Observations Pertaining to the Effect of Chemical Transformation in Cloud on the Anthropogenic Aerosol Size Distribution

Airborne observations from two case studies of continental cloud, one of summertime cumulus and one of autumn stratus, are examined for evidence of changes in the size of the accumulation-mode aerosol particles due to S(IV) oxidation in cloud. Such changes might affect the efficiency with which the aerosol scatters light and its ability to function as cloud condensation nuclei (CCN), and hence impact climate. In both cases, there were strong anthropogenic contributions to the aerosol, and the measured particles (0.17-2 μm in diameter) were not scavenged with 100% efficiency: the relative number concentrations of measured particles activated in cloud were about 70% for the cumulus and 15%-50% for the stratus. Within the measurement resolution, the apparent effect of cloud processing in the cumulus case was to nearly preserve the shape of the size distribution of the measured aerosol, even after the mass addition of sulfate by S(IV) oxidation. This can be accomplished through the activation of sufficiently small particles to balance the increase in larger particles. In the stratus case, the aerosol size distributions associated with the cloud are bimodal with peaks at 0.23 and 0.35 μm. The second peak coincides closely with the lower size of particles activated in the cloud suggesting that S(IV) oxidation or possibly coalescence scavenging contributed to its formation.