Chemical data quantify Deepwater Horizon hydrocarbonflow rate and environmental distributionThomas B. Ryersona,1, Richard Camillib, John D. Kesslerc, Elizabeth B. Kujawinski d, Christopher M. Reddy d,David L. Valentinee, Elliot Atlasf, Donald R. Blakeg, Joost de Gouwa,h, Simone Meinardig, David D. Parrisha,Jeff Peischla,h, Jeffrey S. Seewaldd, and Carsten Warnekea,haChemical Sciences Division, National Oceanic and Atmospheric Administration Earth System Research Laboratory, Boulder, CO 80305;bApplied Ocean Physics and Engineering Department and dDepartment of Marine Chemistry and Geochemistry, Woods Hole OceanographicInstitution, Woods Hole, MA 02543; cDepartment of Oceanography, Texas A&M University, College Station, TX 77843; eDepartment of EarthScience and Marine Science Institute, University of California, Santa Barbara, CA 93106; fRosenstiel School of Marine and AtmosphericScience, University of Miami, Miami, FL 33149; gDepartment of Chemistry, University of California, Irvine, CA 92697; and hCooperativeInstitute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309

Edited by Philip Gschwend, Massachusetts Institute of Technology, Cambridge, MA, and accepted by the Editorial Board November 29, 2011 (received forreview June 30, 2011)

Detailed airborne, surface, and subsurface chemical measurements, primarily obtained in May and June 2010, are used to quantify initialhydrocarbon compositions along different transport pathways (i.e., in deep subsurface plumes, in the initial surface slick, and in theatmosphere) during the Deepwater Horizon oil spill. Atmospheric measurements are consistent with a limited area of surfacing oil, withimplications for leaked hydrocarbon mass transport and oil drop size distributions. The chemical data further suggest relatively littlevariation in leaking hydrocarbon composition over time. Although readily soluble hydrocarbons made up ∼25% of the leaking mixtureby mass, subsurface chemical data show these compounds made up ∼69% of the deep plume mass; only ∼31% of the deep plume masswas initially transported in the form of trapped oil droplets. Mass flows along individual transport pathways are also derived fromatmospheric and subsurface chemical data. Subsurface hydrocarbon composition, dissolved oxygen, and dispersant data are used to assessrelease of hydrocarbons from the leaking well. We use the chemical measurements to estimate that (7.8 ± 1.9) × 106 kg of hydrocar-bons leaked on June 10, 2010, directly accounting for roughly three-quarters of the total leaked mass on that day. The average envi-ronmental release rate of (10.1 ± 2.0) × 106 kg/d derived using atmospheric and subsurface chemical data agrees within uncertaintieswith the official average leak rate of (10.2 ± 1.0) × 106 kg/d derived using physical and optical methods.

Gulf of Mexico | deepwater blowout | marine hydrocarbon partitioning | oil spill flow rate

Knowledge of the composition,distribution, and total mass of thehydrocarbon mixture (gas plusoil) emitted following loss of the

Deepwater Horizon (DWH) drilling unit isessential to plan mitigation approachesand to assess environmental impacts of theresulting spill. Estimates of DWH hydro-carbon flow rate were originally derivedusing physical and optical methods appliedduring the spill; values were subsequentlyrefined, and an official government esti-mate of oil flow rate was published (1).Analysis of airborne atmospheric chemicaldata provided information on hydrocarbonevaporation into the air and a lower limitto the flow rate (2); however, a more de-tailed description of environmental distri-bution has not been available. Here, wepresent combined atmospheric, surface,and subsurface chemical data to constrainphysical transport pathways, and the re-sulting composition and mass flow rate ofDWH hydrocarbon mixtures along eachpathway, following subsurface releasefrom the leaking well in early to mid-June 2010.Our analysis primarily focuses on the

period following installation of Top Hatno. 4 on June 3 (3), which includes flightsby a chemically instrumented P-3 aircraft(2, 4) and remotely operated vehicle(ROV) sampling of leaking fluid at the

well (5), and ends roughly in late June atthe conclusion of the R/V Endeavor cruise(Fig. S1). The suite of deployed subsur-face, surface, and airborne measurementsoffers spatial, temporal, and chemical de-tail that is unique to this period and tothis spill. We use atmospheric, surface,and subsurface measurements ofhydrocarbons, dissolved oxygen, and dis-persant from throughout this period, aswell as considering additional chemicaldata following closure of the well, to definethe initial compositions, distributions,and mass flow rates of the hydrocarbonmixtures evolving along different pathwaysfollowing release into the marineenvironment.

Results1. Composition Data Constrain Physical Trans-port Pathways. DWH hydrocarbons werereleased at a depth of ∼1,500 m in a high-pressure jet, resulting in gas bubbles andliquid oil droplets with an initial numberand volume distribution that is not yetwell quantified (1). Size and chemicalcomposition of the hydrocarbon bubblesand droplets evolved extremely rapidlyfollowing release from the well (6). Acomplex interplay of physical processesdetermined hydrocarbon-water plumemixing dynamics (7, 8) and affectedthe composition and 3D distribution of

the hydrocarbon mixtures within thewater column, at the surface in the re-sulting oil slick, and in the overlyingatmosphere (2).Prediction of mass fluxes along envi-

ronmental transport pathways followinga deepwater blowout requires accurateunderstanding of time-dependent dynam-ical behavior and evolving chemical com-position along various transport pathways,on time scales of seconds to weeks fol-lowing release. Three observed featuresof the DWH spill offer key insights intomarine transport pathways:

a) Short surfacing time constrains oildroplet size. Visual observations fromresponse vessels suggested a ∼3-h lag

Author contributions: T.B.R., R.C., J.D.K., E.B.K., C.M.R., andD.L.V. designed research; T.B.R., R.C., J.D.K., E.B.K., C.M.R.,D.L.V., E.A., D.R.B., J.d.G., S.M., D.D.P., J.P., J.S.S., and C.W.performed research; T.B.R., R.C., J.D.K., E.B.K., C.M.R., D.L.V.,E.A., D.R.B., J.d.G., S.M., D.D.P., J.P., J.S.S., and C.W. analyzeddata; and T.B.R., R.C., J.D.K., E.B.K., C.M.R., and D.L.V. wrotethe paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission. P.G. is a guest ed-itor invited by the Editorial Board.1To whom correspondence should be addressed. E-mail:thomas.b.ryerson@noaa.gov.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1110564109/-/DCSupplemental.

20246–20253 | PNAS | December 11, 2012 | vol. 109 | no. 50 www.pnas.org/cgi/doi/10.1073/pnas.1110564109

Dow

nloa

ded

by g

uest

on

June

30,

202

0

time between deliberate interventionat the well and the onset of change inthe fresh surface slick. This time corre-sponds to a mean buoyant velocity of0.14 m/s from a depth of 1,500 m andis generally consistent with the 70-minsurfacing time observed during theDeepSpill experiment following an in-tentional release of gas and oil from adepth of 844 m in the North Sea (9).Further, narrow atmospheric plumesobserved under nearly orthogonal winddirections on June 8 and June 10, 2010,by the National Oceanic and Atmo-spheric Administration (NOAA) P-3aircraft (2) indicate that the surfaceexpression was limited to a small arealaterally offset 1.0 ± 0.5 km from thewell, a finding also consistent withobservations from the DeepSpill ex-periment (9). Acoustic Doppler cur-rent profiler data recorded at the wellsite (www.ndbc.noaa.gov/download_data.php?filename=42916b2010.txt.gz&dir=data/historical/adcp2/) in-dicate a net horizontal velocity (inte-grating from depths of 1,200 m to thesurface) of ∼0.03 m/s on June 8 and 10,2010. Combined with the lateral offsetat the surface, this would imply a meanvertical transport time of no more than∼10 h, corresponding to a mean buoy-ant velocity of no less than ∼0.05 m/s.

The 3- to 10-h lag time indicates thatdroplets with approximately millime-ter-scale diameters transported themajority of the surfacing hydrocarbonmass (10, 11) (Fig. S2 A and B). Thisaverage diameter is consistent withvisual observations of droplet size dis-tributions within the near-field plumesource regions, both before and aftershearing of the well riser pipe (5, 12),and approaches the maximum stabledroplet diameter of ∼10 mm (13).

b) Small surfacing area implies a narrowdroplet mass distribution. Gaussian fitsto data in the narrow atmosphericplume of hydrocarbons, with no detect-able volatile hydrocarbon mass outsideof the narrow plume (Fig. 1B) ∼10 kmdownwind of DWH (2), imply thatessentially all the buoyant mass sur-faced within a ∼2-km2 area (Fig. 1 Aand B). This is a robust result, becausethe airborne instruments were suffi-ciently sensitive to have detected andquantified a similar mass of oil surfac-ing over an area of ∼2,000 km2 witha plume signal-to-noise ratio of ∼60for alkanes and ∼25 for aromatics(Fig. S3). The airborne measurementsprovide strong evidence that negligiblemass surfaced outside of the ∼2-km2

area immediately adjacent to the spillsite (Fig. 1 C and D).

c) Atmospheric hydrocarbon relationshipsimply minimal variability in surfacingtimes. Within the atmospheric plume,the tight correlations and single molarenhancement ratios, defined as Δ[XA]/Δ[XB] between pairs of alkanes A andB with different solubility and volatility,and aromatic-alkane pairs of differentsolubility (Fig. 1 C and D), provide fur-ther direct evidence for a narrow dis-tribution of surfacing times. Surfacingtimes appreciably shorter or longer than3–10 h would have resulted in lesseror greater removal of partially solublehydrocarbons, and thus variable atmo-spheric enhancement ratios for a givenhydrocarbon pair. The tight correlationbetween each hydrocarbon pair (Fig. 1)provides further evidence for a narrowmass distribution of large droplets (11).

The available atmospheric observationsthus argue for a single pathway trans-porting the majority of surfacing hydro-carbon mass directly and promptly to thesurface. We conclude that the surface oilslick was fed primarily by this single path-way, with negligible mass transported tothe surface via smaller droplets surfacingafter longer transport times, and thus atgreater distances from the well (Fig. 1A).The available subsurface observations

have been described in detail elsewhere (5,14–22). These reports conclude that the

A

B C

D

Fig. 1. (A) Scale diagram of surfacing hydrocarbon plume dimensions; the atmospheric plume data are consistent with a surface source area of ∼1.6 km indiameter. ppbv, parts per billion by volume. (B) Gaussian fits to hydrocarbon composition data and corresponding full width at half maximum (FWHM) fromcrosswind P-3 aircraft transects of the evaporating plume 10 km downwind of DWH; data from a single transect are shown as an example. (C) Data above thedetection limit [>5 parts per trillion by volume (pptv)] from all DWH plume transects show no evidence for different populations of n-C4 through n-C8 alkanesrelative to n-C9 (different volatilities and solubilities). (D) Data >5 pptv from all transects show no evidence for different populations of C7 and C8 aromaticsrelative to n-alkanes of the same carbon number (similar volatilities but different solubilities).

Ryerson et al. PNAS | December 11, 2012 | vol. 109 | no. 50 | 20247

SPECIA

LFEATURE:PERSP

ECTIV

ED

ownl

oade

d by

gue

st o

n Ju

ne 3

0, 2

020

majority of the subsurface mass was de-tected generally between a depth of 1,000and 1,300 m in concentrated deep hydro-carbon plumes. This finding is consistentwith a physical mechanism that predictsformation of horizontal intrusions, orplumes, of dissolved species and smallundissolved droplets of liquid oil formed inthe turbulent DWH jet (8). Althoughconcentration enhancements outside ofthese plume depths have been reported(e.g., 17, 21), no significant DWH hydro-carbon mass enhancement above or belowthese discrete layers is evident in the sub-surface chemical data to date (5, 14–22).Numerical simulations of this mechanismpredict the observed depth of the deepplumes (8) and further predict additionaldiscrete plumes at shallower depthswith negligible mass compared with thedeep plumes.In the following sections, we interpret

the available chemical data in terms ofa simplified model in which leaked DWHhydrocarbon mass was transported pri-marily along two initial pathways, eitherdirectly into the deep plume or directly tothe surface; after surfacing, further evap-oration into the air occurred (Fig. 1A).

2. Composition Data Quantify Partitioninginto Dissolved, Evaporated, and UndissolvedHydrocarbon Mixtures. Here, we comparethe measured hydrocarbon compositionsof atmospheric and subsurface DWHplume samples with the compositionleaking from the Macondo well; observeddifferences define the extent and natureof alteration attributable to dissolutionand evaporation over time along differenttransport pathways (2). The hydrocarboncomposition of subsurface samples canfurther be altered on multiday time scalesby differential biodegradation duringtransport from the well (14, 16, 17, 19,21). To minimize this confounding effect,the analysis here considers hydrocarboncomposition data from the closest andmost concentrated subsurface samples[i.e., those taken within 5 km of the welland characterized by very large concen-tration enhancements (CH4 >45,000 nano-molar (nM) of seawater or toluene >1,000ng/μL of seawater)].The DWH drilling unit was destroyed

because of uncontrolled high-pressure re-lease of natural gas and liquid oil (3). Thehydrocarbon composition leaking into theGulf of Mexico may have differed fromthe composition measured in the prespillreservoir because of potentially abruptreservoir composition changes associatedwith the blowout, phase separation, frac-tionation, or gas washing (23) within theflowing reservoir during the ensuing 83-d spill. A previous report (2) calculatedthe distribution of gas and oil compoundsbetween the atmosphere and the water

column, and a lower limit to the leakingmass flow rate, by assuming the composi-tion of leaking fluid was unchanged fromthe prespill reservoir composition. Thisassumption resulted in a large uncertaintyin the lower limit flow rate calculated fromairborne atmospheric hydrocarbon dataalone (2). This uncertainty is minimized,and partitioning and mass flow estimatesare improved, by use of composition datafrom a sample of leaking fluid taken dur-ing the spill (5).The hydrocarbon composition of a sam-

ple taken directly within the leaking lowermarine riser package (LMRP) (5) isqualitatively similar (Fig. 2A) to that in-ferred from prespill analysis of reservoirfluid (2). Different values of the derivedgas-to-oil ratio (GOR) result primarilyfrom the different abundances of com-pounds in the gas fraction (i.e., CH4through isomers of C5; Fig. 2A and Fig.S4A). Additional differences are noted buthave a proportionally smaller effect on theconclusions presented here. Analyticaluncertainties of ±5%, with no additionaluncertainty attributable to unspecifiedtreatment of chromatographic unresolvedcomplex material (2) in the analysis of theleaking fluid (figure S2 in ref. 5), signifi-cantly improve the utility of atmosphericdata to determine hydrocarbon distri-butions between the air and the watercolumn and to quantify hydrocarbon massflow rates, as described separately below.Use of the leaking fluid composition (5)

leads to a calculated distribution of DWHhydrocarbons between air and water sim-ilar to that previously derived using theinferred prespill composition (2). Themass fraction of each compound X in air is

The numerator is the slope of a linearregression to X and 2-methylheptanemeasured in the atmosphere, and the de-nominator is the mass abundance of Xrelative to 2-methylheptane in the leakingfluid (5). Here, we normalize to 2-meth-ylheptane, but the results are insensitiveto the choice of undissolved and volatilehydrocarbon for the denominator. Thepresent analysis uses atmospheric hydro-carbon data obtained from ships and theP-3 aircraft between mid-May and the endof June 2010, sampling a much longer timeperiod than the 2 d previously reported(2). The overall picture developed fromthis larger atmospheric dataset and theleaking fluid composition is qualitatively

similar to that reported by Ryerson et al.(2), and is shown graphically in Fig. 2B.The air–water distribution of individualhydrocarbon species reported below ishighly constrained by the chemical data;uncertainties of ±10% in the calculateddistributions are determined by propaga-tion of gas chromatography-flame ioniza-tion detection (GC-FID) calibrationuncertainties of ±5% (5, 24). The generalsimilarity of the atmospheric composi-tion, illustrated by data taken over theperiod of a month, suggests little changein the average composition of the surfac-ing DWH hydrocarbon mixture duringthis period.i) Hydrocarbon mixture remaining subsurface.DWH hydrocarbon transport into thesubsurface resulted from two separateprocesses operating simultaneously duringthe spill (8). The first process involveddissolution of hydrocarbons from large,millimeter-scale diameter buoyant drop-lets during ascent to the surface. Contin-ued buoyant ascent physically transportedthe resulting droplets out of the trappedintrusion (8), leaving behind dissolvedhydrocarbons in the subsurface. The dis-solved hydrocarbon composition is de-termined from observed differencesbetween atmospheric DWH plume com-position measured from surface ships andaircraft (2) and the leaking compositionmeasured directly in the well (5). Dis-solved mass fractions are given by (1 −fraction of X in air) for compounds moresoluble than 2-methylheptane, and theyare set to zero for less soluble species (Fig.2B, Upper, filled red squares). Multiplyingthese mass fractions by leaking fluidmass abundances gives the dissolved mix-

ture composition, which accounted for∼25% of the mass of the leaking mixture.Methane (CH4), ethane (C2H6), propane(C3H8), and isomers of butane (C4H10)accounted for 89% of the dissolvedhydrocarbon mass.The second process transporting

hydrocarbons into the persistent sub-surface plumes involved physical trappingof small droplets of leaking hydrocarbonfluid (8). Trapped small droplets are ex-pected to remain suspended following lossof dissolved hydrocarbons into the sur-rounding seawater (8). We focus on thedeep plume data because subsurfacesamples (5, 14, 16–22) show little evidencefor substantial hydrocarbon mass initially

Fraction of X in air ¼

�Xplume −Xbkgd

2-methylheptaneplume − 2-methylheptanebkgd

��

X2-methylheptane

�fluid

20248 | www.pnas.org/cgi/doi/10.1073/pnas.1110564109 Ryerson et al.

Dow

nloa

ded

by g

uest

on

June

30,

202

0

deposited at depths above 1,000 m or be-low 1,300 m. The relative contributionfrom (a) dissolved hydrocarbon mass and(b) suspended droplet mass in the deepplume is estimated by comparing sub-surface plume chemical composition datawith the composition of the unmodifiedleaking fluid and with its dissolvedfraction below.The deep plume composition is identical

to that of the leaking fluid for the highlysoluble species but begins to differ forless soluble species. Published subsurfacedata on alkanes larger than propane, and

on aromatics larger than toluene (14–17),were examined for samples within 5 kmof the well and for which measuredmethane was >45,000 nM of seawater ormeasured toluene was >1,000 ng/μL ofseawater. These concentrated near-fieldplume measurements (Fig. 3 A–C, bluesquares) are normalized to the most solu-ble measured compound and comparedwith the compositions of dissolved (redcircles) and leaking (gray bars) mixturesdefined above. In each published dataset,the observed pattern of subsurface hydro-carbons relative to measured methane

reported by Joye et al. (17) (Fig. 3A),benzene reported by Camilli et al. (14)(Fig. 3B), or toluene reported by Hazenet al. (16) (Fig. 3C), respectively, approx-imates the composition of just the dis-solved fraction of the leaking mixture. Thedeep persistent subsurface plumes wereprimarily composed of dissolved speciesand were relatively depleted in the moresparingly soluble species. This finding,based on subsurface chemical measure-ments, is qualitatively consistent witha standard oil drop size parameterization(11) in which droplet number decreases

0.016

0.012

0.008

0.004

0.000

Leak

ing

uid

mas

sfr

actio

n

met

hane

etha

nepr

opan

ebe

nzen

eto

luen

eis

obut

ane

cycl

open

tane

n-bu

tane

o-xy

lene

m,p

-xyl

enes

cycl

ohex

ane

ethy

lben

zene

met

hylc

yclo

pent

ane

met

hylc

yclo

hexa

ne1-

met

hyl-4

-eth

ylbe

nzen

en-

pent

ane

isop

enta

ne2,

3-di

met

hylp

enta

ne1-

met

hyl-3

-eth

ylbe

nzen

e2-

met

hylh

exan

e1,

3,5-

trim

ethy

lben

zene

1-m

ethy

l-2-e

thyl

benz

ene

3-m

ethy

lhex

ane

1,2,

4-tr

imet

hylb

enze

nen-

prop

ylbe

nzen

en-

hexa

nena

phth

alen

e3-

met

hylp

enta

ne2,

4-di

met

hylp

enta

ne1,

2,3-

trim

ethy

lben

zene

2-m

ethy

lpen

tane

2,3-

dim

ethy

lbut

ane

2-m

ethy

lhep

tane

3-m

ethy

lhep

tane

n-he

ptan

en-

octa

nen-

nona

nen-

deca

nen-

unde

cane

3

2

1

0

Atm

osphericm

assratio

to2-m

ethylheptane

1.0

0.8

0.6

0.4

0.2

0.0

frac

tion

inai

r 1.0

0.8

0.6

0.4

0.2

0.0

fractiondissolved

Leaking uidF/V EugenieP-3 June 08P-3 June 10R/V JeffersonR/V Pelican

0.14

9>

0.02

8>

0.02

6>

10-4

10-3

10-2

10-1

Pre-

spill

uid

mas

sfr

actio

n(G

OR

=28

19sc

f/st

b)10

-410

-310

-210

-1

Leaking uid mass fraction(GOR = 1600 scf/stb)

methane

naphthalene

3-methylnonane

ethane

propane

n-butane

methylcyclohexane

toluene

1,3,5-trimethylbenzene

benzene

2,4-dimethylhexane

A

B

Fig. 2. (A) Prespill Macondo reservoir hydrocarbon mass fraction (mass of compound per mass of reservoir fluid) (2) plotted vs. leaking fluid hydrocarbon massfraction measured during the spill in mid-June (5). Each data point represents an individual hydrocarbon compound; several are labeled for illustration. Data formethane (CH4) through n-undecane (C11H24) are shown, comprising 38%of the totalmass of the leaking fluid. The dashed line (blue) has a slope of unity; the slope ofa linear-least-squaresfit (red) is, within estimated errors, not significantly different fromunity. Gas-to-oil ratio (GOR) data are given in units of standard cubic feet perstock tank barrel (scf/stb). (B) (Lower) Atmospheric hydrocarbon mass enhancement ratios to measured 2-methylheptane (open symbols) from research vessels andaircraft reflect the undissolvedand volatile componentsof the leakingfluid (graybars). (Upper) Fractions in air (open symbols) are theatmospheric enhancement ratiosnormalized to the expected ratio to 2-methylheptane in the leaking fluid. The dissolved fraction (filled squares) is calculated from the data from June 10, 2010.

Ryerson et al. PNAS | December 11, 2012 | vol. 109 | no. 50 | 20249

Dow

nloa

ded

by g

uest

on

June

30,

202

0

exponentially with increasing diameter,suggesting proportionally little mass can

be transported in the form of suspendeddroplets of liquid oil (Fig. S2B).

However, the actual drop size distribu-tions of the DWH leaks are not known,and may not be well described by thisstandard parameterization. Becausetransport in the subsurface is highly de-pendent on the actual drop size distribu-tion (8), the mass initially suspended in thedeep plumes as small droplets of oil re-mains one of the largest uncertainties inthe DWH hydrocarbon budget to date.Initially, suspended droplets are predicted(8), were positively identified by ROVcameras (14), and are qualitatively con-firmed by published subsurface enhance-ments of sparingly soluble polycyclicaromatic hydrocarbons (15, 16). Theselatter composition measurements, alltaken very close to (within 1 km radius of)the leaking well, are not sufficient toquantify hydrocarbon mass transported inthe form of suspended droplets. No directmeasurements have been presented toquantify this suspended mass to date.To begin to address this uncertainty, we

use chemical data to define the fractionalcontribution of sparingly soluble com-pounds relative to dissolved compoundsfor samples taken in the deep persistentplume. An approximate estimate is affor-ded by further analysis of published data(16) on C10 to C32 n-alkanes from samplestaken within the concentrated deep plumeat varying distances from the well (Fig.3D). These data show a large systematicdepletion (by ∼85%) of heavier n-alkanesrelative to the highly soluble aromaticcompound toluene (C7H8), further dem-onstrating that proportionally little masswas transported into the deep plume in theform of suspended small droplets. Mini-mal biodegradation in these samples isindicated by (n-C17/pristane) and (n-C18/phytane) ratios (Fig. 3D) similar to thosein the leaking fluid. Sparingly soluble n-alkane mass abundances of ∼15% (rangeof 5–25%; Fig. 3D) in the deep plumerelative to the leaking fluid suggests that31% (range of 13–43%) of the subsurfaceplume mass can be accounted for bytransport of hydrocarbons in the form ofinitially suspended droplets. We note thisconclusion is qualitatively consistent withDWH simulations showing that onlysmall droplets were trapped (8), as well aswith extrapolations from standard dis-persed oil droplet size parameterizations(Fig. S2B) suggesting that small dropletsdo not transport the bulk of the mass (11).However, a different drop size distributioncould also be consistent with these ob-servations. More accurate size informationthrough the full range of potential dropsize diameters is needed to constrain theseextrapolations further.ii) Volatile mixture evaporating to the atmosphere.Undissolved volatile and semivolatilehydrocarbons evaporate on characteristictime scales of hours to days after reaching

50

40

30

20

10

0

Leak

ing

and

diss

olve

dui

d

mas

sfr

actio

ns,g

/gx1

0-3

met

hane

etha

ne

prop

ane

isob

utan

e

n-bu

tane

isop

enta

ne

n-pe

ntan

e

cycl

open

tane

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Subsurfacem

assratio

toCH

4

Leaking gas and oilDissolved from buoyant massJoye <5km avg ± 1s (n=7)0.

149

-->

10

8

6

4

2

0

Leak

ing

and

diss

olve

dui

d

mas

sfr

actio

ns,g

/gx1

0-3

benz

ene

tolu

ene

ethy

lben

zene

m,p

-xyl

ene

o-xy

lene

4

3

2

1

0

Subsurfacem

assratio

tobenzene

Leaking gas and oilDissolved from buoyant massCamilli plume data

20

15

10

5

0

Leak

ing

and

diss

olve

dui

d

mas

sfr

actio

ns,g

/gx1

0-3

tolu

ene

ethy

lben

zene

cycl

ohex

ane

met

hylc

yclo

hexa

ne

naph

thal

ene

m,p

-xyl

ene

o-xy

lene

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Subsurfacem

assratio

totoluene

Leaking gas and oilDissolved from buoyant massHazen plume data to July 22

3025201510

n-alkane carbon number

1.0

0.8

0.6

0.4

0.2

0.0

Subs

urfa

cem

ass

enha

ncem

ent

ratio

tom

easu

red

tolu

ene C10-C31 n-alkanes

(sized by concentration)pristane & phytane

A

B

C

D

Fig. 3. (A) Subsurface near-field plume data (blue) from Joye et al. (table 2 in ref. 17), normalized tomeasured methane, compared with the composition of leaking gas and oil (gray) and the composition in-ferred for the mixture dissolved from the promptly surfacing mass (red). The seven most concentratedsamples (CH4 > 45,000 nM) sampled within 5 km of the well were averaged; the isobutane and n-butanedata were transposed, and isomer-specific pentane data were apportioned according to their relativeabundance in the leaking fluid. (B) As in A using subsurface plume data from Camilli et al. (14) normalized tomeasured benzene. (C) As in A using subsurface benzene, toluene, ethylbenzene, and total xylenes (BTEX)plume data >5 μg/L seawater from five separate samples (colored lines and markers) reported in Hazen et al.(16) normalized to measured toluene. (D) As in A using subsurface n-alkane plume data >2.5 μg/L seawaterfrom Hazen et al. (16) normalized to measured toluene. The average and range of (0.15 ± 0.10) used to scalethe dissolved oxygen (DO) observations are shown by the dashed line and shading, respectively.

20250 | www.pnas.org/cgi/doi/10.1073/pnas.1110564109 Ryerson et al.

Dow

nloa

ded

by g

uest

on

June

30,

202

0

the surface (2, 4, 25). The undissolved andvolatile hydrocarbon mixture evaporatingwithin 2–3 h of surfacing (2) was de-termined directly with uncertainties of ±10% (24) using shipborne and airbornemeasurements of CH4 through n-C11. Theevaporated fraction of unmeasured semi-volatile hydrocarbons greater than n-C11 iscalculated (Fig. S5A) using the volatilitydistribution of the oil mixture determinedfrom the chemical composition and the netevaporation measured in the laboratory (4).The sum of volatile and semivolatile masses(Fig. 2B) shows that 14% of the surfacingmixture was both sufficiently insoluble toreach the surface and sufficiently volatile toevaporate from the slick within 1–2 d ofsurfacing. Because not all the leaked massreached the surface, a smaller percentageactually evaporated; this amount isquantified below.Summing the amounts dissolved and

evaporated shows that these processes to-gether reduced the mass of hydrocarbonsin the surface slick by [1 − (0.75·0.86)] =0.36, or approximately one-third, relativeto the slick mass that would have occurredin the absence of these processes. Furtherevaporation of less volatile compoundslikely removed little additional mass fromthe slick after the second day (26). Theevaporating mixture chemical composi-tion is shown graphically in Fig. 4A;n-heptane, n-octane, n-nonane, and

methylcyclohexane were the four mostabundant hydrocarbons by mass in theevaporating mixture.The atmospheric composition data

taken aboard surface vessels and the re-search aircraft, together with the sub-surface composition data, demonstraterelatively little variation in evaporatinghydrocarbon composition from late Maythrough the end of June, 2010 (Fig. 2B).The F/V Eugenie cruise data were takenbefore shearing the broken riser pipe onJune 2 and installation of the LMRP capon June 3. The atmospheric data takensubsequently showed no significant changefollowing this event (Fig. 2B), suggestinglittle change in the composition of thesurfacing hydrocarbon mixture as a resultof this intervention. The absence of at-mospheric CH4 enhancements associatedwith any DWH hydrocarbons in these data(Fig. 2B) confirms earlier reports ofcomplete CH4 dissolution in the sub-surface (2, 18, 19, 21, 22, 27) anddemonstrates that no emissions of CH4 tothe atmosphere were detected through atleast the first 2 mo of the spill. These at-mospheric measurements further demon-strate that leaked benzene (C6H6) wasnearly completely removed in the watercolumn,minimizing its impact at the surface.iii) Hydrocarbon mixture remaining in the surfaceoil slick.Leaked and surfacing hydrocarbonsthat neither dissolved nor evaporated

within the first 1–2 d of surfacing de-termined the initial composition of thepersistent surface oil slick. Slick chemicalcomposition ∼2 d after surfacing is showngraphically in Fig. 4B; n-C17, n-C16, n-C18,and n-C15 were the four most abundanthydrocarbons by mass in the initial surfaceslick. Slick composition inferred from theairborne and shipborne atmospheric datais qualitatively confirmed by GC-FIDanalysis of oil samples taken from R/VEndeavor directly in the surface slick 1.5km horizontally from the well on June 20,2010 (Fig. S5B, Lower).

3. Composition Data Constrain Mass FlowAlong Different Transport Pathways. Thecombined datasets are used to estimate themass flow rates of leaked hydrocarbonsalong each of the identified transport path-ways (Fig. 4D) in early June, 2010, that canbe accounted for by the available chemicalcomposition measurements. These arecompared with the consensus governmentestimate of total mass flow from the well,calculated from the official volume flowrate estimate (1) in barrels of liquid oil(Fig. S1, black circles). Total hydrocarbonmass flow rate, including the gas fraction,is calculated by multiplying the govern-ment estimate of leaked oil volume flow by132.2 kg per stock tank barrel of liquid oiland by a mass ratio of [(gas + oil)/oil] =1.31 ± 0.08 measured at 1 atmosphere and

0.6 ± 0.1

recovered2.1 ± 0.2

Surface ships

deep plumes3.6 ± 0.8

surface slick1.0 ± 0.5

evaporated0.46 ± 0.23

0 meterssea surface

1520 m

cap

avg. release:10.1 ± 2.0

atmosphere

0.0001

0.001

0.01

0.1

mas

s fr

actio

n, g

/g

et

hane

pr

opan

e is

obut

ane

n-bu

tane

m

,p-x

ylen

es

ethy

lben

zene

m

ethy

lcyc

lohe

xane

1-

met

hyl-4

-eth

ylbe

nzen

e 2,

3-di

met

hylp

enta

ne

2-m

ethy

lhex

ane

3-m

ethy

lhex

ane

n-pr

opyl

benz

ene

2-m

ethy

lpen

tane

n-

C6

n-C7

n-

C8

n-C1

0 n-

C11

n-C1

2 n-

C14

n-C1

6 n-

C18

n-C2

0 n-

C22

n-C2

4 n-

C26

n-C2

8 n-

C30

n-C3

2 n-

C34

n-C3

6 n-

C38

0.0001

0.001

0.01

0.1

mas

s fr

actio

n, g

/g

0.0001

0.001

0.01

0.1

mas

s fr

actio

n, g

/g

primarily soluble mixturedetected in subsurface plumes

(~35% of leaking mass)

insoluble, non-volatile mixturedetected in surface slick(~10% of leaking mass)

insoluble, volatile mixturedetected in atmosphere(~5% of leaking mass)

met

hane

benz

ene

tolu

ene

cycl

open

tane

o-xy

lene

cycl

ohex

ane

met

hylc

yclo

pent

ane

n-pe

ntan

eis

open

tane

1-m

ethy

l-3-e

thyl

benz

ene

1,3,

5-tr

imet

hylb

enze

ne1,

2,4-

trim

ethy

lben

zene

2,3-

dim

ethy

lbut

ane

3-m

ethy

lpen

tane

2,4-

dim

ethy

lpen

tane

2-m

ethy

lhep

tane

n-C9

1,2,

3-tr

imet

hylb

enze

nena

phth

alen

en-

C13

n-C1

5n-

C17

n-C1

9n-

C21

n-C2

3n-

C25

n-C2

7n-

C29

n-C3

1n-

C33

n-C3

5n-

C37

n-C3

9

A

B

C

D

Fig. 4. Evaporated hydrocarbon composition after 2 d (A; blue bars), surface oil slick composition after 2 d (B; black bars), and dissolved hydrocarboncomposition (C; red bars). The leaking hydrocarbon composition from CH4 through n-C39 (black line) is shown in each panel for comparison. (D) Schematic (notto scale) of hydrocarbon mass flows in the marine environment; values are calculated for June 10, 2010, in millions of kilograms per day.

Ryerson et al. PNAS | December 11, 2012 | vol. 109 | no. 50 | 20251

Dow

nloa

ded

by g

uest

on

June

30,

202

0

15.6 °C from the Woods Hole Oceano-graphic Institution (WHOI) sample ofleaking fluid (5) (Fig. S1, red circles).i) DWH hydrocarbon mass recovered to the surfaceship. Discoverer Enterprise was the onlysurface ship recovering hydrocarbons inearly June, 2010, via the installed LMRPcap (Top Hat no. 4); liquid oil was col-lected after separation from recovered gas,which was combusted continuously ina flare. Airborne data in the atmosphericCO2 plume downwind of the flare on June10 verify, within error limits, gas and oilrecovery rates reported for DiscovererEnterprise (2). We use the reported valueof 15,402 barrels of liquid oil recovered onJune 10, 2010 (13) and a GOR of 1,600standard cubic feet per stock tank barrelconsistent with the leaking fluid composi-tion (5), and estimate a ±10% uncertaintyto derive a mass flow of (2.7 ± 0.3) × 106

kg/d of hydrocarbons recovered via thecap on June 10, with the gas fraction flaredand the liquid fraction collected in atanker. Flared gas and recovered oilamounts are shown schematically in Fig. 4D.ii) Hydrocarbon evaporation to the atmosphere.The airborne data on June 10, 2010, showa steady-state atmospheric hydrocarbonmass flux of (0.46 ± 0.23) × 106 kg/d (Fig.4D), which is the sum of the directlymeasured hydrocarbon mass evaporatingwithin ∼2–3 h of surfacing (2) plus thelesser volatile hydrocarbon mass evapo-rating within 1–2 d of surfacing as inferredfrom atmospheric aerosol data (4). Theuncertainty of ±50% is primarily attribut-able to uncertainties in the integration ofatmospheric plume hydrocarbon data.These values are indicated in Fig. 4D.iii) Hydrocarbon flow into the surface oil slick.An estimate of mass flow into the surfaceslick is obtained by summing the dissolvedand evaporated masses, and subtractingthis sum from the initially buoyant plumemass [according to the method of Ryersonet al. (2), from the slope of the linear fit(red line) in Fig. S4B] of (2.0 ± 1.0) × 106

kg/d. This estimate suggests that (1.0 ±0.5) × 106 kg/d of leaked hydrocarbons wasproducing the surface slick in early June.Analysis of airborne remote sensing data

from the airborne visible/infrared imagingspectrometer (AVIRIS) instrumentoverflights suggested a lower limit to theaverage daily flow into the surface slick of(0.68–1.30) × 106 kg/d (129,000–246,000barrels of detectable liquid oil remainingon the surface 25 d after the spill began)(28). This value is consistent with the es-timate from P-3 in situ measurements, al-though different amounts of hydrocarbonswere being recovered to the surface onthese two dates. The flow rate into theslick derived from in situ measurements onJune 10, 2010, indicated in Fig. 4B sug-gests a relatively small fraction, roughly13% of the total mass escaping the cap

and leaking into the subsurface, formedthe persistent, visible surface slick. Thislikely contributed to a low bias in early oilleak rate estimates that relied on visualobservations of the surface slick (29).iv) Hydrocarbon flow into the subsurface plume.Subsurface hydrocarbon mass is estimatedusing measurements of dissolved oxygen(DO) deficits in the deep hydrocarbonplumes. Kessler et al. (18, 19) integratedthe detected far-field plume DO deficitsto estimate a total of (3.5 ± 0.5) × 1010

mol of oxygen was consumed during bac-terial respiration of DWH hydrocarbons,using data generated on research cruisesin August through October, 2010, afterflow from the well had ceased. They de-rived a similar value using the observednear-field relationship between DO andthe surfactant di-(2-ethylhexyl) sodiumsulfosuccinate (DOSS) in the deep plumes(18–20). This deficit in DO was sufficientto respire all emitted DWH methane inthe official estimate (1), plus substantialadditional mass of nonmethane hydro-carbons (19). A hydrocarbon mass fluxinto the persistent deep plume of (3.6 ±0.8) × 106 kg/d averaged over the 83-d spillis calculated by scaling the integrated DOanomaly by the mass of the dissolvedcompounds (Fig. 2B), by the estimatedmass of suspended droplets, and by O2respiration stoichiometry appropriate toeach hydrocarbon in this mixture(Table S1).This calculation assumes complete bio-

degradation to CO2 of dissolved hydro-carbons, of which methane (18, 19),ethane (21), propane (21), and isomers ofbutane (17) account for 89% of the mass

(Table S1). It further assumes that by theAugust through September cruise dates,all hydrocarbon mass was biodegraded(Table S1). The biodegraded fraction ofhydrocarbons has not been directly mea-sured, and it is likely to have been negli-gible for the heaviest hydrocarbons; thus,the calculation represents a lower limitto hydrocarbon mass flow into the deepplume. We note that deriving hydrocarbonmass from the observed DO anomaly issensitive to the assumed composition andextent of biodegradation of the subsurfaceplume. Error limits encompassing thesesensitivities are estimated by assuminga range of 5–25% for the heavy n-alkanefractions (Fig. 3D, shaded region), leadingto a range of 13–43% calculated for theplume mass initially transported in theform of suspended oil droplets. Underthese assumptions, the calculated massflow of (3.6 ± 0.8) × 106 kg/d into thesubsurface plumes was the primary flowpath for leaked DWH hydrocarbons, asshown in Fig. 4D, and was composed pri-marily of dissolved species.v) Composition data constrain hydrocarbon releaseinto the environment. A total DO-removingpotential in the deep plume of (0.041 ±0.008) mol of O2 per gram of hydrocarbonis calculated (Table S1) from the deepplume chemical composition above. Di-viding this into the total integrated DOanomaly of (3.5 ± 0.5) × 1010 mol of O2removed over the 83 d of the spill resultsin an average daily environmental hydro-carbon release into the water column of(10.1 ± 2.0) × 106 kg/d (Fig. 5 and TableS1). This hydrocarbon mass flow ratebased on the available chemical data

12

10

8

6

4

2

0

DWH

hydr

ocar

bon

mas

sow

,

kg/d

ayx1

06

directly detected(this estimate,

for June 10, 2010)using chemical data

average ow rate(of cial estimate)

using physicaland optical data

average releaseinto the Gulf

(this estimate)using chemical data

2.7

3.6

1.00.46

recovered+ ared

deep plumes

surface slickevaporated

1.7

8.5

recovered+ ared

releasedinto Gulf

(7.8 ± 1.9)

(10.1 ± 2.0)(10.2 ± 1.0)

Fig. 5. Left-hand bar shows DWH hydrocarbon mass flow, in millions of kilograms for June 10, 2010,along different environmental transport pathways calculated using the chemical composition data. Thecenter bar shows the calculated release into the Gulf averaged over the spill duration, and the right-hand bar shows the official estimate of total hydrocarbon mass flow averaged over the spill duration.

20252 | www.pnas.org/cgi/doi/10.1073/pnas.1110564109 Ryerson et al.

Dow

nloa

ded

by g

uest

on

June

30,

202

0

agrees, within the uncertainties, with theofficial estimate of environmental releaseby subtracting recovered amounts fromthe official average flow rate of (10.2 ±1.0) × 106 kg/d of gas and oil based onphysical and optical data (1).

DiscussionAlthough the totals agree quantitatively,we note that the sum of chemicallydetected mass flows along individualtransport pathways (Fig. 4D) is lower thanthe average environmental release rateinferred from the DO anomaly. Althoughthe simplified model shown in Fig. 1 isgenerally consistent with the availablesubsurface and atmospheric chemicaldata, it does not rule out additional masstransported outside of the deep plumesbut not yet detected in the chemical data.A specific gravity <1 is expected for themixture remaining after removal of solublespecies; thus, dissolution alone is not ex-pected to cause suspended droplets todescend out of the deep plume. A poten-tial transport pathway could instead in-volve gradual ascent, on time scales ofhours to days, after the initial trapping ofsmall hydrocarbon droplets into the deepplume (8), which would distribute thecorresponding hydrocarbon mass intoa larger volume of the subsurface asa function of rise velocity, and thus dropletsize. Absent measured data throughoutthe full range of permitted drop sizes,

a model study is needed to determine whatfraction of the total leaked mass could berepresented by the size range of initiallytrapped droplets that subsequently exitedthe plume on relevant time scales.Analysis of the chemical data provides

an independent estimate of total hydro-carbon mass flow rate against which otherestimates based on physical (1, 12) or op-tical (13, 30) methods can be compared(Fig. 5). Beyond the flow rate, the chem-ical data provide critical information oninitial environmental distribution of thedifferent mixtures resulting from transportof hydrocarbons emitted from the leakingwell (e.g., Fig. 4). The information pro-vided by a cooperative subsurface, surface,and airborne chemical sampling programshould therefore be an integral part ofa systematic response to future deepwaterblowouts. Strategic cooperation duringa response would significantly improve theability to quantify leaking mass and envi-ronmental impacts of future spills, andwould further provide a means to trackand quantify the effects of deliberate in-tervention measures, subsurface disper-sant application, and well and sea-floorintegrity after cessation of flow. With suf-ficient advance preparation, joint airborneand subsurface chemical sampling couldprovide a national rapid-response capa-bility to assess deepwater well leak ratespromptly, especially those in remote andArctic regions (2).

Materials and MethodsLeaking fluid was collected into isobaric gas-tightsamplers by ROV from directly within the LMRP(5). Subsequent analyses of the gas and oil compo-sition were conducted in parallel using GC-FIDperformed by Geomark Research Ltd., AlphaAnalytical Laboratory, and the WHOI, with sim-ilar results (5).

Atmospheric hydrocarbon samples were ac-quired by sampling air into evacuated stainless-steel canisters carried aboard three surface vessels,F/V Eugenie, R/V Pelican, and R/V Thomas Jeffer-son; similar canisters were used on June 8 and 10during two DWH survey flights of a chemicallyinstrumented NOAA P-3 research aircraft (2). Allatmospheric samples taken aboard the vessels andaircraft flights were subsequently analyzed by GC-FID or GC-MS at the University of California atIrvine (24).

ACKNOWLEDGMENTS. T.B.R. thanks C. Brock foruseful discussions on drop size distributionsand A. Ravishankara for critical comments thathelped improve themanuscript. This research wassupported by the National Science Foundationthrough Grant AGS-1049952 (to D.R.B.), GrantsOCE-1042650 and OCE-0849246 (to J.D.K.), GrantOCE-1043976 (to C.M.R.), Grants OCE-1042097and OCE-0961725 (to D.L.V.), Grant OCE-1045811(to E.B.K.), and Grant OCE-1045025 (to R.C.);by U.S. Coast Guard Contract HSCG3210CR0020(to R.C.); and by US Department of Energy GrantDE-NT0005667 (to D.L.V.). The August, Septem-ber, and October research cruises were fundedby the NOAA through a contract with Consoli-dated Safety Services, Incorporated. The NOAAP-3 oil spill survey flights were funded, in part, bythe NOAA and, in part, by a US Coast GuardPollution Removal Funding Authorization tothe NOAA.

1. McNutt MK, et al. (2011) Assessment of Flow RateEstimates for the Deepwater Horizon/Macondo Well OilSpill. Flow Rate Technical Group Report to the NationalIncident Command Interagency Solutions Group. Avail-able at http://www.doi.gov/deepwaterhorizon/loader.cfm?csModule=security/getfile&PageID=237763. AccessedDecember 13, 2011.

2. Ryerson TB, et al. (2011) Atmospheric emissions from theDeepwater Horizon spill constrain air–water partitioning,hydrocarbon fate, and leak rate. Geophys Res Lett 38(L07803), 10.1029/2011GL0467.

3. Graham R, et al. (2010) Deep Water: The Gulf OilDisaster and the Future of Offshore Drilling. NationalCommission on the BP Deepwater Horizon Oil Spill andOffshore Drilling Report to the President. Available athttp://www.gpoaccess.gov/deepwater/deepwater.pdf.Accessed December 13, 2011.

4. de Gouw JA, et al. (2011) Organic aerosol formationdownwind from the Deepwater Horizon oil spill.Science 331:1295–1299.

5. Reddy CM, et al. (2012) Composition and fate of gasand oil released to the water column during theDeepwater Horizon oil spill. Proc Natl Acad Sci USA109:20229–20234.

6. Zheng L, Yapa PD, Chen FH (2003) A model forsimulating deepwater oil and gas blowouts—Part I:Theory and model formulation. Journal of HydraulicResearch 41:339–351.

7. Leifer I, Luyendyk BP, Boles J, Clark JF (2006) Naturalmarine seepage blowout: Contribution to atmosphericmethane. Global Biogeochem Cycles 20(GB3008),10.1029/2005GB002668.

8. Socolofsky SA, Adams EE, Sherwood CR (2011) Formationdynamics of subsurface hydrocarbon intrusions followingthe Deepwater Horizon blowout. Geophys Res Lett 38(L09602), 10.1029/2011GL047174.

9. Johansen Ø, Rye H, Cooper C (2003) DeepSpill—Fieldstudy of a simulated oil and gas blowout in deep water.Spill Science and Technology Bulletin 8:433–443.

10. Chen FH, Yapa PD (2007) Estimating the oil droplet sizedistributions in deepwater oil spills. Journal of HydraulicResearch 133:197–207.

11. Delvigne GAL, Sweeney CE (1988) Natural dispersion ofoil. Oil & Chemical Pollution 4:281–310.

12. Camilli R, et al. (2012) Acoustic measurement of theDeepwater Horizon Macondo well flow rate. Proc NatlAcad Sci USA 109:20235–20239.

13. Lehr W, Bristol S, Possolo A (2010) Oil Budget CalculatorDeepwater Horizon Technical Documentation. OilBudget Calculator Science and Engineering Team Reportto the National Incident Command Interagency Solu-tions Group. Available at http://www.restorethegulf.gov/sites/default/files/documents/pdf/OilBudgetCalc_Full_HQ-Print_111110.pdf. Accessed December 13, 2011.

14. Camilli R, et al. (2010) Tracking hydrocarbon plumetransport and biodegradation at Deepwater Horizon.Science 330:201–204.

15. Diercks A-R, et al. (2010) Characterization of subsurfacepolycyclic aromatic hydrocarbons at the DeepwaterHorizon site. Geophys Res Lett 37(L20602), 10.1029/2010GL045046.

16. Hazen TC, et al. (2010) Deep-sea oil plume enrichesindigenous oil-degrading bacteria. Science 330:204–208.

17. Joye SB, MacDonald IR, Leifer I, Asper V (2011)Magnitude and oxidation potential of hydrocarbongases release from the BP oil well discharge. Nat Geosci4:160–164.

18. Kessler JD, Valentine DL, Redmond MC, Du M (2011)Response to Comment on “A persistent oxygen anomalyreveals the fate of spilled methane in the deep Gulf ofMexico”. Science 332:1033.

19. Kessler JD, et al. (2011) A persistent oxygen anomalyreveals the fate of spilled methane in the deep Gulf ofMexico. Science 331:312–315.

20. Kujawinski EB, et al. (2011) Fate of dispersantsassociated with the Deepwater Horizon oil spill.Environ Sci Technol 45:1298–1306.

21. Valentine DL, et al. (2010) Propane respiration jump-starts microbial response to a deep oil spill. Science330:208–211.

22. Yvon-Lewis SA, Hu L, Kessler J (2011) Methane fluxto the atmosphere from the Deepwater Horizonoil disaster. Geophys Res Lett 38(L01602), 10.1029/2010GL045928.

23. Whelan J, Eglinton L, Cathles, L, III, Losh S, Roberts H(2005) Surface and subsurface manifestations of gasmovement through a N-S transect of the Gulfof Mexico. Marine and Petroleum Geology 22:479–497.

24. Colman JJ, et al. (2001) Description of the analysis ofa wide range of volatile organic compounds in wholeair samples collected during PEM-tropics A and B. AnalChem 73:3723–3731.

25. Fingas M (1996) The evaporation of oil spills: Predictionof equations using distillation data. Spill Science andTechnology Bulletin 3:191–192.

26. Fingas MF (1997) Studies on the evaporation of crudeoil and petroleum products: I. The relationshipbetween evaporation rate and time. J Hazard Mater56:227–236.

27. Valentine DL (2010) Measure methane to quantify theoil spill. Nature 465:421.

28. Clark RN, et al. (2010) A Method for QuantitativeMapping of ThickOil Spills Using Imaging Spectroscopy.USGS Open-File Report 2010-1167. Available at http://pubs.usgs.gov/of/2010/1167/. Accessed December 13,2011.

29. US Geological Survey (2010) Deepwater HorizonMC252 Gulf Incident Oil Budget: Government Esti-mates—Through August 01 (Day 104) US GeologicalSurvey Report to the National Incident Command.Available at http://www.usgs.gov/foia/budget/08-02-2010...Deepwater%20Horizon%20Oil%20Budget.pdf.Accessed December 13, 2011.

30. Crone TJ, Tolstoy M (2010) Magnitude of the 2010 Gulfof Mexico oil leak. Science 330:634.

Ryerson et al. PNAS | December 11, 2012 | vol. 109 | no. 50 | 20253

Dow

nloa

ded

by g

uest

on

June

30,

202

0