Collection methods and measurement techniques

PM_2.5 samples for the year 2013 were collected using a Partisol-Plus Model 2025 Sequential Air Sampler (Rupprecht and Patashnick Co., Inc., Albany, NY) on PTFE 2.0-μm Teflon filters by the Puerto Rico Environmental Quality Board (PR-EQB), which maintains a PM monitoring stations network at various locations in the Island as part of their monitoring program (Fig. ​(Fig.1).1). The air sampler maintains a flow rate of 1 m³/h through a single filter. Dates for sample selection were classified into three different groups, each covering 4 months, starting from January to April (S1), May to August (S2), and September to December (S3) for Bayamon and Humacao. For the other stations (Guayanilla, Ponce and Guayama) two period were included, Jan-May and August to December. S2 was of major interest given that it is associated with the passing of African dust events which are seasonal and enriched particulate matter with both organic and inorganic compounds (Jimenez-Velez et al. 2009). The sampling sites for this study were the following urban sites: Ponce (n = 99 filters) and Bayamón (n = 44 filters); and the rural stations: Humacao (n = 43 filters), Guayama (n = 94 filters), and Guayanilla (n = 94 filters). These sites can be observed in Fig. ​Fig.11 and were selected based on location (urban or rural) and availability of the samples at the time. Filters from all the stations were collected throughout the year 2013 and stored at a temperature of 4° C. The PR-EQB was in charge of and kept custody of the samples before the commencement of this project. The storage following the collection of the samples was done following their established EPA guidelines. Samples were afterward refrigerated in our facilities for further analysis.