Aerosol particles in the atmosphere play a fundamental role in the formation of cloud droplets. Moreover, cloud-processing of aerosols is an important component of the changes observed in aerosol composition and size. In this study we present measurements of the physical and optical properties of interstitial and droplet residual particles in clouds made at a tropical mountain site. The measurements were made with the Aerosol Particle Spectrometer with Polarization Detection (APSPD) that was deployed at the mountaintop observatory of Pico del Este in Puerto Rico, between 25 August and 10 September, 2011 as part of the Puerto Rico African Dust and Cloud Study (PRADACS).
We evaluate the optical signatures and equivalent optical diameters of the sampled aerosol with respect to the origin and meteorological history of the air mass in which the aerosol were embedded. Additionally, the aerosol properties are related to the liquid water content present in the cloud.
The origin of the air mass is the most important factor that modulates the optical signatures, volume size distributions and total volume concentration of the sampled aerosol. Differences in the aerosol volume concentrations were significant (up to a factor of 4) between air masses originating in Africa, the North Atlantic and the South Atlantic. In addition, African aerosols have a much higher ratio of residual to interstitial volume concentration, suggesting that these particles had a higher fraction of cloud condensation nuclei than in the other air masses. Finally, the amount of precipitation along the air mass trajectories impacts the aerosol volume concentration, as does the residence time of the air mass in the moist, oceanic boundary layer, below 500 m.