|Working Group I: The Scientific Basis|
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5.6 Investigations Needed to Improve Confidence in Estimates of Aerosol Forcing
and the Role of Aerosols in Climate Processes
Atmospheric measurements have lagged behind awareness of the importance of
aerosols in climate. There is a great challenge in adequately characterising
the nature and occurrence of atmospheric aerosols and in including their effects
in models to reduce uncertainties in climate prediction. Because aerosols: (i)
originate from a variety of sources, (ii) are distributed across a wide spectrum
of particle sizes and (iii) have atmospheric lifetimes that are much shorter
than those of most greenhouse gases, their concentrations and composition have
great spatial and temporal variability. Satellite-based measurements of aerosols
are a necessary but not sufficient component of an approach needed to acquire
an adequate information base upon which progress in understanding the role of
aerosols in climate can be built. Below we outline measurements and process
level studies that are necessary to reduce uncertainties in both direct and
indirect forcing. These studies and observations are needed both to improve
the models on which climate forcing rely and to check our understanding of this
forcing as aerosol concentrations change in the future.
I. Systematic Ground-Based Measurements
There is a need for countries of the world to develop and support a network
of systematic ground-based observations of aerosol properties in the atmosphere
that include a variety of physical and chemical measurements ranging from local
in situ to remotely sensed total column or vertical profile properties. The
Global Atmospheric Watch programme of the World Meterological Organization is
but one player in organising routine aerosols measurements on a stage that includes
other international organisations (e.g. International Atmoic Energy Agency,
IAEA) as well as national research programmes (e.g. national environmental agencies,
national atmospheric research agencies).
It is recommended that, at all levels, emphasis be placed in developing a
common strategy for aerosol and gas measurements at a selected set of regionally
representative sites. One possible model is to develop a set of primary, secondary
and tertiary aerosol networks around the world. At the primary stations, a comprehensive
suite of aerosol and gas measurements should be taken that are long-term in
scope (gaseous precursors are an essential part of the aerosol story since much
aerosol mass is formed in the atmosphere from gas-to-particle conversion). At
secondary stations, a less comprehensive set of observations would be taken
that would provide background information for intensive shorter term process-oriented
studies. It would be desirable to co-locate vertical profiling networks that
involve complex instrumentation such as lidars with these baseline stations.
Tertiary stations may include stations operated by national research programmes
that are related to urban aerosol issues and human health.
These measurements should be closely co-ordinated with satellite observations
of aerosols. The types of measurements should include in situ size-segregated
concentrations of aerosol physical properties such as number and mass but also
chemical properties such as composition and optical properties. Total column
properties such as aerosol optical depth, Angstrom coefficient, CO and O3
add value to these data sets in evaluating the simulation of aerosols as active
constitutents in climate models.
II. Systematic Vertical Profile Measurements
There is a paucity of systematic vertical profile measurements of size-segregated
or even total atmospheric aerosol physical, chemical and optical properties.
For these parameters, no climatological database exists that can be used to
evaluate the performance of climate models that include aerosols as active constituents.
The COSAM model comparison (Barrie et al., 2001) had to use vertical profile
observations from a few intensive aircraft campaigns of only a few months duration
to evaluate climate model aerosol predictions. Such measurments would be best
co-located with the ground-based network stations. Since they involve routine
aircraft surveillance missions and are costly, the development of robust, sensitive
lightweight instrument packages for deployment in small aircraft or on commercial
airliners is a high priority. Both continuous real time measurements and collection
of aerosols for post-flight analysis are needed.
The network design needs to be systematically developed and implemented. One
possible model is to conduct observations at pairs of stations around –
and downwind of – major aerosol sources types such as industrial (Europe,
North America, Asia), soil dust (Sahara or Asian), biomass burning (Amazon or
southern Africa) and sea salt (roaring forties of the southern Pacific Ocean
III. Characterisation of Aerosol Processes in Selected Regions
There is a need for integrated measurements to be undertaken in a number of
situations to enhance the capability to quantitatively simulate the processes
that influence the size-segregated concentration and composition of aerosols
and their gaseous precursors. The situations need to be carefully selected and
the observations sufficiently comprehensive that they constrain models of aerosol
dynamics and chemistry. The International Global Atmospheric Chemistry (IGAC)
programme in its series of Atmospheric Chemistry Experiments in the roaring
forties of the southern Pacific (ACE-1), the outflow from North Africa and Europe
to the eastern North Atlantic and the 2001 study in Southeast Asia and downwind
in the Pacific (ACE-Asia) are examples of attempts to do this that require support
and continued adjustment of experimental design to match outstanding questions.
Such studies need to be conducted in industrial continental and neighboring
marine, upper-tropospheric, Arctic, remote oceanic and dust-dominated air masses.
Closure of aerosol transport and transportation models as well as direct forcing
closure studies should be an integral part of these studies.
IV. Indirect Forcing Studies
There is a need for several carefully designed multi-platform (surface-based
boat, aircraft and satellite) closure studies that elucidate the processes that
determine cloud microphysical (e.g., size-distributed droplet number concentration
and chemical composition, hydrometeor type) and macrophysical properties (e.g.
cloud thickness, cloud liquid-water content, precipitation rate, total column
cloud, albedo). A second goal would be to understand how aerosols influence
the interaction of clouds with solar radiation and precipitation formation.
These studies should take place in a variety of regions so that a range of aerosol
types as well as cloud types can be explored. Emphasis should be placed on reducing
uncertainties related to scaling-up of the processes of aerosol-cloud interactions
from individual clouds (about 1 to 10 km) to the typical resolution of a climate
model (about 100 to 500 km). Can sub-grid parametrizations of cloud processes
accurately represent cloud-radiation interactions and the role that aerosols
play in that interaction? Answering this question will require that process
studies be performed in conjunction with a range of model types such as models
that include a detailed microphysical representation of clouds to models that
include the parametrizations in climate models.
V. Measurements of Aerosol Characteristics from Space
An integrated strategy for reducing uncertainties should include high quality measurements of aerosols from space. At the time of this report, only measurements from AVHRR and POLDER were available. The latter instrument may yield measurements of aerosol optical depth over land, but it was operational for less than a year. High quality satellite measurements together with systematic comparisons with models and the process-level studies noted above should allow us to reduce the uncertainties in current aerosol models. Systematic comparison of models that include an analysis of the indirect effect with satellite measurements of clouds and with data gathered from process-level studies will also reduce uncertainties in indirect effects and in projected climate change.
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