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A new method of analysis for S(IV) is described incorporating a counter-flow virtual impactor for collection and evaporation of cloud droplets with an SO2 analyzer. This technique allows analysis of aqueous S(IV) concentration in real-time and overcomes some sampling or analysis problems of conventional sampling with bulk cloud water collectors and wet chemical analysis methods. The technique is demonstrated by the correlated responses of the S(IV) instrument and instruments measuring physical properties of clouds while passing in and out of clouds. Measured aqueous S(IV) concentrations ranged from the detection limit (0.1 nmol per m3 of air) to 1.8 nmol m-3 in clouds in Ohio. S(IV) molar concentrations (moles/l of cloud water) were not calculable from these measurements, but can be calculated when a measurement of the liquid water content of the collected cloud water is made simultaneously.

Tropospheric aerosols are calculated to cause global-scale changes in the earth's heat balance, hut these forcings are space/time integrals over highly variable quantities. Accurate quantification of these forcings will require an unprecedented synergy among satellite, airborne, and surface-based observations, as well as models. This study considers one aspect of achieving this synergy--the need to treat aerosol variability in a consistent and realistic way. This need creates a requirement to rationalize the differences in spatiotemporal resolution and coverage among the various observational and modeling approaches. It is shown, based on aerosol optical data from diverse regions, that mesoscale variability (specifically, for horizontal scales of 40-400 km and temporal scales of 2-48 h) is a common and perhaps universal feature of lower-tropospheric aerosol light extinction. Such variation is below the traditional synoptic or 'airmass' scale (where the aerosol is often assumed to be essentially homogeneous except for plumes from point sources) and below the scales that are readily resolved by chemical transport models. The present study focuses on documenting this variability. Possible physical causes and practical implications for coordinated observational strategies are also discussed.

A generalized reformulation of the Kohler theory to include the effect of soluble gases and slightly soluble aerosol substances is presented. A single equation is derived that takes into account 1) the Kelvin effect; 2) the Raoult effect caused by highly soluble aerosol material (salt); 3) increase in droplet radius due to an undissolved, insoluble, or slightly soluble core; 4) contribution of solute into the droplet by a slightly soluble substance; and 5) contribution of hygroscopic material into the droplet by a soluble trace gas allowed to deplete from the gas phase because of the condensational growth of the droplets (assuming a monodisperse size distribution). Model calculations are presented for a system in which the aerosol is composed of a slightly soluble CaS[O.sub.4] core coated with ammonium sulfate, and the gas phase contains HN[O.sub.3]. It is shown that the resulting equilibrium curves allow the occurrence of stable, unactivated droplets with radii up to about 10 [[micro]meter] at realistic ambient conditions. The equilibrium curves show in some cases local minima and maxima, the reasons for and consequences of which are discussed. The results of this study suggest that a new definition for 'activated droplet' is needed.

Calculations of direct climate forcing by anthropogenic aerosols commonly use radiative transfer parameters, including asymmetry parameter g. One method of obtaining the asymmetry parameter of a particle population is to convert measured values of the hemispheric-to-total-scatter ratio (backscatter ratio b) into their corresponding g values. We compare a conversion derived from Mie calculations with one derived from the Henyey-Greenstein (HG) phase function to show that the HG method systematically overestimates g for typical size distributions of accumulation-mode aerosols. A delta-Eddington radiative transfer calculation is used to show that a 10% overestimation of g can systematically reduce climate forcing as a result of aerosols by 12% or more. Mie computations are used to derive an empirical relationship between backscatter ratio and asymmetry parameter for log-normal accumulation-mode aerosols. This relationship can be used to convert the backscatter ratio to the asymmetry parameter, independent of geometric mean diameter [D.sub.gv] or complex refractive index m, but the conversion requires knowledge of the breadth [[Sigma].sub.g] of the size distribution. Key words: Climate forcing, aerosols, radiative transfer parameters, backscatter ratio, asymmetry parameter, light scattering.