Sources and Impacts of Particles on Stratocumulus Clouds in the Southeast Pacific

The southeast Pacific Ocean is covered by the world's largest stratocumulus cloud layer, which has a strong impact on ocean temperatures and climate in the region. The effect of anthropogenic sources of aerosol particles such as power plants, urban pollution and smelters from Chile and Peru on the stratocumulus deck was investigated during the VOCALS (VAMOS Ocean-Cloud-Atmosphere-Land Study) field experiment. Aerosol measurements below and above cloud were made with a ultra-high sensitivity aerosol spectrometer and analytical electron microscopy. In addition to more standard in-cloud measurements, droplets were collected and evaporated using a counterflow virtual impactor (CVI), and the non-volatile residual particles were analyzed.

Many flights focused on the gradient in cloud properties along an E-W track from near the Chilean coast to remote areas offshore. Mean statistics from seven flights and about forty individual legs were compiled. Consistent with a continental source of cloud condensation nuclei, below-cloud aerosol and droplet number concentration generally decreased from near shore to offshore. This applied for particles larger than 0.05 and 0.1 µm in diameter, but not for total particles larger than 0.01 µm diameter. This suggests pollution contributed aged accumulation-mode aerosols to the stratocumulus layer, but fresher nuclei-mode particles were generated from other sources as well. Liquid water content and drizzle concentration tended to increase with distance from shore, but exhibited much greater variability.

Aerosol number concentration in the >0.05 and >0.1 µm size ranges was correlated with droplet number concentration, and anti-correlated with droplet effective radius. These variables were especially well correlated on individual flights with near constant liquid water content (LWC), but were also statistically significant for the data set as a whole. The impact of aerosol pollutants was to increase droplet number and decrease droplet size within a region extending about 1000 km offshore. Cloud droplets were more numerous and smaller near shore, and there was less drizzle. However, calculated cloud albedo was not necessarily higher near shore, because the cloud liquid water path actually tended to be smaller near shore than offshore. This variation in liquid water path confounds detection of the indirect effect of aerosol particles on clouds.

Analysis of the droplet residual measurements showed that not only were there more residual nuclei near shore, but that they tended to be larger than those offshore. Single particle analysis over a broad particle size range has been used to reveal types and sources of CCN. Clean marine regions will be contrasted with polluted near shore regions.

Differences in the size distribution of droplet residual particles and ambient aerosol particles were observed due to the preferential activation of large aerosol particles and, possibly, in-cloud coalescence and aqueous chemical reactions. By progressively excluding small droplets from the CVI sample, we were able to show that the larger drops, which initiate drizzle, contain the largest aerosol particles. Measured residual droplet distributions will be compared to those from a microphysical model to determine impacts of coalescence, entrainment and mixing on the aerosol and droplet distributions.