First oceanic measurements of ethanol and propanol



[1] The role of the ocean in the cycling of ethanol and propanol remains unanswered due to a total lack of data on their concentrations in seawater. We report the first measurements of these low molecular mass alcohols made during two field experiments in the Atlantic Ocean. Ethanol concentrations ranged from 2–33 nM. Our analytical technique allowed the separation of the two propanol isomers and concentrations of 1- and 2-propanol varied between 2–22 nM and 1–19 nM respectively. Global extrapolation of the sea - air fluxes indicate that the ocean may be an important source for these compounds to the atmosphere and suggest that the magnitude of the fluxes may be enhanced in natural upwelling regions.

1. Introduction

[2] The measurement of ethanol and propanol, two Oxygenated Volatile Organic Compounds (OVOCs) in sea water has so far been restricted due to a lack of suitable analytical methodology, their presence at trace (nanomolar, nM) concentrations and also their high solubility. Consequentially, the role of the ocean in acting as a source and/or sink for ethanol and propanol is unanswered. These organic species are important in atmospheric chemistry. They are able to alter the oxidative capacity of the troposphere by influencing levels of both ozone and hydroxyl radicals [Atkinson and Arey, 2003; Jaeglé et al., 2001; Singh et al., 2000]. We present the first data on ethanol and propanol concentrations in sea water. We also assess whether 1- or 2-propanol is dominant by the chromatographic separation of the two isomers. Because we did not make atmospheric measurements, published atmospheric data are used to estimate the direction and magnitude of the flux of these compounds between the ocean and the atmosphere. The implications of the air-sea flux on global budgets of OVOCs is also discussed.

[3] Samples were collected and analysed during two separate field experiments (refer to Figure 1). The first was the Surface Ocean - Lower Atmosphere Study - The Impact of Coastal upwelling ON the air-sea exchange of climatically important gases (SOLAS - ICON) cruise in the Mauritanian Upwelling off North Africa during May–June 2009. Measurements were also made on Atlantic Meridional Transect (AMT) 19, from UK to Chile between October and December 2009.

Figure 1.

Sampling locations on SOLAS-ICON and AMT19 (plus cruise track). Inset shows ICON position with proximity to coast and shelf contour.

2. Methods

[4] All samples were collected from CTD (Conductivity, Temperature, Depth) casts using stainless steel sprung 20 litre Niskin bottles and transferred immediately into brown glass bottles (volume 330 mL) via Tygon™ tubing. Samples for analysis of OVOCs were always collected first from the Niskins in order to minimise contamination by air. Samples were run within 30 minutes of sampling and were not filtered as we found filtering resulted in OVOC contamination.

[5] Details of the method used to extract and quantify ethanol and propanol in sea water are discussed in full elsewhere (R. Beale et al., manuscript in preparation). Briefly, ethanol and propanol are purged from the seawater sample by sparging with a flow of pre-cleaned nitrogen gas. The humid gas flow is subsequently trapped on a solid sorbent material (Unicarb™ contained within Markes Tubes) at reduced temperature (−15°C) and the moisture removed by a period of dry purging with nitrogen. Desorption to a gas chromatograph is achieved by heating to 200°C for a 30 second period and by flushing the trap with nitrogen gas. OVOCs are separated on a Porabond U capillary column (Varian®) and detected by a Flame Ionisation Detector (FID) (method sections are similar to those described by Hopkins et al. [2003] and Williams et al. [2004]). Sequential sparging has shown 80% purge efficiency for the carbonyl compound acetone, and Henrys Law solubility values (R. Sander, available at∼sander/res/henry.html, 1999) predict that for ethanol and propanol the efficiency will be considerably less. However, water standards were analysed by the same technique, enabling quantitative calibration. Standards were prepared by solvent addition to seawater with subsequent serial dilution and were corrected for both nitrogen background and original seawater concentration (pre-spiked). This provided a peak area response created from the standard addition alone. Limits of Detection (LOD) were 2, 1 and 1 nM for ethanol, 1-propanol and 2-propanol respectively. Precision of the method is estimated at 30% for ethanol and within 10% for both propanol isomers.

3. Results

[6] The majority of the sampling during AMT19 took place in the Northern Atlantic gyre (33°N to 10°N), at either pre-dawn (0400) or solar noon (1300). Only two surface samples were >LOD for ethanol at 15 and 34 nM (Table 1), both were from solar noon casts. Surface water concentrations for 1-propanol averaged 3 nM with a range of <LOD to 10 nM. Levels of 2-propanol were significantly lower than those observed on ICON (discussed below) with only one sample above the LOD, at a value of 1.3 nM.

Table 1. Summary of Sea State Saturations and Fluxes Measured on ICON and AMT19 for Ethanol, 1-Propanol and 2-Propanola
OVOCCruisebTime of CastWater (Cw)Air (Ca) (ppbV)Saturationc (%)Fd (μmol m−2 day-1)
  • a

    All measurements recorded are from surface samples (<5 m).

  • b

    Latitiude/longitude coordinates in parentheses.

  • c

    Refer to equation (1). Where 100% = equilibrium and >100% implies super-saturation (oceanic source).

  • d

    Where F is the calculated flux rate, refer to equations (2)(4). Negative result indicates gas transfer from ocean to atmosphere.

 13:00Not detectedNot detected0.0100.2
 16:00Not detectedNot detected0.0100.2
(33.3°N/31.2°W)Pre-dawnNot detectedNot detected0.0100.2
(32.1°N/31.6°W)Solar NoonNot detectedNot detected0.0100.2
(30.4°N/34.3°W)Solar Noon374150.011017−0.9
(29.1°N/35.6°W)Pre-dawnNot detectedNot detected0.0100.1
(10.3°N/32.0°W)Solar Noon831340.012787−4.2
(17.2°S/24.6°W)Solar NoonNot detectedNot detected0.0100.1
 16:00Not detectedNot detected0.0100.2
(32.1°N/31.6°W)Solar NoonNot detectedNot detected0.0110.2
(30.4°N/34.3°W)Solar Noon6230.01225−0.1
(29.1°N/35.6°W)Pre-dawnNot detectedNot detected0.0100.1
(10.3°N/32.0°W)Solar Noon248100.011117−1.4
(17.2°S/24.6°W)Solar Noon4220.01144−0.1
(32.1°N/31.6°W)Solar NoonNot detectedNot detected0.0100.2
(30.4°N/34.3°W)Solar Noon2010.01730.02
(10.3°N/32.0°W)Solar NoonNot detectedNot detected0.0100.1
(17.2°S/24.6°W)Solar NoonNot detectedNot detected0.0100.1

[7] The average concentration of ethanol sampled in surface water during ICON was 8 nM with a range from <LOD to 33 nM. The mean concentration of 1-propanol was 5 nM with a range of <LOD to 22 nM. Surface concentrations for 2-propanol averaged 7 nM and all samples for this compound were above the LOD and ranged between 2 and 19 nM.

4. Discussion


[8] Those casts where ethanol was undetectable in surface water on ICON were always sampled in the afternoon (1300 or 1600 hrs) but the highest concentrations of ethanol were observed in darkness at predawn (0300 hr) casts (Figure 2a). The largest average concentration for 1-propanol was also observed in pre-dawn samples (11 nM) (Figure 2b). Finally, levels of 2-propanol also suggest the presence of a diel cycle (Figure 2c), similar to that of the other two alcohols.

Figure 2.

Trend in surface OVOC concentration with time, ICON 09: (a) ethanol, (b) 1-propanol and (c) 2-propanol (error bars representing standard error are shown where multiple data points were collected).

[9] The location of the SOLAS ICON experiment was in a highly productive region where the action of wind parallel to the West African coast causes cold, deeper, nutrient rich water to be upwelled to the surface where these nutrients are used by phytoplankton and bacteria [Arísteguil et al., 2003]. The experiment was based around the use of sulphur hexafluoride (SF6), as a Lagrangian tracer [Jickells et al., 2008]. The on-board analytical detection of SF6 ensured that sampling was carried out from close to the centre of each labelled patch until the response from the tracer became too low to detect reliably. In total, three patches were marked with SF6. Patch 1 was very high in primary productivity with observed values between 1260–8280 (average of 4580) mgC m−2 d−1 compared to 1350–3790 (average of 2250) mgC m−2 d−1 for Patch 3. Patch 2 sub-ducted and hence no samples were taken from this region.

[10] Mean ethanol surface concentrations sampled from the 09:00 casts were lower in Patch 1 at 4 nM compared to 12 nM in Patch 3. This corresponds to higher ethanol surface measurements in an area of relatively lower primary production suggesting that marine organisms may be removing ethanol from seawater. Comparing the two tracer labelled patches of upwelled water analysed during the experiment (09:00 casts), results for 1-propanol were higher in Patch 1 (average of 6 nM), a region of higher primary production. Although still high in production, Patch 3 was lower than Patch 1 and the 1-propanol levels observed dropped to below the detection limit (<1 nM).

[11] Levels of 2-propanol were similar to those detailed for 1-propanol with an average measurement of 6 nM in Patch 1. However, while 1-propanol values dropped in Patch 3, 2-propanol surface levels remained stable with an average concentration of 6 nM observed for samples from the 09:00 cast.

4.2. AMT19

[12] In contrast to the biologically rich ICON environment, primary production varied considerably on AMT19, with values as high as 600 mgC m−2 d−1 in the equatorial Atlantic and as low as 110 mgC m−2 d−1 through the oligotrophic Northern Gyre.

[13] Ethanol was detected only twice in the Atlantic surface water suggesting lower production in these low-nutrient, oligotrophic environments. The differences in ethanol concentration observed between research campaigns is discussed further in section 4.3. Concentrations of 1-propanol were also lower and ranged from <LOD to 4 nM. An increased level of 10 nM was recorded at a latitude of 10°N correlating with the start of more temperate waters found near the equatorial upwelling. This increased value is within the 1-propanol data range recorded on ICON in the upwelling region. Interestingly, 2-propanol values did not show this same increase. This may suggest different production or consumption rates to those encountered on ICON. In contrast to ICON, only trace amounts of 2-propanol were detected during AMT19, with all measurements <2 nM. As low productivity dominated the samples analysed this suggests a possible link between increased 2-propanol concentrations and very high productivity water, such as that sampled on ICON.

4.3. Sea State Saturation

[14] Because we did not measure air concentrations of OVOCs on the cruises, we have to resort to using literature values to estimate air-sea fluxes from our seawater concentration data. Atmospheric ethanol has been measured in remote locations over the Atlantic and Pacific regions but rarely detected and always less than 50 ppt [Singh et al., 2000, 2001]. Measurements of 2-propanol at Trinidad Head, California have been reported with a median value of 17 ppt [Millet et al., 2004]. Concentrations of 2-propanol in remote areas are expected to be even lower than this value. No atmospheric measurements of 1-propanol have been found in the literature. For the purpose of the following calculations, we have assumed concentrations of 10 ppt for ethanol and both propanol isomers.

[15] The direction of the flux across the air – sea interface can be determined by calculating the percentage saturation state of the water with respect to the atmosphere.

equation image

where H is the Henrys Law value at equilibrium and Cw and Ca are the concentrations in water and air respectively. Using our assumed air concentrations and the seawater concentrations reported here, we determined whether surface water was super- or under-saturated (Table 1).

[16] Ethanol fluxes during the ICON cruise appear to have been both into and out of the ocean, depending on the time of day. The data set suggests that the ocean source is at its strongest during the night and a sink after 09:00. Ethanol super-saturation implies an active production process that must be biological to occur in the dark. The absence of ethanol in samples from the 13:00 and 16:00 casts indicates rapid loss via either photochemical degradation or biological oxidation, not transfer to the atmosphere, as the oceans are under-saturated during the day. The more dominant process is likely to be photochemical as biological removal would also occur at night and we see little evidence for this in our data set. In contrast, the measurements recorded on AMT19 show no ethanol is present pre-dawn (oceanic sink) and that the mean concentration recorded at solar noon (12 nM), corresponds to an oceanic source. Ethanol super-saturation in surface water during the daylight and under-saturation in the dark suggests that the dominant production source is photochemical, possibly from the degradation of Dissolved Organic Matter (DOM) in less productive areas of the surface ocean.

[17] The direction of 1-propanol flux on ICON was variable, making the ocean in the upwelling region both a source and a sink at different times. Atmospheric concentrations would have had to have been as high as approximately 110 ppt for the ocean to be a consistent sink. The concentrations of 1-propanol in Patch 1, with the highest productivity levels (09:00) were consistently super-saturated, whilst in patch 3 (09:00), the ocean was under-saturated and therefore acting as a net sink. Highest super-saturation states were seen in the water sampled at pre-dawn suggesting a net biological production process at night. The ocean was only a source again at 13:00, which might imply photochemical production in surface water at this time. Saturation calculations for 1-propanol on AMT19 also suggest temporally reversing fluxes. The averaged values for both pre-dawn and solar noon (latitudes between 33°N – 29°N) show saturation states at 130% and 110% respectively. This implies no significant production process in the oligotrophic environment was active enough to force a significant flux out of the ocean. The highest saturation was measured at 10°N, suggesting super-saturation in the surface water. This correlates with a region of higher productivity located in the equatorial Atlantic.

[18] There is a significant change in the direction of the flux between ICON and AMT19 for 2-propanol. During ICON the concentrations of 2-propanol found in the surface water were high enough to force the direction of the gas exchange into the atmosphere, making the ocean a source of this OVOC. However, the concentrations recorded on AMT19 were significantly lower and the flux was into rather than out of the ocean. Even at the highest concentration detected on AMT19, the assumed flux would have been close to equilibrium. From this data set we can hypothesise that the highly productive upwelling environment may have caused enhanced 2-propanol concentrations in the surface water via biological production mechanisms which were not present in the oligotrophic regions of the AMT19 track.

5. Direct Flux Calculations

[19] The air-sea fluxes (F) were calculated from our measured sea water concentrations (Cw) and published atmospheric values (Ca) using equation (2) [Liss and Slater, 1974]:

equation image

where 1/Kw is equal to 1/kw + 1/Hka and H denotes the Henrys Law constant (corrected for salinity and temperature). kw and ka are gas transfer coefficients for both water and air respectively. Both terms are required for gas transfer calculations of the OVOCs due to their high solubility in water leading to transfer resistances in both phases being important. The coefficients are calculated using equation (3) [Duce et al., 1991] and equation (4) [Nightingale et al., 2000];

equation image
equation image

where u10 is the average wind speed normalised to 10 metres above sea level (calculated over the period of each respective cast), MW is the molecular weight of the OVOC compound, Scw is the Schmidt number of the gas of interest at the temperature and salinity of the sample (ratio of the kinematic viscosity of seawater (ν) and molecular diffusivity (D) of the gas in seawater; 820, 938 and 972 for ethanol, 1- and 2-propanol respectively) and Sc600 is the Schmidt number of CO2 at 20°C in freshwater [Nightingale, 2009].

[20] The mean ethanol flux during ICON period was −0.6 μmol m−2 day−1 with a range of −3.9 to 0.2 μmol m−2 day−1. A negative flux is indicative of release by the ocean to the atmosphere. The highest ethanol flux to the atmosphere observed was at pre-dawn and therefore at night, and was −3.9 μmol m−2 day−1. The mean flux during AMT19 was approximately −0.7 μmol m−2 day−1 (oceanic source) with a range of −4.0 to 0.2 μmol m−2 day−1.

[21] The fluxes during ICON of 1-propanol ranged from −2.7 to 0.2 μmol m−2 day−1, with a mean oceanic flux of −0.3 μmol m−2 day−1 (oceanic source). Fluxes of 1-propanol during AMT19 ranged from −1.3 to 0.2 μmol m−2 day−1.

[22] The mean 2-propanol flux observed during ICON was −0.6 μmol m−2 day−1 and values ranged between −0.02 and −2.6 μmol m−2 day−1 (oceanic source), with the highest source flux measured at pre-dawn. The mean flux of 2-propanol during AMT19 was 0.1 μmol m−2 day−1 (oceanic sink). Values ranged between −0.001 to 0.1 μmol m−2 day−1. This suggests that high primary productivity in the nutrient rich, upwelled water might be an important factor in the production of 2-propanol when compared to oligotrophic water encountered on AMT19.

[23] We aimed to determine whether the oceans were of importance when compared to published land sources by extrapolating our admittedly sparse dataset to the global scale (using the average flux values from each campaign, a value of 3.6 E+8 km2 (M. Pidwirny, Introduction to the oceans, Fundamentals of Physical Geography, available at, 2006) for surface area of Earth covered by water and relevant molecular weight of gas).

[24] The mean ethanol flux determined during ICON and AMT19 suggests a total annual oceanic source of 4 Tg yr−1. Our roughly estimated global source figures can be compared to land-based sources by Singh et al. [2004] of 2 Tg yr−1 (anthropogenic and biomass burning) and 6 Tg yr−1 (biogenic emissions). Extrapolation from ICON for 1-propanol release from the ocean is estimated at 3 Tg yr−1. This net oceanic source decreases to 2 Tg yr−1 when the flux from AMT19 is utilised, which may be more representative of the global oceans than the upwelling region encountered on ICON. Our data for 2-propanol shows contrasting results. Annual emissions calculated from ICON indicate a source of 5 Tg yr−1. When the AMT19 results are used, this reverses to a net global sink of 1 Tg yr−1.

6. Conclusion

[25] These are the first measurements of ethanol and propanol in sea water so there are no other data for comparison. We have seen evidence that these compounds may exhibit a diel signal, with surface concentrations decreasing during the day. We also estimate the first air-sea fluxes for these alcohols and use them to extrapolate to a global scale. Our data suggests that the ocean could represent an important source of these compounds to the atmosphere. Recent work [Naik et al., 2010] has examined known sources and sinks of ethanol and concluded that there seems to be a missing source required to explain the abundance of atmospheric ethanol measured over remote oceans. Our results indicate that marine ethanol could potentially be this missing source. Elevated oceanic fluxes for propanol (specifically 2-propanol) from ICON, emphasise the potential importance of upwelling regions for increased OVOC release to the atmosphere. Super-saturation in surface water at night, has also highlighted that both ethanol and propanol may be produced by some marine organisms.

[26] These estimates are based on sparse data, but they do suggest that oceans are important in the atmospheric budgets of these compounds. We have highlighted the importance of the ocean in the cycling of these OVOCs but, with little atmospheric data and our small data set, further work is required to corroborate our findings and reduce the associated uncertainty.


[27] Thanks to the officers and crew on board RRS Discovery (ICON) and RRS James Cook (AMT19). Thanks to Claire Widdicombe (PML) for primary production data. This work was funded by the UK SOLAS project (NE/C517192/1) and OCEANS 2025, PML's NERC funded core research programme. This is contribution number 195 of the AMT programme.