Environmental films are formed from atmospherically transported particulate and semi-volatile species that accrete on solid surfaces over weeks and months.To study the film morphology and properties that will allow assessment of environmental transformations, we analyzed collected sample films in rural and urban areas of Iowa on multiple substrates and characterized using bright field microscopy (BFM), computer controlled scanning electron microscopy (CCSEM) with energy dispersive x-ray spectroscopy (EDS), total dissolved solids analysis (TDS), and time of flight secondary ion mass spectrometry (ToF-SIMS). BFM analysis of the surface coverage and particulate per mm²show similar results on the two substrates. Most of the particulate have diameters between 10-30µm. CCSEM/EDS data shows the two substrates had similar particle types: dust, pollen/ biotic activity, and aggregates. Particles with diameters of 15µm are 15% biotic while 30µm are 40% biotic. The trend is opposite in dust particles with 45% of the 15µm particles versus 9% of the 30µm particles. We analyzed the water extract of the film (TDS). Surface chemistry can affect film mass by as much as 25%. ToF-SIMS shows spatial distributions of the water-soluble species like NH₄⁺ and pH active species like NO₃^- and SO₄^2-. Gold supported films have defined chemical domains, resembling particles which suggests reduced chemical mixing in these films vs silicon-supported films which show less definition in domains Both surfaces have similar surface area, but the adsorbed species on the gold surface are more isolated. This study shows that more hydrophilic surfaces may form homogeneous films while less hydrophilic surfaces, like metals, preserve the shape of deposited materials.

Uchechukwu Grace Akporere

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