Allan, J.D., et al., 2007. Clouds and aerosols in Puerto Rico — a new evaluation. Atmos.
Chem. Phys. Discuss. 7, 12573–12616.
The influence of aerosols, both natural and anthropogenic, remains a major area of
uncertainty when predicting the properties and behaviour of clouds and their influence
on climate. In an attempt to better understand warm cloud formation in a tropical
5 marine environment, a period of intensive measurements using some of the latest developments
in online instrumentation took place in December 2004 in Puerto Rico.
Simultaneous online measurements of aerosol size distributions, composition, hygroscopicity
and optical properties were made near the lighthouse of Cape San Juan in
the north-eastern corner of the island and at the top of East Peak mountain (1040m
10 a.s.l.), the two sites separated by 17 km. Additional measurements of the cloud droplet
residual and interstitial aerosol properties were made at the mountain site, accompanied
by measurements of cloud droplet size distributions, liquid water content and the
chemical composition of cloud and rain water samples.
Both aerosol composition and cloud properties were found to be sensitive to wind
15 sector. Air from the east-northeast (ENE) was mostly free of anthropogenic influences,
the submircron fraction being mainly composed of non-sea salt sulphate, while that
from the east-southeast (ESE) was found to be moderately influenced by populated
islands upwind, adding smaller (<100 nm), externally mixed, carbonaceous particles
to the aerosol that increased the number concentrations by over a factor of 3. This
20 change in composition was also accompanied with a reduction in the measured hygroscopicity
and fractional cloud activation potential of the aerosol. At the mountain
site, the average cloud droplet concentrations increased from 193 to 519 cm−3, median
volume diameter decreased from 20 to 14 μm and the liquid water content increased
from 0.24 to 0.31 gm−3 when the winds shifted from the ENE to ESE. Larger numbers
25 of interstitial particles were recorded, most notably at sizes greater than 100 nm, which
were absent during clean conditions. The average size of the residual particles and
concentrations of cloudwater nitrate, sulphate and insoluble material increased during