Aerosol Particle Activation observed inside Clouds at a Mountain Site on Puerto Rico

The phase partitioning of aerosol particles remaining in the interstitial phase and those transferred into the droplet phase have been measured during several days in December 2004 to study the particle activation at the East Peak mountain site (1000 m asl) on Puerto Rico.

For this purpose an interstitial inlet (upper cut size 5 µm) and a counterflow virtual impactor (CVI, lower cut size 5 µm) were operated in order to sample and separate non-activated particles and cloud drops. Inside the CVI, the drops were evaporated releasing dry residual particles that are related to the original cloud condensation nuclei (CCN). Concentration, number size distribution, content of soluble substances and black carbon (BC) of the interstitial and residual particles are analyzed by condensation particle counters (CPC), a scanning mobility particle sizer (SMPS), an aerosol mass spectrometer (AMS) and particle soot absorption photometers (PSAP). The sampled liquid water content was determined by Lyman-alpha hygrometers and a dew point mirror.

Results obtained from the CVI are compared to in-situ cloud microphysical measurements of LWC and drop concentration and to the chemical analysis of cloud water samples. Particle activation diameters below 100 nm were found inside clouds which possess LWC above 0.3 g m-3 and low drop concentration. The residual particles show a CCN mode above 100 nm and interstitial particles were mostly smaller than 100 nm. Sulfate was identified as the main CCN compound whereas nitrate, ammonium and organic matter were not observed in the CCNs above the AMS detection limit. Black carbon could be detected in the residual particles but were mainly found in the interstitial particles.