, and measure
variables that cause or modulate ozone change. In addition
to ozone, other key variables include especially ClO,
aerosols, NOx,
water vapor, temperature, and meteorological
tracers; (2) Obtain sufficiently specific information for
quantitative models of composition of this region under a
wide range of conditions.
With changing concentrations of CFCs and their substitutes,
with a possible large increase in high altitude aircraft
operations, and with probable changes in temperature and
dynamical structure arising from continued increases of
greenhouse gases, changes in the ozone concentration in this
layer are likely to occur well past the year 2000.
Scientists in 2010 will need to have enough information on
ozone and ancillary trace species to understand basic ozone
changes.
The combination of stratospheric chemistry measurements on
the EOS CHEM payload is well suited to address this
question. The constituents to be measured in this region of
the atmosphere include O3,
ClO, NO2, ClONO2,
HCl, HNO3, N2O5, CFCs, H2O,
and CH4, as well as
aerosols and
temperature. These measurements will extend through the
tropopause region into the upper troposphere, and will have
vertical resolutions of 2-3 km (MLS) and 1 km (HIRDLS).
These instruments are complementary in that HIRDLS will have
high resolution in longitude as well as latitude, while MLS
will be unaffected by aerosol loading and will be able to
make measurements in high aerosol or cloudy regions which
are not accessible to HIRDLS. In addition, the SAGE III
proposed for launch in a lower inclination orbit will
provide much-more-definitive information on aerosol
distribution and properties and important baseline
measurements on a number of the key constituents in the
lower stratosphere and cloud-free portions of the upper
troposphere with high vertical resolution and very high
sensitivity and precision. Ground-based measurements and in
situ measurements will be necessary to provide calibration
verification for the satellite retrievals. Ground-based
profilers will also be needed to provide more detailed local
vertical structure information where profiling capability is
available.
One of the most demanding problems is that of quantifying
stratosphere-troposphere exchange under a wide range of
circumstances. For mid-latitudes, this will require a
combination of in situ aircraft observations for
measurements on horizontal scales of the order of 100 km -
500 km and vertical scales down to the order of 100 meters.
While there exist many aircraft mesoscale studies of the
tropopause region, the combination of aircraft measurements
with the satellite capabilities mentioned above and modeling
capabilities that are now evolving, particularly the
capability for modeling lagrangian trajectories, should make
possible new breakthroughs in quantitative understanding of
stratosphere-troposphere exchange in the EOS time frame.
Assuming that the base EOS CHEM capabilities become
available in 2003, two additional science issues arise: (1)
What UV flux measurement capabilities are required? (2)
Should EOS CHEM include measurements of OH or other HOx
radicals that could be obtained by enhancement of the MLS
instrument?
The answer to the first question is that UV flux
measurements must be available at the same time that EOS
CHEM measurements are obtained since it is impossible to
obtain closure on the chemistry without this measurement.
Although, on long time scales like the solar cycle, UV flux
variations are well correlated with proxies such as the 10.7
cm flux, this is not necessarily true on the shorter time
scale of the solar rotation. The current plan to fly
SOLSTICE on some MTPE mission in the post-2000 time frame is
endorsed. There is a need to ensure that SOLSTICE
measurements be available when EOS CHEM is flying.
It was the consensus of the group that it is very important
to obtain OH (and possibly HO2)
measurements, and that the
unique opportunity to do this with MLS on EOS CHEM should be
used. Although there are a few in situ aircraft or
long-path
balloon measurements of OH in the stratosphere, the MLS
measurement of OH is the only foreseen opportunity to obtain
global measurements of this important radical. The absence
of global OH measurements is a serious gap in the post-UARS
time frame until EOS CHEM since: (1) OH controls the
conversion of CH4 to
H2O, (2) reactions of
HOx radicals are
the most important loss mechanisms for ozone in both the
lowest and highest regions of the stratosphere, (3)
reactions with OH control the rate of oxidation of sulfur
gases (SO2, OCS)
to sulfate aerosol, (4) OH is in
competition with heterogeneous chemistry in controlling the
transfers between radical and reservoir species in both the
NOy and Clx
systems (e.g., OH plus NO2
to produce
HNO3 in
competition with both the reverse reaction [OH + HNO3] and
the reaction ClO plus NO2
to produce chlorine nitrate, and
OH plus HCl to produce free chlorine and water).
Although OH is often specified in models in terms of
concentrations of other species, it is necessary to measure
the dependence of OH on these concentrations of other
species over a wide range of situations in order to validate
the applicability of models. OH may be a "well-behaved"
constituent under a wide range of circumstances, as must be
assumed in models in the absence of measurements to the
contrary, but it is essential that this assumption be
tested.
4.2 What controls the concentration of ozone in the mid-
and upper stratosphere?
Although changes in the ozone concentration in this region
are of less importance for changes in the total column ozone
and, therefore, less important for UV fluxes at the surface,
they are still significant. There are important gaps in our
understanding of the processes controlling ozone
concentration.
Moreover, changes in ozone concentration in this region
affect the thermal and dynamical structure of the
stratosphere, which can feed back to changes in structure
and composition of the lower stratosphere. Gaps in our
understanding of the feedback processes include: (1) a
continuing discrepancy between models and observation in the
"photochemical region" around 40 km; (2) incomplete
understanding of the transition between gas phase and
heterogeneous chemistry in the lower stratosphere (25-35
km).
The MLS, HIRDLS, and SAGE III satellite measurements
described above also apply to this region, and will provide
key global information for both monitoring and process
studies. Because of the central role of HOx
chemistry, the augmented MLS capability to measure OH
and possibly HO2)
described above will be very important for closing the
unresolved questions for this layer. In situ measurements
and profiles from ground stations will also be important in
this layer for validating satellite retrievals.
5. MAJOR TROPOSPHERIC CHEMISTRY PROBLEMS IN THE YEAR
2000
AND BEYOND.
We examine here how a proper combination of space-, ground-,
and aircraft-based platforms can be used in an optimal way
for addressing critical tropospheric chemistry problems in
the next decade.
5.1 What factors control the concentrations of the major
greenhouse gases, water vapor, CO2,
methane, N2O, HCFCs, and
ozone?
Continuous observation of trends of the major greenhouse
gases is essential for an assessment of human influence on
climate. For CO2,
methane, N2O, and HCFCs,
lifetimes are
sufficiently long to allow thorough mixing in the
troposphere. Monitoring of trends for these gases is best
achieved at low cost with a limited network of ground-based
stations (as presently implemented by the NOAA/CMDL
network). By contrast, water vapor and ozone have shorter
lifetimes and hence considerable spatial variability in the
atmosphere in general. The radiation budget at the surface
of the Earth and in the lower troposphere is particularly
sensitive to water vapor in the upper troposphere. The
photochemical formation of ozone in the upper troposphere
makes a very important but uncertain contribution to the
total tropospheric ozone budget. Thus, long-term global
measurements of both ozone and water vapor on time and space
scales that can be related to synoptic activity and to
tropical mesoscale convection systems are of great
importance. Such measurements are very difficult to obtain
by in situ or surface-based techniques, but there is
excellent potential for obtaining them from the combination
of MLS and HIRDLS on EOS CHEM. Indeed, recent measurements
from the UARS satellite have demonstrated the utility of MLS
for obtaining upper tropospheric water vapor.
Interpretation of trends in greenhouse gases requires a
mechanistic understanding of their sources and sinks.
Chemical issues related to the origin of ozone and to the
oxidation of methane and HCFCs are particularly relevant to
a space-based program and are discussed in the next section.
Gas exchange with the biosphere and with the ocean are
critical processes for CO2, methane, and N2O;
quantifying
the exchange fluxes has proven to be exceedingly difficult
because a large number of variables are involved and these
vary greatly in both space and time. These variables control
concentrations of inorganic carbon, carbon dioxide, and
other gases in the surface waters of the ocean. We expect
that advances in our understanding will be largely driven by
surface measurements from ships and buoys and eddy
correlation measurements from towers and aircraft, with some
valuable additions from ground- and aircraft-based
measurements of isotopic ratios. The role of space-based
measurements will be largely limited to providing
information on surface properties and land use change.
Space-based measurements can contribute to our understanding
of the tropospheric methane budget. Both MOPITT and TES can
measure methane concentrations from space with 1%
sensitivity, and this information may prove useful for
identifying large sources of methane.
5.2. What controls the concentrations of tropospheric
oxidants, including, in particular, ozone and OH?
Ozone, OH, and other oxidants such as H2
O2 are produced in
the troposphere by a complicated ensemble of photochemical
reactions involving nitrogen oxides, CO, hydrocarbons, and
water vapor. The chemistry involved is not yet fully
understood, but large advances are expected over the next
decade. Emerging questions focus on the role of
heterogeneous chemistry (reactions in aerosols and clouds)
and the origin of NOx.
Further progress will require
advances in chemical instrumentation and well-designed field
experiments to study the chemistry on a small scale. It is
unlikely that space-based measurements can play much role in
the progress of this science.
Determination of the global trend of OH concentrations is of
particular importance as reaction with OH is the main
removal pathway for a large number of trace gases. Mass
balances on methylchloro-form measured in surface air have
been particularly useful in providing a surrogate
measurement of the global mean OH concentration. This
measurement will become increasingly reliable over the next
decade as methylchloro-form is phased out by the Montreal
protocol, thus removing the difficulty of estimating
emissions. Over the longer term horizon, HCFCs can provide
an excellent surrogate to replace methylchloroform. The
tropospheric lifetimes of the major HCFCs are sufficiently
long to allow thorough mixing; surface measurements at a
limited network of sites, as presently conducted by
NOAA/CMDL, are adequate.
Space-based measurements can play a critical role in our
understanding of tropospheric oxidants by global mapping of
the oxidant precursors (NOx,
CO, hydrocarbons, water vapor,
in addition to ozone itself). All these species have short
atmospheric lifetimes and hence show considerable spatial
and temporal variability. Aircraft have so far been the
platform of choice for mapping the distribution of oxidant
precursors, but aircraft measurements are necessarily
limited in space and time. Space-based measurements are the
only practical approach for global observation. As can be
seen in Table 2, sufficient resolution can be achieved from
space for global mapping of CO (MOPITT, TES), NO and HNO3
(TES), water vapor (SAGE III, TES), and ozone (SAGE III,
TES). Continuous global observation of oxidant precursors
from space takes on particular importance as source
distributions of these precursors are expected to change
substantially over the next decades due to growth of
aircraft emissions, land use change and industrial
development in the tropics, and changing patterns of
agriculture.
Transport from the stratosphere is a significant source of
tropospheric ozone and NOx.
The magnitude of the
cross-tropopause flux is still uncertain, and the mechanisms
for stratosphere-troposphere exchange are the subject of
debate, which is likely to continue into the next decade.
Much of stratosphere-troposphere exchange appears to take
place at the mesoscale, and is therefore best investigated
at the process level by in situ aircraft measurements
(the
projected ~1 km vertical resolution of satellite
measurements is not sufficient by itself). It is, however,
likely that the forcing of stratosphere-troposphere exchange
takes place on a larger scale. Global mapping from satellite
of tracers of stratosphere-troposphere exchange (e.g., N
2O,
CH4, HCFCs,
H2O) can provide useful
constraints for testing
the simulation of cross-tropopause transport in global
meteorological models.
5.3 What are the sources and properties of the
tropospheric
aerosol?
Scattering of solar radiation by aerosols cools the surface
of the Earth. It has been argued that the negative radiative
forcing caused by the increase in anthropogenic sulfate
aerosols over the past century could have largely offset the
positive forcing from greenhouse gases in some regions.
Reliable assessment is, however, hampered by our poor
knowledge of aerosol properties. Specific issues relate to
the chemical composition, size distribution, and optical
properties of the aerosol; its nucleation, growth, and
removal; its global distribution and the magnitude of human
influence; and the role of aerosols in modifying the
formation and microstructure of clouds. It is likely that
many questions will remain at the process level in the next
decade, and in situ measurements offer the best means
to
address them.
Space-based measurements must, however, play a critical role
in quantifying aerosol effects on climate by providing a
global mapping of aerosol optical depth along with
indicators of other aerosol properties (size distribution,
chemical composition). Of particular importance is the
identification of temporal trends in global aerosol
concentrations as driven for example by human activity,
volcanic eruptions, windblown soil dust, or large fires.
Preliminary studies using AVHRR data indicate particularly
high aerosol optical depths over the oceans downwind of the
arid continents, suggesting that soil dust (which interacts
with both shortwave and longwave radiation) may be of
particular radiative interest. Such information could not
have been obtained by other means. Measurements from the EOS
instruments (MODIS, MISR, EOSP) will improve considerably on
the AVHRR data by global mapping of the aerosol optical
depth and by polarization measurements (EOSP) from which
size distribution and aerosol phase information can be
retrieved.
Table 2. EOS Trace Species Instruments