Processing of Aerosol Particles by Tradewind Cumuli at a Mountain Top in Puerto Rico

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 shallow clouds made at a tropical site. The measurements were made with a new optical sensor, the Aerosol Particle Spectrometer with Polarization Detection (APSPD) that was deployed at the mountaintop observatory of Pico de Este (PE) in Puerto Rico, between 25 August and 10 September 2011 as part of the Puerto Rico African Dust and Cloud Study (PRADACS).

Shallow cumulus clouds associated with the trade winds from the central Atlantic are very common in Puerto Rico. These warm clouds typically have cloudbase at approx. 500 m amsl and a vertical extent of about 2 km. The mountaintop observatory at PE is located over 1 km amsl, so in situ instrumentation can sample the low- and mid-levels of the clouds as they reach the site. The measurements were made of cloud residual and interstitial particles in the size range from 0.5 to 20 µm. The hypotheses were that particle volume concentration and optical signatures depend on air mass origin, the amount of precipitation that forms along the air mass trajectory, the number of hours that the air is close to the ocean surface (below 500 m) and the concentration of liquid water content (LWC) in the mountaintop cloud.

The optical signatures determined from the polarized light scattered in the APSPD's laser beam allowed the identification of three types of particles. Two of the signatures matched test particles of pollen and iron oxide measured in the laboratory and the third signature is of a particle type that remains to be identified with future studies. The signatures are related to the particle shape that is in turn a function of particle composition. The measurements were stratified by the three particle shapes, their volume size distributions and total volume concentrations, and then evaluated under the hypothesized conditions.

Air mass origin was the condition that produced the largest difference in both the volume concentrations and the ratio of residual to interstitial concentrations. Air masses from Africa produced the largest volume concentrations and residual to interstitial ratios. Air masses that had spent more time below 500 m had lower particle volume concentrations than those that stayed aloft, presumably due to more removal of the particles by sedimentation to the surface. Likewise, air masses with a history of precipitation formation along their trajectory had much lower volume concentrations than those that formed little precipitation. This difference is also most likely a result of particle removal by precipitation. The primary impact of cloud LWC is on the removal of the hygroscopic particles, first by activating them as cloud droplets then removing them as the droplets sediment.

The next stage of this study will be to compare the optical signatures with measurements made with an aerosol mass spectrometer that was operated during the same period. More extensive laboratory measurements are also in progress to catalogue a larger library of optical signature that will be used to better quantify measurements by the APSPD and other similar spectrometers that measure polarized light scattering from individual particles.