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 region).

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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