African dust incursions in the Caribbean Region: dust concentrations, chemical composition, and fungal spore concentrations

Abstract

African dust can be transported over thousands of kilometers through the Atlantic Ocean reaching the Caribbean region, Central Am&eacute;rica, and parts of the southern USA mostly in the summer months. Dust emissions from North Africa vary on time and space scales. This project was motivated by the gaps in knowledge related to the long-range transport of African dust, their spatial and time variations in concentrations, chemical characterization, and impacts to climate and human health at the Caribbean region. The goal of this project was to determine dust concentration and chemical composition of African dust particles as well as the possible transport of fungal spores on African dust particles from North Africa to the Caribbean island of Puerto Rico. The specific objectives were to determine (1) the origin of air masses sampled and the presence of African dust incursions, (2) the concentration of African dust at a marine and an urban stations in Puerto Rico, (3) the chemical characterization (water-soluble ions) of aerosols during African dust incursions, and (4) the relative abundance of fungal spores during African dust incursions at a marine and a tropical montane cloud forest stations in Puerto Rico. The hypotheses were (1) high concentrations of mineral dust during African dust incursions in Puerto Rico can be responsible for exceedances in the 24-h PM<span style="font-size:10px">10</span> guidelines of the WHO, (2) there are differences in the aerosol chemical composition between background days and days with African dust incursions, and (3) fungal spores are present in African dust incursions. Aerosol samples were collected at a marine station (Cabezas de San Juan (CSJ)) and at an urban station (Facundo Bueso (FB)) in Puerto Rico, San Juan. Fungal spores were collected at the tropical montane cloud forest station (Pico del Este (PDE)) at El Yunque National Forest in R&iacute;o Grande, Puerto Rico, and at an urban station (Guillermo Arbona (GA)) in San Juan, Puerto Rico. Aerosol samples were collected using Stacked-filter Units (SFUs) and High-volume (Hi-Vol) samplers, while fungal spore samples were collected using a Burkard fungal spore trap. Dust concentrations were determined by combusting the filter at 500&deg;C and then weighting the ash residue that was assumed to be mineral dust. The presence of African dust on each aerosol and fungal spore sample was confirmed using dust forecast models, air mass back trajectories, and satellite images. With that information we were able to classify samples as background or African dust.&nbsp;<br /> Analysis of PM<span style="font-size:10px">10</span> data from Cabezas de San Juan in Fajardo during the 2008 to 2017 period evidenced an increase in PM<span style="font-size:10px">10</span> concentrations during the summer months, May to August, that coincided with the periods of African dust incursions at the Caribbean region. The first set of data reported in Puerto Rico for dust concentrations and their correlations to aerosols chemical composition and fungal spore concentrations at a marine (CSJ) and urban (FB) stations were presented. Average dust concentrations of 5 &plusmn; 2 &mu;g/m3 during background days were reported both at CSJ and FB stations; during African dust days average dust concentrations were 26 &plusmn; 19 &mu;g/m&sup3; at CSJ and at 18 &plusmn; 5 &mu;g/m&sup3; at FB. Significant difference between background and African dust days (p value of 5.8 x 10^-3 at CSJ and 4.3 x 10^-7 at FB) were observed. A significant dust event incursion was reported on May 22-25, 2017 where the high dust concentrations of 58.13 &mu;g/m&sup3; on May 22, and 61.13 &mu;g/m&sup3; on May 24 exceeded the PM<span style="font-size:10px">10</span> 24-h WHO guidelines of 50 &mu;g/m&sup3;. This showed how the presence of African dust incursions in Puerto Rico can affect the air quality of the island. Fungal spore concentrations were measured at PDE during the summer 2011 (June 11 to July 31, 2011), and a significant difference in the average fungal spore concentrations between background and African dust days was obtained with a p value of 2.9 x 10^-2 suggesting that African dust does not contribute to fungal spores at PDE. At GA station during the summer of 2017, the average fungal spore concentration during background days was 1.3 times higher than for African dust days. Basidiospores and ascospores were the most abundant fungal spores identified and counted at PDE and GA stations contributing up to 83 % of the total fungal spores. Ascospores were the most abundant spores at PDE while basidiospores dominated at GA. Results obtained suggested that fungal spores measured comes from local origin and that the amounts of fungal spores that could be transported on African dust is minimal. Fungal spores released depend on temperature and high relative humidity as well as precipitation. African dust incursions cause an increase in temperature and a decrease in relative humidity and precipitation causing a reduction in local fungal spores. Chemical analyses of aerosols samples confirmed the presence of mineral dust. An increase in average concentrations of Ca²⁺, Mg²⁺, and K⁺ during African dust days (0.55 &plusmn; 0.32 &mu;g/m&sup3;, 0.52 &plusmn; 0.13 &mu;g/m&sup3;, 0.17 &plusmn; 0.05 &mu;g/m&sup3;) were observed in comparison with background days (0.08 &plusmn; 0.06 &mu;g/m&sup3;, 0.40 &plusmn; 0.16 &mu;g/m&sup3;, 0.12 &plusmn; 0.05 &mu;g/m&sup3;). Nss-Ca²⁺ concentrations reported on background days were close to zero while an average concentration of nss-Ca²⁺ of 0.39 &plusmn; 0.29 &mu;g/m&sup3; was reported during African dust days representing 71% of the total [Ca⁺²] reported. The calcium concentrations reported during African dust days were attributed to the presence of African dust. The determination of dust concentrations using the Hi-Vol sampler was successfully performed for both the marine (CSJ) and urban (FB) stations. Our first hypothesis was confirmed, higher concentrations of dust (that exceed the PM<span style="font-size:10px">10</span> 24-h WHO guidelines) are evidence of the impact African dust has on air quality and the risk it could represent to public health in Puerto Rico. For that reason and due to changes in emissions patterns year to year, long-term sampling and measurements need to continue. The differences in the chemical composition between background and African dust days showed the impact African dust has on the aerosol composition confirming our second hypothesis. The results of the fungal spore concentrations at the tropical rain forest (PDE) and at the urban station (GA) during background and African dust days suggested that African dust plumes do not carry a significant amount of fungal spores to our region rejecting our third hypothesis. Studies on fungal spores in African dust incursions are needed to have a better understanding of African dust impact on climate processes in the Caribbean region.