Host surface orientation impacts environmental film accumulations.

Two environmental films were passively collected in different orientations (vertical or horizontal) at the same location over two months. We characterized these films using bright field microscopy, total dissolved species analysis, pH analysis, vibrational interfacial spectroscopy, and contact angle goniometry. Results show that horizontal films have significantly higher surface coverage than the vertical samples (+50%). The vertical and horizontal films also show different particle morphologies but the particle size distributions are not statistically different. Vertical surfaces have smaller, less compact particulate suggesting particle adsorption depends on the surface area in contact with the parent substrate. Horizontal surfaces also generate more total dissolved solid material per unit area when washed with water (+61%). The dissolved solids from the vertical substrate are more acidic per unit mass, suggesting increased pH active species like nitrate, sulfate, or organic acids. Vibrational spectroscopy provides evidence of nitrates and sulfates in both films, but spectroscopic profiles show these ions are present in different forms. Contact angle goniometry measurements show horizontal films are more hydrophilic than vertical films, despite being deposited on the same substrate material. We also report significantly different hydrogen bonding environments for condensed water between the two films. Our results suggest that environmental films deposited on vertical vs horizontal surfaces will have significantly different characteristics, informing models for deposition and impacts to human and environmental health.