A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change (1996) / Chapter Skim
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2 ELEMENTS OF A RESEARCH PROGRAM FOR AEROSOL FORCING OF CLIMATE
2 ELEMENTS OF A RESEARCH PROGRAM FOR AEROSOL FORCING OF CLIMATE
Pages 35-88
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From page 35... ...
To accomplish this goal requires improved methods of representing aerosol radiative forcing in global climate models. The purpose of this chapter is to recommend the components of an aerosol research program required to achieve that goal.
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From page 36... ...
Long-term systematic monitoring of aerosol properties can provide data to test not only process models but also global climate models. Intensive field campaigns provide an opportunity to measure a comprehensive set of chemical and physical parameters that govern features of aerosol forcing.
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Indeed, by its very definition, radiative forcing means that no atmospheric process has altered the climate state used to calculate the change in radiative flux caused by aerosol. Aerosol forcing calculations are typically performed in an AGCM, but employ only the
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From page 38... ...
by no means complete, of some global models currently being used to study aerosol forcing is given in Table 2.1, where the atmospheric general circulation and chemical models are listed separately. The chemical transport models listed in Table 2.1B cover hemispheric to global scales and are suitable for coupling to AGCMs.
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From page 39... ...
Links are also associated with the land and surface modules in climate models through the representation of dry deposition processes. Wet removal processes in the ACTM require precipitation information from the AGCM.
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This latter component is especially important for determining the light absorption coefficient and single-scatter albedo (a measure of the relative magnitudes of aerosol scattering and absorption) of anthropogenic aerosols.
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From page 41... ...
Attempts to calculate indirect climate forcing by anthropogenic aerosols have all been based on predicted geographical distributions of anthropogenic sulfate aerosol (Boucher and Rodhe, 1994; Chuang et al., 1994; Jones et al., 1994; Boucher and Lohmann, 19951. These approaches empirically relate cloud drop number concentrations, and thus cloud optical properties, to sulfate aerosol mass concentration derived from a chemical transport model.
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From page 42... ...
Recommended Research on Global Climate Modeling of Aerosol Radiative Forcing Global climate modeling of aerosol radiative forcing requires the following: 1. major advances in the representation of indirect climatic effects of aerosols in global climate models by treating in a fundamental manner the relationship between aerosol mass and aerosol number, aerosol number and CCN number, CCN number and cloud drop number concentration (CDNC)
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Optical Properties The basic quantities needed to describe the direct interaction of aerosol particles with solar radiation are the aerosol optical depth 6~) , single scat
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~. Wet and dry deposition FIGURE 2.2 Direct and indirect forcing mechanisms associated with sulfate aerosols.
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. The aerosol optical depth is the vertical integral of the aerosol extinction coefficient Oe(~)
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From page 46... ...
and the degree of variability of these optical properties in both space and time, which is linked mainly to variabilities in aerosol properties, especially size distribution. There is a clear need for laboratory determinations of the refractive index and density of pure and mixed-composition aerosol particles as a function of relative humidity.
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From page 47... ...
laboratory and theoretical determinations of the refractive indices of pure and mixed-composition aerosol particles, compared with bulk (e.g., filter samples) and in situ measurements of size-resolved composition and refractive index of atmospheric aerosols, to determine the accuracy of current theoretical treatments of refractive index of mixed-composition aerosols for use in radiative forcing calculations; 2.
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From page 48... ...
of particle size distributions between 3- and 10-nm diameter can be used to identify events of new particle production in both the marine boundary layer and the free troposphere. These measurements, along with data for gas-phase aerosol precursors, permit computations of aerosol nucleation and growth rates.
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From page 49... ...
The first is a change in the physical size of aerosol particles as relative humidity changes, the second involves a change in aerosol index of refraction as water content of the aerosol changes, and the third is associated with a shift in particle size relative to the Mie scattering efficiency curve. All three effects combine to produce a change in radiative forcing of a given aerosol as a function of relative humidity.
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From page 50... ...
The extent to which ambient aerosols consist of "hydroscopic" and "nonhygroscopic" fractions in different regions, such as marine and continental, needs to be evaluated, and the dependence of these hydroscopic properties on chemical composition needs to be determined. Ultimately this information will serve as input to global aerosol models that have prescribed relative humidity fields and to GCMs that predict relative humidity fields.
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From page 51... ...
3. Measure evolution of the atmospheric aerosol size distribution under well-defined conditions that allow one to assess the extent to which theoretical growth rate predictions conform with observations.
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From page 52... ...
In the immediately preceding section dealing with aerosol process models, specific questions have been raised about parameterizations for wet and dry removal processes. The goal of the present section is to outline research on sources and sinks needed to improve models of both direct and indirect radiative forcing by particles.
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From page 53... ...
Besides illustrating the importance of focusing on aerosol number rather than merely on mass, the data outlined in the previous paragraph can also illustrate types of research on aerosol sources and sinks that must be pursued to elucidate aerosol radiative forcing. In particular, if one tries to understand these data, layers of research questions about both aerosol sources and sinks appear.
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From page 54... ...
and if results such as these reveal mostly that air masses typically quite rapidly attain an "equilibrium" CN concentration, dictated by natural sources; again, a host of obvious research questions appear at this level (e.g., dealing with gas-to-particle conversions. Currently, such questions as these have not been answered adequately; it is therefore clear both that such questions must be addressed if aerosol radiative forcing uncertainties are to be reduced and that to answer such questions it is essential not only to obtain particle size information but also to examine sources and sinks simultaneously.
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~. Consequently, there are few to no network data available to define wet and dry deposition of particles as a function of their size, which is critical for the radiative forcing problems.
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of all chemicals (e.g., sulfur dioxide) that are important for modeling aerosol radiative forcing.
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, CN concentrations continued to be in excess of 104 cm~3. Additional studies, in which inert tracer gases and complete particle size distributions (and CCN and IFN spectra' are mea ~, ~
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. Recommended Process Research on Aerosol Sinks To improve modeling of aerosol radiative forcing, additional and more complete wet and dry deposition field studies are especially required.
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From page 59... ...
for dry deposition to "real-world" collectors, from forests in inhomogeneous terrain to the oceans under a variety of conditions. To achieve the essential goal of obtaining particle size-specific dry deposition velocities, developments in technology (e.g., for eddy-flux measurements J and techniques (e.g., for measuring monodisperse particles actually deposited)
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From page 60... ...
Each of these modes of action requires different physical and chemical aerosol properties and hence different types of aerosol particles. This variety in mode of action is no doubt a major source of the inconsistency in ice nuclei concentration observed with different techniques (Vali, 1975~.
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From page 61... ...
Field programs should be undertaken in which current or prospective ice nucleus measurement techniques are tested against field observations to determine if they provide correct predictions of initial ice concentrations in clouds. Implicit in this is an investigation of which mode or modes of action of ice nuclei dominate ice initiation (quite possibly, no single mode dominates under all conditions)
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From page 62... ...
Aerosol Process Models Currently, the aerosol component of global climate models involves a prespecified aerosol size distribution that generally is allowed to vary only with relative humidity. Aerosol size is an important determinant of optical properties, cloud nucleating properties, and wet and dry removal rates, and a more fundamentally based treatment of aerosol size is a long-term goal in global climate models.
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From page 63... ...
These processes and differences in aerosols are not presently represented in aerosol models but may be needed to properly represent the response of the aerosol system to decreased or increased emission of anthropogenic aerosols. Representation of aerosol mass distributions in current climate models has shown that direct radiative forcing is not overly sensitive (i.e., order of 20 percent)
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From page 64... ...
At present, the only available atmospheric aerosol models that explicitly include both size and chemical composition resolution have been developed in the context of urban and regional air pollution (Pilinis and Seinfeld, 1988; Wexler and Seinfeld, 1991; Pandis et al., 1993; Wexler et al., 1994~. Aerosol process model development and evaluation against ambient data are, in any event, needed to test the understanding of microscale chemistry and physics relevant to radiative forcing at the global scale.
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From page 65... ...
relationts) between aerosol optical depths and aerosol properties; (4)
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From page 66... ...
, and reaction rates, and the dependent variables are the time-dependent chemical and microphysical properties of the aerosol particles. In vertical column closure experiments for aerosol optical depth, radiative forcing by aerosol is calculated (with suitable assumptions)
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From page 67... ...
A maximum in relative humidity can occur at the top of the boundary layer, with concomitant effect on aerosol size. Higher concentrations of sea salt and proximity to surface sources and sinks can affect the ability of surface and shipboard samplers to represent even the lower hundred meters of the atmosphere.
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From page 68... ...
Satellite platforms are able to measure aerosol properties over larger spatial scales than surface and airborne platforms. Satellites can therefore help define the region and global context of in situ measurements.
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From page 69... ...
Two examples of multiplatform field campaigns would be those addressing the linkages between sources of anthropogenic SO2 and sulfate aerosol, and between organic aerosols and soot from biomass burning and radiative forcing. The oxidation rates and conversion efficiencies of SO2 by homogeneous and heterogeneous mechanisms are critical input parameters for calculating sulfate aerosol column burdens in aerosol-climate models.
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From page 70... ...
-sulfate and organics; mass size distributions of aerosol chemical species; number size distributions from 3-nary to 10-,um diameter; and · dynamical factors such as entrainment rates, turbulent transport to and from the surface, and mixing depths. The partitioning of SO2 oxidation products and organics between new particle production and particle growth affects the submicron aerosol size distribution and, in turn, the impact of these particles on forcing.
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From page 71... ...
Aerosol monitoring programs to date have focused almost exclusively on extensive aerosol properties, and systematic observations of intensive aerosol properties are sorely lacking. Consequently, despite the enormous effort that has gone into studying aerosols, much remains to be learned about the spatial distributions, seasonal variability, and long-term trends of the radiative, microphysical, and chemical properties of atmospheric aerosols that determine their effects on climate.
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From page 72... ...
With explicit recognition of the different uses of extensive and intensive aerosol properties, specific objectives of the aerosol monitoring component are (1' to determine spatial distributions of relevant extensive aerosol properties on a global scale, along with their temporal trends and seasonal cycles; and (2) to determine means, variabilities, and trends of relevant intensive aerosol properties for key aerosol types.
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From page 73... ...
Aerosol optical properties are strongly dependent on particle size, which in turn is strongly dependent on relative humidity. For certain types of climatological observations, both relative humidity and the size range of particles sampled must be controlled if reproducible and provable comparable results are to be obtained.
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From page 74... ...
provide high horizontal and temporal resolution, but the measurements are limited to columnar measurements of aerosol optical depth over ocean where the surface albedo is relatively constant and well known. In addition, the retrieval process for this technique requires substantial modeling of the surface optical properties and the aerosols themselves.
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From page 75... ...
. 6 Longitude -85.5 -~.9 -~.2 -~.6 -82.9 -~.3 -81.7 -81.1 -80.5 -79.9 -79.4 -78.7 -78.2 -77.6 -77.1 -76.5 -76.0 -7S.S ·75.0 ·7d~ 5 ·7i.0 Accurate in situ observations of aerosol provide essential constraint to satellite retrievals and result in the best depiction of aerosol properties.
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76 c: c: Cal .° _ ~ _ 3 5 cat 3 0 .C)
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77 lo _.
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From page 78... ...
Surface-based remote sensing techniques permit measurements of aerosol optical depths and vertical profiles of aerosol back
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The recommended measurements can provide a continuous time series of all intensive aerosol properties needed for calculating aerosol radiative forcing, except angular scattering function (or asymmetry parameter) and humidity dependence of aerosol light absorption.
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These different strategies for continuous measurements of intensive versus extensive aerosol properties dictate different station densities. A high-density network is needed for aerosol optical depth, whereas a limited number of sites, each located in an area dominated by a different aerosol type, is appropriate for monitoring intensive aerosol properties.
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From page 81... ...
To characterize aerosol optical depth distributions over North America, where even shorter characteristic autocorrelation distances are expected because of the proximity to anthropogenic sources, would require a network of at least 30 stations. The now-defunct aerosol optical depth network operated by the National Oceanic and Atmospheric Administration (NOAA)
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From page 82... ...
Measuring the surface radiation budget together with aerosol properties is needed, also, to evaluate effects of changes in aerosol on cloud optical properties. Satellite observations of cloud albedo and the radiation budget at the top of the atmosphere, plus surface-based observations of aerosols and the radiation budget, are expected to permit initial estimates of the sensitivity of cloud optical properties to below-cloud aerosol.
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From page 83... ...
Recommended Surface-Based Monitoring Programs We recommend establishing a dual-density network of surface-based stations for continuous aerosol monitoring, consisting of a high-density network of cat 30 stations for aerosol optical depth and a low-density network of 7 stations (Table 2.4) to provide detailed information on means, variability, and trends of key aerosol radiative, chemical, and microphysical properties (Table 2.3)
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From page 84... ...
As a minimum, size-resolved samples and scattering coefficients of the aerosol should be collected in fixed, predetermined regions along the track, with subsequent analyses for total mass, major ions, and light absorption. If logistical constraints allow, measurements of aerosol optical depth and aerosol light scattering and absorption coefficients should also be obtained.
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From page 85... ...
In addition, we recommend continuing the long-term record of vertical profiles of aerosol size distribution at Laramie, Wyoming. We recommend that a minimum of one aircraft make weekly flights to systematically make vertical profile measurements of a subset of aerosol properties.
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From page 86... ...
Relatively constant values of p7: in the boundary layer, with lower values aloft, would support the assumption that surface-based measurements represent optical properties of the part of the vertical column that dominates the direct aerosol radiative forcing; conversely, layers aloft with higher values of p~ would indicate that surface observations may be unrepresentative. Although methods exist for estimating aerosol extinction profiles from lidar measurements of 0~ (Kovalev, 1993)
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From page 87... ...
An aerosol collection device optimized for low weight and power would make it possible to sample aerosols above the boundary layer from an inexpensive kite or balloon at a much lower cost than flying a research aircraft. In view of the need for data on concentrations above the surface, the development of lightweight particle samplers needs to be pursued.
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From page 88... ...
laboratory, theoretical, and field research on aerosol optical properties; 3. identification of aerosol molecular composition, particularly the organic fraction; 4.
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