Ethylene Glycol Emissions from On-road Vehicles - ACS Publications

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Ethylene Glycol Emissions from On-road Vehicles Ezra C. Wood, W. Berk Knighton, Edward Charles Fortner, Scott C Herndon, Timothy B. Onasch, Jonathan P Franklin, Douglas R. Worsnop, Timothy R Dallmann, Drew Gentner, Allen H. Goldstein, and Robert A. Harley Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 20 Feb 2015 Downloaded from http://pubs.acs.org on February 21, 2015

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Environmental Science & Technology

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Ethylene Glycol Emissions from On-road Vehicles

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Ezra C. Wood1*, W. Berk Knighton2, Ed C. Fortner3, Scott C. Herndon3, Timothy B. Onasch3,

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Jonathan P. Franklin3###, Douglas R. Worsnop3, Timothy R. Dallmann4#, Drew R. Gentner4##,

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Allen H. Goldstein4,5, Robert A. Harley4

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1

Department of Chemistry, University of Massachusetts, Amherst MA

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2

Department of Chemistry and Biochemistry, Montana State University, Bozeman MT

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3

Aerodyne Research, Inc., Billerica MA

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4

Department of Civil and Environmental Engineering, University of California, Berkeley CA

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5

Department of Environmental Science, Policy and Management, University of California,

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Berkeley CA

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*Corresponding Author.

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[email protected]

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413-545-4003 (phone)

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# now at Center for Atmospheric Particle Studies, Carnegie Mellon University,

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Pittsburgh PA

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## now at Department of Chemical and Environmental Engineering, School of Forestry and

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Environmental Studies, Yale University, New Haven CT

1

ACS Paragon Plus Environment


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### now at Department of Civil and Environmental Engineering, Massachusetts Institute of

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Technology, Cambridge MA

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Abstract.

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Ethylene glycol (HOCH2CH2OH), used as engine coolant for most on-road vehicles, is an

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intermediate volatility organic compound (IVOC) with a high Henry’s Law Coefficient. We

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present measurements of ethylene glycol (EG) vapor in the Caldecott Tunnel near San Francisco

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using a proton transfer reaction mass spectrometer (PTR-MS). Ethylene glycol was detected at

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mass to charge ratio 45 - usually interpreted as solely coming from acetaldehyde. EG

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concentrations in bore 1 of the Caldecott Tunnel, which has a 4% uphill grade, were

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characterized by infrequent (~once per day) events with concentrations exceeding ten times the

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average concentration, likely from vehicles with malfunctioning engine coolant systems. Limited

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measurements in tunnels near Houston and Boston are not conclusive regarding the presence of

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EG in sampled air. Previous PTR-MS measurements in urban areas may have overestimated

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acetaldehyde concentrations at times due to this interference by ethylene glycol. Estimates of EG

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emission rates using the Caldecott Tunnel data are unrealistically high, suggesting that the

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Caldecott data are not representative of emissions on a national or global scale. EG

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emissions are potentially important because they can lead to the formation of secondary organic

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aerosol following oxidation in the atmospheric aqueous phase.

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3



(TOC art)

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Introduction.

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Vehicular emissions of organic compounds contribute greatly to ambient concentrations of

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primary pollutants (e.g., volatile organic compounds, particulate matter) and to the formation of

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secondary pollutants such as ozone and secondary organic aerosol (SOA).

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secondary pollutants affect climate and have adverse effects on public health.

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considerable uncertainty regarding the sources and atmospheric transformations of organic

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aerosol: models under predict SOA concentrations and oxygen-to-carbon (O/C) ratios and over

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predict aerosol volatility. 1, 6

1-3

Both primary and 4-5

There is

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Although most vehicular organic compound emissions are from evaporation and incomplete

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combustion of hydrocarbon fuels and lubrication oil,

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vehicular source of organic compound emissions. The most common type of engine coolant used

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is a 50% by volume solution of ethylene glycol (1,2-ethanediol, HOCH2CH2OH) in water. In

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light-duty vehicles, approximately 5 to 15 liters of engine coolant are pumped at high pressure (2

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bar) and temperature (up to 130° C) past the engine cylinders and through the vehicle’s radiator.

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Leaks of coolant can develop in numerous places within a vehicle, e.g. the water pump’s shaft

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seals, the engine head gaskets, the radiator cap, the rubber transport hoses, and the radiator itself.

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Liquid EG can leak from a vehicle onto the road and subsequently evaporate or can evaporate

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directly from a malfunctioning cooling system (e.g., as part of the visible mist emitted by an

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overheating vehicle).

2, 7

engine coolant is another potential

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Besides its use as an engine coolant, EG is also used in numerous industrial purposes such as the

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production of polyethylene terephthalate (PET) bottles (~45% of global use)8 and for aircraft 5



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deicing. The US EPA’s 2011 Toxic Release Inventory lists airborne EG emissions of 0.8 Gg/yr

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from point sources in the U.S., 9 but this is a lower limit given that not all EG-emitting facilities

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are required to report emissions. To our knowledge, vehicular emissions of EG have not been

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quantified in any emission inventory.

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In the atmosphere, ethylene glycol can be oxidized either in the gas phase or in the aqueous

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phase following uptake to a cloud, fog, or wet aerosol particle. Atmospheric oxidation of gaseous

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EG by OH is rapid (rate constant 1.5 × 10-11 cm3 molecule-1 s-1)10, corresponding to an

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atmospheric lifetime due to reaction with 106 molecules cm-3 OH of 19 hours. Glycolaldehyde

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(HOCH2CHO), glycolic acid (HOCH2COOH), and formaldehyde are presumably the main

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reaction products by analogy with larger diols.

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glycolaldehyde are formaldehyde and carbon monoxide, and the main products of the reaction of

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glycolaldehyde with OH in the gas phase are glyoxal and formaldehyde. 12 Gas-phase chemistry

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and gas-particle partitioning of EG and its oxidation products do not lead to SOA formation.13

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The high Henry’s Law coefficients of EG, glycolaldehyde, glyoxal, and glycolic acid (ΚH = 103

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to 5×105 M atm-1)14-15, however, lead to facile uptake into the atmospheric aqueous phase,

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wherein oxidation leads to highly oxidized products such as glyoxylic acid, oxalic acid

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oligomers. 17 Following liquid water evaporation, these highly oxidized products can form SOA

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with high oxygen/carbon ratios. 18 Due to current estimates of industrial and vehicular emissions,

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EG is not currently considered a significant SOA precursor.

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The major products from photolysis of

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and

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In this paper we present observations of unexpectedly high concentrations of EG in a California

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highway tunnel and discuss possible emission mechanisms and atmospheric implications. 6


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Experimental Methods.

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Tunnel Sampling.

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Pollutant concentrations in bore 1 of the Caldecott Tunnel near Oakland, CA were measured

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from 22 July 2010 to July 28 2010 by a combination of real-time (1-second time resolution) and

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integrated measurement devices housed in the East Fan Room or the Aerodyne Mobile

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Laboratory (AML).

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the roof of the tunnel. Bore 1 of the Caldecott Tunnel (1 km long) has a 4% uphill grade and the

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preceding 4 km of road (Route 24) have grades of 4 to 6.5%. Diesel-fueled vehicles accounted

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for 1 to 4% of the total traffic with the rest being gasoline-powered. 20 Ambient temperatures in

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nearby Oakland ranged from 12 to 21° C. Details on the sampling configuration and

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measurements are described elsewhere.

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through a 1-2 µm PTFE filter and 34 meters of 0.95 cm (3/8”) inner diameter perfluoroalkoxy

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(PFA) tubing at a flow rate of 11 standard liters per minute (SLPM). The inlet was periodically

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flooded (overblow) upstream of the filter with dry zero air (AirGas) to monitor instrument

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baselines. The AML also periodically measured outdoor air near the laboratory itself by

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disconnecting the long sampling tube. Acetaldehyde (CH3CHO) and more than 200 other

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individual volatile organic compounds (VOCs) and intermediate volatility organic compounds

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(IVOCs) were measured in the East Fan Room by gas chromatography with mass spectrometry

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and flame ionization detection. 21 VOC and IVOC measurements are averaged to 60 minutes and

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have 2σ uncertainties of 10%.

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The AML also sampled air in the Washburn tunnel outside Houston, TX during two round-trip

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transits on 22 May 2009 as part of the Study of Houston Atmospheric Radical Precursors

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The sampling point for all measurements was through an access grate at

20-21

Briefly, the AML sampled gas-phase compounds

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(SHARP) campaign. The AML conducted similar mobile measurements in the Central Artery I-

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93 Tunnel in Boston, MA on 16 January 2008. Total sampling times (with a 1 m inlet) were 4

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minutes in Houston and 9 minutes in Boston. Ambient temperatures were 21°C in Houston and

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-5° C in Boston.

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PTR-MS measurements.

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Fast 1-second measurements of VOCs were made with an unmodified proton transfer reaction

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mass spectrometer (PTR-MS, Ionicon) stationed in the AML. Ethylene glycol with a molecular

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weight of 62 g mol-1 was detected at mass to charge ratio m/z = 45 (Figure 1) following a

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dehydration fragmentation reaction (reactions 1 and 2) as first pointed out by Wisthaler et al.: 22

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H3O+ + OHCH2CH2OH → (HOCH2CH2OH)H+ + H2O

(1)

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(HOCH2CH2OH)H+ → (C2H5O)+ + H2O

(2)

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Figure 1.

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PTR-MS difference mass spectrum of air sampled from the headspace of in-service engine

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coolant. m/z 61 and m/z 75 are not thought to be related to ethylene glycol. 8


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CH3CHO is also detected at m/z 45, thus the total m/z 45 signal observed reflects the sum of

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both compounds. Since CH3CHO was simultaneously measured by in-situ GC-FID, its

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contribution to m/z 45 can easily be removed to calculate the EG concentration. Except at night,

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the GC CH3CHO concentrations rarely exceeded 25% of the m/z 45 values. The only other

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known contributors to m/z 45 are ethylene oxide (H2COCH2), which is an isomer of

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acetaldehyde, and a weak interference from protonated carbon dioxide (CO2H+). Ethylene oxide

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is an intermediate in the conversion of ethylene to ethylene glycol, and while there may be

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industrial sources, there is no evidence that vehicles emit ethylene oxide. The high m/z 45

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concentrations could only have been caused by ethylene oxide if there were numerous vehicles

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transporting and leaking ethylene oxide at high emission rates in the tunnel. Endothermic charge

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transfer to carbon dioxide forming CO2H+ has been observed to occur within the PTR-MS.

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The interference from CO2 is small and no increases in m/z 45 were observed during spikes of

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[CO2] of several hundred ppm in which there was little increase in [CO] or other pollutants.

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Furthermore, CO2 would not produce the slow time response observed during the high m/z 45

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events. Previous PTR-MS measurements in urban areas may have overestimated CH3CHO

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concentrations at times due to this interference by ethylene glycol.

23

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The PTR-MS m/z 45 measurements presented here use the instrumental response of CH3CHO,

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quantified by dilution of a multicomponent gas standard (Apel-Riemer). In order to estimate the

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PTR-MS sensitivity response factor for ethylene glycol, we calculate the reaction rate constants

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for the proton transfer reaction of H3O+ with acetaldehyde and ethylene glycol using classic ion-

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molecule theory, similar to that presented by Zhao and Zhang. 24 These calculations suggest that 9



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the PTR-MS sensitivity response factor for ethylene glycol is ~10% lower than that of

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acetaldehyde. Values for the dipole moment and polarizability for acetaldehyde and ethylene

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glycol are from the CRC Handbook of Chemistry and Physics. 25

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Concentrations determined by the PTR-MS are the difference between measurements of ambient

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air and measurements of VOC-free air. The instrumental background of the PTR-MS was

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checked using the dry zero air overblows and also by periodic sampling (every 67 min) of VOC-

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free air generated with a heated platinum catalyst connected to the PTR-MS by 1 m of 1/8” OD

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PFA tubing. After the onset of high EG events the background signal at m/z 45 increased and

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slowly returned to normal levels indicating adsorption and subsequent desorption of EG

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onto/from the 34 m sampling tube and the internal surfaces of the PTR-MS. This behavior was

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evident by a diminished time response of the m/z 45 signal during dry zero air overblows and

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when the PTR-MS switched to sampling air from its catalyst. When m/z 45 mixing ratios were

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high (over 20 ppb), the 1/e time response in the m/z 45 signal exceeded one minute when the

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PTR-MS switched to sampling catalyst air (102 ± 5 sec on 27 July 2010), compared to ~2

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seconds for other VOCs measured (see Figure SI-1 in the Supporting Information). Similarly,

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m/z 45 levels rapidly decreased during zero air overblows but remained elevated (Figure SI-2 in

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the Supporting Information). Reversible sorption leads to errors in short time scale

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measurements (e.g., 1 Hz) but likely cancels out when longer averaging times are considered

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(i.e., PTR-MS measurements are likely low when [EG] is increasing and high when [EG] is

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decreasing). The time response of the sampling system and PTR-MS to CH3CHO was

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comparable to that other VOCs, evident from standard additions of CH3CHO (Figure SI-3 in the

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Supporting Information). 10


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Several additional diagnostic tests demonstrate that the PTR-MS measurements of EG were

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indeed reflective of tunnel air and not long-term contamination of the filter, sampling tube, or

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internal surfaces of the PTR-MS. If the high m/z 45 measurements from the tunnel (described in

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the Results section) were solely caused by long-term desorption of EG from the sampling tube

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when sampling humid air, then sampling outdoor air would have yielded similarly high (20+

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ppb) m/z 45 readings. At the conclusion of the study on 27 July 2010, the PTR-MS sampled

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outdoor ambient air using the same long sampling tube with a used filter (Figure SI-3 in the

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Supporting Information). In contrast to the 35 ppb measured in the tunnel only 1.5 hours earlier,

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the PTR-MS measured an m/z 45 mixing ratio of only 4 ppb, consistent with normal outdoor

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CH3CHO mixing ratios. The low m/z 45 mixing ratios observed by the PTR-MS at night, which

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at times agreed with the CH3CHO measurements by the GC within the measurement

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uncertainties (see Results section), further demonstrate that the PTR-MS and sampling inlet were

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able to “recover” from the high concentrations of EG sampled during the day.

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The PTR-MS was periodically offline for calibrations and other instrumental maintenance.

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During the seven days of sampling, the PTR-MS was online for 39 out of 84 total hours of

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daytime measurements (07:00 – 19:00) and 70 out of 84 hours of nighttime measurements (19:00

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– 07:00). Daytime and nighttime data were averaged to 1 sec and 14 sec, respectively. Because

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of the uncertainty in the internal PTR-MS response to EG and the ad/desorption of EG to both

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the long sampling tube and the internal surfaces of the PTR-MS, we estimate the 2σ uncertainty

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of the time-averaged EG measurements as (+ 100% / - 50%). This large uncertainty does not

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affect the conclusions of this paper. At the Caldecott tunnel, the contribution of CH3CHO (as 11



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measured by the GC) to the total m/z 45 concentration (usually

Ethylene Glycol Emissions from On-road Vehicles - ACS Publications

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