Abstract
Abstract. African aerosol particles, traveling thousands of kilometers before reaching the Americas and the Caribbean, directly scatter and absorb solar radiation and indirectly impact climate by serving as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs) that form clouds. These particles can also affect the water budget by altering precipitation patterns that subsequently affect ecosystems. As part of the NSF-funded Luquillo Critical Zone Observatory, field campaigns were conducted during the summers of 2013 (23 d), 2014 (11 d), and 2015 (92 d) at Pico del Este, a site in a tropical montane cloud forest on the Caribbean Island of Puerto Rico. Cloud microphysical properties, which included liquid water content, droplet number concentration, and droplet size, were measured. Using products from models and satellites, as well as in situ measurements of aerosol optical properties, periods of high- and low-dust influence were identified. The results from this study suggest that meteorology and air mass history have a more important effect on cloud processes than aerosols transported from Africa. In contrast, air masses that arrived after passing over the inhabited islands to the southeast led to clouds with much higher droplet concentrations, presumably due to aerosols formed from anthropogenic emissions.
Highlights
Great strides have been made in our understanding of aerosol–cloud–radiation interactions; substantial uncertainty remains (Mhyre et al, 2013; Li et al, 2019), in our understanding of the role of mineral dust during such interactions
There are no regular trends in the optical properties that can be linked to daily variations in local emissions, indicating that the observed fluctuations are related to largerscale changes in the air masses
The droplets continue to grow, as evidenced by the increasing median volume diameter (MVD) throughout the residency of this air at Pico del Este (PDE). These conclusions do not support those by Spiegel et al (2014), who concluded that microphysical properties of clouds at PDE were significantly altered by African dust
Summary
Great strides have been made in our understanding of aerosol–cloud–radiation interactions; substantial uncertainty remains (Mhyre et al, 2013; Li et al, 2019), in our understanding of the role of mineral dust during such interactions. Other studies have found that dust can commonly act as a cloud condensation nucleus even from the source, as it often has hygroscopic material, and can influence cloud formation (Twohy et al, 2009) In contrast to these findings, Denjean et al (2015) saw that African dust transported from Africa to the Caribbean Basin remained mainly externally mixed and that dust particles did not take up significant amounts of water when exposed to up to 94 % relative humidity, a similar finding to that of Edwards et al (2021) from African dust transported to Florida. The larger, super-micron dust particles act as giant CCN that can form large cloud droplets that lead to the earlier formation of raindrops (Rosenfeld et al, 2008)
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