The partitioning of cloud particles between activated droplets and unactivated interstitial aerosols is a primary determinant of cloud microphysical, radiative and chemical properties. In the present study, high-resolution aircraft measurements (1 s, ~60 m) of the number concentrations (Namp and Ncd) of accumulation-mode particles (AMP, 0.17 to 2.07 mm dia.) and cloud droplets (CD, 2 to 35 mm dia.), made during ten flights in and around continental stratiform clouds near Syracuse NY in autumn 1984, have been used to study the local and instantaneous nature of cloud particle partitioning throughout the sampled clouds. The partitioning is defined as the activated fraction F (º Ncd/Ntot) of all measured cloud particles (Ntot º Namp + Ncd). F may be interpreted approximately as the AMP activation efficiency which is often assumed to be unity in all clouds. In the present study, F varied over its full possible range (0 to 1), being low especially in cloud edges. Even in the near-adiabatic parts of cloud interior, its variation ranged from 0.1 to 1 over the ten days. Statistically, its value in cloud interior exceeded 0.9 in 36% of the data, but was below 0.6 in 28%. On five of the ten days, stratocumulus clouds were embedded in cool, dry and relatively clean (Ntot less than 600 cm-3) northerly air masses. In such cases, cloud droplet concentration increased approximately linearly with increasing total particle loading, and F in cloud interior was near unity and relatively insensitive to changes in the influencing variables. On the other days, especially in stratus clouds embedded in warm and polluted southerly air masses, F was significantly less than unity, with particles in the smallest size ranges (0.17 to 0.37 mm) activating only fractionally depending on several factors.
An important feature of the clouds sampled in this study was the existence of multiple cloud layers and complex vertical thermal structure on most days. Consequently, our analysis of the dependence of F on influencing cloud variables has been based on data grouped into individual cloud layers. Besides the size of the precursor aerosol, we found total particle loading (Ntot) and the local vertical cooling rate (~ temperature lapse rate in individual layers) to influence F the most. In particular, F decreased with increasing particle loading in excess of about 800 cm-3, and increased nearly linearly with temperature lapse rate. Evidently, the activation process can become self-limiting in stratiform clouds under polluted conditions, in which case, increasing anthropogenic aerosol loading of the atmosphere translates less and less into cloud droplet population. This observation has important implications with respect to cloud radiative forcing, precipitation formation and acidification, and for long range transport of the unactivated aerosols.
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