JOURNAL
Photochemical
OF GEOPHYSICAL
alkene
RESEARCH,
formation
VOL. 103, NO. D5, PAGES 5707-5717, MARCH
in seawater
from
20, 1998
dissolved
organic carbon: Results from laboratory experiments
M. Ratte and O. Bujok•
Institut ffir AtmosphiirischeChemie, Jfilich, Germany
A. Spitzy
Institut ffir Biogeochemieund Meereschemie,Universitat Hamburg, Hamburg, Germany
J. Rudolph2
Institut ffir AtmospharischeChemie, Jfilich, Germany
Abstract. The productionmechanismof light alkenes,alkanes,and isoprenewas
investigatedin laboratoryexperimentsby measuringtheir concentrationsin natural
seawateras a functionof spectralrange, exposuretime and origin, and concentrationof
dissolvedorganiccarbon(DOC). The productionmechanismof alkanesand of isoprene
could not be clarified.Ethene and propene are producedphotochemicallyfrom DOC. The
relevantspectralrange is UV and short-wavelength
visiblelight. Initial productionrates
(upto day10of exposure)
werein therangeof several
pmolL-• h-• (mgDOC)-i; the
correspondingmean quantumyieldsfor the spectralrange of 300-420 nm were about
10-s. Generally,
theproduction
ratesandthe quantum
yieldsfor ethenewereabout2
timesthat of propene.The key factorsin the total columnintegratedoceanicalkene
productionare the solarphotonflux at sea surface,the penetrationdepth of the light into
the ocean(especiallythe relationbetweendifferent light absorbers,i.e., the extinctiondue
to absorptionof DOC), and the wavelength-and DOC-dependentquantumyields.As a
result of the high variabilityof theseparameters,actual local alkene productionrates for a
specificoceanicregion may differ considerablyfrom the globallyaveragedoceanicalkene
production
rates.The latterwereestimated
to be at most5 Mt yr-•.
1.
Introduction
of qualitative and quantitative understandingof the NMHC
production mechanism.Although recent investigationsgave
principalinsightsinto possibleformationpathways[Rattee! al.,
1993, 1994, 1995;Moore et al., 1994;Milne et al., 1995;McKay
et al., 1996],the presentknowledgeis far from a precisequantitative description.It is not yet completelyunderstoodwhich
parametersdetermine the oceanicalkene production.
We carried out several experimentsin the laboratory and
studiedthe formationof NMHC, includingisoprene,in natural
seawater. We investigatedseveral parameters affecting the
NMHC production,especiallythe wavelengthdependenceand
the role of DOC. We will focus on C2- to C3-hydrocarbons,
which representmore than 80% of total NMHC emissions
from the oceans[Plass-Dalmeret al., 1995].
Oceanicdissolvedorganiccarbon(DOC) representsone of
the largestorganic carbonreservoirson Earth [Druffel et al.,
1992]. Due to its photochemicaldegradationby sunlight,a
large number of biologicallylabile and volatile organic compoundscanbe formed [Ferekand/indreae, 1984;Kieberet al.,
1989, 1990;Moppere! al., 1991;Mooreand Zafiriou, 1994;Weiss
et al., 1995;Zuo and Jones,1995]. Wilsonet al. [1970]were the
first to publishexperimentalresultspointingto photochemical
production of unsaturated hydrocarbonsin seawater from
DOC. Alkenes may substantiallyaffect local tropospheric
chemistry,and due to their high reactivityand thus short lifetime their dominant sourcein the remote marine troposphere
is emissionsfrom seawater[Rudolphand Ehhalt, 1981; Bonsanget al., 1988; Plasset al., 1992; Plass-Dalmeret al., 1993;
Donahueand Prinn, 1993]. If of sufficientmagnitude,the oce- 2. Methods
anic sourceof alkenesmay have a substantialimpact on the 2.1. Experimental Vessels, Seawater Used, and Bacterial
troposphericchemistryover remote ocean regions.However, Densities
current estimatesof global oceanicnonmethanehydrocarbons
The concentrations
of NMHC
dissolved in seawater were
(NMHC) emissions
are stillveryuncertain[cf.Plass-Dalmeret
measuredas a function of irradiation time for different specal., 1995,and referencestherein].This is mainly due to our lack
tral regions.For each experiment,severalquartz glassvessels
(HSQ 300) of about 2 L volume were filled with identically
1Nowat Institutffir Stratospharische
Chemie,Jfilich,Germany.
pretreated seawaterand exposedto light of a certain wave2Nowat Centrefor Atmospheric
Chemistryand Departmentof lengthrange. Each vesselservedas one sampleand was anaChemistry,York University,North York, Ontario, Canada.
lyzed (see below) after a certain exposuretime. Details of
Copyright1998 by the AmericanGeophysicalUnion.
vessels(cleaningprocedure,material,and geometry)are given
by Ratte et al. [1993]. Seawatersamplesfrom different regions
Paper number 97JD03473.
in the North Atlantic, the North Sea, and the Arctic Sea were
0148-0227/98/97JD-03473509.00
5707
5708
RATTE
Table
1.
ET AL.'
Dark
ALKENE
FORMATION
IN SEAWATER
Conditionsof All ExperimentsConductedin the PresentStudy
Spectral Irradiation
Range,
nm
PHOTOCHEMICAL
Intensity
W m-2
Photon Flux,
Exposure
10•snm
cm-2 s-•
......
Originof
Seawater
North
Time,
d
Nameof
Experiment
1.8
42
dark
300-420
300-420
30 +_10
30 _+10
5.29
5.29
North Sea 1993
North Sea 1993
1.8
1.8
42
10
degassing
02
300-420
300-420
30 _+ 10
30 _+ 10
5.29
5.29
North
North
1.3
1.8
7
9
UV-1
UV-2
300-420
300-420
30 +_10
30 _+10
5.29
5.29
North Sea 1993 + FA?
North Sea 1993 + FA?
1 (FA)$
2 (FA)$
9
9
FA+ 1A
FA+2A
300-420
30 _+ 10
5.29
Arctic
2.8
3
UV-3
300-420
300-420
300-420
30 +_10
30 +_10
30 _+10
5.29
5.29
5.29
Arctic Sea + FAõ
Arctic Sea + FAõ
Arctic Sea + FAõ
0.5 (FA)$
1 (FA)$
2 (FA)$
5
5
5
FA+0.5B
FA+ lB
FA+2B
320-800
410-540
35 _+ 8
35 _+ 8
North Sea 1991
Arctic Sea
1.3
2.8
410-540
410-540
410-540
35 +_8
35 +_8
35 +_8
Arctic Sea + FAõ
Arctic Sea + FAõ
Arctic Sea + FAõ
0.5 (FA)$
1 (FA)$
2 (FA)$
10.0
8.03
8.03
8.03
8.03
Sea 1993
DOC,*
mgL-•
Sea 1991
Sea 1993
Sea
14
10
9
9
9
VIS-320-800
VIS-410-540
FA+0.5C
FA+ 1C
FA+2C
*Total dissolvedorganiccarbon,measuredby high-temperature
combustionoxidation(HTCO).
?Batch 1.
$DOC consisting
of fulvicacids(FA), nominalamountadded.
õBatch 2.
used in severalexperiments(see Table 1). The water was
storedwithout any pretreatmentin 60 L polyethyleneballoons
at 4øCin the dark and filtered through0.2 txmfiltersbeforethe
experimentsstarted.If not mentionedotherwise,the waterwas
not purgedwith any gasbefore the start of an experiment,in
order to avoid a possiblelossof volatile DOC. The filtration
efficiencywascheckedby epifluorescence
microscopy
(100 mL
subsamplesfrom each quartz glassbottle, fixed with 10 mL
17% formalinand storedat 6øC).Generally,filtrationreduced
10%. DOC was also determined by wet chemical oxidation
combinedwith UV irradiation using a 100 W mercury low
pressurelamp (85% emissionat 254 nm, 15% at 185 nm)
(WCO/UV) usingan instrumentproducedby Gr•intzelCompany,Karlsruhe,Germany,with a detectionlimit of about0.03
mg C/L. For concentrationslarger than 0.2 mg C/L the analytical error is about 10%; for lower concentrationsit is about
40%. By this method only 20-30% of HTCO-DOC was detected.
the number of bacteria in seawater down to about 1 x 104
cells/mL,
compared
to 106-107cells/mLin unfiltered
samples. 2.4. Algal Culture
The developmentof NMHC concentrationsin a nonaxenic
Although the number of bacteria sometimesincreasedduring
the experiment(especiallyin dark samples;UV irradiation batch culture of the diatom Thalassiosira rotula was measured
preventedbacterial growth), the bacterial densitygenerally as a function of time and compared to that in a cell free
remainedabout 1 order of magnitudelower in filtered samples control.Two glassbottlesof 25 L volumewere filled with 20 L
of 0.2/am filtered seawaterto which different nutrients were
comparedto unfiltered seawater.
added; one served as control, the other was inoculated with
2.2.
NMHC
Measurement
6 x 104cells/L.Bothvessels
wereclosed
withpolyethylene
foil
The water samplesof nearly 2 L volume were transferred to avoid contaminationand incubatedfor 9 daysat 14øCin a
from the experimentalvesselsthrough a 0.6 txm glassfiber light:darkcycleof 12:12(fluorescentlamps,PhillipsTLM 65W/
filter to a degassing
device.About 750 mL were usedto flush 33RS, spectralrange320-750 nm). To minimizeany possible
the connection lines, valves, etc.; then 1 L was filled into a lossof dissolvedNMHC into the head spaceof the bottles,
strippingchamberand purgedwith heliumfor 30 min at a flow neither of the subsetswas aerated. In order to preventsedirate of about 100 mL/min. The strippingefficiencieswere in mentationof the algal cellsthe culturewasshakenoncea day;
the rangeof 94-99%. Subsequently,
the NMHC were cryogen- for reasonsof comparabilitythe control was treated in the
ically concentratedfrom the purge gasat liquid nitrogentem- sameway. Subsequentto shaking,a sampleof 2 L was taken
perature and analyzedby flame ionizationdetectorgaschro- from each vesselfor NMHC analysis.The cell densityin the
matography(FID/GC). All materials in contact with the culture was determined every day using an inverted microsamplewere stainlesssteel or glass.The detectionlimits were scope.
below 7 pmol/L, and the reproducibilitiesof the measurements
2.5.
Irradiation
Conditions
were about 10%. The analyticalsystemis describedin detailby
The quartzglassvesselswere placedin a closedbox (about
Rudolphet al. [1989] and Plasset al. [1991].
1 m x 0.5 m x 1 m) internallycoveredwith aluminumfoil.
2.3.
DOC Measurements
Fluorescent
lamps(PhillipsTD/D 18W/96(320-800 nm), PhilThe concentrationsof DOC were measuredby two different lips TL 20W 09/N (300-420 nm), and Phillips TL 20W/52
methods.The total amount of DOC was derived from high- (410-540 nm)) providedirradiation of different, but partly
temperaturecombustionoxidation(HTCO), as describedby overlapping,spectralranges(Figure 1). In the followingthe
PeltzerandBrewer[1993],usinga commercial
analyzer(Ionics). three irradiation regimes are referred to as UV300_420nm,
The detectionlimit is 0.1 mg C/L; the analyticalerror is about VIS320-800nm,and VIS 410_540nm.
RATTE
ET AL.:
PHOTOCHEMICAL
ALKENE
FORMATION
IN SEAWATER
5709
2.2E+14
2E+14
1.8E+14
•,1.6E+14
•, 1.4E+14
1.2E+14
x
1E+14
• 8E+13
• 6E+13
4E+13
•
2E+13
250
300
350
400
450
500
•
550
600
wavelength [nm]
650
700
750
800
Figure 1. Actinic fluxesof fluorescentlampsusedin experiments(thin solid lines;PhillipsTD/D 18W/96
(320-800 nm), PhillipsTL 20W 09/N (300-420 nm), PhillipsTL 20W/52 (410-540 nm)) and meanglobal
solarspectrumusedfor extrapolation(thick solidline).
The spectraldistributionsof the photon fluxeswere measuredwith a spectroradiometer
(resolution,1 nm) describedby
Miiller [1994] and Milllet et al. [1995]. The instrument is
equippedwith a specialsensorand recordedthe actinicflux in
the upperhemisphereof the box (2rr sr).
Additionally,the sphericallyintegratedirradiation intensity
of either the upper or the lower hemisphereof the box was
measuredwith a commercialsolarimeter(Kipp and Zonen,
rangeof measurement250-2500 nm) and a filter radiometer
developedby Junkermanet al. [1989], respectively.Thereby,
the total sum of irradiation reachingthe vesselswas recorded,
regardlessof being directlyirradiated or reflected.These measurementsreveal a contributionof 83% from the upper hemisphereand 17% from the lower hemisphereto the total irradiationintensityinsidethe box.The spectraldistributionof the
irradiation was assumedto be identical in both hemispheres.
The total irradiationintensitiesvariedby _+30%dependingon
the horizontaland verticalpositionand the number of vessels
placedin the box. Figure 1 givesthe mean photon fluxes.The
experimentstook place under permanent illumination. The
½=
X2A
(X)a(X)[DOC]
dX
1
where
P
A (•)
a(X)
[DOC]
molecules
produced,
moleccm-3 s-•;
meanactinicflux,photons
cm-2 s-• nm- •'
absorption
coefficient,
L cm-• (mgDOC)-•;
DOC concentration
(measured
withHTCO),mgL-l;
X wavelength,nm.
From the linear increaseof alkene concentrationsin the early
phaseof the experiments,it can be concludedthat (initially)
there is no lossof NMHC and thusthe alkeneproductionrate
is identical
to the rate of increase.
Generally, the absorptioncoefficientof DOC showsan exponential decreasewith increasingwavelength.Depending on
the origin of DOC, the individual absorptionspectraof the
seawaterusedin our experiments
varied(Figure2). They were
measuredin the wavelengthrange from 250 to 400 or 500 nm
box was ventilated, but there was no thermal control. The
with a resolutionof 5 nm or 10 nm, respectively(Kontron
temperatureinsidethe box was about 26øC;the sampletemPhystechspectrometer,type Uvikon 860, and Perkin Elmer
perature was not measured.Dark sampleswere completely
spectrometer,type551). The numberof photonsabsorbedwas
coveredwith aluminumfoil and alsoplaced in the box.
calculatedfor 1 nm intervalsasproductof the photon flux, the
correspondingabsorptioncoefficient,and the concentrationof
2.6.
Rates of Increase
DOC. The following assumptionsor simplificationswere
The experimentslastedbetween7 and 42 days.Initial rates made'The meanpath lengthof light throughthe water column
of increasewere calculatedby linear regressionfor the initial is 20 cm; while passingthese 20 cm, the irradiation intensity
phaseof each experiment(up to day 10). The significance
of decreasesdue to wavelengthdependent absorptionby DOC;
regressioncoefficientswas statisticallytestedaccordingto Sa- this was consideredwhen calculatingthe total number of phochs[1992].For better comparability,the ratesof increasewere tons absorbed(the total loss of light intensitydue to DOC
standardized
to a DOC concentration
of 1 mgL- • (measured absorptionwas at most 20%). We found evidencefor a dewith HTCO). Due to the errorsof regressioncoefficientsand creaseof the DOC absorptioncoefficientand the concentraHTCO measurements, the total error of these standardized tion of a certain DOC fraction within the first 10 days of
rates of increase is about 20%.
irradiation(seebelow).Sincethiswasnot further investigated
and quantified,in a first approximationconstant(initial) DOC
2.7. Calculation of Quantum Yields
absorptioncoefficientsand DOC concentrations
were usedfor
Mean quantumyieldsfor alkeneproductionin the different calculationof quantum yields. The resultingquantumyields
therefore will be lower limits.
wavelengthrangeswere calculatedaccordingto
5710
RATTE ET AL.' PHOTOCHEMICAL
ALKENE FORMATION IN SEAWATER
0.06
4 ß
0.05
,m0.04
0.03
3 o
ß
o
o
ß
o
o
o
o
o
c 0.02
o
o
ß--
0
0
• 1;rlrlrlrl
ß
o Ooo
o
ß
ß
•0.01
ßß Clio
Cl
Cl
Cl
Cl 000000000
O0
0 ßßßßß
•A
0
240
I
I
260
280
ß ß
ßß ClClCl
CI•cl •Oßß
I
I
300
320
340
360
380
400
I
I
420
440
IIl
460
480
500
wavelength [nm]
Figure2. Absorption
spectra
of seawater
usedandof fulvicacidsadded(absorption
coefficients
per mg
Dec L-J). Sample
1 (opensquares)
isNorthSea1993;sample
2 (triangles)
isArcticSea;sample
3 (circles)
is fulvicacids,batch1; sample4 (solidsquares)is fulvicacids,batch2.
As a consequence
of the uncertainties
of the differentvari- irradiatedfor another26 dayswithout any treatment(subset
ablesinvolvedin (1), errorpropagation
leadsto a totalerrorof degassing-continued).
the resultingquantumyieldsof about40-50% with the dominatingerrorbeingthe uncertainty
of the photonflux.
3.
Results
2.8.
Description of Experiments
3.1.
NMHC Concentrations in Algal Culture
In the culture of Thalassiosira
rotula, increasingetheneconGenerally,the concentrations
of NMHC dissolved
in seawater were measuredas a function of spectralrange, exposure centrationswere found, whereasthe cell-free control showed
time, andoriginandconcentration
of Dec (Table 1).
In someexperiments
anextractof fulvicacids(FA) fromthe
CongoRiver was addedto the seawaterbeforeirradiation
started.Generally,1 mg FA corresponded
to 0.5 mg Dec
approximately
constantetheneconcentrations
(Figure 3a).
The increasefollowedthe exponentialgrowthphaseof the
algae(days3-5) with a time shiftof 1 day.When the exponentialgrowthphaseof thealgaehadpassed
(after6 days),the
(measured
byHTCe). In thefollowing,
all resultsarerelated ethene concentrationremained constant at about 120 pmol
added. Two different batches
L -•. The propeneconcentration
(not shown)exhibiteda very
to the nominal amount of Dec
of FA extractwere usedwhich were characterizedby different
similartime dependence,
with maximumvaluesof about40
absorptionspectra(Figure2).
The light-independent
NMHC production
wasmeasured
in
filteredseawaterkept dark for 42 days.The effectof oxygen
concentration
on NMHC productionrateswasinvestigated
in
pmolL-•. We haveno explanation
for theincreased
celldensityat day9. The ethane(Figure3b) andpropane(notshown)
an independent
experiment,
namedexperiment
02. About25
L of seawaterwas purgedwith nitrogen(10 hours,100 mL
isopreneboth in the cultureand in the control,increasing
concentrations
showedirregularvariationsbothin the control
and in the culture, that is, no relation to cell density. For
concentrations
upto a fewtensof pmolL- • werefound(Fig-
min-•) untilthe02 concentration
wasbelow
0.1mgL-• (mea- ure 3c).
suredaccording
to Winkler'smethod,[Parsons
et al., 1984]).
3.2.
Quartz glassvesselswere filled with this nearlyoxygen-free
NMHC
Concentrations
and Rates of Increase
water or with seawaterpurgedwith syntheticair for the same
in Filtered Seawater Samples
time(oxygen
concentration
about8 mg/L)andirradiated
with
In filteredseawater
keptdarkfor 41 daysthe concentrations
of NMHC generally
remainedat a verylowlevelcompared
to
UV300-420nmfor 10 days.
A further experimentnamed"degassing"
startedwith degassedseawater(purgedwith syntheticair, 17 hours,1.5 L
min-•). Twenty-two
quartzglassvessels
with seawater
were
exposed
to UV300_420n
m. During27 daysof exposure,
eight
samples
weretakento measureNMHC concentration
(subset
degassing-initial).
At day27 the contentof sevenvessels
was
concentrationsobtained in irradiated samples(experiment
"dark",Table 2). The resultsobtainedin unfilteredseawater
kept darkup to 8 days[Ratteet al., 1993]wereverysimilar
(Table 2). Nevertheless,
in somecasesthe concentrations
roughlydoubledthroughout
the courseof a darkexperiment,
implyinga smalldarkproduction.
The highestrateof increase
in darksamples
was0.2pmoletheneL-• h-• (rag
transferredinto a stainlesssteel container and again purged measured
to at most10%of themeasured
rates
withsynthetic
air (17hours,1.5L min-•) in orderto remove Dec)-•, corresponding
volatileNMHC. Subsequently,
thesevessels
wererefilled,and of increasein irradiatedsamples.Thereforethe ratesof inthe irradiationwas continuedfor another26 days(subsetde- creaseobtainedin irradiatedsampleswere not correctedfor
gassing-degassed).
The remainingsevenvessels
of the setwere dark productionrates.
RATTE
ET AL.'
PHOTOCHEMICAL
ALKENE
FORMATION
IN SEAWATER
5711
The NMHC concentrationsin irradiated seawatersamples Table 2. Nonmethane Hydrocarbon Concentrationsin
dependedon substance,
spectralrange,and originandconcen- SeawaterSamplesKept CompletelyDark
tration of DOC. For ethene and propene after 10 days of
Exposure
irradiationwith UV3oo_42o
nm,concentrationsof severalthouTime, h
Ethene
Propene
Ethane
Propane
Isoprene
sandsof pmolL -1 werereached(Figure4a (onlyetheneis
shown),squaresand diamonds).The individualrates of increasefor one substancediffered to someextent,dependingon
the origin of seawaterand DOC (Table 3). For North Sea
water, ratesof increaseof about12 (ethene) and 5 (propene)
pmolL -1 h-1 (mg DOC)-• wereobtained;
the ratesof increase measured
in Arctic
Sea water
were
about
40%
en-
hanced(ethene:16pmolL-• h-1 (mgDOC)-•; propene:
7.5
pmolL- • h- • (mgDOC)-•). Underirradiation
withVIS32o_
ExperimentDark,FilteredSeawamr
0
33
73
143
239
359
552
986
170'
51
61
60
64
68
not shown).The correspondingrates of increasewere only
12
15
13
18
143'
29
25
4
2
8
4
4
136'
24
24
6
7
8
8
4
6
Ratte et al. [1993]Expedment 1, UnfiltemdSeawater
800n,•the maximumconcentrationsafter 10 dayswere only a
fewhundred
pmolL- • (Figure4a,ethene:triangles;
propene:
14
67*
18
24
23
19
21
0
55
27
21
94
138
180
62
70
83
98
32
30
30
48
19
28
31
33
36
8
10
11
11
16
n.d.
n.d.
n.d.
n.d.
n.d.
Ratte et al. [1993] Experiment2, UnfilteredSeawater
4.00E+06
200
180
3.50E+06
160
0
53
96
144
48
59
63
62
25
34
30
29
13
39
27
27
8
15
11
12
n.d.
n.d.
n.d.
n.d.
3.00E+06
140
120
[]
lOO
80
[]
6o
[]
[]
2.50E+06
-eø_
Valuesare in pmol L -•. For the Ratteet al. [1993]experiments,
North Atlanticseawater
wasused(DOC = 1.7 mg L-q). Here n.d.
2.00E+06 •t
denotes
1.50E+06 •.
*Not consideredfor regressionanalysis(statisticaltest for outliers
[Sachs,1992]).
not determined.
1.00E+06
40
[]
[]
[]
5.00E+05
I
I
I
I
I
I
I
I
I
O.00E+00
1
2
3
4
5
6
7
8
9
20
exposure time [d]
b
concentrations
remain nearlyconstant(Figure 4a, circles)and
no statisticallysignificantrates of increasewere found (Table
3.50E+06
3). In eachexperimentthe etheneformationrate wasroughly
3.00E+06
twice that of propene.
For ethane and propane the concentrationswere in the
2.50E+06
eø_..
4.00E+06
250
200
150
2.00E+06•t
lOO
1.50E+06
•
1.00E+06
50
5.00E+05
O.00E+00
1
2
3
4
5
6
7
8
9
10
exposure time [d]
4.0E+06
_•' 22
•
21)
.6,.18
•' •s
• 14-
about 15% of those for UV3oo_42On
m (Table 3, ethene: 1.8
pmolL- • h-• (mgDOC)-•; propene;
0.8pmolL--• h-• (mg
DOC)-I). In thecaseof VIS41o_54o
nmtheetheneandpropene
3.5E+06
rangeof a fewtensto 150pmolL- • regardless
of irradiation
regime(Figure4b), with an occurrenceof severaloutliers(due
to scaling,they are not all shownin Figure 4b). Sincewe have
no explanationfor thisphenomenonexceptcontamination,the
outlier values were eliminated before regressionanalysis.In
most casesthe obtainedrates of increasewere statisticallynot
significant.
Isopreneconcentrationswere only measuredin two experiments usingUV300_420n
m. Generally, the isopreneconcentrations were much lower than those of other hydrocarbons
(about5-25pmolL-•, Figure4c).Thehighest
rateof increase
obtained
was0.02pmolL- • h- • (mgDOC)- • (Figure4c,open
•
•
...o///
'"-o--
3.0E+06The
=.s•+os
[
•
addition
of fulvic acids to natural
seawater
and subse-
quentirradiationwith UV3oo_420
nmled to enhancedformation
rates of ethene (Figure 5a) and propene (not shown)com1.5E+06[
•
8 •o
• •
pared to seawatercontainingonly natural oceanicDOC. The
rates of increaseper milligram DOC as fulvic acidswere very
E•6
I o
I
o
1.0E+06
similar to those per milligram oceanicDOC. They increased
.• 4
5.0E+05
2
0
I
•
•
I
•
• O.OE+00 linearly with amount of fuivic acidsadded (Table 3). irradiation with glS41o_54On
m caused a statisticallysignificantin0
I
2
3
4
5
6
7
8
9
10
exposure time [d]
creaseof etheneand propeneconcentrations
in seawaterwith
fulvic
acids
added,
as
opposed
to
seawater
containingonly
Figure 3. (a) Ethene, (b) ethane,and (c) isopreneconcennm (Table 3). The
tration in a culture of the diatom Thalassiosira
rotula (solid oceanicDOC and irradiatedwith VIS41o_54o
m were
squares)and in a cell-freecontrol(open squares)as function rates of increase from fulvic acids under VlS41o_54On
of time; circles denote cell number.
about 6-7 timeslower than thosefor UV3oo_420
nm'In the case
• 12
I
2.0E+06
•
5712
RATTE
ET AL.: PHOTOCHEMICAL
ALKENE
3.3.
8000
FORMATION
IN SEAWATER
DegassingExperiment
Ethene and propenewere nearlycompletelyremovedby the
• 7000
• 6000
degassing
procedure
(ethene:from6126pmolL-1 downto 12
pmolL-l; propene:
from3700pmolL-1 downto 8 pmolL-l).
õ •ooo
Subsequentirradiation of the degassedsamplesevoked an
• 4000
increase
= 3000
•a
1000
0
5
10
:
:
:
:
20
25
30
35
40
45
centration
exposure time [d]
of alkene
concentrations.
For ethene
the rate of in-
creaseafter degassingwas about 30% lower than the initial
rate of increasebut exceededthat of continuedirradiationby
about30% (Table 3). For propenethe ratesof increasein the
different experimentalsubsetswere very similar (Table 3).
Neither the alkanesnor isopreneshowedany systematic
con-
3.4.
increase.
Quantum Yields
300
Thequantum
yieldswerein therangeof 10-7 to 10-9. The
250
200
150
& &ß
•oo
ß I•&oß
50
o
&
ß ß ß ß
0
0
5
10 15exposure
20 t•m5e
[d]30 35 40 45
5O
quantumyields obtainedfor a certain spectralrange varied
slightlydependingon the originof seawaterand of DOC (Table 3). In mostcases,the quantumyield for etheneproduction
wastwice that for propeneproduction.For DOC of the same
origin,the quantumyieldsfor VI832o_8o
onmwere very similar
to thosemeasuredfor UV3oo_420
nm'In contrast,the quantum
yieldsfor UV3oo_42On
m generallyexceededthose obtainedin
VlS41o_54Onm,
with the extentof the differencedependingon
the origin of DOC. For example, the relation of quantum
yieldsin UV300_420
nm/VIS410_540
nmvariedbetweena factorof
about 2 (FA in Arctic Sea water) and a factor of about 100
(Arctic Seawater) for ethene(Table 3).
45
3.5.
•E 40
ß•- 35
o
==3o
•
25
õ 2o
ß
ß
ß
ß
=.1o
'--
ß
5
ß
ß
ii
ß
I
0
i
•
5
10
15
20
25
30
35
40
45
exposure time [d]
Figure 4. Concentrationof (a) ethene,(b) ethane,and (c)
isoprenein filtered, irradiatedseawatersamplesas a function
of irradiationtime. Squaresand diamondsdenoteexposureto
UV light (solid squares:experimentdegassing;
open squares:
experimentUV-1; open diamonds:experimentUV-2; filled
diamonds:experimentUV-3); trianglesdenote exposureto
VIS32o_8oonm;
and circlesdenote exposureto VIS41o_54On
m.
The concentrationof isoprenewas only measuredin the experimentsUV-2 and degassing.For namesand conditionsof
experiments,seeTable 1.
DOC Concentrations and Absorption Coefficients
The HTCO DOC measurementsexhibited no statistically
significantchangeduringexposuretime, either in samplesirradiatedwith UV3oo_42On
m (Figure 6a, open symbols)or in
those kept dark (not shown). Nevertheless,there occurred
elevatedHTCO DOC concentrationsafter day 40. Unfortunately,the experimentswere terminatedjust at this time, and
the statisticalevaluationof the existingdata set doesnot indicate that the changeis statisticallysignificant.In contrast,the
DOC concentrationsmeasuredby WCO/UV exhibited a statisticallyhighlysignificantdecreaseof 75% within 41 daysif the
sampleswere irradiatedwith UV3oo_42On
m (Figure 6a, solid
symbols).In dark samplesthe WCO/UV DOC concentrations
showed
a slightdecrease
(initialconcentration:
0.29mg
finalconcentration
after41 days:0.23mg L-1, not shown);
however,due to the smalldatabase(n = 3) this couldnot be
judgedstatistically.
The absorptionmeasurementsclearlyreveal a decreaseof
absorptioncoefficientthroughoutthe courseof an experiment
under UV3oo_42Onm,
indicatinga changein DOC concentration and/orcomposition
due to irradiation(Figures6b, 7a, and
7b). If the sampleswere kept dark, the absorptionspectra
changedby lessthan 10% within 41 days(not shown).
of ViS41o_54o
nm a linear relationshipbetweenrate of increase
and amountof fulvicacidsaddedwasobviousonlyfor propene
4.
Discussion
and lessevidentfor ethene(Table 3).
Accordingto the existingliterature,NMHC in seawatermay
In contrastto seawatercontainingonly oceanicDOC, the
concentrations
of ethaneand propaneexhibiteda continuous be formed by two different mechanisms:They may be of bioincrease in the subsets with fulvic acids added and irradiated
logicalorigin,for example,directlyreleasedby phytoplankton,
with UV3oo_42On
m (Figure 5b, only ethaneshown).However, or they may be producedphotochemicallyfrom dissolvedorno dependenceon fulvicacidconcentrationwasobserved.The ganiccarbon,with the DOC beingprobablyof biologicalorigin
concentrationsof isoprenewere not affected by addition of [Wilsonet al., 1970;Hannah et al., 1977;Schobertand Elstner,
fulvic acidsand remained constantthroughoutthe exposure 1980;Lee andBaker,1992;McKay et al., 1996;Ratteet al., 1993,
1994;Riemeret al., 1996].The resultsobtainedin the present
time (Figure 5c).
RATTE
ET AL.: PHOTOCHEMICAL
ALKENE
FORMATION
IN SEAWATER
5713
Table 3. Alkene ProductionRates and QuantumYields Obtainedin the PresentPaper
Rate of Increase,
Spectral
Region,
nm
300-420
pmolL-• h-• (mgDOC)-•
Origin of
Water/DOC
Experiment
UV- 1
UV-2
UV-3
FA+ 1A
FA + 2A
FA+0.5B
North
North
Arctic
Sea 1991
Sea 1993
Sea
FA+2B
FA
FA
FA
FA
FA
02 < 0.1 mg L -•
02 = 8 mg L -•
North
North
Sea 1993
Sea 1993
North
North
North
North
Arctic
Sea
Sea
Sea
Sea
Sea
FA+
lB
(batch 1) in North Sea 1993:•
(batch1) in North Sea 1993:•
(batch2) in Arctic Sea{}
(batch2) in Arctic Sea{}
(batch 2) in Arctic Sea{}
Ethene
Propene
12.1
11.2
16
5.3
5.3
7.5
12.7'
12.2'
14.0'
13.8'
13.0'
5.9*
5.4*
8.6*
6.9*
10.1'
13.4
11.9
7.2
7.4
QuantumYield, 10-8
Ethene
8.5
8.1
17
Propene
3.7
3.8
7.9
3.5?
4.0?
1.87
1.07
2.4½
1.67
1.87
1.17
1.07
1.9½
9.5
8.5
5.1
5.3
Degassing
-initial
-degassed
-continued
320-800
VIS-320-800
410-540
VIS-410-540
FA+0.5C
FA+IC
FA+2C
1993
1993
1993
1991
FA (batch2) in Arctic Sea
FA (batch2) in Arctic Sea
FA (batch 2) in Arctic Sea
7.3
5.1
3.7
1.8
0.021 n.s.
4.1
4.3
3.7
0.8
0.032 n.s.
3.2*
2.2*
1.8'
0.8*
0.8*
0.8*
5.2
16
7.6
13.0
2.9
14
7.6
5.8
0.11
0.16
1.07
0.7-½
0.6-½
0.3½
0.3-½
0.3-½
FA denotesfulvic acids;n.s. denotesstatisticallynot significant.
*Only productionfrom fulvic acidsadded,that is, differencefrom controlwithout FA added.
.½Quantum
yield of productionfrom fulvic acidsadded.
•Control: UV-2.
õControl: UV-3.
study also point to different production mechanismsfor alkanes,for ethene and propene, and for isoprene.
For alkanes,no consistentpicture could be obtained. The
alkane concentrationsgenerally exhibited irregular fluctua-
sanget al., 1992;Mooreet al., 1994;Milne et al., 1995;McKayet
al., 1996].
In the following,we will focuson the photochemicalformation pathwayleadingto the productionof etheneandpropene.
tions and several outliers, the reason for which could not be The resultsmightbe influencedby oxygenconsumption,DOC
identified. From our experimentswe therefore cannot con- consumption,or a possiblephotochemicalalkene destruction.
cludehow alkanesare producedin seawater,exceptthe genThe rates of increasefor ethene and propene were very
eral statementthat the factors determiningtheir concentra- similar regardlessof oxygenconcentration(experiment 02,
tions are different from those for ethene, propene, and Table 3). Experimentalartifactsdue to changingoxygenconisoprene.
centrationsthroughoutthe courseof an experimenttherefore
Our experimentswith cell-free seawaterclearly indicate a seemto be negligible,at leastfor the rangeof oxygenconcenphotochemicalproductionof ethene and propene from dis- trations
investigated
(0.1-8 mgL-•). Generally,
bothaeration
solvedorganic carbon. Also, in a culture of diatoms the con- with syntheticair or purging with N 2 before irradiation not
centrationsof ethene and propene increasedcompared to a only affect oxygenconcentration,but alsomay causea lossof
cell-free control. However, in spite of an exponentialgrowth volatile compounds,that is, possiblealkene precursors,and
rate of the algae,the rates of increaseof alkene concentrations thus may affect the rates of increasefor alkenes.When comwerelow (ethene:0.9 pmolL-• h-•; propene:0.2 pmolL-• paringrates of increasein degassedand not degassedsamples
h-1) compared
to thoseobtained
in filteredseawater
samples (degassing-initial,02, UV-2, Table 3), no clear tendencycan
(seeTable 3). Moreover,in view of the spectralrangeusedin be seen,and in mostcasesthe differencesfell within the range
this experimenta major part of thesealkenesprobablyare of of error of the individual rates of increase. Moreover, the
photochemicalorigin. Sincein the cell-free control no increase absorptionspectrameasuredjust before and after degassing
of alkene concentrationswas observed,a photochemicalalk- were very similar (not shown).
ene production in the algal culture must be based on DOC
The tapering off of alkene concentrationswhich occurred
releasedby the algae.This experimenttherefore demonstrates under long time exposurewith UV3oo_42On
m (Figure 4a) may
the indirect role of phytoplanktonin the alkene production be due to a depletion of alkene precursorswith increasing
processrather than a direct biologicalalkeneproduction.
irradiation time or a photochemicalalkene destruction.We
For isoprenewe found increasingconcentrationsin a culture found hintssupportingboth explanations.Generally,the DOC
of the marine diatom Thalassiosirarotula, but also in the cell-
measurements
free control. The origin of the isopreneproducedin this experiment therefore cannot be clearly identified. However, the
extentof photochemicalisopreneproductionin filtered irradiatedsampleswasfoundto be 2 ordersof magnitudelower than
that of ethene and propene. Recent findingsof other authors
provide evidencethat isopreneis not producedphotochemically,but biologicallyduringthe phytoplanktonlife cycle[Bon-
whichcan be detectedby WCO/UV and absorptionmeasurement (Figures 6a and 6b). Different DOC concentrations
would explain the slight difference between the rates of increaseof ethenein the subsetsdegassing-initial
and degassingdegassed(Table 3), which startedwith identical (very low)
alkene concentrations.Since the propene productionrate remains unaffectedin these subsets,this may indicate the exis-
reveal a decrease of a certain fraction of DOC
5714
RATTE
ET AL.: PHOTOCHEMICAL
ALKENE
10000
8000
6000
ß
[]
2000
o
o
i
I
I
i
2
4
6
8
10
exposure time [d]
150
• 125
E
'•' 100
._
=
75
õ 5o
;
IN SEAWATER
spectralrange and of the origin of seawaterand of DOC. The
effect of Dec dependson the spectralrange:In UV3oo_42o
nm
the productionrates for ethene and propene in seawatercontaining Dec from the North Sea, from the Arctic Sea, or as
fulvicacidswereverysimilar(Table 3). In contrast,in VIS41o_
540nmthe productionratesbasedon Dec from the Arctic Sea
water and on fulvicacidsdifferedby a factorof 100 (Table 3).
However, the differencesbetweenthe correspondingproduction efficiencies,measured as quantum yields, were less extreme. Dependingon the origin of Dec they varied between
a factor of 2 and a factor of 10. Generally,the ratesof increase
and the quantumyieldsfor ethenewere about 2 timesthoseof
propene(Table 3).
The quantum yields obtained in the present study are not
wavelengthresolved,but are mean values for a broad wavelengthrange.Actually, they are only valid for the corresponding emissionspectrumfor which they were calculated.Since
the emissionspectraof the fluorescentlampsdiffer considerablyfrom the real solaractinicflux (Figure 1), the meanquantum yields obtained in our experimentscan only provide a
12000
4000
FORMATION
25
rough estimatefor the quantumyields under natural solar
0
I
i
I
I
2
4
6
8
irradiation. As far as we know, up to now no quantumyields
for photochemicalalkene productionare availablein the literature.The only data we are awareof are wavelength-specific
quantumyieldsobtainedby D. D. Riemer et al. (unpublished
data, 1997),who measuredphotochemicalalkeneproduction
10
exposure time [d]
=o40
.=_o
30
õ 20
0
3
ß
0.5
0.45
• •o
2.5
ß
o
o
0.4
._•
0
I
I
I
I
2
4
6
8
0.35
10
o
oo
exposure time [d]
1.5
to the alkene
concentration
and therefore
would
be
more important,the higher the alkene concentrations
were.
However, regardlessof possibleprecursorconsumptionor
alkene destructionin our experiments,the taperingoff of alkene concentrationswas first detectable after 8 to 10 days of
irradiation.
Therefore
the initial
increase measured
o
0.3
o
o
0.25
o
1
0.15
0.1
0.5
0.05
FA+ 1A, additionof 2 mgL- • of fulvicacids,corresponding
to
1 mgL- • Dec; solidsquares:
experiment
FA+ 2A, additionof
4 mgL -• of fulvicacids,corresponding
to 2 mgL- • Dec).
tional
ß
o
o
Figure 5. Concentrationof (a) ethene, (b) ethane, and (c)
isoprene in seawaterwith fulvic acids added and in control
without additionof Dec (open circles:experimentUV-2), as
a functionof exposureto UV light (open squares:experiment
tence of different DOC poolsfor ethene and propeneproduction or a slowerrate of Dec conversioninto propenethan into
ethene. On the other hand, when comparingthe rates of increasein subsetswith identical Dec pools but very different
alkeneconcentrations
(subsetdegassing-degassed
comparedto
degassing-continued,
Table 3), for ethenethe rate of concentration increasein seawaterobviouslydependson the overall
concentrationlevel: The higher the startingconcentration,the
lower the further increase.This may be explainedby a photochemicalalkene destruction.Such a processwould be propor-
o
0
I
I
I
i
I
I
I
I
5
10
15
20
25
30
35
40
0
45
exposure time [d]
0.014
'•' o.o12iI
E 0.01
v• 0.008
ß
• o.oo6
ß
/
ß
• 0.004
• 0.002
0
0
5
10
15
20
25
30
35
40
45
exposure time [d]
6. DOC concentrations (open symbols' hightemperaturecombustionoxidation;solid symbols:wet chemi-
Figure
in our ex-
caloxidation/UV)and(b) absorption
coefficientat • = 300nm
perimentswill not be influencedby this problem.
The alkene productionrate was found to be a function of
in seawatersamplesas a function of exposuretime to UV3oo_
420nm(experimentdegassing).
RATTE
ET AL.'
PHOTOCHEMICAL
ALKENE
FORMATION
rates from DOC of different origin for individualwavelengths
between280 nm and 366 nm. They found quantumyieldsfor
IN SEAWATER
5715
0.03
etheneandpropeneproduction
in the rangeof 10-8 to 10-7
dependingon the origin of DOC, that is, in the samerangeas
the quantumyieldsobtainedin the presentstudy.The highest
quantumyieldswere obtainedfor Gulf streamwater; the lowest were found in water of the Banana River. Generally, the
quantumyieldsexhibitedan exponentialdecreasewith increasing wavelength and became nearly zero at wavelengthsof
about 420 nm (Figure 8; the only exceptionto exponential
decreaseoccurredfor propenein Gulf streamwater). Suchan
exponentialwavelengthdependenceof quantum yield would
explainour experimentalfindingof statisticallysignificantproduction rates under VIS32o_8oon
m in contrastto those under
VIS41o_54onm:
The alkene production rates found in VIS32o_
800nmthen mostprobablyare due to the wavelengthrange up
to 420 nm. We will use the data of Riemer
•' 0.025
0.02
0.015
•
• 0.005
o
250
I
I
300
350
400
wavelength [nm]
et al. to calculate
0.12
alkeneproductionratesin the ocean(seebelow).
Since emissioninto the atmosphereis the dominant loss
processfor alkenesin seawater[Plasset al., 1992;Ratte et al.,
1993],estimatesof the oceanicsourcestrengthfor alkenescan
be basedon productionrates. The oceanicalkene production
0.1
0.08
0.06
rates can be calculatedaccordingto
d[alkene](X)/dt dX : A(A, /)a(A)[DOC]q(A)
0.01
0.04
(2)
where
0.02
A (X, l)
actinicfluxat wavelength
X anddepthl, nm- •
0
cm-2 s-1.
250
a(X) absorption
coefficient,
L cm-• (mgDOC)-•;
[DOC] DOC concentration,
mgL •'
350
400
wavelength [nm]
½(X) quantumyield.
Assuming
A(X, l) = A,o,(X) exp (-E(X)I)
300
(3)
Figure 7. (a) Absorption spectra(from top to bottom) of
seawatersamplesbefore irradiation and after 27 daysand after
42 daysof irradiation with UV3oo_42On
m (experimentdegassing).(b) Absorptionspectraof seawaterwith fulvicacids(FA)
added before irradiation (solid lines, A: FA+2A (i.e., 2 mg
whereA •,,,•is actinicflux at oceansurfaceand E(X) is extinc- L • DOC asFA added),B: FA+iA (i.e., 1 mgL-• DOC as
FA added), and after 9 daysof irradiation with UV3oo_42On
m
tion at wavelengthX, we obtain
(dashedlines,C: FA+2A, D: FA+IA).
d[alkene](X)/dt dX -A,o,(X) exp
ß[DOC]½(X)
(4)
Integrationof (4) leadsto a column-integrated
productionrate
Poolper unit of ocean area:
hours) of the actinicflux at sealevel at latitude 33øwas averagedfor eachwavelengthinterval separatelyin order to obtain
a global mean actinic flux spectrum. By this procedure the
wavelength-dependent
weightingfactor of the averagingprocedure was considered.The actinic flux was determined every
10 nm includingabsorptionof 03 and NO2 (with the absorption spectragivenby WMO [1985])aswell asRayleighandMie
scatteringfor clear skyconditions.In a first approximationthe
albedo of sea surfacewas neglected,and all photonsreaching
the sea surface were assumedto penetrate into the water
Pc(,•
=f d[alkene](X)/dt
dXdl
:Aso•(X)a(X)
ß[DOC]/E(A)q(A)
If in a first approximationlight extinction is assumedto be
causedonlyby DOC absorption,that is,E = a(X)[DOC], (5)
can be simplified to
column.
To sumup, for calculatingglobalalkeneproductionrateswe
assume(1) all photonsreachingthe seasurfacepenetrateinto
the water and (2) DOC is the only absorbingcomponent.The
Hence all photonspenetratingthe sea surfaceare assumedto resultingnumber of photonsthat are relevant for alkene procontributeto alkene production,and production rates can be ductiontherefore is an upper limit.
calculated based on a total ocean surface of 361 x 106 km 2.
The wavelengthinterval of 290-420 nm was assumedas the
A mean actinicflux (24 hours) reachingthe ocean surface relevant spectralrange for alkene production. In a first ap(Figure 8) was calculatedas follows:Startingwith the extra- proach,
meanquantum
yieldsof 8.1x 10-s (ethene)and3.8x
terrestric flux [World MeteorologicalOrganization (WMO)
10-s (propene)wereused,asobtainedin experiment
UV-2
1985],the number of photonspassingthrough an atmosphere (Figure 8). As alreadypointed out in section2.7, thesequanof globalmean conditions(U.S. StandardAtmosphere,1976) tum yields are lower limits, which will partly compensatefor
was calculatedfor different solar zenith anglesusingthe pho- the overestimationof the number of photonsabsorbed.
To investigatehow the irradiation conditionsbased on our
ton flux model by ROth [1992]. Then, the diurnal cycle (12
Pco,(/•)---Asol(/•)q0(/•)
(6)
5716
RATTE
ET AL.: PHOTOCHEMICAL
ALKENE
FORMATION
IN SEAWATER
1.4E+14
8.00E-07
7.00E-07
1.2E+14
:
ß
flux
1E+14 • ?
._o
6.00E-07
o
=. 5.00E-07
ß
quantum
yiel•d
(Riemer, unpu_bl.
data )
8E+13co:
•
•J
• 4.00E-07ß
ß•.
6E+13 o..=
3.00E-07
4E+13 .c:_ce
=2.00E-07
' /
r,•a4e•n
p
1.00E-07
quantum
yield
I,,•/,=•,•
=
•,••.=.
(own
measure•e•'ts
r•'du•'tion
rate2
cr
•'x
O.00E+00
250
rate1
I
270
2E+13
o
290
310
330
350
370
390
410
430
450
wavelength [nm]
Figure 8. Mean globalsolarfluxat seasurface(thinsolidline,calculated
according
to ROth[1992],seetext),
quantumyieldsfor etheneproduction
(squares
denotemeasurements
of D. D. Riemeret al. (unpublished
data,1997)in fourdifferentwatertypes(a fit to thesedataisgivenbythedashedline);lowerstraightdashed
line:meanquantumyieldobtainedin the presentstudyin experiment
UV-2), andthe resultingglobaloceanic
etheneproduction
rates(thicksolidlines;production
rate 1 isbasedon meanquantumyield;production
rate
2 is basedon data of Riemer et al.).
assumption
compareto real oceanicconditions,
we calculated
the penetrationdepthsof light for the spectralregionof 300420 nm. AssumingDec to be the only absorbingcomponent,
a mean global solar flux (see above),a mean oceanicDec
per year. Therebythe contributionof ethene is about 40%,
corresponding
to 0.9 Mt yr-• with an upperlimit of 2.2 Mt
yr-•. The globaloceanicC2 and C3 alkeneproduction
rates
estimatedin the presentstudycloselycorrespondto thesedata
concentration
of 1.5+_0.5mgL- • [Druffeletal., 1992;DeBaar and thus confirm the conclusionof Plass-D•lmeret al. [1995]
et al., 1993; Thomaset al., 1995], and a Dec absorptionspectrum asmeasuredfor North Seawater (experimentUV-2), the
corresponding
penetrationdepths(l/e) varybetween1 m and
12 m. They are in the samerangeas thosegivenin the literature; for example,the meanpenetrationdepthfor 350 nm can
that the global oceanicsourcestrengthfor alkenesis much
lower than previouslyestimated.In fact, it seemsnegligible
comparedto globalvolatileorganiccarbonemissions
of about
1150Mt C yr- • [Guenther
et al., 1995].Nevertheless,
depend-
ing on localconditionssuchassolarflux andbiologicalactivity,
varybetween0.25m (coastalwater,type9) and15m (Sargasso individuallocalproductionratescandiffer from globaloceanic
Sea,openoceanwater, type I) [GordonandMcCluney,1975] sourcestrength.In the presentinvestigationthe mostimpor(water typesas classifiedby Jerlov[1976]).Therebyfor the tant factorsaffectingalkeneproductionrates and thuslocally
valuesfrom the literature the correspondingattenuationcoefrestrictedatmospheric
chemistryare identifiedandquantified.
ficientsare sumparameterswhichdo not distinguish
between
absorptionby Dec and by other absorbers.
The extrapolationof globalalkeneproductionratesbased 5. Conclusions
on the quantumyieldsobtainedin the presentstudyresultsin
The present investigationprovidesa quantitativerelation
0.93/xgm-2 h-• (corresponding
to 2.95Mt yr-•) for ethene betweenthe photochemical
productionrate of alkenes,Dec
and0.66 /xgm-2 h-1 (corresponding
to 2.07Mt yr-•) for concentration,and actinicflux. The productionis determined
propene.For comparison
we usedthe quantumyieldsmeaby the concentration
andreactivityof Dec, the incominglight
suredby D. D. Riemer et al. for four different water types
intensityin the near UV and shortvisiblelight, and the wave(unpublished
data,1997),for whicha meanfit wascalculated
length-specific
quantumyields. Only if Dec is the primary
asshown
in Figure8 (forethenethecorrelation
coefficient
R2
absorberwouldthe total column-integrated
alkeneproduction
rate
be
independent
of
the
light
penetration
depth.Otherwise,
point).Thisresults
in production
ratesof 0.26/xgm-2 h-1 for
the relation betweenthe different light absorberswill play a
ethene(corresponding
to 0.84Mt yr-•) and0.31/xgm-2 h-1
key role in the alkene productionrate. Dependingon water
for propene(corresponding
to 0.99Mt yr-•).
Figure8 showsthe contributionof the differentwavelengths type and biologicalactivitythe contributionof Dec to actual
to etheneproduction.Under the assumptionof wavelength- light extinctionmay be highlyvariable.We thereforewould
resolvedquantumyields(productionrate 2 in Figure 8), the expectpotentiallyhigh alkeneproductionin oceanwaterwith
of reactiveDec, but otherwiselow light
wavelengthrangearound330 nm canbe identifiedto be most high concentrations
extinction.Important Dec sourcesare riverineinput and biefficient for alkene production.
Plass-D•lmeret al. [1995] estimatedglobaloceanicC2 to C4 ological activity, which both are also sources for lightemissionratesof 2.1 Mt per yearwith an upperlimit of 5.5 Mt absorbingphytoplanktonand particles.Thereforethe alkene
was0.92;for propeneit wasonly 0.64 due to an outlyingdata
RATTE
ET AL.:
PHOTOCHEMICAL
ALKENE
productionrate is not strictlyproportionalto the DOC concentrationnor to the biologicalactivity.
Acknowledgments. We greatly appreciate the assistance of
A. Kraus and M. M/iller
with characterization
of irradiation
conditions
and of E. P. R6th with modelinga globalactinicflux. Many thanksto
D. Riemer for providingunpublisheddata and for interestingdiscussions.We are gratefulto A. Dickmeisand F. Haegel for assistance
with
WCO/UV DOC analytics and absorption measurements and to
B. Oligschlegerfor assistance
with one of the experiments.
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(ReceivedJune 10, 1997;revisedNovember5, 1997;
acceptedNovember26, 1997.)