Variable ethanol concentrations and stable carbon isotopes reveal anthropogenic ethanol contributions to rainwater

The recent rapid increase in ethanol use as a biofuel and emission to the atmosphere may impact air quality and oxidizing capacity in the atmosphere. However, the global models show large uncertainties on sources of atmospheric ethanol. Concentrations and compound-specific stable carbon isotope ratios of ethanol (δ13Cethanol) in rainwater were measured in samples collected in six countries and 16 different locations over a two-year period (n = 270 for concentration; n = 111 for δ13Cethanol). Ethanol concentrations ranged from below the limit of detection (<28 nM) to 12 μM, while δ13Cethanol values ranged between −31 and −11‰. The higher the percent ethanol in automotive fuel, the higher the volume-weighted average ethanol concentration in rain, with the exception of Asian countries where there may be significant alternative sources. The wide range of δ13Cethanol values indicates that mixtures of sources contribute to atmospheric ethanol and the relative proportion of sources differs among rain events. Another explanation of variable δ13Cethanol values is partial oxidation of ethanol to acetaldehyde in the atmosphere. In Wilmington NC, USA, air mass back trajectory appeared to be the primary factor controlling ethanol concentrations and δ13Cethanol values with air masses originating over the Atlantic Ocean containing significantly 13C-enriched ethanol with lower concentrations compared to air masses from the continental interior (p < 0.03). A two-endmember isotope mixing model was applied for samples collected in Wilmington in order to quantify contributions from biofuels and natural biogenic emissions. The δ13Cethanol value of the biogenic endmember was determined to be −33‰ using measurements of ethanol isotopic composition in local plants combined with land coverage of C3 and C4 plants estimated by geographic information system with USDA Crop Data Layer, National Land Cover Dataset, and National Wetland Inventory. The relative contribution of biofuel ethanol in rainwater ranged from 11% to 97% at the Wilmington site, with a mean and standard deviation of 76 ± 17% (n = 97). The average concentrations of biofuel-sourced ethanol were similar among air mass back trajectories (p > 0.15), indicating that biofuel ethanol in rainwater is locally supplied during rain droplet formation. The results of this study are significant because they shed new insight into the role of anthropogenic emission of ethanol to the atmosphere. This study is particularly relevant as ethanol production and usage as a biofuel continues to increase globally.