Variability of ethanol and acetaldehyde concentrations in rainwater

Ethanol and acetaldehyde concentrations were measured in 52 rain events collected between January 25, 2011 and March 4, 2012 in Wilmington, North Carolina, USA. Ethanol concentrations ranged from 23 nM to 908 nM with a volume weighted average concentration of 192 ± 20 nM while acetaldehyde ranged from 23 nM to 909 nM with a volume weighted average concentration of 193 ± 25 nM. There was a great deal of variability in the abundance of ethanol and acetaldehyde between rain events driven primarily by temporal and air mass back trajectory influences. The ratio of ethanol to acetaldehyde was at a minimum during periods of peak solar intensity underscoring the importance of alcohol oxidation by a photochemically generated oxidant such as hydroxyl radical in the gas and/or aqueous phase. Ethanol and acetaldehyde concentrations were not strongly correlated with rain amount suggesting that gas-phase concentrations were not significantly depleted during the storm or that they were resupplied during the course of the rain event. The concentration of ethanol and acetaldehyde were correlated with nitrate and non-sea salt sulfate suggesting the importance of terrestrial and anthropogenic inputs at this location. Comparison of future ethanol and acetaldehyde concentrations in rainwater to the data presented in this study will help delineate potential consequences of these labile oxygenated volatile organic compounds (OVOCs) on the chemistry of the troposphere as the United States transitions to more ethanol blended fuels. Aqueous phase impacts of increasing ethanol concentrations will be particularly significant to the oxidizing capacity of atmospheric waters because of its reactivity with OH and HO2 radicals in solution. Increased rainwater concentrations could also have significant ramifications on receiving watersheds because of the biogeochemical lability of the alcohol.