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Rainwater ethanol concentrations were measured for one year (June 2013-May 2014) in central (Elon, NC) and coastal (Wilmington, NC) North Carolina, allowing for a comparison of the effects of coastal and marine rain on ethanol concentration and deposition both at the coast and 250 km inland. Rain samples were collected on an event basis and analyzed using enzyme oxidation and headspace solid-phase microextraction (HS-SPME). The volume-weighted average ethanol concentration at Elon (609 ± 116 nM) was higher than at Wilmington (208 ± 21 nM). Rainfall influenced by air masses originating over the Atlantic Ocean has previously been observed to be lower in ethanol concentration than terrestrial rain at the Wilmington location, and this was true during this study as well. Lower-ethanol marine and coastal air masses did not affect the concentration of ethanol in Elon rain, 250 km from the coast. This is likely due to the rapid supply of locally emitted ethanol to air masses moving over the land. No difference in rainwater ethanol concentrations was observed for Elon rain based on air mass back trajectories, most likely because all the rain was impacted by both anthropogenic and biogenic terrestrial sources typical of most inland areas. Seasonal variation in ethanol concentrations was significant in the inland location with elevated ethanol concentrations observed in fall; no seasonal variation was observed in coastal location rain. This study presents for the first time the different drivers for ethanol concentrations in rainwater from a coastal and a proximal inland location.

The Henry's law constant for ethanol in seawater was experimentally determined to be 221 ± 4 M/atm at 22 °C compared with 247 ± 6 M/atm in pure water. The salting out coefficient for ethanol was 0.13 M−1. In seawater ln(KH) = -(12.8 ± 0.7) + (5310 ± 197)/T where KH is in M atm−1 and temperature is in K. This plus the salting out coefficient allow calculation of KH for any estuarine or sea water between 1 and 35 °C. High concentrations of dissolved organic carbon do not affect KH values in fresh or seawater. Nearshore surface waters were usually undersaturated with respect to gas phase ethanol except when air concentrations decreased, whereas surface seawater 40 km from shore was supersaturated. The percent saturation in surface waters is driven primarily by changes in air concentrations because these change quickly (hours) and more extensively than surface water. This study allows calculation of ethanol saturation states from air and surface water concentrations which is a necessary step to define the role of surface oceans in the global biogeochemical cycling of ethanol both now and in the future as use of ethanol biofuel continues to grow.