Weekly, composite wet deposition samples were collected from National Atmospheric Deposition Program (NADP) National Trends Network Sites in the Denver-Boulder metropolitan area,  Front Range foothills near Boulder, and Rocky Mountain National Park (ROMO), Colorado in 2017 and 2018.  Samples were filtered (0.45 micron [mm] polyethersulfone and glass fiber) to obtain solid-phase components assumed to be washed from the atmosphere.  A wide variety of different types and sizes of washout particles were observed under magnification on these filters, including soil, minerals, fragments of plants and insects, pollen, microplastics, various forms of carbonaceous materials, and others. Some of the wet-deposited particles have physical appearances and characteristics of tiny fragments of tires, which have been described in previous studies as black, elongated, cylindrical or crescent shaped particles with rough surface textures resembling aggregated pieces of different materials.

Suspected tire particles were analyzed with a FEI Quanta 450 field emission gun Scanning Electron Microscope (SEM) equipped with an Oxford Instruments X-Max 50 Energy Dispersive Spectrometer (EDS).  Analysis conditions were varied to optimize imaging and qualitative chemical analysis. EDS spectra obtained from these particles, which are comprised mostly of carbon, indicated the presence of metallic components found in tire rubber, such as zinc, copper, and iron.  Morphology and EDS spectra of particles obtained from actual worn tire rubber were similar to the atmospherically deposited particles.  Equipment rinse blanks were collected and were free of the suspected tire particles.  More work with these particles is needed to confirm that the wet-deposited particles are indeed tire fragments, such as Fourier-transform infrared spectroscopy (FTIR) analysis of the rubber components of the particles. 

While it is not surprising to find the suspected tire particles in atmospheric washout obtained from urban NADP sites, the presence of these particles at the remote, high-elevation, Loch Vale site (NADP ID CO98) in ROMO is alarming and poses new questions about particulate transport.  The CO98 site is located in wilderness at approximately 3,159 m above MSL.  This suggests that tire particles may be a pervasive contaminant from atmospheric transport as the closest road to CO98 is 3 km to the northeast, and the closest town is 14 km away on the opposite side of the Continental Divide.