References

ACPD

Atmospheric Chemistry and Physics Discussions

ACPD

1680-7375

Copernicus GmbH

Göttingen, Germany

10.5194/acpd-7-12573-2007

Clouds and aerosols in Puerto Rico – a new evaluation

Allan

J. D.

¹ ² Baumgardner

³ Raga

G. B.

³ Mayol-Bracero

O. L.

⁴ Morales-García

⁴ García-García

³ Montero-Martínez

³ Borrmann

⁵ ⁶ Schneider

⁶ Mertes

⁸ Walter

⁶ Gysel

¹ ¹¹ Dusek

⁷ ¹² Frank

G. P.

⁷ ⁹ Krämer

¹⁰

School of Earth, Atmospheric & Environmental Science, University of Manchester, Manchester, UK

Composition Directorate, National Centre for Atmospheric Science, Leeds, UK

Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico

Institute for Tropical Ecosystem Studies, University of Puerto Rico, San Juan, Puerto Rico

Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany

Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany

Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany

Leibniz-Institute for Tropospheric Research, Leipzig, Germany

Department of Physics, Lund University, Lund, Sweden

Institute for Chemistry and Dynamics of the Geosphere 1: Stratosphere, Forschungszentrum Jülich, Jülich, Germany

now at: Paul Scherrer Institut, Villigen, Switzerland

now at: Utrecht University, Utrecht, The Netherlands

24 08 2007

7 4 12573 12616

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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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 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 (1040 m 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 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 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 g m−3 when the winds shifted from the ENE to ESE. Larger numbers 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 polluted conditions. Previous studies in Puerto Rico had reported the presence of a significant non-anthropogenic organic fraction in the aerosols measured and concluded that this was a factor controlling the in situ cloud properties. However, this was not observed in our case. In contrast to the 1.00±0.14 μg m−3 of organic carbon measured in 1992 and 1995, the organic matter measured in the current study of 0.17±0.35 μg m−3 is many times lower, most of which can be attributed to anthropogenic sources. During clean conditions, the submicron aerosol was observed to be almost entirely inorganic, an observation supported by the hygroscopicity measurements. This suggests that organic aerosols from marine sources may not be completely ubiquitous (either spatially or temporally) in this environment and requires further investigation to quantify their true extent and implications, with more extensive, longer-term sampling in conjunction with back trajectory analyses.

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