The Effect of Water Surface Simulated Rain Drop Impacts on Water to Air Chemical Transfers of Perfluorinated Carboxylic Acids (PFCAs)

Perfluorinated carboxylic acids (PFCAs) are anthropogenic environmentally ubiquitous surfactants that tend to concentrate on water surfaces. This investigation looked at the effect of simulated rain on the atmospheric concentration of a suite of PFCAs (C2 -- C12) above the bulk water system. Increased air concentrations of all PFCAs were detected during simulated rain events. Long chain PFCAs (≥C8) were found to be much more concentrated in the air above the bulk water system than their short chain counter parts (≤C7). Therefore we conclude that simulated rain events generated aqueous aerosols enriched in PFCAs. Changing needle gauges in the rain chamber modified drop sizes in the experiment. Drops were observed to be 5.7 mm 6.6 mm and 7.3 mm in diameter. The results of the drop size experiment showed no significant variation (p<0.05) in air concentration when drop sizes were modified. Further investigation using high-speed photography attempted to visualize the simulated raindrop to determine if the ejected aerosols originated from the drop itself or the body of water the drop was impacting. Lighting issues made it impossible to determine the origin of the ejected aqueous aerosols. Drop sizes calculated from the high speed photography for 18 gauge (G) and 22 G (4.92 mm and 3.19 mm in diameter) were not in agreement with calculated drop sizes (7.3 mm and 5.7 mm) most likely due to the fact calculations from the photographs were based on perfectly spherical drops. The smallest observed drops ejected during the photography experiments were 0.5 mm in diameter and reached maximum heights of 0.05 m. We conclude that the magnification was not sufficient to observe the small (0.01 mm) drops with greater ejection heights and residence times in the air that were expected from previous measurements. In summary results suggest that simulated rain drop impacts on water surfaces cause an upward flux of PFCAs from water to air.