A
Illustrations of Recommended Research, Emphasizing Recent Literature

SMIC, Study of Man's Impact on Climate (1971). Inadvertent Climate Modification. Report of the Study of Man's Impact on Climate. MIT Press, Cambridge, Mass., 308 pp.:

We recommend the compilation of figures on global particle production rates that are accurate to a factor of 2 or better …. All studies should give information on mass fluxes as well as size distribution in the range between 0.01 and 10 µm radius.

We recommend that the transformations of atmospheric trace gases which lead to particle formation be studied. [These studies] should include the collection of better data on the distribution of these gases in the atmosphere and on their life cycles and residence times.

We recommend periodic measurement (for example, at intervals of 2 years) of the major sources of particles which are man-made and over which he can exert control.

We recommend more comprehensive studies of the relative importance of the principal removal mechanisms of particles and gases by precipitation, of particles by impaction, sedimentation, and diffusion at the ground, and of gases by absorption at the ground. This has a particularly important bearing on computation of residences times in the atmosphere.

We recommend that suitable methods be developed to measure the particle size distribution below 0.1 µm radius and to study its modification in clean and polluted atmospheres.



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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change A Illustrations of Recommended Research, Emphasizing Recent Literature SMIC, Study of Man's Impact on Climate (1971). Inadvertent Climate Modification. Report of the Study of Man's Impact on Climate. MIT Press, Cambridge, Mass., 308 pp.: We recommend the compilation of figures on global particle production rates that are accurate to a factor of 2 or better …. All studies should give information on mass fluxes as well as size distribution in the range between 0.01 and 10 µm radius. We recommend that the transformations of atmospheric trace gases which lead to particle formation be studied. [These studies] should include the collection of better data on the distribution of these gases in the atmosphere and on their life cycles and residence times. We recommend periodic measurement (for example, at intervals of 2 years) of the major sources of particles which are man-made and over which he can exert control. We recommend more comprehensive studies of the relative importance of the principal removal mechanisms of particles and gases by precipitation, of particles by impaction, sedimentation, and diffusion at the ground, and of gases by absorption at the ground. This has a particularly important bearing on computation of residences times in the atmosphere. We recommend that suitable methods be developed to measure the particle size distribution below 0.1 µm radius and to study its modification in clean and polluted atmospheres.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change We recommend the spatial distributions of particle concentrations and trends with time be monitored on a global basis. A network of about 100 stations is required to give representative data for the whole atmosphere. For all optical measurements an accuracy of 5 percent is required to obtain meaningful data. Because of the wide range of particle sizes, different methods have to be used simultaneously, in particular: … [transmissivity] … [horizontal extinction] … [trends for particles < 0.1 µm] … [oceans: continue electrical conductivity measurements] Trends in the concentration of ice and cloud nuclei should be continuously monitored. Very careful selection and perhaps improvement of existing methods are necessary. We recommend increased research on the refractive index (including absorption) of atmospheric particles in relation to their composition, origin, size, and shape and its change with increasing pollution for short-and long-wave radiation. Also, more data on particle growth with humidity are needed to understand the influence on the refractive index with relative humidity. We recommend comprehensive comparative studies of the radiation fields for clean and polluted atmospheres in order better to identify the effects of short-and long-wave radiation on the atmosphere and its modification with increasing pollution. We recommend measurements of albedo and other radiative properties of clouds and fogs in unpolluted and polluted areas, in conjunction with sufficiently complete measurements of cloud microstructure. These very important measurements require operation of instrumented aircraft. We recommend studies of the effects of pollution on the refractive index of cloud droplets and ice crystals. We recommend theoretical study of the integrated effects of pollution on radiative properties of clouds and fogs. We recommend that comprehensive field studies directed toward the resolution of the question of the effect of particle concentration on frequency, type, and intensity of clouds and precipitation be designed and implemented. We recommend that at suitable time intervals (about 2 years) the refractive index and the detailed size distribution of particles be determined at selected places in both clean and polluted air. We recommend that objective methods for monitoring cloud cover by satellites or other means and for monitoring changes of cloud cover over large areas of the atmosphere be developed and implemented.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change We recommend that at suitable intervals (for example, 5 years) the optical properties of clouds (reflectivity, transmissivity, emissivity, and absorption) be determined in areas of increasing air pollution. We recommend monitoring of [sulfur and nitrogen gases] … in unpolluted areas, with an accuracy of ±10 percent. Discontinuous measurement at about 10 baseline stations and 100 regional stations is required in order to improve the understanding of the life history of these gases and of the particle formation processes in which they are involved. We recommend that high priority be given to efforts to determine the humidity at which cirrus cloud forms. In particular, the question of whether cirrus forms through sublimation or by means of the liquid phase should be answered. We recommend that information [be] collected on the fundamental physical properties of cirrus clouds. These include water content, particle concentration, and distribution of sizes and shapes of crystals. We recommend that the optical properties of cirrus clouds, in both solar and infrared radiation, be investigated. We recommend that high priority be given to monitoring trends, if any, in cirrus cloudiness and characteristics. For this purpose, objective methods are needed in order to distinguish between cirrus and lower clouds. Hobbs, P.V., H. Harrison, and E. Robinson (1974). Atmospheric effects of pollutants. Science 183, 909-915: It is our opinion … that coupled effects between particles and clouds are likely to outweigh direct albedo effects from the particles themselves. Most critical of all, do the pollutants affect aerosols which play a role in cloud processes? If so, the structure and distribution of clouds may be affected by the pollutants, thereby causing changes in precipitation and optical scattering. Research priority should be given to this area. Junge, C.E. (1975). The possible influence of aerosols on the general circulation and climate and possible approaches for modeling. Chapter 10 of The Physical Basis of Climate and Climate Modeling. GARP Publication Series No. 16, World Meteorological Organization, Geneva: Understanding of the indirect influence [of aerosols] on the short-and long-wave radiation budget by modification of cloud micro-and macrostructure, i.e., albedo, cloud cover, etc. Problem practically open. The available data indicate that the indirect effects of aerosols on water clouds may be very important for changes in the radiation budget, perhaps more than the direct effects of aerosols. Unfortunately, no attempts to estimate these effects have been made so far.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Shaw, G.E. (1987). Aerosol as climate regulators: A climate-biosphere linkage? Atmos. Env. 21, 985-986: Of the particles in the atmosphere, those around a few tenths of a µm are noteworthy. Firstly, they are relatively immune to removal and therefore remain suspended for long periods, and secondly, they interact strongly with sunlight. The collective system of sub-µm aerosols, therefore, constitutes an enormously sensitive climate regulating machine. In this regard, it is interesting to note that the present quantity of biologically produced sulfate aerosol is nearly that needed to opalize the atmosphere. This might suggest that the sulfur aerosol system plays a role in climate …. We should try and find evidence pro or con regarding the past record of atmospheric transparency to sunlight. Charlson, R.J., J.E. Lovelock, M.O. Andreae, and S.G. Warren (1987). Oceanic phytoplankton, atmospheric sulfur, cloud albedo, and climate. Nature 326, 655-661: There are significant gaps in our knowledge of this proposed feedback system. Most importantly, we need to understand the climatic factors affecting DMS [dimethyl sulfide] emission. Because some species produce much more DMS than others, we must include the necessary understanding of controls on phytoplankton species abundance. We also need to understand the relationship between DMS concentration in the air and the CCN population, through the intervening aerosol physical processes. Knowing how the area of cloud cover is influenced by CCN [cloud condensation nuclei] is also important. Charlson, R.J. (1988). Have the concentrations of tropospheric aerosol particles changed? Pp. 79-90 in The Changing Atmosphere, F.S. Rowland and I.S.A. Isaksen (eds.). Wiley, New York: In order to understand and be able to predict the aerosol particle concentrations and effects of the future, especially with regard to their climatic role as CCN, we will require more and substantially different measurements than are presently being made. Full aerosol characterizations (including chemical, physical, optical, and cloud nucleating characteristics) should be conducted systematically in areas that are frequently downwind of industrial and other key source regions. Chemical data are required for understanding chemical effects and for relating to sources. Physical data are needed both to understand the atmospheric processes and turnover times of the aerosol and to understand physical effects. These measurements should be undertaken with the intent of establishing the extent of anthropogenic increases of aerosol concentration, to document trends, and eventually to be able to forecast effects.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Bigg, E.K. (1986). Discrepancy between observation and prediction of concentrations of cloud condensation nuclei. Atmos. Res. 20, 82-86: Obviously we need to know how reliable these measurements are and how well they relate to droplet growth in the free atmosphere before we can have full confidence in the calculations. Gras, J.L. (1990). Cloud condensation nuclei over the southern ocean. Geophys. Res. Lett. 17, 1565-1567: [T]o answer questions regarding possible long-term climate effects of CCN concentration changes, either through anthropogenic activities or from marine DMS emissions, far better climatologies than are currently available are required from a range of remote oceanic regions. Thorough intercomparison is clearly an essential feature for these programs. Grassl, H. (1988). What are the radiative and climatic consequences of the change in concentration of atmospheric aerosol particles? Pp. 187-199 in The Changing Atmosphere, F.S. Rowland and I.S.A. Isaksen (eds.). Wiley, New York: The strong dependence of the local planetary albedo change on aerosol particle size distribution change and soot content underlines the need for more reliable input parameters and more sophisticated models before the sign and the relative magnitude of an aerosol climate signal … may be given with higher reliability …. Further steps in aerosol research related to climate should be simultaneous measurements of aerosol particle parameters and optical cloud parameters, as well as the introduction of aerosol transport into atmospheric general circulation models. Crutzen, P.J., and M.O. Andreae (1990). Biomass burning in the tropics: Impact on atmospheric chemistry and biogeochemical cycles. Science 250, 1669-1678: Because of the great importance of biomass burning and deforestation activities for climate, atmospheric chemistry, and ecology, it is clearly of the utmost importance to improve considerably our quantitative knowledge of these processes …. Biomass burning is also an important source of smoke particles, a large amount (maybe all) of which act as CCN or can be converted to CCN by atmospheric deposition of hygroscopic substances. The amount of aerosols produced from biomass burning is comparable to that of anthropogenic sulfate aerosol. Through this process, the cloud microphysical and radiative processes in tropical rain and cloud systems can be affected with potential climatic and hydrological consequences. Kaufman, Y.J., and T. Nakajima (1993). Effect of Amazon smoke on cloud microphysics and albedo—Analysis from satellite imagery. J. Appl. Meteorol. 32, 729-744: [T]he presence of dense smoke (an increase in the optical thickness from 0.1 to 2.1) can reduce the remotely sensed drop size of continental cloud

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change drops from 15 to 9 µm. Due to both the high initial reflectance of clouds in the visible part of the spectrum and the presence of graphitic carbon, the average cloud reflectance at 0.64 µm is reduced from 0.71 to 0.68 …. [H]igh concentration of aerosol particles [from biomass burning] causes a decrease in the cloud-drop size and … darkens the bright Amazonian clouds …. [I]t is possible to explain the reduction in the cloud reflectance … for smoke imaginary [index of refraction] of -0.02 to -0.03. The increase in the average cloud-top temperature as a function of the smoke optical thickness indicates the possibility that reduction of convection due to the smoke absorption and reflection of sunlight caused a decrease in the updraft speed and in the amount of liquid water available to form the cloud …. [T]he results indicate that smoke reduces rather than increases the reflectivity of clouds in the topics, in contrast to previous assumptions …. [T]o better understand the interaction of aerosol particles with clouds, more information about the details of their size distribution and time evolution are required. Novakov, T., and J.E. Penner (1993). Large contribution of organic aerosols to CCN concentrations. Nature 365, 823-826: The apparent ability of organic aerosols to serve as CCN can have two possible … explanations …. [H2SO4 condensed on organics, or the organics are CCN]. The present data, however, are insufficient to argue for either of these possibilities. Hansen, J.E., and A.A. Lacis (1990). Sun and dust versus greenhouse gases: An assessment of their relative roles in global climate change. Nature 346, 713-719: We conclude that the lack of global aerosol data makes it impossible at present to determine the net anthropogenic aerosol forcing of the climate system …. Satisfactory quantitative analysis of the net climate forcing owing to anthropogenic aerosols will be difficult, because of the inhomogeneous distribution of the aerosols. It will be necessary to monitor global tropospheric aerosol properties, and carry out in situ case studies under a broad variety of conditions. Such data could yield the direct aerosol climate forcing, and in conjunction with global cloud data, it could also allow evaluation of aerosol-cloud interactions. Aerosols are the source of our greatest uncertainty about climate forcing …. Tropospheric aerosols are difficult to monitor because of their spatial inhomogeneity, but they are a crucial variable because of the strong anthropogenic influence on their amount. Not only will it be necessary to monitor the aerosols but also to have continued global cloud observations, because of possible interactions between aerosols and clouds …. [U]ntil the research on aerosols is carried out (a difficult task), we do not even know the direction of the change of climate forcing on decadal time scales which would be caused by a modification of fossil-fuel use.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Langner, J., and H. Rodhe (1991). A global three-dimensional model of the tropospheric sulfur cycle. J. Atmos. Chem. 13, 225-263: Many assumptions have been made in deriving the model and further work is needed to narrow the uncertainties in model parameters. Further improvements in our understanding of the circulation of sulfur species through the global atmosphere require that the uncertainties be narrowed of, in particular, the following fluxes/factors: the emission of DMS from the oceans, the oxidation pathways of DMS, the liquid phase oxidation of SO2, and the wet removal of aerosol sulfate. [They ignored cloud venting of sulfate and their dry deposition velocity for sulfate has essentially zero experimental support.] Possible approaches to this challenge include field campaigns in marine environments under well-defined meteorological conditions with simultaneous measurements of the various sulfur species and H2O2 at different heights … supported by models that describe boundary-layer mixing, surface exchange, cloud processes, and chemistry. Savoie, D.L., J.M. Prospero, S.J. Oltmans, W.C. Graustein, K.K. Turekian, J.T. Merrill, and H. Levy II (1992). Sources of nitrate and ozone in the marine boundary layer of the tropical North Atlantic. J. Geophys. Res. 97, 11575-11589: A recent synthesis of field measurements and modeling results … indicates that the nitrogen and sulfur transport simulated by the models is far weaker over the open ocean than that indicated by our measurements or those of other investigators …. Such models are likely to play a major role in predicting how the world's weather and climate will change as a consequence of man's activities. However, to serve that purpose they must first be able to correctly simulate the transport from the major source regions as well as the resulting concentration fields for current conditions. Langner, J., H. Rodhe, P.J. Crutzen, and P. Zimmermann (1992). Anthropogenic influence on the distribution of tropospheric sulfate aerosol. Nature 359, 712-716: Little information is available to verify the calculated changes in the tropospheric distribution of sulfate and fluxes of n.s.s. [non sea-salt ] sulfate presented here. This is especially true regarding concentrations of non-sea salt sulfate in air …. The best quantitative information comes from ice-core records obtained in Greenland showing an enhancement by a factor of 2-4 over the last hundred years ….

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Charlson, R.J., J. Langner, H. Rodhe, C.B. Leovy, and S.G. Warren (1991). Perturbation of the Northern Hemisphere radiative balance by backscattering from anthropogenic sulfate aerosols. Tellus 43AB , 152-163: Large remaining questions and problems: The first main category concerns the refinement of the model. Better knowledge of [the mass scattering coefficient] and [the hemispheric backscattered fraction], their variability and dependence on controlling factors, would increase confidence in the calculated effects … [model improvements: solar zenith angle, time of day, surface albedo, clear sky transmission]. Seasonal variation of the [sulfate burden] should be added …. Finally, the calculated radiative effect should be compared to measurements, both ground and satellite based. Improvements in the uncertainty of natural and anthropogenic emission fluxes of gaseous sulfur compounds are also needed, as are improvements in modeling the transformation and removal processes. We cannot be as quantitative in comparing the direct effect with the possible indirect CCN effect of anthropogenic sulfate… A mere 20% increase of CCN in the NH [Northern Hemisphere] is calculated to yield a cooling of ca. 1 W m-2 (Wigley, 1989). While there certainly is potential for the anthropogenic sulfate to have increased the CCN number concentration, there are as yet no data and there is no agreed-upon theory relating number concentration of CCN to mass concentrations of sulfate. Lelieveld, J., and J. Heintzenberg (1992). Sulfate cooling effect on climate through in-cloud oxidation of anthropogenic SO2. Science 258, 117-120: Earlier estimates of the sulfate climate forcing were based on a limited number of sulfate-scattering correlation measurements from which a high sulfate-scattering efficiency was derived. Model results suggest that cloud processing of air is the underlying mechanism. Aqueous phase oxidation of SO2 into sulfate and the subsequent release of the dry aerosol by cloud evaporation render sulfate a much more efficient scatterer than through gas-phase SO2 oxidation. On the basis of aircraft measurements over southern Sweden …, Charlson et al. … adopted an empirical value of alpha = 8.5 m2 g-1. This mass-scattering coefficient, derived from a correlation between sulfate amount and light-scattering, implies that sulfate is very efficient in scattering solar radiation. In these measurements, however, 40-60% of the light-scattering material measured was not sulfate. Hence, a causal relation between aerosol chemical composition and scattering properties was not demonstrated. [W]e conclude that the mean climate forcing by sulfate in this part of the globe [NH: 0.5-2.0 µg m-3 sulfate] may be about -0.5 to -1.0 W m-2, to a large extent caused by in-cloud oxidation of anthropogenic SO2 [versus the -1.1 W m-2 of Charlson et al., 1991].

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Kiehl, J.T., and B.P. Briegleb (1993). The relative roles of sulfate aerosols and greenhouse gases in climate forcing. Science 260, 311-314: To define better the direct forcing due to sulfate aerosols, more comprehensive and simultaneous observational data are needed on the chemical, physical, and radiative properties of the aerosol. These data are needed for a range of different geographic locations, because the sulfate characteristics are no doubt linked to the chemical environment (e.g., NH3 sources). [R]ecent estimates of the climate forcing due to smoke … use the same simple radiative model of Charlson et al., where the aerosol specific extinction is assumed to be independent of wavelength. Most aerosols exhibit a decrease in extinction with wavelength. Thus, the radiative effects of smoke have probably been overestimated …. The remaining aerosols of importance are those composed of elementary carbon. Estimates of the spatial effects of these aerosols on the climate system are urgently needed. Box, M.A., and T. Trautman (1994). Computation of anthropogenic sulfate aerosol forcing using radiative perturbation theory. Tellus 46B, 33-39: It has been argued by many authors over the years that anthropogenic aerosols are likely to have a cooling effect on the earth's surface temperature, by reflecting sunlight back to space …. [T]o quantify that forcing, we need to complete a three-stage calculation: … (estimate) the mass loading of anthropogenic aerosols …; this must be converted to an optical model; and then the radiative effects of this optical model must be evaluated. In this paper we have concentrated on the second and third of these stages, to determine the forcing produced per unit of model aerosol. We have found that the perturbation factor due to an additional aerosol loading with dry sulfate aerosols, as estimated by Charlson et al. (1991) is too large, by a factor of between 2 and 3. The main reason for this is that the spectral dependence of the aerosol optical properties is ignored in the method used by Charlson et al. (1991). However, when the effects of humidity were taken into account, our final results are very close to those of Charlson et al. …. This is in sharp contrast to the very recent calculations of Kiehl and Briegleb (1993), despite their use of a fully realistic sulfate optical model, which appears quite similar to ours. However, one key difference is in their handling of the effects of humidity, where they use a single factor to rescale the specific extinction. Our calculations … show that this rescaling is itself wavelength dependent …. Although we believe that our results are the most accurate to date, it is clear that subtle details of the sulfate aerosol model, including the effects of humidity, can have a significant effect on the final result.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Twomey, S. (1991). Aerosols, clouds, and radiation. Atmos. Env. 25A , 2435-2442: It is regrettable that present-day monitoring programs include aerosols only to the extent of the ''Aitken count"; measurements of cloud-nucleating particles are not especially difficult. At the present time, little is known about the major gas-to-particle route(s) for sulfur, etc., in the cleaner parts of the atmosphere; far more anthropogenic sulfur is being cycled annually through the atmosphere than is converted from gas into new small particles, and this anthropogenic sulfur is further augmented by the biogenic sulfur injections discussed by Charlson et al. (1987) and others. Particle production does not appear to be sulfur-limited, and one cannot rule out the possibility that some other trace gas, or photon supply, might be the limiting factor. Clearly many more field measurements and laboratory experiments are called for, rather than endless repetitions of computer simulations that are closely related to each other and parameterize in very similar ways. Satellite measurements capable of giving not just cloud cover (and cloud-top temperature) but also some information about cloud microphysics would be valuable, especially if the measurements continued unaltered over long periods of time. Parenthetically, it should be pointed out that Schwartz (1988) argued that since mean albedos seemed to be about equal for Northern and Southern Hemispheres (despite there being perhaps 3-10 times more sulfur injected into the atmosphere of the Northern Hemisphere, albedo modifications by man-made emissions could be discounted. [The figure] shows [however] that it is the clean regions that are most susceptible to albedo increase: Schwartz['s] estimated 6 megatons yearly of anthropogenic S could easily have more impact in the clean regions of the SH [Southern Hemisphere] than 150 megatons in the NH. Schneider, S.H. (1994). Detecting climatic change signals: Are there any "fingerprints"? Science 263, 341-347: As for the aerosol forcing, nobody has tried to produce a regional map of CCN-induced (or soot-induced) cloud albedo changes …. Kaufman, Y.J., R.S. Fraser, and R.L. Mahoney (1991). Fossil fuel and biomass burning effect on climate—Heating or cooling? J. Climate 4, 578-588: [T]o decrease the uncertainty as to the effect of coal and oil burning on climate, there is a need to verify experimentally the relation between the presence of pollution and the corresponding change in cloud characteristics. Since the effect of pollution on clouds is usually much smaller than the variability in the cloud characteristics due to dynamic effects, the relation must be based on statistical studies of cloud characteristics and aerosol density. Satellite imagery can be used to study thousands of clouds simultaneously …, parallel to studies of the surrounding aerosol ….

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Because of the nonlinearity of the relation between cloud albedo and aerosol density, in addition to detailed studies of cloud microphysics from aircraft or balloons, and its relation to aerosol concentration, there is need to study the relation between the aerosol concentration and the average cloud albedo from satellite imagery. Leaitch, W.R., G.A. Isaac, J.W. Strapp, C.M. Banic, and H.A. Wiebe (1992). The relationship between cloud droplet number concentrations and anthropogenic pollution: Observations and climatic implications. J. Geophys. Res. 97, 2463-2474: Much of the current discussion on this issue is quite speculative due to a lack of observational data describing how the CDNC [cloud drop number concentrations] are affected by changing pollution. These observations offer support for the importance of the issue of pollution, cloud microphysics, and cloud albedo. They should underscore the need for more global measurements of cloud microphysics and chemistry, particularly in remote regions, to help address the issue of global climate change. Kaufman, Y.J., and M.-D. Chou (1993). Model simulations of the competing climatic effects of SO2 and CO2. J. Climate 6, 1241-1252: [Not addressed: Induced changes in cloud lifetime] The major uncertainties concern the relationship between the SO2 emission and the CN [condensation nuclei] production, the relationship between the CCN concentration and the cloud optical thickness, the value of the background CCN concentration, and the vertical distribution of the SO2 derived CCN. In order to fully assess the impact of anthropogenic SO2 on climate, we need to improve our understanding of these processes. Leaitch, W.R., and G.A. Isaac (1994). On the relationship between sulfate and cloud droplet number concentration. J. Climate 7, 206-212: Scatter in the data makes it impossible to constrain model parameters; however, the comparisons suggest that there may not be a universal relationship, and that the uncertainties involved in trying to model this process are large. It is important to emphasize that the sensitivity of Nd [the number density of cloud droplets] to sulfate may be greater at lower sulfate concentrations …. Although our understanding of the effect of intense pollution on cloud microphysics is by no means satisfactory, efforts should be focused on obtaining more data concerning the effect of anthropogenic sulfate, and other aerosol species, at concentrations more typical of global sulfate concentrations [i.e., <50 nEq m-3]. There is a clear need to study the relationships among Nd, CCN, and sulfate in detail before parameterizations of this nature can be properly

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Hoell, J.M., Jr., D.D. Davis, G.L. Gregory, R.J. McNeal, R.J. Bendura, J.W. Drewry, J.D. Barrick, V.W.J.H. Kirchhoff, A.G. Motta, R.L. Navarro, W.D. Dorko, and D.W. Owen (1993). Operational overview of the NASA GTE CITE-3 airborne instrument intercomparisons for sulfur-dioxide, hydrogen-sulfide, carbonyl sulfide, dimethyl sulfide, and carbon-disulfide. J. Geophys. Res. 98, 23291-23304: In summarizing these results, we note that the CITE 3 data base provides a comprehensive data base from which one may begin to analyze various sulfur budget issues. The data base confirms existing sulfur observations and theories while, at other times, provides emphasis to reconsider other issues. For example, the data clearly confirm (1) the importance of the ocean as a source of DMS and the rapid oxidization of DMS after transport from the marine mixing layer; (2) the continental source of H2S, CS2, and SO2 as compared to a marine source; (3) the existence of a COS [carbonyl sulfide] latitudinal gradient (decreasing southward) estimated to be about 1.8 pptv/deg; (4) the higher concentration and variability of all sulfur gases in the NH, thus verifying the importance of NH anthropogenic emissions to global budgets; and (5) in the tropical Atlantic regions, devoid of major anthropogenic influences, photochemistry results in a net loss of ozone (3 to 5 pptv) during the day with a tendency for the marine mixed layer to be replenished at night via subsidence. On the other hand, the data base raises important questions. For example, (1) In terms of global sulfur budgets, is the transport and influence of NH air more important than originally viewed? (2) Is the COS budget complete, or are there missing source terms? (3) In view of the lower observed mixing ratios of H2S and CS2 in the tropical Atlantic (respectively, a factor of 3 and 10 lower than earlier data), what is the role and importance of the oceans as a source for these gases compared to continental sources? [Also: A host of improvements to instrumentation are needed.] Lin, X., and W.L. Chameides (1993). CCN formation from DMS oxidation without SO2 acting as an intermediate. Geophys. Res. Let. 20, 579-582: [T]o determine if such a chemical scheme does in fact exist [DMS-> SO3], more detailed studies on the kinetics of DMS oxidation under conditions typical of the MBL [marine boundary layer] are needed. Kulmala, M., A. Laaksonen, P. Korhonen, T. Vesala, T. Ahonen, and J.C. Barrett (1993). The effect of atmospheric nitric acid vapor on CCN activation. J. Geophys. Res. 98, 22949-22958: Our simulations show that enhanced nitric acid concentrations can affect cloud droplet distributions by increasing the number concentrations and decreasing the mean size of the droplets. This can cause considerable changes to the radiative properties of low clouds. Furthermore, these effects can probably enhance cloudiness. First, it is likely that the smaller droplet size will decrease precipitation … so that the clouds will have

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change longer lifetimes. Second, the cloud formation can take place at smaller saturation ratios of water vapor. Third, with increased HNO3 concentrations the disappearance of the cloud droplets due to evaporation is slower. In marine regions the nitric acid concentrations are usually several orders of magnitude lower than those needed to significantly change the numbers of CCN, [but] other vapors [for example MSA (methanesulfonic acid)] may cause changes in CCN of marine clouds …. Further studies are needed to quantify the potential effect of the phenomena described in this study. Lawrence, M.G. (1993). An empirical analysis of the strength of the phytoplankton-DMS-cloud-climate feedback cycle. J. Geophys. Res. 98, 20663-20673: An understanding of the coupling between DMS flux and CN levels is tied up in the intricacies of the oxidation pathways leading from DMS to particulate sulfate. The examination of the long-term regional trends in DMS flux and CN measurements … revealed indications of a coupling, though it could not be discerned from the data whether the coupling was linear or nonlinear in nature. Since the nature of this relationship represented the most significant uncertainty in the model, … continued research into the mechanisms relating DMS emissions and CCN levels should help to reduce the uncertainties in the estimate. Such research should also help to illuminate the partitioning between homogeneous and heterogeneous oxidation pathways, which determines the relative roles of direct and indirect effects of enhanced aerosol and CCN levels. Slinn, W.G.N. (1992). Structure of continental clouds before the industrial era: A mystery to be solved (Opinion). Atmos. Env. 26A, 2471-2473: Which then raises a minor mystery: Is the creation of new CCN in the marine atmospheric boundary layer (MABL) negligible? If this investigation has been correct, most CCN are created in the free troposphere (FT), but that follows because the FT has ˜ 10 times the volume of the atmospheric boundary layer (ABL). Yet, when the FT CCN mix back down to the ABL, there would be negligible simultaneous dilution of the FT's bank of CCN. So, where's the proof that most MABL CCN aren't created in the FT? Clarke, A.D. (1993). Atmospheric nuclei in the Pacific midtroposphere: Their nature, concentration, and evolution. J. Geophys. Res. 98, 20633-20647: Horizontal transects totaling over 35,000 km at about 9- to 10 km altitude [over the remote North and South Pacific] exhibited variability of approximately 3 orders of magnitude in both aerosol mass and number concentrations over spatial scales ranging from 1 to 1000 km. At these altitudes an approximate inverse relationship between ultrafine concentrations and the

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change surface area of the larger aerosol was evident. Regions having lowest aerosol mass were characterized by aerosol thermal volatility indicative of a predominately sulfuric acid composition and by very high concentrations of ultrafine nuclei, indicative of recent homogeneous nucleation. These conditions were frequently observed but were conspicuously evident above clouds over the ITCZ [Inter-Tropical Convergence Zone]. The clean, free troposphere appears to be a significant source region for new tropospheric nuclei. The relationship of atmospheric nuclei, CCN, and clouds is clearly a complex and coupled system. The importance of clouds to the global climate and the dependence of cloud properties on available CCN merits understanding of this system. The observations presented here demonstrate why boundary layer studies alone are unlikely to provide an adequate understanding of the system and make it clear that aircraft studies of the troposphere involving coordinated gas and aerosol measurements are essential. These studies should include deliberate efforts to assess the interdependent ways that gases, surface derived aerosols, existing aerosol surface area, environmental conditions, and clouds may interact to control both the CCN spectrum and related cloud properties. Karl, T.R., G. Kukla, V.N. Razuvayev, M.J. Changery, R.G. Quayle, R.R. Heim, D.R. Easterling, and C.B. Fu (1991). Global warming: Evidence for asymmetric diurnal temperature change. Geophys. Res. Lett. 18, 2253-2256: At present we lack an adequate understanding of the causes of differential changes in the mean and extremes of maximum and minimum temperatures. This is a fundamental characteristic of recent climate variation over a large portion of the NH land mass, and it must be better understood before we can confidently project the climate …. This will require rigorous atmospheric chemistry and climatological monitoring and analysis efforts. Additionally, improved modeling efforts are required which would consider the combined impact of changes of greenhouse gases, surface characteristics, and aerosols on the diurnal cycle in the atmospheric boundary layer. Karl, T.R., P.D. Jones, R.W. Knight, G. Kukla, N. Plummer, V. Razuvayev, K.P. Gallo, J. Lindseay, R.J. Charlson, and T.C. Peterson (1993). A new perspective on recent global warming: Asymmetric trends of daily maximum and minimum temperature. Bull. Am. Meteorol. Soc. 74, 1007-1023: Strong evidence exists for a widespread decrease in the DTR [daily temperature range] over the past several decades in many regions of the globe. There are many possible climatic factors that affect the DTR, but indications are that cloud cover, including low clouds, has increased in many areas that have a decrease in the DTR. The increases in cloud cover could be indirectly related to the observed global warming and increases of greenhouse gases, related to the indirect effects of increases in aerosols, simply a manifestation of natural climate variability, or a combination of all three. A robust answer regarding the cause(s) of the decrease in the DTR will require efforts in several areas.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change First, an organized global effort is required to develop relevant and homogeneous time series of maximum and minimum temperature along with information on changes of climatic variables that influence the DTR (such as cloudiness, stability, humidity, thermal advection, and snow cover). Second, improvements in the boundary-layer physics and treatment of cloud within existing GCMs are critically important. Third, the treatment of both anthropogenic tropospheric aerosols and greenhouse gases must be realistically incorporated into GCMs with a diurnal cycle. Fourth, measurements need to be made to help clarify the role of aerosols. Finally, imaginative climate change detection studies that link the observed climate variations to model projections will be required to convincingly support any relation between anthropogenic-induced changes and the DTR. Hansen, J., M. Sato, and R. Ruedy (1995). Long-term changes of the diurnal temperature cycle: Implications about mechanisms of global climate change. Atmos. Res., in press: Tropospheric aerosols alone cannot provide the continentally located forcing, i.e., they are not capable of damping the diurnal cycle as much as observed. Only an increase of continental cloud cover, possibly a consequence of anthropogenic aerosols, can damp the diurnal cycle by an amount comparable to observations. Needed Observations. The climatic implications of such cloud and aerosol changes can be discerned only if the changes are known globally on decadal time scales. The required long-term precision of measurements required to interpret decadal climate change are a cloud cover change [of] 0.4% as a function of cloud height (Δp = 5 mb) and a tropospheric aerosol optical depth change [of] 0.01 …. Although such precisions are not attained by existing or flight-scheduled satellite instruments, the capabilities have been demonstrated with relatively inexpensive long-lived planetary instruments. Specifically, a Michelson interferometer … has been shown to be capable of the cloud measurements in a multilayered atmosphere (Carlson, B.E., A.A. Lacis, and W.B. Rossow, 1993: "Tropospheric gas composition and cloud structure of the Jovian North Equatorial Belt." J. Geophys. Res. 98, 5251-5290). Similarly, a photopolarimeter has been demonstrated to be capable of such precise aerosol measurements (Travis, L.D., 1992: "Remote sensing of aerosols with the EOS Polarimeter," Proc. Polarization and Remote Sensing, Vol. 1747, 154-164, SPIE, San Diego), including accurate determination of aerosol and cloud microphysical properties. Without such data, long-term global climate forcings will remain unknown and interpretation and projection of any observed global climate change will be impossible.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Coakley, Jr., J.A., R.L Bernstein, and P.A. Durkee (1987). Effect of ship-stack effluents on cloud reflectivity. Science 237, 1020-1022: Because continental environments have significantly higher concentrations of CCN and lower relative humidities, a similar study should be undertaken to determine the effects of stack effluents from industrial centers on continental stratiform clouds. Albrecht, B.A. (1989). Aerosols, cloud microphysics, and fractional cloudiness. Science 245, 1227-1230: Increase in aerosol concentrations over the oceans may increase the amount of low-level cloudiness through a reduction in drizzle—a process that regulates the liquid water content and the energetics of shallow marine clouds. The resulting increase in global albedo would be in addition to the increases due to enhancement in reflectivity associated with a decrease in droplet size and would contribute to a cooling of the earth's surface. [No recommendations are made, but there is the obvious complication for climate models: the need to predict CCN concentrations.] Ackerman, A.S., O.B. Toon, and P.V. Hobbs (1993). Dissipation of marine stratiform clouds and collapse of the MBL due to the depletion of CCN by clouds. Science 262, 226-229: Marine stratiform clouds overlie about a third of the Earth's oceans and play a prominent role in the Earth's radiative heat balance …. It has been estimated that the global cooling that would result from a 4% increase in the area covered by marine stratocumulus would offset the expected warming from a doubling of atmospheric CO2 concentrations …. Extensive sheets of marine stratocumulus are a climatological feature of the eastern regions of subtropical oceans, where the PBL [planetary boundary layer] is capped by a strong temperature inversion produced by large-scale subsidence. The vertical mixing that supplies moisture to these clouds and maintains the depth of the boundary layer is generally driven by cloud-top radiative cooling …. Clearly an understanding of the processes that determine the lifetime and albedo of marine stratocumulus is of critical importance to an understanding of the Earth's climate system. Albrecht … argued that increased CCN concentrations, which decrease cloud droplet sizes and reduce drizzle, can increase the fractional coverage of marine stratiform clouds because drizzle can regulate the liquid-water content during the lifetime of a cloud …. We have found, through numerical studies, that clouds themselves may reduce particle concentrations to such an extent that the clouds dissipate and, as a consequence, the boundary layer collapses. [No explicit suggestions, but obviously the theory should be tested, e.g., what entrainment of CCN from aloft?]

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Baker, M.B., and R.J. Charlson (1990). Bistability of CCN concentrations and thermodynamics in the cloud-top boundary layer. Nature 345, 142-145: We … require simultaneous observational studies of the microphysical structure, albedo, and cloud-topped boundary layer structure, together with both background and anthropogenically enhanced CCN concentrations, to test the present model …. Baker, M.B. (1993). Variability in concentrations of CCN in the marine cloud-topped boundary layer. Tellus 45B, 458-472: [The] eventual dependence of cloud cover on N [the CCN number concentration] is not exactly as envisaged by Albrecht (1989),1 who suggested that the increased drying due to precipitation at low N might remove cloud. In fact, precipitation is self-limiting; when clouds get thin, the precipitation stops. [Our results] suggest that the link between cloud cover and N may result from the fact that N plays a role in controlling the heating profile in the cloud boundary layer. If this is correct, then those "equilibria …" that correspond to net in-cloud heating would in reality be short lived; ensembles of stratocumulus clouds might pass through these states, but only those clouds in which there is net cooling would be long lasting. A simple two-box model of the clear and cloud system suggests that the CCN number concentration in the clear part of the system is greater than in the cloudy part …. Measurements focusing on these points will be extremely useful in understanding the controls on cloud cover in the marine boundary layer. Ackerman, A.S., O.B. Toon, and P.V. Hobbs (1994). Reassessing the dependence of CCN concentration on formation rate. Nature 367, 445-447: We find no evidence for bistability [in the CCN concentration]. However, we find that CCN concentrations are generally strongly dependent on their production rate, so that changes in the latter would influence the Earth's albedo. Falkowski, P.G., Y. Kim, Z. Kolber, C. Wilson, C. Wirick, and R. Cess (1992). Natural versus anthropogenic factors affecting low-level cloud albedo over the North Atlantic. Science 256, 1311-1313: Cloud albedo depends on column-integrated liquid water content and the density of CCN …. A comparison of two independent satellite data sets suggests that, although anthropogenic sulfate emissions may enhance cloud albedo immediately adjacent to the east coast of the U.S., over the central North Atlantic Ocean the variability in albedo can be largely accounted for by natural marine and atmospheric processes that probably have remained relatively constant since the beginning of the industrial revolution. 1    Albrecht, B.A., 1989. Aerosols, cloud microphysics and fractional cloudiness. Science 245, 1227-1230.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change [No recommendations but some reader questions: These results are for low-level stratus (e.g., fog); what about for other-level clouds? What albedos for individual meteorological events? Correlation shown, but ''correlation does not mean causation!"] Kim, Y., and R.D. Cess (1993). Effect of anthropogenic sulfate aerosols on low-level cloud albedo over oceans. J. Geophys. Res. 98, 14883-14885: [W]e examined satellite-measured low-level cloud albedo off the east coasts of North America and Asia at midlatitudes where anthropogenic sulfate sources are large and aerosols are transported eastward over the oceans by prevailing westerlies. The satellite data demonstrate enhanced cloud albedo near the coastal boundaries where sulfate concentrations are large. Similar trends are absent over ocean regions of the southern hemisphere that are removed from anthropogenic sulfate sources. [No research recommendations, but their results stimulate some obvious questions from the reader: What influence from natural aerosol (from continents and from the "roaring forties")? What correlation with daily meteorology?] Parungo, F., J.F. Boatman, H. Sievering, S.W. Wilkison, and B.B. Hicks (1994). Trends in global marine cloudiness and anthropogenic sulfur. J. Climate 7, 434-440: A statistical analysis … shows a significant positive trend (4.2% increase from the 1930 baseline) in total oceanic cloud amount in the period between 1930 and 1981. The increase of total cloud amount for the NH (5.8%) was twice that for the SH (2.9%). The more consistent 30-yr (1952-1981) data show that the change in cloud amount (1952 base) was 1.5% for the globe, 2.3% for the NH, and 1.2% for the SH. The analysis shows that the greatest cloud amount increase was for altocumulus and altostratus clouds and that this increase was most pronounced at midlatitudes (30-50°N) …. The trend and the pattern of cloud amount variations appear to be in accord with the temporal trend and geographic distribution of SO2 emissions. It is hypothesized that sulfate particles, converted from SO2, may modify cloud droplet spectra, causing affected clouds to be more colloidally stable than unaffected clouds. The longer residence times of affected clouds could cause increases of cloud frequency and cloud amount …. Sinha, A., and K.P. Shine (1994). A one-dimensional study of possible cirrus cloud feedbacks. J. Climate 7, 158-173: Early parameterizations [for climate models of the optical properties of cirrus clouds], in terms of ice water content, IWC (or path, IWP) of the

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change radiative properties of cirrus clouds relied upon assumptions about the shapes and orientations of cirrus ice crystals. The differences between these assumptions and the great variety of actual sizes, shapes, and orientations of ice crystals inferred from aircraft observations was claimed to account for the persistent discrepancies between the observed dependence of the emissivity … and the shortwave albedo… on the IWC and the parameterized dependence …. Aside from such theoretical obstacles, Stephens (1987) highlighted some of the problems regarding observational determinations of ice water content when he in turn ascribed the above discrepancies to the inability of measurements to account for the porosity of ice crystals. It has subsequently been found that perhaps the major hindrance to observational studies is the difficulty in obtaining reliable estimates of the numbers of small crystals (i.e., less than about 20 μm). These may play an important role in determining the optical properties of cirrus clouds …. Indeed small crystals may dominate the radiative characteristics of very high, cold cirrus …. [T]he magnitude of the ice water content feedback is seen to depend substantially on the inclusion of the effects of ice crystals whose characteristic dimension is less than 20 μm …. This result underlines the need to develop instrumentation to accurately measure the number distributions of ice crystals in cirrus clouds within this size range. Charlson, R.J., S.E. Schwartz, J.M. Hales, R.D. Cess, J.A. Coakley, Jr., J.E. Hansen, and D.J. Hofmann (1992). Climate forcing by anthropogenic aerosols. Science 255, 423-430: Rates of new particle formation and of the time evolution of the size distribution of aerosols, which affect their optical and cloud-nucleating properties, are highly dependent on the rate of production of condensable material and on the concentration and size distribution of aerosol particles already present. A description of these rates is uncertain to orders of magnitude and is a major unsolved problem. The indirect (cloud) forcing is more difficult to address than the direct forcing. Additional key phenomena that must be considered are (i) the relations between mass and number concentrations and composition of precloud aerosol concentrations and the number, concentration, and size distribution of cloud droplets formed in air containing the aerosol; and (ii) the dependence of aerosol perturbation to cloud radiative forcing on the nature of the cloud fields. The nature of the relation between CCN concentrations and sulfate concentrations, which is likely to be nonlinear …, is not established …. The knowledge required to evaluate the influence of anthropogenic aerosols on cloud lifetimes, atmospheric dynamics, and the hydrological cycle includes the dependence of precipitation formation processes on cloud microphysical properties, such as number concentration and size distributions

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change of cloud droplets. These secondary phenomena are considerably more difficult to quantify than the radiative properties, and additional research is required to accurately describe them in climate models. Because anthropogenic aerosols are short-lived in the atmosphere and their sources are geographically localized, their concentration is not uniformly distributed. It is therefore necessary to specify pertinent aerosol properties as functions of location and time …. Vertical distribution information is required …. Horizontal distribution information is essential …. Temporal distribution information is needed …. Better quantification of aerosol radiative forcing and of its relation to anthropogenic emissions requires globally representative measurements of aerosol and cloud properties. Long-term monitoring of the aerosol optical depth and of effective particle size can be achieved by satellites equipped with photopolarimeters …. Simultaneous monitoring of global cloud properties …. However, evaluation and improvements of inferences drawn from satellite measurements require that the measurements are tied to concurrent ground-based measurements of optical properties and to in situ surface and aircraft measurements of chemical and microphysical properties …. The large uncertainties in the magnitude and geographical distribution of aerosol forcing … indicate the need for substantial research to improve the description of aerosol forcing. Among the components of such a research program would be a new set of satellites dedicated to questions of radiative forcing and able to provide the needed global coverage and high frequency of sampling. Connection of this global data set to local and column integral optical, chemical, and microphysical properties requires a set of ground-based observing sites carefully located to delineate the key characteristics of air masses that are either influenced or unaffected by anthropogenic sulfate. Understanding the chemical and physical processes that produce the aerosol and control its relevant properties requires both laboratory and field studies. These research activities must be integrated. Ground-based, airborne, satellite, and laboratory data acquisition activities should be coordinated, and modeling is needed at a variety of scales, from the mesoscale to the global scale, to aid in the design of measurement strategies, to provide a framework for data analysis, and to examine the climatic impact of anthropogenic aerosols. Key questions remain regarding the climate influence of anthropogenic aerosol and its relation to the climate influence of increased concentrations of greenhouse gases: To what degree do such physically, altitudinally, geographically, and temporally different forcings compensate each other? Do such disparate forcings produce identical but opposite meteorological and climatological response? Have the radiative effects of sulfate aerosol decreased the magnitude of warming from anthropogenic greenhouse gases or delayed its onset or both? Or are there other climatic influences of anthropogenic aerosol that might have offset the radiative influences [of aerosols]? To what extent will reductions in SO2 emissions for control of acid deposition exacerbate the greenhouse effect?

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Penner, J.E., et al. (1994b). Quantifying and minimizing uncertainty of climate forcing by anthropogenic aerosols. Bull. Am. Meteorol. Soc. 75, 375-400: It is crucial that a research program be established to quantify the climate forcing by anthropogenic aerosols …. The prime components of such a program are (1) laboratory analyses and process studies, (2) surface based (land and ship) observations both for continuous and intensive periods, (3) airborne observations of key aerosol and chemical variables as well as radiation, (4) satellite observations of upwelling radiation and atmospheric extinction, and (5) models to connect estimates of aerosol sources to estimates of aerosol properties and to the geographically dependent observations. A research strategy has been developed here to quantify the direct forcing by anthropogenic aerosols. If aggressively implemented, this strategy promises to allow these effects to start to be incorporated into climate models within the next several years. To enable the development of a similar strategy for quantifying the indirect forcing by anthropogenic aerosols, a less well-structured program of basic research is required, wherein exploratory measurements of aerosol effects on clouds are made on a smaller scale. Much of the effort directed toward quantifying the direct effect of anthropogenic aerosols on climate forcing will benefit subsequent programs to quantify the indirect effects of anthropogenic aerosols. Garratt, J.R. (1994). Incoming shortwave fluxes at the surface—A comparison of GCM results with observations. J. Climate 7, 72-80: The comparisons suggest that much of the excess surface net radiation found in GCMs [general circulation models] at continental surfaces (for 22 inland locations, by about 20 percent or 11 W m-2 on average, with significant seasonal and regional variations) is probably the result of excess incoming shortwave fluxes being calculated [by about 6 to 9 percent or 9 to 18 W m-2 on average (for 4 models: Commonwealth Scientific and Industrial Research Organization, Colorado State University, Geophysical Fluid Dynamics Laboratory, and U.K. Meteorological Office)] …. In the case of the incoming shortwave flux at the surface, observations over a period of time are affected … by [among other things] aerosol loading in the atmosphere …. Such effects are not simulated in most GCMs, so that there will be a tendency for model fluxes to be higher than those observed. Karl, T.R., R.W. Knight, G. Kukla, and J. Gavin, 1995. Evidence for radiative effects of anthropogenic sulfate aerosols in the observed climate record. In Aerosol Forcing of Climate, R. Charlson and J. Heintzenberg (eds.). Dahlem Konferenzen 1994, Wiley, New York: The detection of an anthropogenic sulfate aerosol forcing on the surface air temperature is supported by a new analysis of daily surface air temperatures, cloud amount, and SOx emissions in North America. Within the contiguous U.S.A., SOx emissions increased by about 8 Tg between 1950

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change and 1970 and decreased by a similar amount from 1970 to 1990. The daily surface air temperature maxima under both clear sky and overcast conditions cooled by about 0.9°C from 1950 to 1970 and warmed by a similar amount thereafter. The results are consistent with the expected direct, and also possible the indirect impact of SOx emissions on surface air temperature …. The possible interference of factors affecting aerosol formation and transport, interaction with clouds, as well as the role of natural variations of atmospheric/ocean circulation require further assessment. IPCC (1995). The effect of tropospheric aerosols. Excerpted from Radiative Forcing of Climate Change. The 1994 Report of the Scientific Assessment Working Group of IPCC, Summary for Policymakers, Intergovernmental Panel on Climate Change: New estimates of both the direct and indirect effect of anthropogenic aerosols in the troposphere have become available since IPCC [Intergovernmental Panel on Climate Change] 1992. In order to compare different aerosol effects it is useful to express them in terms of globally averaged values of radiative forcing. The direct radiative forcing due to increases in sulphate aerosol since 1850, averaged globally, is estimated to lie in the range -0.25 to -0.9 W m-2. The direct effect of aerosol from biomass burning is estimated to lie in the range -0.5 to -0.6 W m-2. Calculations of the indirect effect of aerosols are at an early stage. Preliminary results suggest that the radiative effect of aerosols on cloud radiative properties is probably a negative forcing and may be of similar magnitude to the direct effect. Note that in the global average the total aerosol induced radiative forcing is negative, but the absorption of solar radiation by carbonaceous aerosols may cause local positive radiative forcing. It is interesting to compare these estimates with the direct radiative forcing due to increases in greenhouse gases since pre-industrial times (+2.1 to +2.8 W m-2), although it is unlikely to be appropriate to add the negative global radiative forcing of aerosols to the positive global radiative forcing of greenhouse gases.