Observations of VOCs in the Colorado Front Range during FRAPPÉ

Rebecca Hornbrook1,*, Eric Apel1, Alan Hills1, Don Blake2, Nicola Blake2, Jason Schroeder2, Alan Fried3, Petter Weibring3, Dirk Richter3, Jim Walega3, Andy Weinheimer1, Deedee Montzka1, Meghan Stell1, John Orlando1, Geoff Tyndall1, Teresa Campos1, Brian Heikes4,

Victoria Treadaway4, Dan O’Sullivan5, Greg Huey6, David Tanner6, Ron Cohen7, Frank Flocke1, Gabi Pfister1, and the FRAPPÉ science team

1NCAR, Boulder, CO, *rsh@ucar.edu; 2University of California, Irvine, CA; 3University of Colorado, Boulder, CO; 4University of Rhode Island, Kingston, RI; 5United States Naval Academy, Anapolis, MD; 6Georgia Institute of Technology, Atlanta, GA; 7Univeristy of California , Berkeley, CA.

NO2 6.79 NO2 1.97 butane 0.74 CO 0.27 CH4 0.24 HCHO 0.39 isoprene 0.94

NO 4.23 butane 0.75 propane 0.70 HCHO 0.26 HCHO 0.23 CO 0.33 HCHO 0.20

CO 0.78 pentane 0.67 CH3CHO 0.57 CH4 0.26 CO 0.21 CH4 0.20 CH4 0.18

HCHO 0.73 CH3OOH 0.65 CH3OOH 0.42 CH3CHO 0.24 propane 0.17 CH3OOH 0.18 CO 0.17

SO2 0.60 propane 0.62 HCHO 0.41 CH3OOH 0.19 CH3CHO 0.15 CH3CHO 0.11 MVK 0.16

CH3CHO 0.55 isopentane 0.51 pentane 0.40 propane 0.15 ethane 0.13 NO2 0.08 CH3OOH 0.12

isoprene 0.44 NO 0.49 NO2 0.39 butane 0.12 butane 0.09 methanol 0.07 MBO 0.06

ethanol 0.30 CO 0.37 CO 0.32 NO2 0.09 isobutane 0.09 H2O2 0.07 CH3CHO 0.05

CH3OOH 0.29 butenes 0.32 butenes 0.28 methylcyclohexane 0.08 methanol 0.06 CH3COOH 0.05 H2O2 0.05

CH4 0.25 2-methylpentane 0.29 isopentane 0.28 pentane 0.08 methylcyclohexane 0.06 PAN 0.04 methanol 0.05

ForestedWeld CountyDenver High NOx Weld RifleUintah Foothills

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Overview The summer 2014 Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) was an airborne and ground-based field study designed to characterize and understand summertime air quality in the Colorado Front Range, where air quality issues persist and National Ambient Air Quality Standards (NAAQS) ozone levels are frequently exceeded in summertime. The study sought to answer the following questions: (1) What are the factors controlling surface ozone? (2) Are current emission controls sufficient to reduce O3 below the NAAQS?

As part of the project, measurements of many key VOC species were observed on board the NSF/NCAR C-130 using the TOGA and whole air canisters, crucial for characterizing emissions and photochemical processing in the Front Range, as well as the air transported into the region.

VOC tracers from several sources/types:

• Biogenic VOCs and oxidation products • Anthropogenic VOCs • Oil and Gas Tracers • Long-lived Halogenated VOCs • Short-lived Halogenated VOCs • OVOCs, including HCHO • DMS • Alkyl Nitrates • Biomass burning tracers (HCN, CH3CN)

TOGA The NCAR Trace Organic Gas Analyzer is a fast online gas chromatograph/ mass spectrometer (GC/MS) capable of simultaneous measurements of 50+ VOCs every 2 minutes.

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40003000200010000

n-butane, pptv

Uintah 2.7 s-1

2.1 s-1

Rifle

High NOx Weld

11.0 s-1

Denver

16.5 s-1

Weld 6.8 s-1

Foothills 1.9 s-1

Forested 2.1 s-1

OH Reactivities OH reactivity contributions by individual trace gases were calculated for samples from several FRAPPÉ study regions.

Total alkane mixing ratio by carbon number, and (b) total alkane OH reactivity by carbon number. Alkane OH reactivities in Denver and high-NOx Weld regions are greatest at C5, while the lower-NOx Weld region is centered around C4.

Ten largest contributing species to OH reactivity by region.

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Alkane Carbon Groups

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Weld Denver Forested Foothills

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Alkane Carbon Groups

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a b

Summary/Next Steps • [Toluene]/[Benzene] and iC5/C5 are useful for source attribution of

O&NG vs. typical urban and industrial emissions. iC4/C4 ratios, on the other hand, can be used as an indicator of O&NG versus only NG extraction, and to identify emissions from different shale plays and/or extraction regions.

• O&NG activities in Weld County contribute significantly to the OH

Alkane OH Reactivity Contributions 42.0

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iC5/C5

(points sized by isopentane)

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1.00.80.60.40.2

iC4/C4

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FRAPPÉ DC3 (< 3 km) NOMADSS ( < 2 km)

Butane and Pentane Ratios Ratios of isopentane/n-pentane (iC5/C5) are useful for contrasting Oil & Natural Gas (O&NG) (0.8-1.0) vs. urban emissions (1.5-2.5). Similarly, ratios of isobutane/n-butane (iC4/C4) are useful for differentiating between different O&NG extractions and regions.

References/Acknowledgements The authors thank NSF, NASA and the Colorado Department of Public Health and Environment (CDPHE) for funding of FRAPPÉ, NOMADSS and DC3.

Baker et al., Atmos. Environ., 42, 1 doi:10.1016/j.atmosenv.2007.09.007, 2008. Swarthout et al., JGR, 118, 10,614-10,637 doi:10.1002/jgrd.50722, 2013.

Benzene and Toluene Emission ratios [VOC]/[CO] of benzene and toluene vary significantly between Denver and Weld County. Both are correlated with n-butane emissions where CO is low, but in the Denver area, [toluene]/[benzene] ratios are much higher than in Weld County.

A comparison of the total alkane OH reactivity observed in the different FRAPPÉ regions to previous ground-based (BAO Tower in Weld County, Swarthout et al., 2013; U.S. cities, Baker et al., 2008) and airborne studies (NOMADSS, DC3), demonstrates the massive

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impact of alkane emissions on the OH reactivity in Weld County.

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reactivity in the region, and much of the reactivity is from the C4-C6 alkanes, typically not well represented in regional and global models.

• We plan to use a box model to estimate the O3 production that can be attributed to the O&NG activities in the front range.

(points sized by isobutane)

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54321

[toluene]/[benzene]

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benzene, pptv

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O&NG, [CO] < 125 ppbvslope = 0.0197 ± 0.0002intercept = 15.6 ± 0.3

r2 = 0.822

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n-butane, pptv

O&NG, [CO] < 125 ppbslope = 0.0248 ± 0.0004intercept = 9.1 ± 0.5r

2 = 0.654