Ethylene Glycol Emissions from On-road Vehicles - Environmental

Subscriber access provided by UNIV PRINCE EDWARD ISLAND

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

Environmental Science & Technology

1

Ethylene Glycol Emissions from On-road Vehicles

2 3 4

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

5

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

6

Allen H. Goldstein4,5, Robert A. Harley4

7 8

1

Department of Chemistry, University of Massachusetts, Amherst MA

9

2

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

10

3

Aerodyne Research, Inc., Billerica MA

11

4

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

12

5

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

13

Berkeley CA

14 15

*Corresponding Author.

16

[email protected]

17

413-545-4003 (phone)

18 19

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

20

Pittsburgh PA

21

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

22

Environmental Studies, Yale University, New Haven CT

1

ACS Paragon Plus Environment


23

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

24

Technology, Cambridge MA

2


Page 2 of 31

Page 3 of 31


25

Abstract.

26

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

27

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

28

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

29

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

30

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

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

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

40

aerosol following oxidation in the atmospheric aqueous phase.

41

3



(TOC art)

42 43 44

4


Page 4 of 31

Page 5 of 31


45

Introduction.

46

Vehicular emissions of organic compounds contribute greatly to ambient concentrations of

47

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

48

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

49

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

50

considerable uncertainty regarding the sources and atmospheric transformations of organic

51

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

52

predict aerosol volatility. 1, 6

1-3

Both primary and 4-5

There is

53 54

Although most vehicular organic compound emissions are from evaporation and incomplete

55

combustion of hydrocarbon fuels and lubrication oil,

56

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

57

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

58

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

59

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

60

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

61

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

62

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

63

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

64

overheating vehicle).

2, 7

engine coolant is another potential

65 66

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

67

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



Page 6 of 31

68

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

69

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

70

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

71

quantified in any emission inventory.

72 73

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

74

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

75

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

76

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

77

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

78

reaction products by analogy with larger diols.

79

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

80

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

81

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

82

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

83

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

84

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

85

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

86

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

87

EG is not currently considered a significant SOA precursor.

11

The major products from photolysis of

16

and

88 89

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

90

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


Page 7 of 31


91 92

Experimental Methods.

93

Tunnel Sampling.

94

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

95

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

96

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

97

Laboratory (AML).

98

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

99

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

100

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

101

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

102

measurements are described elsewhere.

103

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

104

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

105

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

106

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

107

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

108

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

109

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

110

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

111

have 2σ uncertainties of 10%.

112

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

113

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

19

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

20-21

Briefly, the AML sampled gas-phase compounds

7



Page 8 of 31

114

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

115

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

116

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

117

-5° C in Boston.

118 119

PTR-MS measurements.

120

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

121

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

122

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

123

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

124 125

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

(1)

126

(HOCH2CH2OH)H+ → (C2H5O)+ + H2O

(2)

127 128

Figure 1.

129

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

130

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


Page 9 of 31


131 132

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

133

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

134

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

135

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

136

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

137

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

138

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

139

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

140

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

141

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

142

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

143

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

144

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

145

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

146

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

147

concentrations at times due to this interference by ethylene glycol.

23

148 149

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

150

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

151

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

152

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

153

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



154

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

155

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

156

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

Page 10 of 31

157 158

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

159

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

160

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

161

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

162

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

163

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

164

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

165

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

166

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

167

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

168

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

169

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

170

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

171

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

172

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

173

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

174

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

175

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

176

Supporting Information). 10


Page 11 of 31


177 178

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

179

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

180

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

181

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

182

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

183

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

184

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

185

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

186

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

187

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

188

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

189

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

190

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

191 192

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

193

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

194

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

195

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

196

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

197

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

198

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

199

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



Page 12 of 31

200

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

Ethylene Glycol Emissions from On-road Vehicles - Environmental

Recommend Documents