Mercury (Hg) is a persistent environmental contaminant and can accumulate and concentrate in food webs as methylmercury (MeHg), presenting a health risk to humans and wildlife. Multiyear monitoring and modeling studies have shown that atmospheric Hg in litterfall is an important form of Hg deposition to forests. Litterfall consists primarily of leaves with some amounts of twigs, bark, flowers, seeds, fruits, and nuts. Atmospheric Hg accumulates in the forest canopy material and transfers to the forest floor in litterfall. This data set is derived from monthly litterfall collected at the National Atmospheric Deposition Program (NADP) Mercury Deposition Network (MDN) site at El Verde, Puerto Rico, identification number PR20, during 2014-2016. The U.S. Geological Survey (USGS) provided passive litterfall collectors to the PR20 MDN site operator who systematically deployed them in a forest plot near the MDN equipment. Each collector was a 0.25 square meter box made of plastic lattice with a 20-micrometer mesh, polyester, netting liner. The operator retrieved the litterfall in the collectors on a monthly schedule and shipped the subsamples to the USGS Mercury Research Laboratory where concentrations of Hg and MeHg and litterfall dry mass and sample moisture were determined using published methods. More information about the litterfall Hg monitoring field and laboratory methods and quality assurance is available from the NADP at http://nadp.sws.uiuc.edu/newissues/litterfall/
