Indirect Effect of African Dust Particles on Cloud Microphysical and Chemical Properties in a Tropical Montane Cloud Forest in the Caribbean

Huge amounts of African dust travels thousands of kilometers from the Sahara and Sahel regions across the Atlantic Ocean to the Caribbean, northern South America and southern North America. However, not much is understood about how the aging process that takes place during transport changes dust properties, and how it affects cloud's composition and microphysics. In order to improve our understanding of the role of long-range transported African dust (LRTAD) in cloud formation processes we had field campaigns measuring dust physical and chemical properties in summers of 2013, 2014 and 2015 and fall 2015 as part of the Puerto Rico African Dust and Cloud Study (PRADACS) and of the Luquillo Critical Zone Observatory (LCZO). Measurements were performed at the tropical montane cloud forest (TMCF) of Pico del Este (PE, 1051 masl) and at the nature reserve of Cabezas de San Juan (CSJ, 60 masl). In both ground stations we monitored meteorological parameters (e.g., temperature, wind speed, wind direction). At CSJ, we measured light absorption and scattering coefficients at three wavelengths (467, 528 and 652 nm). At PE we collected cloud and rainwater for chemical analyses and monitored cloud microphysical properties (e.g., liquid water content, droplet size distribution, droplet number concentration, effective diameter and median volume diameter). Soluble ions, insoluble trace metals, pH, conductivity, total and dissolved organic carbon, and total and dissolved nitrogen were measured for cloud and rainwater. Microphysical and chemical data from summers of 2013 and 2014 suggest that dust may be acting as a cloud condensation nuclei and hindering cloud formation processes. In periods when we see a decrease in dust intensity we also see a decrease in droplet number concentration and an increase in droplet effective diameter, as well as a decrease in ion concentration. We investigate if there are any correlations with the origin of the air masses. Detailed results from all campaigns will be presented at the meeting.