Impact of Long-Range Transported African Dust on Cloud Water Chemistry at a Tropical Montane Cloud Forest in Northeastern Puerto Rico

ABSTRACT

Bulk and drop size fractionated cloud water was collected at a Caribbean tropical montane cloud forest (TMCF) in northeastern Puerto Rico in the summer months of 2010–2012 and winter months of 2011 as part of the Puerto Rico African Dust and Cloud Study (PRADACS). We studied how cloud water chemistry in a Caribbean TMCF was affected by long-range transported African dust (LRTAD). Using HYSPLIT trajectories and enrichment factor analysis, the air masses influencing clouds at Pico del Este were identified as dust, marine, and dust with anthropogenic influence. Samples were analyzed for pH, conductivity, water-soluble ions, and metals. Na⁺ and Cl^– comprised the main water-soluble ions (60–80%) suggesting a strong marine influence. A 0.1–10% contribution of anthropogenic (nss-SO₄^2–) and 0.2–13% contribution of mineral dust (nss-Ca²⁺) sources to the cloud chemical composition was observed. Primary aerosols (i.e., mineral dust and sea-salt) were enriched in large cloud water droplets (LCWD) and secondary aerosols (i.e., anthropogenic particles) were enriched in small cloud water droplets (SCWD). As a result, pH was found to be higher in LCWD due to the neutralizing capacity of nss-Ca²⁺ and lower in SCWD due to the presence of acidifying species (nss-SO₄^2–, NO₃^–, and organic acids). Fe and Al, indicators for mineral dust were similarly distributed across the cloud droplet size spectrum. Our results show that LRTAD events modulate bulk and size-fractionated cloud water chemistry, potentially influencing cloud microphysical properties and processes.