Impact of Changes in Oil and Gas Production Activities on Air Quality

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Impact of Changes in Oil and Gas Production Activities on Air Quality in Northeastern Oklahoma – Ambient Air Studies in 2015, 2016, and 2017 Buddhadeb Ghosh Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05726 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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Environmental Science & Technology

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Impact of Changes in Oil and Gas Production

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Activities on Air Quality in Northeastern Oklahoma

3

– Ambient Air Studies in 2015, 2016, and 2017

4

Buddhadeb Ghosh*

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Phillips 66 Research Center, Phillips 66, Highway 60 and 123, Bartlesville, Oklahoma 74003,

6

United States (Email: [email protected]; [email protected])

7

ABSTRACT: Three ground-based ambient air studies were conducted in February through

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March of 2015, 2016, and 2017 at the Phillips 66 Research Center in Northeastern Oklahoma.

9

C2-C12 non-methane hydrocarbons (NMHCs) were measured using whole air sampling/gas

10

chromatography-mass spectrometry. In 2016 and 2017, online methane and ethane

11

measurements were also conducted. Strong methane-ethane correlation identified oil & gas

12

(O&G) upstream/midstream operations to be the primary methane source. C2-C5 alkanes were

13

the dominant NMHCs whose average mixing ratio peaked in 2016 before dropping in 2017. This

14

observation is attributed to regional O&G upstream operations, which peaked in 2015. Mean

15

mixing ratios of C2-C5 alkanes ranged from 0.99-16.99 ppb. Measured ratios of i-C5/n-C5 were

16

0.97±0.03, 1.18±0.04, and 1.06±0.02 in 2015, 2016, and 2017, respectively indicating O&G

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upstream/midstream operations to be their primary source. Photochemical age was estimated

18

using hexane/propane ratio. Emission ratios of NMHCs at zero photochemical age were

ACS Paragon Plus Environment

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calculated, which resembled the composition reported in the literature for natural gas field

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condensate tank flashing. Back-trajectory analysis showed that hydrocarbon-rich plumes came

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from south/west directions where O&G upstream/midstream operations are abundant. High OH

22

reactivity values were calculated from C2-C6 alkanes mixing ratios with average reactivity for the

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three years being 1.55, 1.88, and 1.16 s-1. This indicates that VOC emissions from O&G

24

operations may contribute to ozone production.

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


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42 43

INTRODUCTION: Volatile organic compounds (VOCs) can be emitted into the atmosphere

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from different sources. In an urban environment, vehicle exhaust can be a major source of VOCs,

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along with industrial and oil and gas (O&G) sources.1 Elevated levels of C2-C5 alkanes typically

46

indicate emissions from O&G upstream/midstream operations, which include oil and gas

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production and natural gas/natural gas liquids (NGL) processing but exclude downstream

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processes such as refining, hereafter referred to as O&G operations.2-6 Alkenes and aromatics

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usually originate from vehicle emissions in urban areas.1,

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propene have also been found to be emitted from petrochemical facilities in places like Houston.7

51

Among aromatic compounds, benzene is usually emitted from vehicle exhaust.1 However, other

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fossil fuel sources may also contribute to benzene emissions.4,

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tracers of different emission sources can be used to identify these sources. Ethane and propane

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are emitted from O&G operations and are well known tracers for such sources,2,

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unsaturated compounds such as ethyne and ethene as well as carbon monoxide, are produced

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during combustion processes and are tracers for vehicle exhaust.1, 4

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Certain VOCs such as benzene and 1,3-butadiene are hazardous to human health .12, 13All non-

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methane hydrocarbons (NMHCs) can undergo photochemical oxidation in the presence of NOx

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to produce ozone.14 Photochemical ozone has traditionally been an urban issue relevant for cities

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like Los Angeles and Houston where vehicular and industrial emissions are the primary sources

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of VOCs.7, 14 In recent years, however, high levels of wintertime ozone have been observed in

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rural areas in Wyoming, Utah, and Colorado where the primary VOC sources are O&G

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operations.15-17 Given the health hazards and air quality impact of VOC emissions, it is important

6

Certain alkenes like ethene and

8-10

Correlation with chemical

4, 6, 11

while


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to identify VOC emission sources and their role in regional air quality, which can be useful in

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creating sound environmental regulations.

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Unlike many NMHCs, over 50% of methane emissions in the US originate from a variety of

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biogenic sources such as animal agriculture, landfills, and wetlands,18 while natural gas and

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petroleum systems account for 31% of methane emission as methane is the primary component

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of natural gas.18 Because methane is an important greenhouse gas, distinguishing its biogenic

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emission sources from O&G sources can be useful in environmental policymaking. Methane

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emissions from O&G sources are accompanied by other light alkanes such as ethane and propane

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while biogenic emissions of methane contain little additional light alkanes. This can be exploited

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to identify methane emissions from O&G sources.19-21

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An air research program was developed at the Phillips 66 Research Center (PRC) in 2014 to

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study regional air quality. To that end, three ground-based field studies were conducted during

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February-March of 2015, 2016, and 2017 at PRC (36.74oN, 96.00oW) in Bartlesville in

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Northeastern Oklahoma. NMHCs in ambient air were analyzed using whole air sampling

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followed by offline analysis. During the 2016 and 2017 studies, methane and ethane were also

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measured using online instrumentation. Oklahoma has a high concentration of O&G wells;

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however, there has been only a handful of air quality studies conducted in this region.1,

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Importantly, the two studies conducted in this region by Baker et al.1 and Katzenstein et al.3 took

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place prior to the significant increase in O&G operations over the last decade. The current study

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aims to bridge the gap. The goals of the current study were (1) measure VOCs observed in

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ambient air and identify their primary emission source and (2) understand the air quality impact

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of VOC emissions. The results from 2015, 2016, and 2017 campaigns are reported here.

3


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METHODS:

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PRC is located northeast of Anadarko Basin, north/northwest of Arkoma Basin, and near the

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center of Cherokee Platform Basin. Given the proximity to O&G fields, there is a high

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abundance of oil and gas wells west and south of this site (Figure 1).

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Ambient Air Sampling:

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Ambient air was sampled at the ground level using evacuated 6-L canisters (Entech Instruments)

92

equipped with flow controllers (Entech Instruments) for time-integrated sampling. The canisters

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were also equipped with timers containing solenoid valves (Nutech) for automated sampling at

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predetermined times. During the 2015 campaign, a combination of two-hour and six-hour flow

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controllers was used for whole air sampling to collect a total of 238 samples during the study

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period between February 23 and April 5, 2015. During the 2016 and 2017 campaigns, only two-

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hour time integrated sampling was conducted throughout the study period stretching from

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February 25 to April 1 in 2016 and February 2 to March 11 in 2017. A total of 375 and 388

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canisters, respectively, were collected during these time periods.

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Offline Analysis of Whole Air Samples using GC-MS

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A preconcentrator (7200, Entech Instruments) was used for sample preparation prior to analysis

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by gas chromatography-mass spectrometry (GC-MS). Air samples were prepared using three

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stages of trapping in the preconcentrator and subsequently injected directly into the GC column

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inside an Agilent 7890 GC interfaced with an Agilent 5975 MS. A 100% dimethylpolysiloxane

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DB-1 column of 60 m x 0.32 mm O.D., 1-µm film thickness (Agilent J&W) was used in the GC.

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The instrument parameters were optimized for analysis of C2-C12 hydrocarbons. Electron impact


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(EI) ionization mass spectra of the eluted compounds were recorded by the mass spectrometer.

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Identification of major constituents was performed via manual interpretation of the

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corresponding mass spectra, facilitated by comparison with NIST standard reference spectra.22

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The instruments were calibrated using a commercially available PAMS calibration mix (Restek).

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The details of sample preconcentration and analysis are discussed in the Supporting Information.

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Online Analysis of Ambient Air

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In 2016 and 2017, methane, ethane, CO, CO2, N2O, and H2O were measured in real time using a

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dual quantum cascade laser (QCL) trace gas analyzer (Aerodyne) operating at 1 Hz. The

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instrument works on the principle of tunable infrared laser differential absorption spectroscopy

116

(TILDAS), which has been described in detail elsewhere.21, 23-25 Only the salient features relevant

117

to the current study are described here. The instrument employed two tunable QCLs in the mid-

118

IR region. The first laser operated at a wavelength range near 2230 cm-1 and detected CO, CO2,

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N2O, and H2O while the second laser, operating near 3000 cm-1, detected methane and ethane.

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The instrument was zeroed every 30 min by flowing ultra-zero air (Airgas) through the cell. The

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ultra-zero air contained a trace level of CO, which was removed with a sorbent prepared in

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house.

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Meteorological Data

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Meteorological data were collected using a meteorological station (Davis) installed on top of a

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building onsite (12 m AGL).

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RESULTS AND DISCUSSION:


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Use of Ethane to Identify Methane Emission Sources. As discussed earlier, a strong

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correlation between ethane and methane mixing ratios would imply that methane has originated

129

from O&G sources while a lack of correlation would indicate the presence of biogenic methane.

130

Methane and ethane mixing ratios were measured during the 2016 and 2017 campaigns at 1 Hz

131

resolution and later averaged to 10 s for convenience of data handling. The scatter plot of ethane

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and methane (Figure 2) demonstrates strong correlation with an r2 of 0.87 in 2016 and 0.73 in

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2017 for the complete dataset representing the entire study periods. This indicates that methane

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plumes were emitted from O&G sources. As methane mixing ratios are typically two orders of

135

magnitude higher than ethane mixing ratios in the ambient air, the enhancement ratio of ethane to

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methane is represented in percentage. The enhancement ratios obtained from their scatter plot

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were 5.1±0.007% and 3.93±0.009% for 2016 and 2017 (error bars represent 2 standard

138

deviations of the linear fit). The average ethane content was 4.9% and 3.8% of methane in 2016

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and 2017. The datasets have been color coded with the corresponding date and time. The total

140

dataset is comprised of individual plumes with strong correlations (r2>0.95) but widely varying

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ethane content ranging between a few percent to over 20%. High ethane content indicates “wet”

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emission sources (i.e. a methane source containing condensable higher alkanes, C2, C3, C4, etc.)

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whereas low ethane content signifies “dry” sources. An order of magnitude variation in ethane

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content indicates that methane plumes originated from a wide variety of O&G sources. A small

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segment of data shows negligible ethane content of 2.41 Baker et al conducted a similar study between 1999 and 2005 in 28 US cities

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and obtained an average i-C5/n-C5 ratio of 2.27.1 A comparison with these literature values

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strongly indicates that the NMHC encountered in this study originated from O&G operations.

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Wind Directions and Mixing Ratios

227

Southern wind predominated during the campaigns in all three years. The second most

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predominant wind contribution came from northeast in 2015 and 2017 and from northwest in

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2016. Several clean air masses as well as plumes with high abundance of NMHC were

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encountered during the studies. Time series of C1-C5 alkanes are provided in Supporting

231

Information (Figure S3, S4, and S5). In all three years, elevated NMHC levels were observed

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almost exclusively in the southern/western wind (Figure 5). A more detailed understanding of

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regional air transport was obtained by calculating 72 hour back trajectories for the PRC at 1 m

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AGL using NOAA Air Resources Laboratory Hybrid Single Particle Lagrangian Integrated

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Trajectory Model (HYSPLIT).42,

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agreement with the local meteorological data showing hydrocarbon-rich plumes traveled through

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south and west directions of PRC. Wind coming from south and west would travel through

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regions with a high density of O&G operations (Figure 1) and consequently will be enriched in

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light alkanes. This is consistent with observations in this study.

240

Estimation of Emission Ratios using Photochemical Age Calculation

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The results (Figure S6, Supporting Information) are in


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The atmospheric decay of a VOC can be attributed to its chemical conversion and dilution. The

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ratios of any two VOCs emitted from the same source, however, will primarily be controlled by

243

their atmospheric reactivities as they will be similarly affected by dilution.44 Their ratios will

244

change during transport as more reactive species will decay faster than less reactive species. The

245

measured ratio of two VOCs in the atmosphere, therefore, will depend on their residence time in

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the atmosphere, which is called their photochemical age. Photochemical age can be estimated

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using a simple model provided the following assumptions are true:44-46

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I.

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VOCs are emitted from sources with the same composition upwind of the measurement location

250

II.

Background air has a negligible concentration of VOCs

251

III.

VOCs react exclusively with OH radicals

252

These assumptions have reasonable validity for the dataset used in this study. Within the

253

uncertainties of measurement, VOCs appear to share a common source, O&G operations, as

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demonstrated by the strong correlation among C2-C5 VOCs, pentane ratios, and wind patterns.

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Background air has negligible contribution to the VOC levels, as shown in the time-series data in

256

Figures S3, S4, and S5 in Supporting Information. Reactions with OH would be the dominant

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loss mechanism for the VOCs under consideration; i.e., alkanes and cycloalkanes.

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Considering the above assumptions, atmospheric decay of the ratio of two VOCs, A and B, can

259

be described as:

260

݈݊ ቀ

[஺] | ቁ [஻] ௧ୀ௧

[஺]

= ݈݊ ቀ[஻] |௧ୀ଴ ቁ − ሺ݇஺ − ݇஻ ሻ[ܱ‫ݐ∆]ܪ‬

(1)

261 262

kA and kB = Rate coefficients for reaction of OH radical with A and B

263

[A]t and [B]t = Ambient mixing ratios of A and B at the measurement location at time t


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[A]0 and [B]0 = Mixing ratios of A and B at the emission source at time zero

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∆t = Photochemical age

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Light alkanes were the dominant class of compounds encountered during this study. Therefore,

267

the ratio of hexane and propane were used to estimate photochemical age. The above equation

268

can be re-written as:

269

∆‫= ݐ‬

ଵ

[ைு]ሺ௞ಹ೐ೣೌ೙೐ ି௞ುೝ೚೛ೌ೙೐ ሻ

[ு௘௫௔௡௘]

[ு௘௫௔௡௘]

× ቂ݈݊ ቀ[௉௥௢௣௔௡௘] |௧ୀ଴ ቁ − ݈݊ ቀ[௉௥௢௣௔௡௘] |௧ୀ௧ ቁቃ

(2)

270 271

As shown in equation 2, photochemical age can be determined using the observed ratio of

272

hexane and propane, ([Hexane]/[Propane])t=t, and the ratio at which they were emitted from the

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source at time zero, ([Hexane]/[Propane])t=0, i.e. their emission ratio. The emission ratio can be

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approximated to equal the observed ratio for a fresh plume with a low photochemical age.45, 46 As

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the OH reaction rate coefficient for hexane (kHexane=5.2ᵡ10-12 cm3 molecule-1 s-1) is

276

approximately five times higher than that of propane (kPropane=1.1ᵡ10-12 cm3 molecule-1 s-1),47

277

their ratio decays with aging. In the absence of any prior knowledge on emission ratio of hexane

278

and propane, their highest observed ratio was approximated as the emission ratio. Error

279

associated with choosing the hexane/propane emission ratio in this way would introduce an error

280

in analysis, which is discussed in detail in the supporting information (Figure S7, Supporting

281

Information).

282

Once the photochemical age is calculated for each sample, the ratio of a VOC with propane can

283

be plotted against the photochemical age. Such plots for butane and 2-methylpentane for 2015,

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2016, and 2017 are shown in Figure 6 (a.1, b.1, and c.1). The reactivity of butane (kOH=2.4ᵡ10-12

285

cm3 molecule-1 s-1) is similar to propane. Therefore, the ratio of butane and propane decays


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slowly, as explained by equation 1. In contrast, 2-methylpentane (kOH=5.2ᵡ10-12 cm3 molecule-1 s-

287

1

288

Effect of photochemical aging on VOCs is also found in their scatter plots, color-coded with

289

corresponding photochemical age (Figure 6). The scatter plots of butane with propane (a.2, b.2,

290

and c.2) demonstrate strong linearity as butane and propane decay similarly with photochemical

291

aging. Other compounds with reactivities similar to butane (i.e., ≤C5 alkanes) also exhibit tight

292

correlations with propane. However, scatter plots for 2-methylpentane with propane (a.3, b.3,

293

and c.3) display some non-linearity. Data points at higher photochemical age exhibit a lower

294

ratio of 2-methylpentane and propane as 2-methylpropane has a much shorter atmospheric

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lifetime than propane and gets preferentially removed over time. Other compounds with high

296

reactivity similar to 2-methylpentane (i.e., cycloalkanes and ≥C6 alkanes) also demonstrate

297

similar behavior.

298

The intercept of a linear fit to a plot of VOC/propane ratio against photochemical age (Figure 6;

299

a.1, b.1, and c.1) corresponds to the ratio at zero photochemical age; i.e., at the emission source.

300

The emission ratios for selected alkanes from the study were calculated and are provided in

301

Figure 7 and Table S1 (Supporting Information). Emission ratios reported for Uintah Basin by

302

Western Regional Air Partnership (WRAP)48 and measured by Warneke et al.49 are also provided

303

for comparison. The emission ratios estimated in this work qualitatively match with the

304

composition of natural gas field condensate tank flashing. As Utah/Colorado and Oklahoma

305

regions are parts of different basins, the crude/natural gas composition of Uintah Basin is not

306

expected to be identical to those found in the Oklahoma basins (Figure S8, Supporting

307

Information). Acknowledging this difference, the resemblance of the composition calculated in

308

this work to that reported by WRAP is noteworthy. Therefore, this analysis predicts that a

) is considerably more reactive than propane and its ratio with propane has a much faster decay.


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significant portion of emissions encountered in this study originated from natural gas field

310

condensate tank flashing, with other sources like oil tank flashing and conventional gas/gas well

311

pad possibly playing a small role.

312 313

OH Reactivity

314

OH-radical-initiated oxidation of VOCs leads to the formation of tropospheric ozone. While the

315

extent of ozone formation depends on multiple factors including NOx concentration, the potential

316

of a VOC to form ozone can be estimated by calculating its OH reactivity.

317

The OH reactivity of a VOC “A” (ROH-A) is obtained from its mixing ratio ([A]) and OH reaction

318

rate coefficient (kOH-A):

319

R ୓ୌି୅ = ݇ைுା஺ [A]

(3)

320

Using mixing ratios measured in this work and rate coefficients reported in the literature,26 OH

321

reactivity was calculated for C2-C6 alkanes (Table S2, Supporting Information). As these alkanes

322

are primarily emitted from O&G operations, their OH reactivity helps in understanding the

323

potential impact of O&G operations on ozone production. The minimum OH reactivity values

324

from C2-C6 alkanes in 2015, 2016, and 2017 were 0.32, 0.43, and 0.44 s-1, which signify

325

relatively cleaner background conditions. Average OH reactivity values from this class of

326

compounds were 1.55, 1.88, and 1.16 s-1 in these years while the maximum values were 14.8,

327

18.9, and 6.48 s-1. These values are similar to the OH reactivity of 1.81 s-1 reported for total

328

alkane at BAO.6 This is also comparable to the total OH reactivity of 7.25 s-1 reported for total

329

VOCs in Houston and Galveston Bay.47 Results from this work indicate that alkane emissions


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from O&G activity can potentially contribute to ozone production in the region. Given the longer

331

atmospheric lifetimes of alkanes, they will likely contribute to ozone production downwind.

332 333 334

ASSOCIATED CONTENT

335

Supporting Information

336

A description of measurement location, a detailed discussion of the analysis of sampled air, a

337

graph showing diurnal profiles of propane during the study, a graph of mean C2-C4 alkane

338

mixing ratios measured in this work and reported in literature along with O&G production

339

metrics, a graph of C2-C5 alkane time series in 2015, a graph of C1-C5 alkane time series in 2016,

340

a graph of C1-C5 alkane time series in 2017, a description of back-trajectory analysis, a figure of

341

back-trajectories calculated for representative polluted plumes in 2015,2016, and 2017, a detailed

342

discussion on the effect of uncertainties in input parameters in photochemical age estimation, a

343

graph of butane/propane and 2-methylpentane/propane plotted against photochemical age

344

calculated with different values of ([hexane]/[propane])0, a table of calculated emission ratios of

345

selected alkanes and propane at zero photochemical age in 2015, 2016, and 2017, a graph

346

showing composition of crude oil from Colorado and Oklahoma regions, and a table of

347

calculated OH reactivity for C2-C6 alkanes in 2015, 2016, and 2017. This information is available

348

free of charge on the ACS Publications website at http://pubs.acs.org/.

349

AUTHOR INFORMATION


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350

Corresponding Author

351

Buddhadeb Ghosh. Telephone: 001-918-977-4632. Email: [email protected],

352

[email protected]

353 354

Notes

355

The author declares no competing financial interest.

356

ACKNOWLEDGMENTS

357

The author thanks Joost A. de Gouw and Jessica Gilman (NOAA) for helpful discussions. The

358

author thanks Ryan Fitzpatrick, Dan Nielsen, and Jeff Bean (Phillips 66) for help with writing

359

macros for data analysis. The author thanks Neal McDaniel (Phillips 66) for synthesizing a

360

sorbent for removal of CO from ultra-zero air needed for zeroing instrument. The author thanks

361

Rebecca McFarland, Jennifer Murphy, and Irby Bailey (Phillips 66) for help with the field work.

362

The author gratefully acknowledges the NOAA Air Resources Laboratory (ARL) for the

363

provision of the HYSPLIT transport and dispersion model and READY website

364

(http://www.ready.noaa.gov) used in this publication.


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TOC/Abstract Art


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FIGURES

Figure 1. Location of the study site, Phillips 66 Research Center, in northeast Oklahoma is shown using a black star. The site is located northeast of Anadarko basin, north/northwest of Arkoma basin, and near the center of Cherokee Platform basin. The site is located at the eastern edge of a large area with high concentration of oil and gas wells (drilling data and underlying map courtesy of Energy Information Administration, EIA).


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Figure 2. Scatter plots of ethane (a tracer for O&G operations) and methane mixing ratios in 2017 (a) and 2016 (b) showing strong overall correlation with r2 values of 0.73 and 0.87, respectively. The average ethane/methane enhancement ratios (ER) in 2017 and 2016 are 3.93±0.009% and 5.1±0.007% (error bars represent 2σ standard deviation of the linear fit). The datasets consist of segments of data with varying ethane content while small segments of data show no ethane to methane correlation signifying biogenic contribution.


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Figure 3. Box plots displaying maximum, 75th, 50th, 25th, and minimum mixing ratios for major VOCs from 2015 (a.1), 2016 (b.1), and 2017 (c.1). Mean mixing ratios for this work as well as Southwestern United States (2003),3 Oklahoma City (2007),1 Houston (2009),47 and BAO (2013)2 are provided for comparison. Scatter plots of ethane, butane, and pentane with propane are also shown for 2015 (a.2), 2016 (b.2) and 2017 (c.2). High r2 values for these scatter plots indicate strong correlation among C2-C5 alkanes and consequently a common source for them.


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Figure 4. Scatter plots of 2-methylbutane and pentane measured during this study in 2015, 2016, and 2017 that resulted in enhancement ratios (ER) of 0.97±0.03, 1.18±0.04, and 1.06±0.02, where the error bars represent 2σ uncertainty. These results are similar to results reported from studies of O&G emission sources (ER = 0.885 from Gilman et al.2 and ER = 1.21 from Simpson et al.37) and are significantly different from urban emission sources (ER = 2.27, Baker et al.1).


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Figure 5. Wind rose plots depicting the relationship of mixing ratios for propane, a marker for O&G operations, and wind directions for campaigns in 2015, 2016, and 2017. High propane mixing ratios were primarily observed in wind coming from the directions of O&G operations in south and west.


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Figure 6. Ratio of butane and 2-methylpentane with propane plotted against photochemical age (a.1, b.1, and c.1) demonstrate the effect of their reactivities on the evolution of their ratios. The intercept of a linear fit to the plots provide the emission ratio at zero photochemical age. The color-coded scatter plots of butane (a.2, b.2, and c.2) and 2-methylpentane (a.3, b.3, and c.3) against propane demonstrates how photochemical aging affects the linearity of these plots.


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Figure 7. Emission ratios of VOCs with propane estimated at zero photochemical age from data collected during 2015, 2016, and 2017. For comparison, emission ratios reported in WRAP inventory and by Warneke et al. (2014) are also provided.49


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Table 1. Summary of Mixing Ratios (in ppb) for Major VOCs from 2015, 2016, and 2017 of this Work and Literature.

2015 (n=237) Compound

2016 (n=375)

2017 (n=388)

OK*

SWUS†

Mean

Median

Max

Mean

Median

Max

Mean

Median

Max

Mean

Mean

-

-

-

2136

2014

4073

2148

2049

4160

1810

1850

Ethane

16.04

7.75

163.4

16.47

8.02

164.1

13.42

9.16

97.04

4.42

3.4

Propane

11.48

6.03

125.1

16.99

6.71

245.1

6.90

5.41

49.07

3.17

1.6

2-methylpropane

4.23

1.43

64.76

3.91

1.42

81.89

1.94

1.21

15.67

1.7

0.29

Butane

4.06

2.13

38.57

5.55

2.29

79.48

3.20

2.46

31.73

0.6

0.59

C2-C4 Alkanes‡

33.45

15.39

352.9

41.37

19.29

516.8

24.57

17.91

134.9

9.89

5.88

2-methylbutane

1.82

0.91

17.81

2.18

1.19

21.73

1.15

0.92

9.98

1.22

Pentane

1.53

0.65

18.44

1.89

1.16

16.45

0.95

0.71

9.37

0.7

Cyclopentane

0.31

0.27

1.35

0.46

0.41

1.53

0.36

0.35

0.70

-

2-methylpentane

0.47

0.27

5.07

0.47

0.32

3.41

0.41

0.37

2.18

-

Hexane

0.55

0.31

6.51

0.62

0.43

3.96

0.45

0.40

2.59

0.26

Methylcyclopentane

0.47

0.30

4.64

0.36

0.24

2.56

0.42

0.39

1.91

-

Benzene

0.39

0.29

3.75

0.35

0.32

1.33

0.36

0.34

1.37

0.19

Methane

n = Number of ambient air samples collected. Total mixing ratio of C2-C4 alkanes, i.e. ethane, propane, 2-methylpropane, and butane. * Oklahoma City, Oklahoma- Baker et al.1 † Southwestern United States (Texas, Oklahoma, and Kansas)- Katzenstein et al.3 ‡


26

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