Environmental films are particulate and semi-volatile species deposited on solid surfaces. The adsorbate-surface interactions dictate deposition. The accumulation changes active surface area increasing adsorption of semi volatile organic species, NO2, and SO2. Previous work showed surface hydrophilicity affects film chemical composition and physical form. It is not understood how this changes accumulation of films in the environment. To study this, we analyzed films collected in a rural area of Iowa on gold (less hydrophilic) and silicon (more hydrophilic) substrates. Films supported by both surface types are 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). If hydrophilicity makes the anticipated impact the large particulate to be the similar but the smaller components would be different. This is because surface interactions are not enough to overcome the force of gravity favoring large particulate deposition. BFM analysis of the surface coverage and particulate per mm2 show 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. The aggregate fraction slightly increases from 42% at 15µm to 50% at 30µm. We analyzed the water extract of the film (TDS). The gold substrate film dissolved 25% more mass causing a lower pH. ToF-SIMS showed the distributions of the water-soluble species like NH4+ and pH active species like NO3- and SO42-. Gold supported films have defined chemical domains, resembling as-deposited particles. This could be explained by less chemical mixing as evidenced by TDS and pH. The silicon-supported films show more overlap. Both surfaces have similar active surface area but the adsorbed species on the gold surface are more isolated. This study shows that more hydrophilic surfaces, like windows, form homogeneous films while less hydrophilic surfaces, like metals, preserve the deposited materials. Different building materials create surfaces with unique contributions to local atmospheric chemistry.

Chemical differences of passively collected environmental films on surfaces with different hydrophilicity

ACS Spring 2022

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