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Although the importance of the aerosol contribution to the global radiative budget has been recognized, the forcings of aerosols in general, and specifically the role of the organic component in these forcings, still contain large uncertainties. In an attempt to better understand the relationship between the background forcings of aerosols and their chemical speciation, marine air samples were collected off the windward coast of Oahu, Hawaii, during the Rough Evaporation Duct project (RED) using filters mounted on both the Twin Otter aircraft and the Floating Instrument Platform (FLIP) research platform. Laboratory analysis revealed a total of 17 species, including 4 carboxylic acids and 2 carbohydrates that accounted for 74% [+ or -] 20% of the mass gain observed on the shipboard filters, suggesting a possible significant unresolved organic component. The results were correlated with in situ measurements of particle light scattering ([[sigma].sup.sp]) at 550 nm and with aerosol hygroscopicities. Principal component analysis revealed a small but ubiquitous pollution component affecting the [[sigma].sup.sp] and aerosol hygroscopicity of the remote marine air. The Princeton Organic-Electrolyte Model (POEM) was used to predict the growth factor of the aerosols based upon the chemical composition. This output, coupled with measured aerosol size distributions, was used to attempt to reproduce the observed [[sigma].sup.sp]. It was found that while the POEM model was able to reproduce the expected trends when the organic component of the aerosol was varied, due to large uncertainties especially in the aerosol sizing measurements, the [[sigma].sup.sp] predicted by the POEM model was consistently higher than observed.

Simultaneous field measurements of aerosol and cloud droplet concentrations and droplet diameter were performed at a maritime site on the coast of Washington State. The aerosol and droplet spectra were compared for estimating cloud condensation nucleus concentration ([N.sub.ccn]) as the number of particles with diameters greater than 80 nm, that is, [N.sub.ccn] = N([D.sub.p] > 80 nm). Several analytical approaches were developed and applied to the data, including a stratification of the observations into periods of high and low liquid water content (LWC) based on a threshold value of 0.25 g [m.sup.-3]. The aerosol data were corrected for inertial losses of cloud droplets at the inlet using wind speed and droplet size; this correction improved the measured relationships between [N.sub.ccn], and droplet number concentration ([N.sub.d]). These measurements, when coupled with the range of possible aerosol chemical compositions, imply a cloud supersaturation of 0.24%-0.31% at the Cheeka Peak sampling site during periods of high LWC. The observations of droplet and aerosol spectra supported Twomey's cloud brightening hypothesis in that [N.sub.ccn] was highly correlated ([r.sup.2] = 0.8) with [N.sub.d] in apparent 1:1 proportions. For the investigated range (50 [cm.sup.-3] < [N.sub.d] < 600 [cm.sup.-3]) droplet effective diameter ([D.sub.eff]) was very sensitive to variation in [N.sub.ccn] for 50 [cm.sup.-3] < [N.sub.ccn] < 200 [cm.sup.-3], somewhat sensitive for 200 [cm.sup.-3] < [N.sub.ccn] < 400 [cm.sup.-3], but not very sensitive to variation in aerosol number for [N.sub.ccn] > 400 [cm.sup.-3]. A model was applied to the aerosol and droplet data to predict droplet size, as [D.sub.eff], from [Mathematical Expression Omitted] and LWC. Predicted values for [D.sub.eff] agreed ([r.sup.2] = 0.8) with [D.sub.eff] determined directly from the cloud droplet spectra, suggesting that this approach should be useful in climate modeling for predicting cloud droplet size from knowledge of [N.sub.ccn] and LWC.

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.