International Global Atmospheric Chemistry Program
Focus on Atmospheric Aerosols:

Direct Aerosol Radiative Forcing
INTRODUCTION
Perturbation of the earth's radiative budget due to scattering and absorption of solar and terrestrial radiation by anthropogenic aerosols is called direct aerosol radiative forcing.  In major industrialized regions, the radiative forcing of climate by anthropogenic sulfate aerosols has been calculated to exceed in magnitude the forcing by anthropogenic carbon dioxide, although the signs of the two forcings are opposite.  Consequently, much attention has been given in recent years to improving the model calculations of the direct aerosol radiative forcing, and to quantifying the uncertainties of the estimates.  At present, there is a great need for observational data on aerosols relevant to aerosol forcing of climate. 
The objective of this activity of IGAC's Focus on Atmospheric Aerosols is:
	to determine, primarily through observations, the magnitude, uncertainty, chemical sources, and temporal and spatial variations of the direct radiative climate forcing by aerosols of various types (e.g., sulfates, organics, mineral dust).
The emphasis on an observationally-based determination is designed to complement the model-based determinations of aerosol forcing contained within IGAC's Global Integration and Modelling activity.  Both approaches use radiative transfer models, but with different inputs to the radiative transfer calculations.  The observational approach is based on regional- and global-scale measurements of aerosol properties to evaluate the direct aerosol radiative forcing.  In contrast, the modelling approach uses chemical transport models and aerosol microphysical models to evaluate the global distribution of aerosol size distribution and chemical composition, from which the required aerosol radiative properties are calculated.  Both approaches require assumptions about the behavior of the system, and the advantage of employing both is that their assumptions are very different.  Taken together, these two separate approaches will allow an assessment of the confidence that can be placed on the results.  The two approaches are not completely independent, however, as the observational results will also be used to develop model parameterizations and to validate model predictions.  Conversely, modelling will play a key role in the design of experiments and interpretation of their results.
Achieving an observationally-based determination of the direct aerosol radiative forcing requires integration of remotely-sensed and in-situ observations, from satellite, aircraft, and surface-based platforms.  The necessary components, and the specific tasks for this activity, can be summarized as:
1.	Satellite-based remote inference of aerosol radiative properties;
2.	Surface-based remote observations of aerosol radiative properties;
3.	In-situ observations of aerosol radiative, chemical, and microphysical properties;
4.	Closure studies to test the combined ability of measurements and models to describe aerosol radiative properties and aerosol-induced radiative flux perturbations;
5.	Integration of the multi-platform observations to determine the direct aerosol radiative forcing.
TASK 1.	SATELLITE-BASED REMOTE OBSERVATIONS
There are many existing and planned satellite platforms that include sensors that are sensitive to aerosols.  In some cases, determination of aerosol properties is part of the primary mission of the sensor; in others, aerosols interfere with the primary measurement.  In all cases, there is a need to develop and validate the algorithms used to calculate aerosol properties from the satellite-based observations.  Generally, these are ill-posed retrievals requiring assumptions that must be justified with data.  The aim of this element is to assemble an international group, with representatives from the various satellite science teams, whose goal is to
	devise and implement a coordinated strategy for obtaining the surface-based and in-situ observations needed to validate the algorithms for retrieving aerosol properties from the satellite observations.  
Clearly, the surface-based remote observations in Task 2 will play a key role in this validation, but additional observations will also be required.
TASK 2.	SURFACE-BASED REMOTE OBSERVATIONS
Although satellites are required to provide global-scale coverage, remote aerosol observations from the surface provide invaluable data for evaluating trends and validating the satellite observations.  Surface-based observations are particularly important on the continents, where satellite retrieval of aerosol properties is particularly difficult.  There are many instruments, including hand-held or tracking sun photometers, shadowband radiometers, and sky scanning radiometers, that are currently being used to determine aerosol optical depth and related aerosol properties.  The WMO's Global Atmosphere Watch is attempting to establish a network of stations that will determine, among other things, aerosol optical depth.  Likewise, the Baseline Surface Radiation Network of the World Climate Research Program has as a secondary objective the determination of aerosol optical depth.  The goal of this task is not to duplicate these efforts, but rather to
	encourage, advise and coordinate national and international programs so that the surface-based remote aerosol observations can be used for validation of satellite observations and for regional-scale determination of direct aerosol radiative forcing, and
	supplement the national and international programs with additional sites where a more comprehensive suite of aerosol and radiation measurements are obtained.
A more comprehensive suite of measurements will provide multiple, independent observations of aerosol radiative properties and their effects on radiative fields.  These observations can be used to evaluate the validity of the approaches being used in the established networks, as well as providing a basis for closure experiments (Task 4).  The desired sensors include sky radiometers, shadowband radiometers, sun photometers, and lidars.
TASK 3.	IN-SITU OBSERVATIONS
Remote sensing methods can be used to determine the global-scale distribution and temporal variability of aerosol optical depth, and can provide additional information on the column-average scattering phase function and vertical profile of aerosol extinction or backscattering.  In-situ observations, from the surface, aircraft, and balloons are a necessary complement to these methods because they provide measurements of the aerosol single-scatter albedo, phase function, and humidity-dependence of aerosol radiative properties, as well as establishing the connection between the chemical and microphysical properties of the particles (e.g., composition, size distribution, shape) and their radiative properties.  As in Task 2, there are existing national and international programs for obtaining such measurements, and the goals of this task are to
	advise and coordinate national and international programs for observations of aerosol properties so that their results can be used in evaluation of direct aerosol radiative forcing, and
	supplement the national and international programs with additional platforms where a more comprehensive suite of aerosol and radiation measurements are obtained.
The more comprehensive measurements include the scattering phase function, hygroscopic growth characteristics, and state of mixture of compositions and shapes as a function of particle size.  A detailed chemical characterization of the aerosol is also required to link the radiative effects of the aerosol particles to their chemical sources.  Observational plaforms include fixed, ground-based sites for characterizing a range of different aerosol types, supplemented with ship- and aircraft-based transects to characterize the horizontal and vertical distributions of the key aerosol properties.
One of the primary objectives of the Baseline Surface Radiation Network (BSRN) is to measure the earth radiation budget at the surface, in order to provide a database for validation of satellite-based earth radiation budget sensors (ERBE, CERES, SCARAB).   Given the role of aerosols in perturbing the earth radiation budget, the BSRN has recommended that measurements of aerosol optical depth also be obtained at BSRN sites.  Another objective of this task is to coordinate the in-situ observations of aerosols with programs focused on the surface radiation budget, so that a combined aerosol/radiation data set can be obtained.
TASK 4.	CLOSURE STUDIES
This element will combine elements from Tasks 1-3 in focused experiments in order to
	evaluate the consistency of models and measurements of aerosol radiative properties and of radiative transfer through a turbid atmosphere, using column closure experiments.
Some of the specific parameters to be studied include the surface and top-of atmosphere radiation budgets, the radiative flux perturbation by an aerosol layer, the aerosol optical depth, and the aerosol scattering phase function.  Additional closure experiments of aerosol scattering and absorption coefficients are needed and will be a component of the Aerosol Characterization and Process Studies (ACAPS) activity.
The primary approach for this task is to co-ordinate international, multi-platform, multi-sensor field programs that are designed to compare measured radiative properties and fluxes with values calculated from observed aerosol properties.  Two such programs that are currently planned are the Aerosol Characterization Experiments (ACE) and the Tropospheric Aerosol Radiative Forcing Experiment (TARFOX).  This task also includes evaluation of the uncertainties of algorithms for retrieving aerosol radiative and microphysical properties from remote-sensing techniques.
TASK 5.	INTEGRATION 
Integration of the various parts of this Activity will yield its key product, an observationally-based evaluation of global-scale, direct aerosol radiative forcing, along with its uncertainties and chemical causes.  The components of this integration are (a) merge the data sets obtained with remote-sensing techniques (aerosol radiative properties) with in-situ determinations of aerosol chemical, microphysical, and radiative properties for different aerosol types, to determine the global scale distribution of aerosol radiative properties; (b) use radiative transfer models to calculate the resulting radiative forcing; (c) compare the calculated forcing with measurements of the earth radiation budget; (d) assess remaining uncertainties; and (e) recommend future research priorities.
Existing sets of observations from previous field programs can be very useful for achieving the goals of this IGAC activity.  Analysis of such data sets and merging of their results with the results of future measurement programs is planned, with particular attention paid to the complications introduced by the absence of standardized aerosol sampling and analysis protocols.  (The ACAPS activity includes development of standardized methods, and new experiments focused on direct aerosol radiative forcing will take advantage of methods developed by ACAPS.)
LINKAGES
Other IGAC activities will be conducting research that relates to this activity, including aerosol process and closure studies (Aerosol Characterization and Process Studies), Aerosol-Cloud Interactions, radiative effects of stratospheric aerosols (Stratospheric Aerosols), and characterization of aerosols in specific regions (Biomass Burning Experiment, Polar Atmospheric and Snow Chemistry, NARE, APARE, ...).  Close coordination will also be maintained with the Global Integration and Modelling activity, so that the experiments in Tasks 1-4 are optimally designed and their results are used for model validation.
Outside of IGAC, the obvious candidates for close coordination are the WMO/GAW and WCRP/BSRN programs.  On a national scale, the U.S. Department of Energy's ARM program is conducting experiments that are closely related to tasks 2-4.
COORDINATING COMMITTEE
Conveners:
T. Nakajima (Japan)	teruyuki@ccsr.u-tokyo.ac.jp
J. Ogren (USA)	johno@cmdl.noaa.gov
Members:
H. Gordon (USA)*	gordon@phyvax.ir.miami.edu
R. Hoff (Canada)**	rhoff@dow.on.doe.ca
M. King (USA)*	king@climate.gsfc.nasa.gov
P. Pilewskie (USA)	pil@ra.arc.nasa.gov
P. Russell (USA)	philip_russell@qmgate.arc.nasa.gov
G.-Y. Shi (China)*	shigy@bepc2.ihep.ac.cn
L. Stowe (USA)*	lstowe@orbit1i.nesdis.noaa.gov
D. Tanre (France)*	Didier.Tanre@univ-lille1.fr
M. Wendisch (Germany)*	wendisch@tropos.de
* pending formal approval of the IGAC Executive Committee
** invited
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This document is available via anonymous ftp in ASCII and Word for Windows 2.0 formats at ftp.cmdl.noaa.gov/aerosol/igac.