Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.

[1] Knowledge of the size-and composition-dependent production flux of primary sea spray aerosol (SSA) particles and its dependence on environmental variables is required for modeling cloud microphysical properties and aerosol radiative influences, interpreting measurements of particulate matter in coastal areas and its relation to air quality, and evaluating rates of uptake and reactions of gases in sea spray drops. This review examines recent research pertinent to SSA production flux, which deals mainly with production of particles with r 80 (equilibrium radius at 80% relative humidity) less than 1 mm and as small as 0.01 mm. Production of sea spray particles and its dependence on controlling factors has been investigated in laboratory studies that have examined the dependences on water temperature, salinity, and the presence of organics and in field measurements with micrometeorological techniques that use newly developed fast optical particle sizers. Extensive measurements show that water-insoluble organic matter contributes substantially to the composition of SSA particles with r 80 < 0.25 mm and, in locations with high biological activity, can be the dominant constituent. Order-of-magnitude variation remains in estimates of the size-dependent production flux per white area, the quantity central to formulations of the production flux based on the whitecap method. This variation indicates that the production flux may depend on quantities such as the volume flux of air bubbles to the surface that are not accounted for in current models. Variation in estimates of the whitecap fraction as a function of wind speed contributes additional, comparable uncertainty to production flux estimates.

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The Atmospheric Radiation Measurement (ARM) Program, supported by the U.S. Department of Energy, is a major new program of atmospheric measurement and modeling. The program is intended to improve the understanding of processes that affect atmospheric radiation and the description of these processes in climate models. An accurate description of atmospheric radiation and its interaction with clouds and cloud processes is necessary to improve the performance of and confidence in models used to study and predict climate change. The ARM Program will employ five (this paper was prepared prior to a decision to limit the number of primary measurement sites to three) highly instrumented primary measurement sites for up to 10 years at land and ocean locations, from the Tropics to the Arctic, and will conduct observations for shorter periods at additional sites and in specialized campaigns. Quantities to be measured at these sites include longwave and shortwave radiation, the spatial and temporal distribution of clouds, water vapor, temperature, and other radiation-influencing quantities. There will be further observations of meteorological variables that influence these quantities, including wind velocity, precipitation rate, surface moisture, temperature, and fluxes of sensible and latent heat. These data will be used for the prospective testing of models of varying complexity, ranging from detailed process models to the highly parameterized description of these processes for use in general circulation models of the earth's atmosphere. This article reviews the scientific background of the ARM Program, describes the design of the program, and presents its status and plans.

Abstract.Sulfur chmnistry has been incorporated in the National Center [or Atmospheric Research Community Climate Model in an internally consistent manner with other parameterizations in the model. The model predicts mixing ratios of dimethylsulfide (DMS), SO2, SO•-, H202. Processes that control the mixing ratio of these species include the emissions of DMS and SO2, transport of each species, gas-and aqueous-phase chemistry, wet deposition, and dry deposition of species. Modeled concentrations agree quite well with observations for DMS 2-and H202, fairly well for SO2, and not as well for SO42--. The modeled SO4 tends to underestimate observed SO•-at the surface and overestimate observations in the upper troposphere. The SO2 and SO•-species were tagged according to the chernical production pathway and whether the sulfilr was of anthropogenic or biogenic origin. Although aqueous-phase reactions in cloud accounted for 81% of the sulfate production rate, only ,.050-60% of the sulfatc burden in the troposphere was derived from cloud chemistry. Because cloud chemistry is an important source of sulfate in the troposphere, the importance of H•O2 concentrations and pH values was investigated. When prescribing H202 concentrations to clear-sky values instead of predicting H202, the global-averaged, annual-averaged in-cloud production ot • sulfate increased. Setting the pH of the drops to 4.5 also increased the in-cloud production of sulfatc. In both sensitivity simulations, the increased in-cloud production of sulfate decreased the burden of sulfate because less SO2 was available for gas-phase conversion, which contributes •nore efficiently to the tropospheric sulfate burden than does aqueous-phase conversion.