Role of sulfate aerosols in modifying the cloud albedo: a closure experiment

At a remote mountain-top location in the southeastern US, measurements were made to estimate the contribution of anthropogenic aerosols to the cloud albedo. 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. A non-linear relationship between CCN and sulfate mass was obtained with a lowered sensitivity of CCN at high values of sulfate. Differences in N and sulfate from polluted to less polluted type air masses were much larger than that in Reff. This could be due to the variability in cloud liquid water content (LWC) as Reff is more related to LWC and cloud thickness than is N. The variability in cloud liquid water path (LWP) results in the optical depth being more sensitive to changes in Reff than to N for differences in cloud pollutant content. As part of a “closure experiment”, the cloud albedo calculated from in situ measurements for a 3-year period (1993–1995) 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 LWP suggest the importance of determining the contribution of cloud dynamic feedbacks on the indirect effect.