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Methanol and ethanol concentrations in a Greenland ice core
Atmospheric Environment 217 (2019) 116948
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Methanol and ethanol concentrations in a Greenland ice core b
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J. David Felix , J. Alan Roebuck , Ralph N. Mead , Joan D. Willey , G. Brooks Avery Jr. , Robert J. Kiebera,∗ a b
Department of Chemistry and Biochemistry, University of North Carolina at Wilmington, Wilmington, NC, 28403-5932, USA Department of Physical and Environmental Science, Texas A&M University Corpus Christi, Corpus Christi, TX, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Ethanol Methanol Ice core
The first methanol and ethanol concentrations in a Greenland ice core are reported for 14 annual samples ranging from 1686 to 1947 AD. Concentrations ranged from 83 nM to 1666 nM and 16 nM–424 nM for methanol and ethanol, respectively. Low median methanol and ethanol concentrations from 1686 to 1745 result primarily from natural emissions during a cold climate period. These concentrations provide a baseline alcohol level in deposition before impacts from various sources (e.g. naturally recovering plant populations, anthropogenic sources, and changing atmospheric oxidation chemistry) increased alcohol concentrations in the atmosphere. Concentrations of both alcohols increased after 1745 possibly due to increased biogenic and anthropogenic sources. Because both of these alcohols react with hydroxyl radicals, this increase has important implications for changing atmospheric oxidation chemistry. Samples were also analyzed for various other ice core components (e.g. aldehydes, anions, H+, DOC) to investigate possible correlation with alcohol concentrations. These ice core data are significant because they can be used to determine the extent temporal trends in alcohol concentrations were due to changes in atmospheric chemistry and varying source type and magnitude.
1. Introduction Methanol (CH3OH) and ethanol (CH3CH2OH) are volatile organic compounds in the atmosphere that play important roles in atmospheric oxidation chemistry. Their primary source is terrestrial plants with methanol production occurring in plant leaves (Fall and Benson, 1996) and ethanol production via fermentation in plant leaves and roots (Kimmerer and Macdonald, 1987; Schade and Goldstein, 2001, 2002). The alcohols are then emitted from plants via the stomata. Other less significant sources of these alcohols include industrial activity, biomass burning, vehicle biofuels, plant decay, oceans and oxidation of hydrocarbons (Jacob et al., 2005; Beale et al., 2010; Naik et al., 2010; Millet et al., 2012). After emission, methanol can lead to ozone (O3), carbon monoxide (CO), and formaldehyde (HCHO) formation and a decrease in hydroxyl radical (.OH) concentrations (Tie, 2003). Ethanol is a precursor of acetaldehyde (CH3CHO) and peroxyacetyl nitrate (PAN) and a sink for .OH (Millet et al., 2010). These reactions have impacts on atmospheric oxidation chemistry which in turn has implications for atmospheric composition, air pollution, aerosol formation, and greenhouse radiative forcing (Thompson, 1992). Recent precipitation measurements indicate that the global wet depositional flux of ethanol is 2.4 Tg yr−1 based upon analyses of 211
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wet deposition samples collected at 12 locations globally (Felix et al., 2017). A similar study of methanol in precipitation collected in Wilmington NC, USA (Felix et al., 2014) suggested that the global methanol wet depositional sink may be somewhat higher, closer to 20 Tg yr−1. Analysis of temporal trends in gas phase ethanol concentrations indicate that they closely mimic those observed in precipitation samples collected at the same location suggesting rainwater may act as a proxy for ethanol changes occurring in the gas phase (Kieber et al., 2017; Willey et al., 2019). Despite the documented impacts on atmospheric chemistry and the importance of wet and dry deposition as a sink of the alcohols, studies are non-existent regarding the paleo atmospheric abundance of methanol and ethanol. To shed light on past trends in atmospheric concentrations of these labile alcohols, a series of Greenland ice core samples were analyzed ranging in age from 1686 to 1947 AD. Although volatile organic compounds such as formaldehyde and organic acids have previously been analyzed in ice cores (Staffelbach et al., 1991; Fuhrer et al., 1993; Fuhrer and Legrand, 1997; Xinqing et al., 2001; Largiuni, 2003: Legrand et al., 2013; Preunkert and Legrand, 2013), these studies have suggested that there are still significant levels of unquantified organic species in ice core samples (Legrand et al., 2013; Preunkert and Legrand, 2013). The current study quantifies a portion of
Corresponding author. E-mail address: [email protected] (R.J. Kieber).
https://doi.org/10.1016/j.atmosenv.2019.116948 Received 6 June 2019; Received in revised form 15 August 2019; Accepted 1 September 2019 Available online 10 September 2019 1352-2310/ © 2019 Elsevier Ltd. All rights reserved.
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these unquantified species by presenting the first methanol and ethanol measurements in an ice core. Formaldehyde, H2O2, and 17O–SO4 concentrations in ice cores have previously been used as a proxy to investigate paleo atmospheric oxidation chemistry (Gillet et al., 2000; Alexander, 2004; Sofen et al., 2010; Levine et al., 2011; Staffelbach et al., 1991). The novel alcohol data presented in the current study provides an important tool in the interpretation of past atmospheric redox conditions and also sheds significant insight on the understanding of the impact of temperature and anthropogenic activities on biogenic trace gas emissions. Finally, alcohol concentrations in the ice core allow reporting of pre-industrial methanol and ethanol levels in deposition which are requisite to understanding anthropogenic source contribution as contemporary methanol and ethanol emissions increase.
the original precipitation events and from dry deposition during periods when precipitation was not occurring. This assumption is likely valid as recent studies suggest that rainwater ethanol concentrations are close to equilibrium with local atmospheric gas phase concentrations and have increased in proportion to recent increased air concentrations (Willey et al., 2019). There is a possibility that alcohol levels in the ice core may have been altered post-deposition through a process such as photochemical production in the snow from organic precursors. This question has been investigated for other highly soluble gases such as formaldehyde and acetaldehyde (Hutterli et al., 1999, 2002; Dominé and Shepson, 2002; Hutterli, 2003; Grannas et al., 2004; Domine et al., 2010) and could be applied to study alcohol deposition and post-depositional formation as well. Gao et al. (2012) observed that illuminated Arctic snow from remote, rural, and urban areas released formaldehyde but didn't release methanol. Diffusion of ambient Greenland air post-deposition followed by dissolution of alcohols in the ice could also contribute to the concentration of alcohols in the core although the lack of a temporal trend of other highly soluble gases analyzed such as formaldehyde and acetaldehyde suggest this is most likely not a dominant process.
2. Methods 2.1. Core processing The GISP-2 G2 Greenland (72°35′ min N 38°28’ min) ice core was obtained from the National Ice Core Laboratory (NICL), Denver, CO, USA. The core is a de-accessed core that was cut at NICL and divided into yearly samples by using data from previously collected Summit, Greenland cores. The 14 samples span ~261 years and the lower sections are assumed to be within ± 4 years of the date reported. The core was shipped overnight from NICL and was immediately cut upon arrival with a stainless steel saw in a laminar low flow hood fitted with a high efficiency particulate air (HEPA) filter. A 6 × 6 cm section was cut from the center of the 10 cm diameter core in order to limit contamination present in the outer layers of the core. ~50% of the outer portion of the core was removed in order to obtain a contaminant free portion of the core (Saigne et al., 1987; Legrand et al., 1988; Legrand et al., 1993; Jauhiainen et al., 1999). The inner core was allowed to melt at room temperature in covered 4L beakers that had been baked at 450 °C in a muffle furnace for a minimum of 4.5 h to remove organics prior to use. A beaker of MilliQ deionized water (MQ) next to the melting core samples in the hood served as a blank. The MQ water was processed in the same way as the actual core samples, and analyzed via the same method as the actual core. The melted core samples and the MQ blank were both at room temperature when analyzed, so any interactions between lab air and samples should have been similar for blanks and core samples. Analyses were performed as soon as the ice core was fully melted. The blank values for formaldehyde, acetaldehyde, methanol, and ethanol were 4.8 nM, non detect, 38.1 nM and non detect respectively, similar to the Milli-Q blank values reported when running modern rain samples in the same laboratory. The blank concentrations were subtracted from the measured sample concentrations prior to use. The blank concentrations were less than 4% of the average concentrations for each analyte except formaldehyde, where it was 9.8% of the average concentration. The data and blank controls used during sampling suggest contamination is not a major factor in these data because: (1) concentrations of highly water soluble alcohol oxidation products (aldehydes) did not increase at any one level in the core; (2) concentrations of other highly soluble gases (formaldehyde) did not increase and are consistent with previous studies of a Greenland ice cores spanning the same time range (Staffelbach et al., 1991; Anklin and Bales, 1997); (3) drilling fluid was not used to obtain this core; (4) neither butyl acetate (a possible contaminant from drilling fluid) nor any of the common laboratory contaminants benzene, ethylbenzene, styrene, carbon tetrachloride, and 2-methylfuran were detectable in the core; (5) methanol and ethanol concentrations fall exactly into the low end of the range of concentrations observed in contemporary precipitation (Felix et al., 2014; Kieber et al., 2014). For the purpose of the discussion and due to the relatively high solubility of the alcohols, it is assumed the majority of the alcohol present in samples is from alcohol dissolved in the precipitation during
2.2. Alcohol and aldehyde analysis Formaldehyde and acetaldehyde concentrations in core samples were determined by derivatization with 2,4-dinitrophenylhydrazine followed by separation and detection by HPLC (Kieber et al., 1999). Samples and standards were reacted with 2,4-dinitrophenylhydrazine for 1 h in the dark forming a hydrazone, which was separated from interfering substances by HPLC and quantified by UV detection at 370 nm. Derivatized samples (100 μL) were injected onto a reversed phase Kinetex100 mm × 4.60 mm 2.6μ C18 Phenomenex column with a 100 Å pore size at 10 °C. The mobile phase was a 1:1 mixture of 0.1% trifluoroacetic acid (TFA) in acetonitrile and 0.1% TFA in DIW at a flow rate of 1.00 mL min−1. Methanol and ethanol were determined by oxidation of the alcohol to formaldehyde (or acetaldehyde) via alcohol oxidase obtained from the yeast Hansenula sp. (Kieber et al., 2013). The enzyme was prepared by dissolution of 100 units of alcohol oxidase in 5 mL of 0.1M potassium phosphate buffer (pH 9.0). The sample (1000 μL) was combined with 10 μL of buffer, 100 μL of an enzyme working reagent (0.18 units mL−1) and allowed to react at 40 °C for 40 min (120 min for ethanol) before addition of 10 μL of DNPH. The concentration of methanol (or ethanol) was determined after HPLC analysis by the difference in formaldehyde (or acetaldehyde) concentration in samples with and without added enzyme. This method has a detection limit for methanol and ethanol of 6 nM and 10 nM, respectively. All ice core samples were run in triplicate (n = 3). The average standard deviations of samples run in triplicate were ± 8.6 nM for methanol, ± 1.2 nM for ethanol and ± 0.6 nM for both acetaldehyde and formaldehyde. 2.3. Supporting analyses Organic carbon content in filtered ice core samples were determined with a Shimadzu TOC 5000 carbon analyzer (Shimadzu, Kyoto, Japan) equipped with an ASI 5000 autosampler. pH was analyzed immediately after melting using a Ross electrode with low ionic strength buffers. Inorganic anions (Cl−, NO3−, and SO42−) were analyzed using ion suppressed chromatography. 3. Results The range of methanol and ethanol concentrations are 83 nM–1666 nM and 16 nM–424 nM, respectively (Table 1). The range of methanol concentrations in the ice core fell within the low end of concentration ranges reported for contemporary rain events at a coastal U.S. site (methanol: < 6 nM–9300 nM) (Felix et al., 2014). The range in 2
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Table 1 Concentration of analytes in the GISP-2 G2 Greenland ice core. The year column in the table represents the approximate age of the ice ± 4 yrs. All analytes were measured in triplicate (n = 3). Year (AD)
Methanol (nM)
Ethanol (nM)
Formal (nM)
Acetal (nM)
DOC (μM)
H+ (M)
Cl−(μM)
NO3−(μM)
SO42−(μM)
1686 1700 1724 1745 1765 1785 1804 1824 1844 1865 1885 1905 1926 1947
103 111 83 381 1199 1324 1562 1591 1666 1493 1446 1422 1412 1266
26 18 16 44 138 172 247 239 239 179 290 242 332 424
72 53 50 57 46 69 46 41 36 55 34 41 32 50
115 34 52 55 114 61 123 105 74 33 184 505 119 141
76.3 14.6 15.2 22.6 42.6 14.9 25.2 31.9 25.2 46.8 29.0 43.9 NA 37.4
6.3 E−07 1.5 E−06 1.3 E−06 1.0 E−06 1.0 E−06 4.0 E−06 1.3 E−06 1.8 E−06 2.0 E−06 1.6 E−06 4.0 E−07 6.3 E−07 1.3 E−06 1.3 E−06
1.93 2.07 3.42 2.92 15.82 5.56 2.82 3.37 2.72 8.62 1.82 4.38 2.73 1.77
2.47 3.45 5.42 15.65 13.43 2.85 2.46 2.21 1.59 2.82 1.58 3.17 2.5 4.38
0.7 0.46 0.53 0.68 1.67 3.42 0.55 0.91 0.42 0.57 0.35 1.38 0.63 1.07
4. Discussion
ethanol concentrations in the ice core was similar to the range in the yearly volume weighted average concentration of the alcohol measured at the Wilmington site between 2010 and 2015 (ethanol 126 nM–345 nM) (Willey et al., 2019). The increase in rainwater concentrations at the Wilmington site from 2010 to 2015 is very similar to the increase in ethanol from 242 to 424 nM from 1905 to 1947 in the ice core data. The range of formaldehyde and acetaldehyde concentrations were 32 nM–72 nM and 33 nM–505 nM, respectively. The range of acetaldehyde concentrations in the ice core fall within concentration ranges reported for contemporary rain events at a coastal U.S. site (23 nM–909 nM) but the range of formaldehyde concentrations fall well below the range of the contemporary concentration reported at the coastal U.S. site (100 nM–5500 nM) (Felix et al., 2014; Kieber et al., 1999). The sum of acetaldehyde, formaldehyde, ethanol and methanol concentrations make up on average 6% of the total DOC in the ice core samples. There was a large spike in sulfate (3.42 μM) in the ice core in 1785 possibly because of the Icelandic Laki volcanic eruption which occurred in 1783. Methanol and ethanol were significantly correlated with each other (p < 0.001) likely because both have a primary biogenic source (Table 2). Formaldehyde was negatively correlated (p < 0.05) with methanol. There was no correlation between ethanol and acetaldehyde in the ice core samples presented in Table 2. In our earlier study with current rain we also found that there was no correlation between the concentration of ethanol and acetaldehyde in rainwater at the Wilmington NC location which we suggest is because inputs other than simple oxidation of ethanol are responsible for determining the concentration of the aldehyde in precipitation (Kieber et al., 2014).
4.1. Methanol and ethanol sources Methanol concentrations showed a significant increasing trend from 1686 through 1844 (Mann –Kendall p < 0.001) but not for the entire time period (p = 0.062) (Fig. 1). Ethanol concentrations increased significantly over the whole time period (Mann-Kendall trend p < 0.001) (Fig. 1). The increase in methanol and ethanol concentrations may be attributed to changes in source type or magnitude. The concentration of both alcohols between 1686 and 1745 are low and consistent likely due to a period of cold climate (Little Ice Age (LIA)) suppressing plant activity that would directly affect emissions of alcohols from plants. Galbally and Kirstine (2002) predicted that lower methanol concentrations would be found in ice cores dating back to a
Table 2 Correlation coefficients between methanol, ethanol, and other ice core components. Bold faced values indicate significance at p < 0.001. Italics indicate significance at p < 0.05.
methanol ethanol formal acetal H+ Cl− NO3−
ethanol
formal
acetal
H+
Cl−
NO3−
SO42-
0.81
−0.56 −0.58
0.29 0.35 −0.33
0.19 −0.01 0.35 −0.40
0.19 0.13 0.06 0.04 0.08
−0.34 −0.38 0.18 −0.15 −0.18 0.48
0.18 0.04 0.43 0.10 0.70 0.39 0.11
Fig. 1. Ice core ethanol and methanol concentrations vs. year (AD). 3
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climatic cold period due to decreasing primary plant productivity. In previous ice core studies other analytes such as CH4 have also had relatively low concentrations during this period. This has been attributed to pre-1750 AD cold climate dampening biogenic activity including wetland biological activity that produces CH4 (Khalil and Rasmussen, 1989; Etheridge, 1998). Methane increases have been linked to an increase in wetland activity (e.g. production of CH4 by microbes) with increasing temperature as the LIA was coming to an end (Blunier et al., 1993; Etheridge, 1998; MacFarling et al., 2006). Ethanol emissions would likely increase with increasing plant activity brought on by the end of the LIA. Plants are reported to increase ethanol emissions after they have undergone a stress such as cold temperatures. For example, Fukui and Doskey (1998) report large ethanol fluxes from grassland after frost events. Ethanol emissions may also be increasing as wetlands are flooded and temperature rises. As plants are flooded in wetlands, the root zone becomes anaerobic and these conditions favor ethanol production in roots which in turn increases ethanol emissions. For instance grasslands, potted plants, and pines trees are reported to increase ethanol emissions after flooding (Fukui and Doskey, 1998; Holzinger et al., 2000; Rottenberger et al., 2008). The influence of temperature dependent increasing biogenic emissions on ethanol concentrations in the ice core is underscored by the strong correlation (p = 0.0092) between levels of the alcohol and the temperature anomaly as described by Moberg et al. (2005). The authors utilized a multi prong approach to reconstruct Northern Hemisphere temperatures by combining low-resolution proxies with tree - ring data. A minimum temperature was observed around AD 1600 during the LIA corresponding to low ethanol concentrations in the ice core. Temperatures rose consistently through the latter half of the 18 century and into the latter half of the 20th century analogous to the rise in ethanol concentrations most likely because the increase in temperature allowed for an increase in biogenic emissions. This may also be the case for increasing methanol concentrations since increasing temperatures can lead to exponentially increasing methanol emissions from plants (Folkers et al., 2008; Hu et al., 2011). Methanol increases much more rapidly than ethanol and this exponential feedback from increasing temperatures may be the cause for the increase in methanol before it levels off at ~1800 AD then begins slightly decreasing (see Fig. 1). Anthropogenic changes in land use during the period after 1750 AD may also have a modest influence on the increasing trend in alcohol concentrations. Land clearing for cropland as a result of the European agricultural revolution (1750–1880 AD) (Chorley, 1981), the European settlement of North America, and the need for biomass to burn leads to varying emission fluxes. Croplands are irrigated which increases the amount of anaerobic root zones, thus increasing the potential for ethanol production in the roots. Total European and North American cropland (Klein Goldewijk et al., 2011) is strongly correlated (n = 13, R = 0.80, p = 0.001) to ethanol concentrations (Fig. 2) suggesting that conversion of natural lands to working croplands may have increased ethanol emissions. In addition to clearing for agricultural purposes, land was also cleared to obtain biomass for burning. Biomass burning occurs naturally and anthropogenically and can be a source of both methanol and ethanol (Jacob et al., 2005; Millet et al., 2012). While biomass burning possibly contributed to the increased concentration of alcohols, the start of the increase in alcohol concentrations and reported increase in biomass burning are offset by ~100 years according to a fire index inferred from analyte concentrations in a Greenland ice core (Savarino and Legrand, 1998). Other than biomass burning, changes in the amplitude of additional minor sources could have added to the increase in alcohol concentrations. For instance, livestock waste was a significant source of CH4 in the pre-industrial period (Subak, 1994) therefore it may also have been an important source of alcohols since they are directly emitted from
Fig. 2. Ice core ethanol vs. Total European and N. American cropland.
livestock waste (Shaw et al., 2007; Sun et al., 2008). The increase in ethanol from 242 to 424 nM from 1905 to 1947 may also be influenced by emission from the use of ethanol as a fuel. By the turn of the 20th century ethanol was used as vehicle fuel as the Ford motor company created engines that ran onethanol, including the Ford Model T (1908) that could run exclusively on ethanol. Ethanol use as a vehicle fuel also increased in the early 20th century because it was a significant fuel employed during World War One (Kovarik, 1998; Solomon et al., 2007). Increases in ethanol in a Greenland ice core resulting from biofuel usage at the turn of the 20th century would, however, be relatively small compared to other sources such as biogenic inputs. 4.2. Methanol and ethanol atmospheric chemistry and oxidation The major sink for both methanol and ethanol is oxidation via the OH radical. Many studies of paleoatmospheric oxidation chemistry predicted a decrease in .OH concentrations during the time period this ice core encompasses (Staffelbach et al., 1991; Thompson, 1992; Sofen et al., 2010). Methane and CO, additional major sinks of .OH, increased over this time period which could decrease the atmospheric concentration of .OH. The decrease in .OH radical would decrease the primary alcohol sink and increase the lifetime thus increasing concentration. Atmospheric oxidation chemistry also affects peroxy radical formation, a precursor to alcohol formation. Methanol and ethanol can form from the peroxy oxidation products of methane and ethane, (e.g. methyl and ethyl peroxy radicals) especially in low NOx environments (Naik et al., 2010). The methyl and ethyl peroxy radical are the most abundant peroxy radicals in the atmosphere (Villenave et al., 1996) and it is possible that an increasing source of peroxy radicals (via increasing methane and ethane emissions) and low NOx environment lead to increases in alcohols via peroxy radical formation. .
5. Conclusion and implications The low median methanol (107 nM) and ethanol (22 nM) concentrations from 1686 to 1745 represent concentrations resulting from primarily natural emissions during a cold climate period. These concentrations provide a baseline alcohol level in wet and dry deposition before impacts from various sources (e.g. naturally recovering plant populations, anthropogenic sources, and changing atmospheric oxidation chemistry) increased alcohol concentrations in the atmosphere. Anthropogenic alcohol emissions are increasing globally as countries attempt to become more self-sustainable by turning to renewable vehicle fuels such as methanol and ethanol. For example China consumed 2.3 billion gallons of methanol in vehicle fuel (Dolan, 2013) and more than 95% of the gasoline sold in the U.S. is blended with ethanol. 4
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Alcohol concentrations from the ice core will allow atmospheric scientists to quantify the magnitude of change from historical to contemporary and future concentrations which will aid in predicting the scale to which increased alcohol emissions will impact future atmospheric chemistry. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank Mark Twickler and the National Ice Core Laboratory for access to the Greenland ice core. We also thank Wesley K. Mickler and H. Douglas Rainey at the Marine and Atmospheric Chemistry Research Laboratory at the University of North Carolina Wilmington for aid in sample analysis. National Science Foundation Grants AGS 1003078 and AGS 1440425 supported this work. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.atmosenv.2019.116948. References Alexander, B., 2004. Impact of preindustrial biomass-burning emissions on the oxidation pathways of tropospheric sulfur and nitrogen. J. Geophys. Res. 109 (D8), D08303. https://doi.org/10.1029/2003JD004218. Anklin, M., Bales, R.C., 1997. Recent increase in H2O2 concentration at Summit, Greenland. J. Geophys. Res. 102 (19) 099-19, 104. Beale, R., Liss, P.S., Nightingale, P.D., 2010. First oceanic measurements of ethanol and propanol. Geophys. Res. Lett. 37 (24). https://doi.org/10.1029/2010GL045534. Blunier, T., Chappellaz, J.A., Schwander, J., Barnola, J.M., Desperts, T., Stauffer, B., Raynaud, D., 1993. Atmospheric methane, record from a Greenland Ice core over the last 1000 years. Geophys. Res. Lett. 20 (20), 2219–2222. Chorley, G.P.H., 1981. The agricultural revolution in Northern Europe, 1750-1880: nitrogen, legumes, and crop productivity. Econ. Hist. Rev. 34 (1), 71–93. Dolan, G.A., 2013. Methanol production and markets: past, present, and future. In: Clary, J.J. (Ed.), The Toxicology of Methanol. John Wiley & Sons, Inc., Hoboken, NJ, USA. Domine, F., Houdier, S., Taillandier, A.S., Simpson, W.R., 2010. Acetaldehyde in the Alaskan subarctic snowpack. Atmos. Chem. Phys. 10, 919–929. Dominé, F., Shepson, P.B., 2002. Air-snow interactions and atmospheric chemistry. Sci. (New York, N.Y.) 297 (5586), 1506–1510. https://doi.org/10.1126/science.1074610. Etheridge, D., 1998. Atmospheric methane between 1000 A.D. and present: evidence of anthropogenic emissions and climatic variability. J. Geophys. Res. 103, 15979–15993. Fall, R., Benson, A., 1996. Leaf methanol- the simplest natural product from plants. Trends Plant Sci. 1 (9), 1360–1385. Felix, J.D., Jones, S.B., Avery, G.B., Willey, J.D., Mead, R.N., Kieber, R.J., 2014. Temporal and spatial variations in rainwater methanol. Atmos. Chem. Phys. 14 (2), 1375–1398. https://doi.org/10.5194/acpd-14-1375-2014. Felix, J.D., Willey, J.D., Thomas, R.K., Mullaugh, K.M., Avery, G.B., Kieber, R.J., Mead, R.N., Helms, J., 2017. Removal of atmospheric ethanol by wet deposition. Glob. Biogeochem. Cycles 31 (2), 348–356. Folkers, a, Hüve, K., Ammann, C., Dindorf, T., Kesselmeier, J., Kleist, E., Kuhn, U., Uerlings, R., Wildt, J., 2008. Methanol emissions from deciduous tree species: dependence on temperature and light intensity. Plant Biol. 10 (1), 65–75. https://doi. org/10.1111/j.1438-8677.2007.00012.x. Fuhrer, K., Legrand, M., 1997. Continental biogenic species in the Greenland Ice Core Project ice core : tracing back the biomass history of the North American continent. J. Geophys. Res. 102, 26735–26745. Fuhrer, K., Neftel, A., Anklin, M., Maggit, V., Institut, P., Bern, U., 1993. C o n t i n u o u s measurements of h y d r o g e n peroxide , formaldehyde , calcium a n d a m m o n i u m concentrations along the new grip ice core from summit , central Greenland. Atmos. Environ. 27 (12), 1873–1880. Fukui, Y., Doskey, P., 1998. Air surface exchange of nonmethane organic componds at a grassland site: seasonal variations and stressed emissions. J. Geophys. Res. 103 (D11), 13153–13168. Galbally, I.E., Kirstine, W., 2002. The production of methanol by flowering plants and the global cycle of methanol. J. Atmos. Chem. 43, 195–229. Gao, S.S., Sjostedt, S.J., Sharma, S., Hall, S.R., Ullmann, K., Abbatt, J.P.D., 2012. PTR-MS observations of photo-enhanced VOC release from Arctic and midlatitude snow. J. Geophys. Res. 117, D00R17. https://doi.org/10.1029/2011JD017152. Gillet, R.W., Van Ommen, T.D., Jackson, A.V., Ayers, G.P., 2000. Formaldehyde and
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Mitloehner, F.M., 2008. Effects of sodium bisulfate on alcohol, amine, and ammonia emissions from dairy slurry. J. Environ. Qual. 37 (2), 608. https://doi.org/10.2134/ jeq2006.0446. Thompson, A.M., 1992. The oxidizing capacity past probable of and atmosphere: earth's the changes future. Sci 256 (5060), 1157–1165. Tie, X., 2003. Biogenic methanol and its impacts on tropospheric oxidants. Geophys. Res. Lett. 30 (17), 1881. https://doi.org/10.1029/2003GL017167. Villenave, E., Lesclaux, R., Bordeaux, I., Cnrs, U.R.A., Cedex, T., 1996. Kinetics of the cross reactions of CH3O2 and C2H5O2 radicals with selected peroxy radicals. J. Phys. Chem. 100, 14372–14382. Willey, J.D., Avery, G.B., Felix, J.D., Kieber, R.J., Mead, R.N., Shimizu, M.S., 2019. Rapidly increasing ethanol concentrations in rainwater and air. NPJ: Climate Atmos. Sci. 2, 3. https://doi.org/10.1038/s41612-018-0059-z. Xinqing, L.I., Xinqing, L., Dahe, Q.I.N., Hui, Z., 2001. Organic acids: differences in ice core records between Glacier 1, Tianshan, China and the polar areas. Chin. Sci. Bull. 46 (1), 80–83.
Preunkert, S., Legrand, M., 2013. Towards a quasi-complete reconstruction of past atmospheric aerosol load and composition (organic and inorganic) over Europe since 1920 inferred from Alpine ice cores. Clim. Past 9, 1403–1416. https://doi.org/10. 5194/cp-9- 1403-2013. Rottenberger, S., Kleiss, B., Kuhn, U., Wolf, A., Piedade, M.T.F., Junk, W., Kesselmeier, J., 2008. The effect of flooding on the exchange of the volatile C 2 -compounds ethanol, acetaldehyde and acetic acid between leaves of Amazonian flood plain tree species and the atmosphere. Biogeosciences 5, 1085–1100. Saigne, C., Kirchner, S., Legrand, M., 1987. Ion chromatographic measurements of ammonium, fluoride, acetate, formate and methanesulphonate ions at very low levels in Antarctic ice. Anal. Chim. Acta 203, 11–21. Savarino, J., Legrand, M., 1998. High northern latitude forest fires and vegetation emissions over the last millenium inferred from the chemistry of a central Greenland ice core. J. Geophys. Res. 103 (D7), 8267–8279. Schade, G.W., Goldstein, A.H., 2001. Fluxes of oxygenated volatile organic compounds from a ponderosa pine plantation. J. Geophys. Res. 106 (D3), 3111–3123. Schade, G.W., Goldstein, A.H., 2002. Plant physiological influences on the fluxes of oxygenated volatile organic compounds from ponderosa pine trees. J. Geophys. Res. 107. Shaw, S.L., Mitloehner, F.M., Jackson, W., Depeters, E.J., Fadel, J.G., Robinson, P.H., Holzinger, R., Goldstein, A.H., 2007. Volatile organic compound emissions from dairy cows and their waste as measured by proton-transfer-reaction mass spectrometry. Environ. Sci. Technol. 41 (4), 1310–1316. Sofen, E.D., Alexander, B., Kunasek, S.a., 2010. The sensitivity of the oxygen isotopes of ice core sulfate to changing oxidant concentrations since the preindustrial. Atmos. Chem. Phys. 10 (8), 20607–20623. https://doi.org/10.5194/acpd-10-20607-2010. Solomon, B., Barnes, J.R., Halvorsen, K.E., 2007. Grain and cellulosic ethanol : history, economics and energy policy. Biomass Bioenergy 31, 416–425. https://doi.org/10. 1016/j.biombioe.2007.01.023. Staffelbach, T., Neftel, A., Stauffer, B., Jacob, D., 1991. A record of the atmospheric methane sink from formaldehyde in polar ice cores. Nature 349, 603. Subak, S., 1994. Methane from the house of tudor and the ming dynasty: anthropogenic emissions in the sixteenth century b. Chemosphere 29, 843–854. Sun, H., Pan, Y., Zhao, Y., Jackson, W.a., Nuckles, L.M., Malkina, I.L., Arteaga, V.E.,
Further reading Burkhart, J.F., Bales, R.C., McConnell, J.R., Hutterli, M., 2006. Influence of North Atlantic Oscillation on anthropogenic transport recorded in northwest Greenland ice cores. J. Geophys. Res. 111 (D22), D22309. https://doi.org/10.1029/2005JD006771. Hastings, M.G., Jarvis, J.C., Steig, E.J., 2009. Anthropogenic impacts on nitrogen isotopes of ice-core nitrate. Sci 324 (5932), 1288. https://doi.org/10.1126/science.1170510. Kirstine, W.V., Galbally, I.E., 2012. The global atmospheric budget of ethanol revisited. Atmos. Chem. Phys. 12 (1), 545–555. https://doi.org/10.5194/acp-12-545-2012. Mullaugh, K.M., Willey, J.D., Kieber, R.J., Mead, R.N., 2012. Dynamics of the chemical composition of rainwater throughout Hurricane Irene. Atmos. Chem. Phys. 12 (10), 26995–27020. https://doi.org/10.5194/acpd-12-26995-2012. Nemecek-marshall, M., Macdonald, R.C., Franzen, J.J., Wojciechowski, C., Fall, R., 1995. Methanol emission from leaves. Plant Physiol. 108, 1359–1368.
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Methanol and ethanol concentrations in a Greenland ice core b
a
a
a
T a
J. David Felix , J. Alan Roebuck , Ralph N. Mead , Joan D. Willey , G. Brooks Avery Jr. , Robert J. Kiebera,∗ a b
Department of Chemistry and Biochemistry, University of North Carolina at Wilmington, Wilmington, NC, 28403-5932, USA Department of Physical and Environmental Science, Texas A&M University Corpus Christi, Corpus Christi, TX, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Ethanol Methanol Ice core
The first methanol and ethanol concentrations in a Greenland ice core are reported for 14 annual samples ranging from 1686 to 1947 AD. Concentrations ranged from 83 nM to 1666 nM and 16 nM–424 nM for methanol and ethanol, respectively. Low median methanol and ethanol concentrations from 1686 to 1745 result primarily from natural emissions during a cold climate period. These concentrations provide a baseline alcohol level in deposition before impacts from various sources (e.g. naturally recovering plant populations, anthropogenic sources, and changing atmospheric oxidation chemistry) increased alcohol concentrations in the atmosphere. Concentrations of both alcohols increased after 1745 possibly due to increased biogenic and anthropogenic sources. Because both of these alcohols react with hydroxyl radicals, this increase has important implications for changing atmospheric oxidation chemistry. Samples were also analyzed for various other ice core components (e.g. aldehydes, anions, H+, DOC) to investigate possible correlation with alcohol concentrations. These ice core data are significant because they can be used to determine the extent temporal trends in alcohol concentrations were due to changes in atmospheric chemistry and varying source type and magnitude.
1. Introduction Methanol (CH3OH) and ethanol (CH3CH2OH) are volatile organic compounds in the atmosphere that play important roles in atmospheric oxidation chemistry. Their primary source is terrestrial plants with methanol production occurring in plant leaves (Fall and Benson, 1996) and ethanol production via fermentation in plant leaves and roots (Kimmerer and Macdonald, 1987; Schade and Goldstein, 2001, 2002). The alcohols are then emitted from plants via the stomata. Other less significant sources of these alcohols include industrial activity, biomass burning, vehicle biofuels, plant decay, oceans and oxidation of hydrocarbons (Jacob et al., 2005; Beale et al., 2010; Naik et al., 2010; Millet et al., 2012). After emission, methanol can lead to ozone (O3), carbon monoxide (CO), and formaldehyde (HCHO) formation and a decrease in hydroxyl radical (.OH) concentrations (Tie, 2003). Ethanol is a precursor of acetaldehyde (CH3CHO) and peroxyacetyl nitrate (PAN) and a sink for .OH (Millet et al., 2010). These reactions have impacts on atmospheric oxidation chemistry which in turn has implications for atmospheric composition, air pollution, aerosol formation, and greenhouse radiative forcing (Thompson, 1992). Recent precipitation measurements indicate that the global wet depositional flux of ethanol is 2.4 Tg yr−1 based upon analyses of 211
∗
wet deposition samples collected at 12 locations globally (Felix et al., 2017). A similar study of methanol in precipitation collected in Wilmington NC, USA (Felix et al., 2014) suggested that the global methanol wet depositional sink may be somewhat higher, closer to 20 Tg yr−1. Analysis of temporal trends in gas phase ethanol concentrations indicate that they closely mimic those observed in precipitation samples collected at the same location suggesting rainwater may act as a proxy for ethanol changes occurring in the gas phase (Kieber et al., 2017; Willey et al., 2019). Despite the documented impacts on atmospheric chemistry and the importance of wet and dry deposition as a sink of the alcohols, studies are non-existent regarding the paleo atmospheric abundance of methanol and ethanol. To shed light on past trends in atmospheric concentrations of these labile alcohols, a series of Greenland ice core samples were analyzed ranging in age from 1686 to 1947 AD. Although volatile organic compounds such as formaldehyde and organic acids have previously been analyzed in ice cores (Staffelbach et al., 1991; Fuhrer et al., 1993; Fuhrer and Legrand, 1997; Xinqing et al., 2001; Largiuni, 2003: Legrand et al., 2013; Preunkert and Legrand, 2013), these studies have suggested that there are still significant levels of unquantified organic species in ice core samples (Legrand et al., 2013; Preunkert and Legrand, 2013). The current study quantifies a portion of
Corresponding author. E-mail address: [email protected] (R.J. Kieber).
https://doi.org/10.1016/j.atmosenv.2019.116948 Received 6 June 2019; Received in revised form 15 August 2019; Accepted 1 September 2019 Available online 10 September 2019 1352-2310/ © 2019 Elsevier Ltd. All rights reserved.
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these unquantified species by presenting the first methanol and ethanol measurements in an ice core. Formaldehyde, H2O2, and 17O–SO4 concentrations in ice cores have previously been used as a proxy to investigate paleo atmospheric oxidation chemistry (Gillet et al., 2000; Alexander, 2004; Sofen et al., 2010; Levine et al., 2011; Staffelbach et al., 1991). The novel alcohol data presented in the current study provides an important tool in the interpretation of past atmospheric redox conditions and also sheds significant insight on the understanding of the impact of temperature and anthropogenic activities on biogenic trace gas emissions. Finally, alcohol concentrations in the ice core allow reporting of pre-industrial methanol and ethanol levels in deposition which are requisite to understanding anthropogenic source contribution as contemporary methanol and ethanol emissions increase.
the original precipitation events and from dry deposition during periods when precipitation was not occurring. This assumption is likely valid as recent studies suggest that rainwater ethanol concentrations are close to equilibrium with local atmospheric gas phase concentrations and have increased in proportion to recent increased air concentrations (Willey et al., 2019). There is a possibility that alcohol levels in the ice core may have been altered post-deposition through a process such as photochemical production in the snow from organic precursors. This question has been investigated for other highly soluble gases such as formaldehyde and acetaldehyde (Hutterli et al., 1999, 2002; Dominé and Shepson, 2002; Hutterli, 2003; Grannas et al., 2004; Domine et al., 2010) and could be applied to study alcohol deposition and post-depositional formation as well. Gao et al. (2012) observed that illuminated Arctic snow from remote, rural, and urban areas released formaldehyde but didn't release methanol. Diffusion of ambient Greenland air post-deposition followed by dissolution of alcohols in the ice could also contribute to the concentration of alcohols in the core although the lack of a temporal trend of other highly soluble gases analyzed such as formaldehyde and acetaldehyde suggest this is most likely not a dominant process.
2. Methods 2.1. Core processing The GISP-2 G2 Greenland (72°35′ min N 38°28’ min) ice core was obtained from the National Ice Core Laboratory (NICL), Denver, CO, USA. The core is a de-accessed core that was cut at NICL and divided into yearly samples by using data from previously collected Summit, Greenland cores. The 14 samples span ~261 years and the lower sections are assumed to be within ± 4 years of the date reported. The core was shipped overnight from NICL and was immediately cut upon arrival with a stainless steel saw in a laminar low flow hood fitted with a high efficiency particulate air (HEPA) filter. A 6 × 6 cm section was cut from the center of the 10 cm diameter core in order to limit contamination present in the outer layers of the core. ~50% of the outer portion of the core was removed in order to obtain a contaminant free portion of the core (Saigne et al., 1987; Legrand et al., 1988; Legrand et al., 1993; Jauhiainen et al., 1999). The inner core was allowed to melt at room temperature in covered 4L beakers that had been baked at 450 °C in a muffle furnace for a minimum of 4.5 h to remove organics prior to use. A beaker of MilliQ deionized water (MQ) next to the melting core samples in the hood served as a blank. The MQ water was processed in the same way as the actual core samples, and analyzed via the same method as the actual core. The melted core samples and the MQ blank were both at room temperature when analyzed, so any interactions between lab air and samples should have been similar for blanks and core samples. Analyses were performed as soon as the ice core was fully melted. The blank values for formaldehyde, acetaldehyde, methanol, and ethanol were 4.8 nM, non detect, 38.1 nM and non detect respectively, similar to the Milli-Q blank values reported when running modern rain samples in the same laboratory. The blank concentrations were subtracted from the measured sample concentrations prior to use. The blank concentrations were less than 4% of the average concentrations for each analyte except formaldehyde, where it was 9.8% of the average concentration. The data and blank controls used during sampling suggest contamination is not a major factor in these data because: (1) concentrations of highly water soluble alcohol oxidation products (aldehydes) did not increase at any one level in the core; (2) concentrations of other highly soluble gases (formaldehyde) did not increase and are consistent with previous studies of a Greenland ice cores spanning the same time range (Staffelbach et al., 1991; Anklin and Bales, 1997); (3) drilling fluid was not used to obtain this core; (4) neither butyl acetate (a possible contaminant from drilling fluid) nor any of the common laboratory contaminants benzene, ethylbenzene, styrene, carbon tetrachloride, and 2-methylfuran were detectable in the core; (5) methanol and ethanol concentrations fall exactly into the low end of the range of concentrations observed in contemporary precipitation (Felix et al., 2014; Kieber et al., 2014). For the purpose of the discussion and due to the relatively high solubility of the alcohols, it is assumed the majority of the alcohol present in samples is from alcohol dissolved in the precipitation during
2.2. Alcohol and aldehyde analysis Formaldehyde and acetaldehyde concentrations in core samples were determined by derivatization with 2,4-dinitrophenylhydrazine followed by separation and detection by HPLC (Kieber et al., 1999). Samples and standards were reacted with 2,4-dinitrophenylhydrazine for 1 h in the dark forming a hydrazone, which was separated from interfering substances by HPLC and quantified by UV detection at 370 nm. Derivatized samples (100 μL) were injected onto a reversed phase Kinetex100 mm × 4.60 mm 2.6μ C18 Phenomenex column with a 100 Å pore size at 10 °C. The mobile phase was a 1:1 mixture of 0.1% trifluoroacetic acid (TFA) in acetonitrile and 0.1% TFA in DIW at a flow rate of 1.00 mL min−1. Methanol and ethanol were determined by oxidation of the alcohol to formaldehyde (or acetaldehyde) via alcohol oxidase obtained from the yeast Hansenula sp. (Kieber et al., 2013). The enzyme was prepared by dissolution of 100 units of alcohol oxidase in 5 mL of 0.1M potassium phosphate buffer (pH 9.0). The sample (1000 μL) was combined with 10 μL of buffer, 100 μL of an enzyme working reagent (0.18 units mL−1) and allowed to react at 40 °C for 40 min (120 min for ethanol) before addition of 10 μL of DNPH. The concentration of methanol (or ethanol) was determined after HPLC analysis by the difference in formaldehyde (or acetaldehyde) concentration in samples with and without added enzyme. This method has a detection limit for methanol and ethanol of 6 nM and 10 nM, respectively. All ice core samples were run in triplicate (n = 3). The average standard deviations of samples run in triplicate were ± 8.6 nM for methanol, ± 1.2 nM for ethanol and ± 0.6 nM for both acetaldehyde and formaldehyde. 2.3. Supporting analyses Organic carbon content in filtered ice core samples were determined with a Shimadzu TOC 5000 carbon analyzer (Shimadzu, Kyoto, Japan) equipped with an ASI 5000 autosampler. pH was analyzed immediately after melting using a Ross electrode with low ionic strength buffers. Inorganic anions (Cl−, NO3−, and SO42−) were analyzed using ion suppressed chromatography. 3. Results The range of methanol and ethanol concentrations are 83 nM–1666 nM and 16 nM–424 nM, respectively (Table 1). The range of methanol concentrations in the ice core fell within the low end of concentration ranges reported for contemporary rain events at a coastal U.S. site (methanol: < 6 nM–9300 nM) (Felix et al., 2014). The range in 2
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Table 1 Concentration of analytes in the GISP-2 G2 Greenland ice core. The year column in the table represents the approximate age of the ice ± 4 yrs. All analytes were measured in triplicate (n = 3). Year (AD)
Methanol (nM)
Ethanol (nM)
Formal (nM)
Acetal (nM)
DOC (μM)
H+ (M)
Cl−(μM)
NO3−(μM)
SO42−(μM)
1686 1700 1724 1745 1765 1785 1804 1824 1844 1865 1885 1905 1926 1947
103 111 83 381 1199 1324 1562 1591 1666 1493 1446 1422 1412 1266
26 18 16 44 138 172 247 239 239 179 290 242 332 424
72 53 50 57 46 69 46 41 36 55 34 41 32 50
115 34 52 55 114 61 123 105 74 33 184 505 119 141
76.3 14.6 15.2 22.6 42.6 14.9 25.2 31.9 25.2 46.8 29.0 43.9 NA 37.4
6.3 E−07 1.5 E−06 1.3 E−06 1.0 E−06 1.0 E−06 4.0 E−06 1.3 E−06 1.8 E−06 2.0 E−06 1.6 E−06 4.0 E−07 6.3 E−07 1.3 E−06 1.3 E−06
1.93 2.07 3.42 2.92 15.82 5.56 2.82 3.37 2.72 8.62 1.82 4.38 2.73 1.77
2.47 3.45 5.42 15.65 13.43 2.85 2.46 2.21 1.59 2.82 1.58 3.17 2.5 4.38
0.7 0.46 0.53 0.68 1.67 3.42 0.55 0.91 0.42 0.57 0.35 1.38 0.63 1.07
4. Discussion
ethanol concentrations in the ice core was similar to the range in the yearly volume weighted average concentration of the alcohol measured at the Wilmington site between 2010 and 2015 (ethanol 126 nM–345 nM) (Willey et al., 2019). The increase in rainwater concentrations at the Wilmington site from 2010 to 2015 is very similar to the increase in ethanol from 242 to 424 nM from 1905 to 1947 in the ice core data. The range of formaldehyde and acetaldehyde concentrations were 32 nM–72 nM and 33 nM–505 nM, respectively. The range of acetaldehyde concentrations in the ice core fall within concentration ranges reported for contemporary rain events at a coastal U.S. site (23 nM–909 nM) but the range of formaldehyde concentrations fall well below the range of the contemporary concentration reported at the coastal U.S. site (100 nM–5500 nM) (Felix et al., 2014; Kieber et al., 1999). The sum of acetaldehyde, formaldehyde, ethanol and methanol concentrations make up on average 6% of the total DOC in the ice core samples. There was a large spike in sulfate (3.42 μM) in the ice core in 1785 possibly because of the Icelandic Laki volcanic eruption which occurred in 1783. Methanol and ethanol were significantly correlated with each other (p < 0.001) likely because both have a primary biogenic source (Table 2). Formaldehyde was negatively correlated (p < 0.05) with methanol. There was no correlation between ethanol and acetaldehyde in the ice core samples presented in Table 2. In our earlier study with current rain we also found that there was no correlation between the concentration of ethanol and acetaldehyde in rainwater at the Wilmington NC location which we suggest is because inputs other than simple oxidation of ethanol are responsible for determining the concentration of the aldehyde in precipitation (Kieber et al., 2014).
4.1. Methanol and ethanol sources Methanol concentrations showed a significant increasing trend from 1686 through 1844 (Mann –Kendall p < 0.001) but not for the entire time period (p = 0.062) (Fig. 1). Ethanol concentrations increased significantly over the whole time period (Mann-Kendall trend p < 0.001) (Fig. 1). The increase in methanol and ethanol concentrations may be attributed to changes in source type or magnitude. The concentration of both alcohols between 1686 and 1745 are low and consistent likely due to a period of cold climate (Little Ice Age (LIA)) suppressing plant activity that would directly affect emissions of alcohols from plants. Galbally and Kirstine (2002) predicted that lower methanol concentrations would be found in ice cores dating back to a
Table 2 Correlation coefficients between methanol, ethanol, and other ice core components. Bold faced values indicate significance at p < 0.001. Italics indicate significance at p < 0.05.
methanol ethanol formal acetal H+ Cl− NO3−
ethanol
formal
acetal
H+
Cl−
NO3−
SO42-
0.81
−0.56 −0.58
0.29 0.35 −0.33
0.19 −0.01 0.35 −0.40
0.19 0.13 0.06 0.04 0.08
−0.34 −0.38 0.18 −0.15 −0.18 0.48
0.18 0.04 0.43 0.10 0.70 0.39 0.11
Fig. 1. Ice core ethanol and methanol concentrations vs. year (AD). 3
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climatic cold period due to decreasing primary plant productivity. In previous ice core studies other analytes such as CH4 have also had relatively low concentrations during this period. This has been attributed to pre-1750 AD cold climate dampening biogenic activity including wetland biological activity that produces CH4 (Khalil and Rasmussen, 1989; Etheridge, 1998). Methane increases have been linked to an increase in wetland activity (e.g. production of CH4 by microbes) with increasing temperature as the LIA was coming to an end (Blunier et al., 1993; Etheridge, 1998; MacFarling et al., 2006). Ethanol emissions would likely increase with increasing plant activity brought on by the end of the LIA. Plants are reported to increase ethanol emissions after they have undergone a stress such as cold temperatures. For example, Fukui and Doskey (1998) report large ethanol fluxes from grassland after frost events. Ethanol emissions may also be increasing as wetlands are flooded and temperature rises. As plants are flooded in wetlands, the root zone becomes anaerobic and these conditions favor ethanol production in roots which in turn increases ethanol emissions. For instance grasslands, potted plants, and pines trees are reported to increase ethanol emissions after flooding (Fukui and Doskey, 1998; Holzinger et al., 2000; Rottenberger et al., 2008). The influence of temperature dependent increasing biogenic emissions on ethanol concentrations in the ice core is underscored by the strong correlation (p = 0.0092) between levels of the alcohol and the temperature anomaly as described by Moberg et al. (2005). The authors utilized a multi prong approach to reconstruct Northern Hemisphere temperatures by combining low-resolution proxies with tree - ring data. A minimum temperature was observed around AD 1600 during the LIA corresponding to low ethanol concentrations in the ice core. Temperatures rose consistently through the latter half of the 18 century and into the latter half of the 20th century analogous to the rise in ethanol concentrations most likely because the increase in temperature allowed for an increase in biogenic emissions. This may also be the case for increasing methanol concentrations since increasing temperatures can lead to exponentially increasing methanol emissions from plants (Folkers et al., 2008; Hu et al., 2011). Methanol increases much more rapidly than ethanol and this exponential feedback from increasing temperatures may be the cause for the increase in methanol before it levels off at ~1800 AD then begins slightly decreasing (see Fig. 1). Anthropogenic changes in land use during the period after 1750 AD may also have a modest influence on the increasing trend in alcohol concentrations. Land clearing for cropland as a result of the European agricultural revolution (1750–1880 AD) (Chorley, 1981), the European settlement of North America, and the need for biomass to burn leads to varying emission fluxes. Croplands are irrigated which increases the amount of anaerobic root zones, thus increasing the potential for ethanol production in the roots. Total European and North American cropland (Klein Goldewijk et al., 2011) is strongly correlated (n = 13, R = 0.80, p = 0.001) to ethanol concentrations (Fig. 2) suggesting that conversion of natural lands to working croplands may have increased ethanol emissions. In addition to clearing for agricultural purposes, land was also cleared to obtain biomass for burning. Biomass burning occurs naturally and anthropogenically and can be a source of both methanol and ethanol (Jacob et al., 2005; Millet et al., 2012). While biomass burning possibly contributed to the increased concentration of alcohols, the start of the increase in alcohol concentrations and reported increase in biomass burning are offset by ~100 years according to a fire index inferred from analyte concentrations in a Greenland ice core (Savarino and Legrand, 1998). Other than biomass burning, changes in the amplitude of additional minor sources could have added to the increase in alcohol concentrations. For instance, livestock waste was a significant source of CH4 in the pre-industrial period (Subak, 1994) therefore it may also have been an important source of alcohols since they are directly emitted from
Fig. 2. Ice core ethanol vs. Total European and N. American cropland.
livestock waste (Shaw et al., 2007; Sun et al., 2008). The increase in ethanol from 242 to 424 nM from 1905 to 1947 may also be influenced by emission from the use of ethanol as a fuel. By the turn of the 20th century ethanol was used as vehicle fuel as the Ford motor company created engines that ran onethanol, including the Ford Model T (1908) that could run exclusively on ethanol. Ethanol use as a vehicle fuel also increased in the early 20th century because it was a significant fuel employed during World War One (Kovarik, 1998; Solomon et al., 2007). Increases in ethanol in a Greenland ice core resulting from biofuel usage at the turn of the 20th century would, however, be relatively small compared to other sources such as biogenic inputs. 4.2. Methanol and ethanol atmospheric chemistry and oxidation The major sink for both methanol and ethanol is oxidation via the OH radical. Many studies of paleoatmospheric oxidation chemistry predicted a decrease in .OH concentrations during the time period this ice core encompasses (Staffelbach et al., 1991; Thompson, 1992; Sofen et al., 2010). Methane and CO, additional major sinks of .OH, increased over this time period which could decrease the atmospheric concentration of .OH. The decrease in .OH radical would decrease the primary alcohol sink and increase the lifetime thus increasing concentration. Atmospheric oxidation chemistry also affects peroxy radical formation, a precursor to alcohol formation. Methanol and ethanol can form from the peroxy oxidation products of methane and ethane, (e.g. methyl and ethyl peroxy radicals) especially in low NOx environments (Naik et al., 2010). The methyl and ethyl peroxy radical are the most abundant peroxy radicals in the atmosphere (Villenave et al., 1996) and it is possible that an increasing source of peroxy radicals (via increasing methane and ethane emissions) and low NOx environment lead to increases in alcohols via peroxy radical formation. .
5. Conclusion and implications The low median methanol (107 nM) and ethanol (22 nM) concentrations from 1686 to 1745 represent concentrations resulting from primarily natural emissions during a cold climate period. These concentrations provide a baseline alcohol level in wet and dry deposition before impacts from various sources (e.g. naturally recovering plant populations, anthropogenic sources, and changing atmospheric oxidation chemistry) increased alcohol concentrations in the atmosphere. Anthropogenic alcohol emissions are increasing globally as countries attempt to become more self-sustainable by turning to renewable vehicle fuels such as methanol and ethanol. For example China consumed 2.3 billion gallons of methanol in vehicle fuel (Dolan, 2013) and more than 95% of the gasoline sold in the U.S. is blended with ethanol. 4
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Further reading Burkhart, J.F., Bales, R.C., McConnell, J.R., Hutterli, M., 2006. Influence of North Atlantic Oscillation on anthropogenic transport recorded in northwest Greenland ice cores. J. Geophys. Res. 111 (D22), D22309. https://doi.org/10.1029/2005JD006771. Hastings, M.G., Jarvis, J.C., Steig, E.J., 2009. Anthropogenic impacts on nitrogen isotopes of ice-core nitrate. Sci 324 (5932), 1288. https://doi.org/10.1126/science.1170510. Kirstine, W.V., Galbally, I.E., 2012. The global atmospheric budget of ethanol revisited. Atmos. Chem. Phys. 12 (1), 545–555. https://doi.org/10.5194/acp-12-545-2012. Mullaugh, K.M., Willey, J.D., Kieber, R.J., Mead, R.N., 2012. Dynamics of the chemical composition of rainwater throughout Hurricane Irene. Atmos. Chem. Phys. 12 (10), 26995–27020. https://doi.org/10.5194/acpd-12-26995-2012. Nemecek-marshall, M., Macdonald, R.C., Franzen, J.J., Wojciechowski, C., Fall, R., 1995. Methanol emission from leaves. Plant Physiol. 108, 1359–1368.
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