Rainwater concentrations

The concentration of ethanol in rainwater was measured in event rain samples collected on the University of North Carolina Wilmington campus during the summer of 2010 and continuously since January of 2011. The annual volume-weighted average concentrations increased slightly from 2010 through 2013, then more extensively from 2013 through 2015, and then dramatically in 2016 (Fig. 1). The increase from 2010 to 2017 is approximately fourfold. The increasing trend (Mann−Kendall Trend Analysis) is significant (Fig. 1, p = 0.043 for eight volume-weighted averages (VWA), p < 0.0001 for raw data, n = 424 rain samples). This trend is not dependent on the high 2016 concentration because when this point is excluded, the Mann−Kendall trend for n = 7 is significant at p = 0.004, and the correlation coefficient increased from R = 0.8678 (p < 0.01) to R = 0.9291 with p < 0.001. The 2016 point is not an outlier (Grubbs test, p = 0.4); therefore, it has been included in all analyses. This increase cannot be explained by changes in temperature because the annual average temperature (18.4 ± 0.8 °C) in Wilmington did not vary in a consistent pattern over this time period. Annual rainfall amounts in Wilmington were greater in 2015 and 2016 due to excessive rain from tropical systems in the summer and autumn, and excessive rain from the 2015−2016 El Niño winter events (https://www.weather.gov/ilm/, 2017). Extensive flooding occurred in Wilmington and surrounding areas in September and October of 2016 due to the historic amounts of tropical rain received (https://www.weather.gov/ilm/, 2017). Four different tropical storms plus Hurricane Matthew delivered significant tropical rain in September and October of 2016, with subsequent flooding. Excessive dilution by the large volume of rain received (11.2 cm from Hurricane Matthew) was not observed during Hurricane Matthew because this rain had a relatively high concentration of 937 nM. Flooding during 2016 likely contributed to the high concentration observed because episodically submerged plants emit large quantities of ethanol to air.17,18,19 The highest gas phase ethanol concentration observed in the current study was collected above submerged vegetation during flooding (>15 ppbv vs. background concentration of ethanol on a nearby undeveloped island (Masonboro Island) with marine air mass back trajectory of 0.12 ± 0.03 ppbv11).

Fig. 1 Volume-weighted annual average rainwater concentration and standard deviations for ethanol in nM vs. year for rainwater collected in Wilmington, NC, USA. The year 2010 contains summer data only (n = 11). The Mann−Kendall Trend Analysis for these data is significant at p = 0.043; the regression line shown has R = 0.8678 and p < 0.01 for n = 8 Full size image

Much of the increase observed in Fig. 1 may be related to increased use of biofuel ethanol as indicated by the number of km driven by vehicles per year in North Carolina, USA where the field site is located20 (Fig. 2), because of evaporation4 and also the release of uncombusted ethanol in vehicle exhaust.5 The amount of ethanol in gasoline in the USA has also increased from 8% in 2009 to 10% in 2011 and beyond (https://www.eia.gov/, 2017). The Wilmington NC population (117,500 in 2016) increased by 10% from 2010 to 2016 which would also contribute to higher atmospheric ethanol concentrations in this region. However, these small changes cannot explain the magnitude or timing of the increase observed. Furthermore, compound-specific-stable carbon isotope analysis revealed that on average 59% of ethanol in rainwater (n = 4, [ethanol] > 1 µM) collected in Wilmington, NC, USA during 2016−2017 was sourced from biofuel ethanol,21 which suggests that anthropogenic activities are dominating the biogeochemical cycling of ethanol in the southeastern USA. Even though North Carolina is not a large ethanol-producing or -consuming state, the 3-day atmospheric residence time for ethanol12,22 allows for transport of ethanol from thousands of km away (Fig. 3) so the North Carolina atmosphere is affected by distant activities.

Fig. 2 Volume-weighted annual average rainwater concentration of ethanol in nM with standard deviations for rainwater collected in Wilmington, NC, USA between 2010 and 2017 vs. billions of km driven by vehicles in North Carolina, USA, per year over the same time frame. For the regression line, R = 0.9517, n = 8 and p < 0.001 Full size image

Fig. 3 Air mass back trajectory analyses (https://ready.arl.noaa.gov/hypub-bin/trajtype.pl?runtype=archive, 2017) at 500 m for the 72 h prior to the diurnal experiments shown in Table 1 Full size image

Atmospheric concentrations

Two extensive gas phase experiments were conducted to complement rainwater measurements in the current study. Previous research at this location revealed high variations in gas phase ethanol concentrations over time periods of hours as a function of time of day;23 therefore, measurements must be taken over at least a diurnal cycle to compare changes in concentrations over time. Two gas phase studies were conducted 5 years apart, both on the UNCW campus, one on July 5, 2011 and one on July 25, 2016. The air mass back trajectories for these two dates were very similar so air mass back trajectory was not a factor in the observed difference between these two experiments (Fig. 3). The results of these two gas phase studies, each of which included multiple readings over 24 h, suggest gas phase ethanol has increased over the last 5 years, perhaps by as much as fivefold (Table 1), similar to the increase observed in rainwater when comparing 2011 with 2016 concentrations (3.3 fold, t test, p < 0.0001). This increase in atmospheric ethanol occurred over a similar time period (2011−2014, the most recent year for which data are available) when highway vehicle emissions of VOCs in North Carolina decreased by 25%, indicating that ethanol is becoming a larger proportion of total VOCs in this area.24 De Gouw et al.14 found a much greater increase (30×) in atmospheric ethanol when comparing atmospheric ethanol concentration in the NE US in 2002 and 2004 with that in Los Angeles in 2010 which they also attribute to greater ethanol biofuel use; the magnitude of the increase observed however probably reflects in part the two different locations used in the study as well as the temporal variability.

Table 1 Average measured concentrations and standard deviations of ethanol in air over 24 h for the dates shown compared with concentrations calculated from rainwater event sample concentrations using Henry’s Law during the month of July in each year in ppbv Full size table

Air−rainwater interaction

The partitioning of ethanol between the gas and aqueous phases can be described using Henry’s Law for a system at equilibrium as follows:

$$\left[ {{\mathrm{EtOH}}\left( {{\mathrm{aq}}} \right)} \right]{{ \,= K}}_{\mathrm{H}}{{P}}_{{\mathrm{EtOH}}},$$ (1)

where brackets indicate aqueous concentration of ethanol in M, K H is the Henry’s Law constant for ethanol at the relevant temperature in M atm−1, and P EtOH is the partial pressure of ethanol (atm) in the gas phase in contact with the aqueous phase. In this application, the aqueous phase is rainwater, and the gas phase is air. Using the measured ethanol concentration in rainwater, and the known Henry’s Law constant for the temperature of the rain event,11,25 the equilibrium atmospheric partial pressure of ethanol can be calculated.

The first diurnal experiment was conducted on July 5, 2011, with 12 air samples collected over 24 h. Even though there was substantial diurnal variation driven by photochemical production of ethanol,23 the 2011 gas phase concentrations were much lower than the concentrations observed during a similar diurnal study on July 25, 2016 with 22 samples collected over 24 h (t test, p < 0.001). Rainwater concentrations from individual events during the month of July in each of the two study years were used along with the appropriate Henry’s Law Constant for the ambient temperature to calculate air concentrations in equilibrium with these rainwater concentrations. The average calculated air concentrations were the same as the measured air concentrations in each year (Table 1, t test, p > 0.1), which indicates rainwater is in equilibrium with gas phase ethanol at the collection location at the times of these experiments. The remarkable agreement between the predicted and measured gas phase concentrations are the first field data indicating that rainwater ethanol concentrations are in equilibrium with the gas phase at the time and location where the rain falls, which was assumed by earlier atmospheric models of ethanol cycling.12,13 Therefore, atmospheric and rainwater ethanol concentrations are coupled in accordance with Henry’s Law, and hence both are likely to increase in the near future.