Compound-specific carbon isotopic composition of ethanol in Brazil

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Energy and the Environment

Compound-specific carbon isotopic composition of ethanol in Brazil and US vehicle emissions and wet deposition Joseph David Felix, Rachel Thomas, Matt Casas, Megumi Shimizu, Gene Brooks Avery, Robert J. Kieber, Ralph N. Mead, Chad Lane, Joan D. Willey, Amanda Guy, and Maria Lucia Campos Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05325 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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Environmental Science & Technology

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Compound-specific carbon isotopic composition of ethanol in Brazil and US vehicle emissions and wet deposition

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*J. David Felix1, Rachel Thomas2, Matt Casas2, Megumi S. Shimizu2, G. Brooks Avery2, Robert J. Kieber2, Ralph N. Mead2, Chad S. Lane3, Joan D. Willey2, Amanda Guy2, M. Lucia A. M. Campos4

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1. Department of Physical and Environmental Science, Texas A&M University – Corpus Christi, Corpus Christi, Texas, USA, 78412

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3. Department of Earth and Ocean Sciences, University of North Carolina Wilmington, Wilmington, NC, USA, 28403

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2. Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, NC, USA, 28403

4. Departamento de Química, Faculdade de Filosofia, Ciencias e Letras de Ribeirão Preto, Universidade de São Paulo, Avenida dos Bandeirantes 3900, 14040-901 Ribeirão Preto, São Paulo, Brazil *Corresponding author: [email protected]

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ABSTRACT Global atmospheric ethanol budget models include large uncertainties in the magnitude

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of ethanol emission sources and sinks. In order to apply stable isotope techniques to constrain

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ethanol emission sources, a headspace solid phase micro-extraction gas chromatograph-

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combustion-isotope ratio mass spectrometry method (HS-SPME-GC-C-IRMS) was developed to

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measure the carbon isotopic composition of aqueous phase ethanol at natural abundance levels (1 to 30

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µM) with a precision of 0.4‰. The method was applied to determine the carbon isotope signatures (δ13C)

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of vehicle ethanol emission sources in Brazil (-12.8 ± 2.4‰) and the US (-9.8 ± 2.5‰), and to measure

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the carbon isotope composition of ethanol in wet deposition (-22.6 to -12.7‰). A two end-member

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isotope mixing model was developed using anthropogenic and biogenic endmembers and fractionation

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scenarios to estimate ethanol source contributions to wet deposition collected in Brazil and US. Mixing

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model results indicate anthropogenic sources contribute two and a half to four times more ethanol to the

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atmosphere than previously predicted in modeled global ethanol inventories. As established and

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developing countries continue to rapidly increase ethanol fuel consumption and subsequent

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emissions, understanding the magnitude of all ethanol sources and sinks will be essential for

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modeling future atmospheric chemistry and air quality impacts.

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INTRODUCTION Global ethanol fuel production and consumption have doubled over the previous decade

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in efforts to reduce greenhouse gas emissions and increase domestic production of fuel.1,2 Sixty-

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four countries currently have active biofuel mandates or targets that are increasing ethanol

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production.3,4

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placed on the effects ethanol production has on a country’s food, water, and energy supply5-7

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while less effort has been applied to considering the atmospheric impacts of increased ethanol

When determining the pros and cons of production mandates, focus is often

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consumption. Studies focusing on atmospheric effects have concentrated on potential decreases

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in greenhouse gases and VOCs, but there are number of adverse atmospheric impacts associated

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with ethanol emissions and combustion byproducts.2,8,9

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Vehicle ethanol emissions can alter the oxidizing capacity of the atmosphere and alter

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radical and NOx cycling as well as NOy speciation.10,11 Ethanol combustion also leads to an

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increase in atmospheric concentrations of the secondary pollutant peroxyacetyl nitrate and a

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variety of VOCs with relatively high ozone reactivities.12 For instance, large increases in

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formaldehyde and acetaldehyde are associated with blended fuel (i.e. gasoline/ethanol)

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emissions, both of which have relatively high ozone reactivities13 and many studies modeling

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hypothetical increases in blended fuel consumption in the US have predicted an increase in

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national average ozone concentrations.2, 14-16 However, depending on the NOx and VOC

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sensitivity of ozone formation in individual regions, some regions in the US may experience

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decreasing ozone concentrations with increasing ethanol fuel consumption.2,14 The increasing

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effect of ethanol fuel emissions on tropospheric ozone production was recently observed in a

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“real world” scenario when Brazil experienced large fluctuations in the price of ethanol. This

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forced the proportion of bi-fuel vehicles (engines capable of running on any proportion of

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gasoline and ethanol) burning gasohol (i.e E27, where EXX is defined as gasoline-ethanol blend

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and XX is % ethanol) instead of ethanol, to rise from 14 to 76%. In São Paulo, Brazil this 62%

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increase in bi-fuel vehicles burning gasohol, resulted in an average of 22% decrease in ground-

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level ozone.17,18 Despite this observed correlation between ethanol fuel consumption and ozone

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levels, most research investigating the impacts of ethanol fuel consumption on air quality are

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modeling efforts based on limited empirical atmospheric ethanol concentration data.



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The lack of atmospheric ethanol concentration data has led to a deficiency in

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comprehension of the sources, sinks, and cycling of ethanol in the atmosphere and has been

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noted in various studies.2,8, 11, 19-23 Subsequently, large discrepancies are reported among

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estimated atmospheric ethanol inventories. For instance, recent ethanol budget estimates range

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from 7 to 56 Tg/yr for the total ethanol source to the atmosphere. The reported major source of

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ethanol (biogenic emissions from plants) ranges from 21% to 92% of total emissions while

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anthropogenic sources (vehicle and ethanol production facility emissions) range from 8% to 34%

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of total emissions.8,10,22,24 The large range in estimated ethanol contribution from anthropogenic

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sources highlights the general lack of knowledge of vehicle ethanol emission contributions to

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atmospheric ethanol concentrations. Delineating the significance of this anthropogenic source to

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atmospheric concentrations is vital when debating the viability of the current and future global

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biofuel mandates.

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One quantitative approach to determining the significance of anthropogenic versus

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biogenic emission contributions to atmospheric ethanol concentrations is to measure the carbon

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isotopic composition of ambient ethanol (δ13CEtOH) in the air or in atmospheric waters (e.g. wet

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deposition, condensates). The δ13C values of ethanol from the major biogenic source (C3 plants)

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(-30.9 ± 4.2‰ (n =15)) are highly distinguishable from the vehicle exhaust emission source

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(direct vehicle sampling: -5.0‰ (n =1); roadside sampling: 10.4 +3.6‰) (n = 25)).21,25 The large

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difference in δ13C signatures between these two sources is ideal for constructing a two

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endmember isotope mixing model for quantifying the relative contribution of biogenic versus

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anthropogenic emission sources to the atmosphere. However, the vehicle emission endmember

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is based on one measurement from a vehicle produced in 1972 that was not equipped with a

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catalytic converter and current combustion efficiency technology and single site roadside


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collections of ambient air used to represent a vehicle fleet.25 For this mixing model approach to

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be effective in estimating source contribution, more adequate endmember source characterization

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needs to be undertaken.

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In order to further constrain this vehicle endmember and apply an isotope mixing model

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to determine ethanol source contributions to the atmosphere this work aims to 1) combine a

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headspace solid phase micro-extraction gas chromatograph (HS-SPME-GC) method optimized

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for the analysis of low aqueous phase ethanol concentration samples with gas chromatograph-

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combustion-isotope ratio mass spectrometry (GC-C-IRMS), 2) characterize the carbon isotopic

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composition of ethanol in vehicle emissions, and 3) determine biogenic and anthropogenic

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emission source contributions to ethanol concentrations in Brazil and US wet deposition,

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countries that account for ~84% of the world’s ethanol fuel consumption and production.26

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MATERIALS AND METHODS

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Rain Sample Collection and Preservation

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Rainwater samples were collected on an event basis in 2016 at the University of North

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Carolina Wilmington (UNCW) campus (Figure 1). The UNCW rainwater collection site (34°

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22.0' N, 77° 86.3' W) is an open area of land of approximately 0.01 km2 containing vegetation

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that is representative of inland coastal regions of southeastern North Carolina including Aristida

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stricta, Quercus laevis, and Pinus palustris. The site is located approximately 8.5 km from the

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Atlantic Ocean. Samples were collected utilizing Aerochem-Metrics (ACM) Model 301

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Automatic Sensing Wet/Dry Precipitation Collectors containing glass beakers. All beakers and

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glassware used for the collection and filtration were rinsed with Millipore MQ deionized water

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(resistivity of 18 MΩ.cm @ 25 °C) and combusted in a muffle furnace at 450°C for a minimum



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of 4.5 hours to eliminate organics prior to use. Immediately after the rain event, samples were

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returned to the laboratory and filtered using 0.2 μm acid-washed Gelman Supor® polysulfonone

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filters. Brazil wet deposition were collected using an automated sampler located at a site on the

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University of São Paulo campus in Ribeirão Preto (21° 11’ S, 47° 48’ W) (Figure 1) and

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underwent the same treatment as the Wilmington rain, except the glassware used for ethanol

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analysis was cleaned with Fenton solution, a mixture of Fe2+ salts with hydrogen peroxide.27 All

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samples were preserved by adding 70 µL of 100 mg/L HgCl2 to 40 mL of sample to eliminate

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variability in ethanol concentrations that could occur during sample collection and analysis. Previous

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stability experiments report ethanol concentration stability for up to 160 days when preserved with 100

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mg/L HgCl2.23

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Vehicle Exhaust Sample Collection Emissions from four vehicles in the US consuming E10 fuel and five Brazilian vehicles

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consuming either E27 or E100 fuel were sampled. A 500 mL fluorinated ethylene propylene

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resin (FEP) bottle was placed on the tailpipe of the vehicle. The bottle’s bottom was removed

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and feathered so that it could be fitted to different tail pipe diameters. The feathered end was then

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sealed on the tailpipe of the car using a hose clamp. On the opposite end, the bottle was attached

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to a 250 mL low-density polyethylene (LDPE) bottle containing 100 mL of synthetic wet

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deposition (SWD, pH 4.5 H2SO4). Exhaust was collected from cars while idling in neutral. The

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exhaust was allowed to bubble through the 100 mL of SWD for ten minutes to ensure ethanol

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concentrations were well above the HS-GC-C-IRMS analysis limit of detection of 1 µM. All

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exhaust was 0.2 µm filtered and immediately preserved with 100 mg/L HgCl2 upon collection.

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Ethanol Concentration Analysis


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A previously published solid-phase microextraction (SPME) ethanol concentration

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analysis optimized for low concentration aqueous phase samples was used to analyze vehicle

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exhaust and wet deposition samples.28 In summary, 3.5 g of NaCl was added to 11.6 mL of

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vehicle or wet deposition sample, which was then adjusted to a final pH of 4.4 by adding 0.4 mL

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of a saturated succinic acid solution. The sample was heated at 50 ºC for 10 minutes prior to a

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20-minute extraction by a 75 µm Carboxen/PDMS SPME fiber in the headspace. Magnetic

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stirring at a rate of 750 rpm occurred during heating and extraction. The fiber was thermally

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desorbed after extraction in the injection port of the GC-FID for concentration analysis. The

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same method was used for ethanol extraction before injection to the GC-C-IRMS.

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Carbon Isotope Analysis of Ethanol A Thermo Trace 1310 GC was equipped with a Restek®-5 (30 m x 0.32 mm x 1 μm) or

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Equity-5 (20 m x 0.32 mm x 0.32 μm) fused-silica capillary column and a Merlin microseal

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adapter was mounted on the GC injection port. A Taylor-Wharton 25 L liquid nitrogen Dewar

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was equipped with a liquid withdrawal device connected to the back of the GC oven by a copper

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pipe to allow for sub-ambient temperatures. Sub-ambient temperatures focused the ethanol upon

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injection leading to improved chromatography and separation.

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After ethanol was extracted via the SPME method described previously, the SPME fiber

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was introduced to the GC-C-IRMS via the Merlin microseal septum encased in the microseal

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adapter mounted on the GC injection port. The oven temperature was held for 3 minutes at 10oC

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before being ramped to 60°C at 10°C/min and heated to the final temperature of 200°C at

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60°C/min and held for 2 minutes. Eluents were carried to instrumentation (e.g.Thermo Isolink II)

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designed for conversion of analytes to CO2 via the helium carrier gas where they were



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combusted at 1000°C in the presence of nickel oxide and platinum wires in an alumina ceramic

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reactor tube. Resulting CO2 was analyzed in continuous-flow mode via an interfaced Thermo

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Delta V plus stable isotope mass spectrometer at the University of North Carolina Wilmington

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Isotope Ratio Mass Spectrometry (UNC-WIRMS) Facility.

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The vacuum system of the IRMS was maintained at a constant operating source pressure

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of 4.9 x 10-8 mbar. Standards and samples were calibrated daily against a working CO2

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reference gas versus the international Vienna PeeDee Belemnite (V-PDB) standard (equation 1). δ13Csample = [(13C/12C)sample / (13C/12C)V-PDB -1] x 1,000

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(1)

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Two ethanol isotope standards were used to apply a two-point normalization to the V-PDB

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scale.29 The standards were purchased from the stable isotope reference program at Indiana

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University's Biogeochemical Laboratories (δ13C values of -10.98 ± 0.02‰ and -27.53 ± 0.02‰).

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Reference standards were then further verified using a Costech 4010 elemental analyzer

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interfaced with a Thermo Delta V Plus stable isotope ratio mass spectrometer (EA-IRMS) at the

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University of North Carolina Wilmington Isotope Ratio Mass Spectrometry (UNC-WIRMS)

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Facility.

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Source Apportionment

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Ethanol source contribution to wet deposition was calculated using a two-endmember

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mixing model (eq. 2). EPA ISOERROR30 was used to report uncertainty due to the variability in

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source signatures and deviation of the δ13CEtOH values in wet deposition samples. The uncertainty

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is reported as the standard error of the mean at 95% confidence level. In order to calculate

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uncertainty, the ISOERROR user must provide mixture δ13CEtOH values with standard deviations

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and source endmember δ13CEtOH values with standard deviations. Our anthropogenic endmember 8 ACS Paragon Plus Environment

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represents both vehicle and ethanol production facility emissions due to the observed overlap of

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vehicle emission and commercially produced δ13CEtOH values (results section) . The

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anthropogenic endmembers for the Ribeirão Preto, SP, BR and Wilmington, NC, USA were

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calculated separately due to differences between ethanol fuel feedstock, fuel content and vehicle

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engines (further detail in discussion section). The biogenic endmember δ13CEtOH value was

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calculated from available literature of δ13CEtOH values emitted from C3 plants (-30.9 ± 4.2‰ (n

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=15)).21,25 C3 plant emissions were chosen to represent the biogenic endmember because C3

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plant biomass contributes ~95% of the world’s total C3 + C4 biomass.31 It should be noted that

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this assumes that C3 plants and C4 plants emit ethanol at similar amounts per biomass and future

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work should focus on quantifying C3 vs. C4 emission rates in an effort to understand the

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significance of their role in ethanol emission. Other endmembers (e.g. biomass burning, ocean,

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C4 plants) are acknowledged, but are assumed to contribute minimally. For instance, Kirstine

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and Galbally 2012 estimate global biomass burning and ocean to contribute 1.9 and 9.5%,

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respectively. Additionally many of the ethanol source δ13CEtOH values have not been thoroughly

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characterized.21

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δ13CEtOH wet deposition =𝑓anthro (δ13CEtOH anthro) + (1-𝑓anthro)( δ13CEtOH biogenic)

(2)

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HYSPLIT Back Trajectory Models

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Air-mass back trajectories were generated for each wet deposition sample using the

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Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT) (Figure 1). The back

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trajectories were calculated starting at the end of the precipitation event for a 72-hour hind-cast

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at an altitude of 500 m. Seventy two hours was chosen because the estimated lifetime of ethanol



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is 2.8 days10,32 and the 500 m level represented the air mass near the well-mixed boundary layer

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likely to contribute more heavily to in-cloud processes contributing to wet deposition.33

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Figure 1: Left: Sampling sites Ribeirão Preto, SP, BR (purple circle) and Wilmington, NC,

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USA (green circle). Middle: Wilmington NC, USA (X) wet deposition event air mass back

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trajectories. Right: Ribeirão Preto, SP, BR (X) wet deposition events air mass back trajectories.

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Maps created via ESRI34.

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RESULTS AND DISCUSSION

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HS-SPME-GC-C-IRMS method

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Ethanol isotope standards (-10.98 ± 0.02‰ and -27.53 ± 0.02‰) were analyzed at

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various concentrations to determine isotope fractionation associated with the SPME process. The

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measured isotopic composition of the isotope standard with the higher δ13CEtOH value ranged

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between -10.2 and -11.2 ‰ with a mean of -10.9 ± 0.4‰ indicating there was no offset from the

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value reported by the manufacturer. Additionally, there was no concentration dependent offset

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variation between 1 and 30 µM (Figure 2). The measured δ13CEtOH of the lower isotope standard

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(-29.9 ± 0.5) was offset from the manufacturer’s given value by an average of -2.4‰, but the

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δ13C offset was consistent over a range of concentrations between 1 and 30 µM and any observed


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variation was not significant (ANOVA, p = 0.4) thus the “offset” does not affect the results when

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using two-point normalization (Figure 2).

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This measured offset of the lower standard is similar to the offset (1.4 to 2.2‰) observed

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by previously reported SPME-GC-C-IRMS ethanol analysis methods35 and has been observed in

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the analysis of several other polar VOCs (

Compound-specific carbon isotopic composition of ethanol in Brazil

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