Role of Sulfate Aerosols in Modifying the Regional Climate through Cloud-Mediated Radiative Forcing

At a remote mountain-top location in the southeastern U.S., measurements were made to estimate the contribution of anthropogenic aerosols to the indirect radiative forcing of the regional climate. The influence of the long-range transport of anthropogenic emissions on cloud microphysical and optical properties at the mountain top site was investigated. The sources of the cloud forming air masses were determined from back-trajectory analysis. Cloud water sulfate content was used as a surrogate for anthropogenic pollution. The effects of particulate sulfate on cloud condensation nuclei (CCN) concentration, cloud droplet number concentration (N), cloud droplet effective radii (Reff) and cloud albedo were analyzed. Although a linear relationship between CCN and sulfate mass was obtained, the presence of other species (e.g. organics which were not measured) as well as differences in size distributions suggest that a non-linear relationship is more realistic, with a lowered sensitivity of CCN at high values of sulfate. Even though average values of N differed by almost 33% from polluted to less polluted type air masses as compared to a 13% difference in Reff, the cloud optical depth was found to be more sensitive to variations in Reff from polluted to less polluted clouds due to the variability in cloud liquid water and cloud thickness. As part of a “closure experiment” the cloud albedo calculated from in situ measurements for a three year period (1993-95) compared well with that inferred from the Advanced Very High Resolution Radiometer (AVHRR) data. This accomplishes the objective of our closure experiment and proves that albedo of non precipitating, thin, isolated clouds can be resolved against the dark forested background by AVHRR. The cloud reflectivity inferred from satellite measurements and that calculated from in situ observations were found to vary with the cloud water sulfate and N. Non-linear increases in satellite inferred cloud albedo with cloud liquid water path suggest the importance of determining the contribution of cloud dynamic feedbacks on the indirect effect.