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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D4, PAGES 9149-9157, APRIL 20, 1996

Emissionsof ethene, , and 1- by a midlatitude forest

A.H. Goldstein,• S.M. Fan, M.L. Goulden,J.W. Munger,and S.C. Wofsy

Departmentof Earthand Planetary Sciences and Division of AppliedSciences HarvardUniversity, Cambridge, Massachusetts

Abstract. Measurementsof nonmethanehydrocarbon concentrations and gradientsabove HarvardForest (42ø32 ' N, 72ø11'W) arereported for Januarythrough December 1993, along with inferredwhole-ecosystem emission rates for ethene,propene, and 1-butene. Emissions were calculatedusing a micrometeorologicaltechnique where the ratio of observedCO2 fluxesand gradientswere multipliedby the observedhydrocarbon gradients. Average r•10 emissionsof ethene,propene, and i-butene duringsu-mmer were 2 ß63 , I . 13, mm-- • 0.4 1 x 1u moleculescm-2 s -1 • respectivelyß Emission ofthese olefins was correlated with incident solar radiation,implying a sourceassociated with photosynthesis.In the northeasternUnited States,summertime biogenic emissions of propeneand 1-buteneexceed anthropogenic emissions,and biogenic emissions of ethenecontribute approximately 50% of anthropogenic sources.Our measurementssuggest that terrestrialbiogenic emissions of C2-C4olefins may be significantfor atmosphericphotochemistry.

Introduction (including ethene,propene, and 1-butene) have been detected from vegetationand in a variety of forested environments Ozone concentrations in the northeastern United States are [Zimmerman et al, 1988; Khalil and Rasmussen,1992; believed to be sensitive to emission rates of biogenic Bonsang et al., 1987; Isadorov et al., 1985]. However, nonmethanehydrocarbons (NMHC) [Fehsenfeldet al., 1992; global budgetsof propeneand 1-butenehave been reportedto McKeen et al., 1991; Rosell et al., 1991; Sillman et al., be dominatedby the ocean, biomass burning, and fuel 1990]. Uncertainties in rates of these emissions introduce combustion[Singh and Zimmerman, 1992]. large uncertaintyin assessmentsof control strategiesfor air In this paperwe report observationsusing an automated pollution, for example, in defining the relative benefits of instrumentto measureNMHC concentrationscontinuously at reductionsin emissionsof anthropogenicNMHC versusNOx two altitudes above a temperate deciduous forest in (NO + NO2). Moreover, peroxy acetyl nitrate (PAN) and Massachusetts,emphasizing ethene, propene, and 1-butene. other organicnitrates, formed as by-productsof oxidation of The fluxes of these biogenic NMHCs from the forest were NMHCs, can facilitate transportof NOx precursorsinto the calculatedby similarity using the observed concentration global tropospherewhere photochemicalozone productionis gradientand concurrent measurements of fluxesand gradients highly efficient. for CO2. We present diurnal and seasonalvariations of Most measurements of terrestrial biogenic NMHC biogenicemissions of ethene,propene, and 1-butene,and we emissionshave focusedon isopreneand terpeneswhich are assessthe regionalsignificance of theseemissions. widely believed to be the most important for atmospheric chemistry[Zimmerman, 1979; Winer et al., 1989; Singh and Zimmerman,1992; Fehsenfeldet al., 1992]. Ethene, a plant Experiment hormone of interest to plant physiologists, has also been Site studied in detail. Sawada and Totsuka [1986] estimated that 74%of thetotal global emission of ethene(35.4 TgC yr -•) Harvard Forest is located in Petersham,Massachusetts (42 ø was from natural sources,89% from terrestrial ecosystems 32' N, 72ø11' W; elevation 340 m), 100 km west of Boston, (78% from vegetation and 22% from soils), and 11% from Massachusetts and 100 km northeast of Hartford, aquaticecosystems. They estimatedaverage ethene emissions Connecticut. There is a highway = 5 km to the north and a for temperatedeciduous forests during the growing seasonof secondaryroad = 2 km to the west. The site is accessibleby a 2.2 x 10m moleculescm -2 s -• Emissionsof lightolefins dirt road which is closed to public traffic. Measurements were made from a 30-m tower, erected in May 1989, extending 9 m above the forest canopy. Instrumentswere •Now at Departmentof EnvironmentalScience, Policy, and housedin a temperaturecontrolled shack located 15 m eastof Management,University of California, Berkeley the tower. Copyright1996 by theAmerican Geophysical Union. The forest is 50 to 70 years old, predominantlyred oak, with red maple, sugarmaple, beech,yellow and white birch, Papernumber 96JD00334. black spruce,hemlock, and white and red pine. The total 0148-0227/96/96JD-00334505.00 deciduousleaf area index was 4.0 (oak, maple, beach, and

9149 9150 GOLDSTEIN ET AL.: ETHENE, PROPENE, AND 1-BUTENE EMISSIONS birch contributed2.0, 1.0, 0.5, and 0.5, respectively)in 1992, normallydownloaded at 6-day intervals. Concentrationswere as measuredby collecting leaves in litter traps surrounding determinedusing relative responsefactors [Ackman, 1964, the tower. The terrain is moderatelyhilly (relief -- 30 m), but 1968; Dietz, 1967] referenced to an internal neohexane there is no evidence of anomalousflow patternsthat would standard(Scott-Martin, National Institute of Standardsand make eddy-flux measurementsat this site unrepresentative Technology traceable +2%) added to every sample by [Moore et at., 1996], and the local energy budgetis balanced dynamicdilution. to within 10% [Goutden et at., 1996]. The accuracyof the systemwas estimatedto be betterthan Continuous measurements of C2-C6 NMHCs were made +18% for hexaneand for hydrocarbonseluting beforehexane, simultaneouslyat 45-min intervals2 and 7 m abovethe forest based on the cumulative uncertainty of the neohexane canopy commencing July 22, 1992. Other trace standard, measurements of standard addition flows, the concentrations and meteorological variables have been integrity of individual compounds in the sampling and measuredcontinuously at this site since 1990, including CO, analysisprocess, and relativeresponse factors. Measurement CO2, 03, NOx, NOy, H20, rainfall, wind speed, wind precisionwas approximately3% at 1 ppbv, 5% at 0.5 ppbv, direction,temperature, and eddycorrelation fluxes of sensible 10% at 0.2 ppbv, and 20% for concentrationsless than 0.1 heat,latent heat, 03, NOy,and CO2 [Wofsy et at., 1993]. parts per billion by volume (ppbv), as determinedby the variance between measurements taken from the same level Flux-Gradient Similarity Method everyfifth injection. The detectionlimit for thesecompounds Direct measurementsof fluxes of ethene,propene, and 1- was approximately0.01 ppbv. buteneby eddy correlationare not currentlypossible, because Compoundseluting after hexane (including , eddy correlationrequires concentrationmeasurements more ,, and ) suffered systematiclosses in rapid than the timescalefor the turbulenteddies that carrythe the analyticalsystem. Standardadditions of isopreneto air flux (1-1000 s at Harvard Forest). Instead,we use a similarity samples showed that isoprene recovery was linearly approachfor determiningthese fluxes for a whole forest, dependenton the amount of water vapor in the air and based on other quantities for which we have both nonlinearly dependenton the amount of isoprene added concentration data and direct measurements of flux. The (recoverydecreased with decreasingconcentration). Isoprene trace gas flux (F) is assumedto be proportionalto the time- data could not be correctedreliably, and the system was averagedconcentration gradient (dC/dz) abovethe forestfor changedin 1995 to eliminateisoprene losses in the trap. For intervalslonger than the time scalefor the slowestsignificant further details see Goldstein et al. [ 1995a]. turbulent events, The analyticalsystem was checkedfor contaminationdaily by runningzero-air blanks. No ethene,propene, or 1-butene F=K dC/dz, (1) was observed. The Teflon samplingtubes were checkedfor contaminationand memoryeffects by introducingzero-air at whereK is the exchangecoefficient for the averaginginterval. the sampleinlets on top of the tower on July 12, 1993. Three Denmead and Bradley [1985] reported that K (as defined measurementswere made over a 2.5-hour period. Small above)for sensibleheat and water vapor were nearly identical quantitiesof etheneand 1-butenewere measuredin the top above a 40-year-old pine forest canopy. In this paper we level (30 and 25 parts per trillion by volume (pptv) computeK using measurementsof flux from eddy correlation respectively),and 1-butene was measuredin the lower level observationsalong with observedconcentration gradients for (30 pptv), indicatingsome memory for thesecompounds. No CO2, H20, and sensibleheat and take the product with the memory was observed for propene at either level. The hydrocarbongradient to define the hydrocarbonflux. The influence of memory effects and systematic differences hydrocarbonfluxes derived using similarity with different between the responseof the dual analysissystem (3%-7%) quantities are in generally good agreement as discussed were eliminated from the gradient data by linearly below. interpolatingthe NULL gradient measuredevery fifth run onto the timeline of the gradient measurements and Measurements subtractingfrom the measuredgradients. Air wasdrawn continuously at 10 L min-1 through 3/8 inch Ethene was used as a reagentin an instrumentmeasuring OD Teflon tubes from two inlets (24 and 29 m) on a 30-m ozone at the site. The effluent from this instrument(30% tower. Samples for analysis were extracted from the inlet etheneat 1 L min-l) wasvented approximately 30 m southeast lines throughtees at the instrumentand passedthrough nafion of the tower. Prevailing winds were from the southwest, dryers (Perma Pure Products) and Ascarite IT (Thomas northwest, and north, nevertheless,reagent ethene was Scientific)traps to remove 03, CO2, and H20. Sampleswere occasionallydetected by the NMHC instrumentas indicated cryogenicallypreconcentrated on dual traps (40 ml min-1 of by single-point enhancements in the time series. air for 10 min onto a bare 1/16 inch OD stainlesssteel tube) Contaminatedmeasurements were removedfor the gradient and injected into a gas chromatographwith dual flame determination by eliminating observations where the ionization detectors (Hewlett Packard 5890 series IT). concentrationwas above 0.8 ppbv, which also eliminated Chromatographicseparation was accomplishedusing 30-m time periodsfor large pollutionevents. Correlationsobserved PLOT GS-Alumina Megabore capillary columns (J & W betweenethene gradients selected in this way and gradients Scientific). Every fifth pair of sampleswas taken from the for 1-buteneand propenesupport the validity of the selection samealtitude (29 m) throughseparate tubes (by switchinga criteria (seebelow). valve near the inlet of the 24-m sampling line) in order to Gradients for concentrationsof CO2 and H20, and for air determinethe NULL for the observedconcentration gradient. temperature, were measured simultaneously with the The measurementsystem could operate continuouslyand hydrocarbongradients. Concentration differences for CO2 unattended for more than 2 weeks, although data were and H20 were measuredusing a differential infrared gas GOLDSTEIN ET AL.: ETHENE, PROPENE, AND 1-BUTENE EMISSIONS 9151 analyzer(LICOR 6262), with air from 29 m passedthrough Individual flux calculations have random the referencecell and air from 24 m throughthe samplecell. errors associated with each of the measurements used to The gradientmeasurements were zeroedafter everysampling calculatethe flux. Hydrocarbonfluxes are determinedby period by filling both cells with air from 29 m. Instrument similaritywith CO2 usingthe equation, gain was determinedby addition of CO2 and dry air to the flow from 29 m. Pressure broadening and dilution = Fc(gdgc) (2) correctionsto the CO2 concentrationdue to the presenceof water vapor were made according to the instrument where F is flux, g is gradient,and hc and c refer to hydro- manufacturer'sspecifications. The standarddeviations of the carbonand CO2, respectively.Assuming that errorsin Fc, ghc, zero gradient measurementswere determinedby comparing and gc are randomand independent,the absolutestandard the NULL gradient measuredevery fifth sampling period deviation (•) for a determinationof Fhc can be calculated (when hydrocarbonNULL gradientswere determined)to the from [Skoog,1985]: zero measurementdirectly following that period. The standarddeviation in the zero measurementsfor CO2 and H20 IJFhc= Fhc [(tJFc/Fc) 2q- (Ogc/gc) 2+ ((Jghc/ghc) 2]1/2 (3) (0.18 ppm and 42 ppm, respectively)was of the order of 20% of the mean midday gradients(-0.9 ppm CO2 and 190 ppm Values for each of theseterms are given in Table 1 for typical H20). Flux determinationswere not attempted when ob- daytime summer conditions. Under these conditions the served gradientswere very small, that is, within 1 standard coefficient of variation ((JFhc/Fhcfor flux determinationsof deviation of zero. Water vapor gradient measurementswere ethene,propene, and1-butene ) is48%, 60%, and 76%, also discarded when the inlet filters on top of the tower respectively. becamewet after rain events. The CO2 fluxes and gradients Randomerrors will vary with ambientconditions including and all the NMHC measurementsare reported as mole magnitude of the flux, atmosphericstability, and absolute fractions relative to dry air at a common temperature, hydrocarbon concentrations. Most of the uncertainty is avoiding the need for density correctionsdue to fluxes of associated with quantifying gradients of CO2 and sensibleheat or H20 [Webbet al., 1980]. hydrocarbonsabove the forest. When fluxes are small or air Temperature gradients were measured using copper- abovethe canopyis being vigorouslymixed, the gradientsare constantinbare fine wire thermocouples(44 gage), placed 1 small and harder to quantify. Precisionof the hydrocarbon m south of the tower at both 29 and 24 m. Significant gradient measurement is a function of the absolute radiation loading occurredduring the daytime, making the concentration thus uncertainties increase when ambient differences between two thermocouplesat the same level concentrationsincrease (owing to biogenic emission or similar in magnitudeto the temperaturegradient between the regionalpollution). We have averagedthe gradient and flux levels (0.1 øC-0.2øC). The temperaturemeasurements worked data to minimize random errors while examining diurnal well at night (1700 to 0800 LT), with the standarddeviation cycles, forcing factors, seasonalityof emissions,and relative between two thermocouplesat the same level of 0.04øC, emissionsin the following discussion. comparedto standard deviations of 0.12 øC during the Systematicerrors in the flux-gradientsimilarity assumption daytime (0800 to 1700 LT). Accurate gradientscould be could occur if the distribution of the sources and sinks for measured over Harvard Forest for CO2 more often than for thesescalars are inhomogeneousin the footprint of the tower, temperatureor H20 owing to radiation loading on the if exchange occurs at significantly different heights in the thermocouplesand to the wetting of sample inlet filters. forest or if mesoscalecirculations strongly affect observed Therefore hydrocarbonfluxes reported here were calculated concentrationgradients. The magnitude of these systematic usingsimilarity with CO2. errors is extremely difficult to evaluate. We checked the Approximately9000 pairs of measurementswere made for validity of our similarity assumptionby comparingexchange each hydrocarboncompound, more than 75% of all 45 min coefficientsdetermined from CO2 fluxes and gradientswith intervalsduring the 12 monthsof data reportedhere. Gaps in those determined from H20 and sensibleheat. There could be the data occurred during the summer of 1993 owing to a additional systematicerrors if the distributionof the sources lightningstrike which disabledthe sonicanemometer (August and sinksfor CO2, H20, and sensibleheat were significantly 9-September 7), a broken gas chromatographiccapillary different from the distribution of the hydrocarbonsources. column (April 22-May 4), intermittent computer failures (April 2-May 7), and a broken samplingpump (June 7-12). Shorter gaps were due to occasionalpower failures and Table 1. Olefin Flux RandomError Analysis routine maintenance of the instruments.

Variable Units Ethene Propene 1-Butene Hydrocarbon Flux Error Analysis Uncertaintyinthe hydrocarbon fluxes can arise from issues Fc 1013molecules cm '2 s-• - 104 - 104 - 104 with the validityof the flux-gradienttechnique, systematic •5Fc 10•3 cm -2 s 'l 16 16 16 measurement errors, and random measurement errors. In the gc ppm -0.87 -0.87 -0.87 followingdiscussion the randomand systematicerrors are •5g½ ppm 0.18 0.18 0.18 evaluatedseparately. Random errors are assessedby ghc ppbv 0.059 0.029 0.014 •(Jghc propagatingthe standard deviations of thegradient and eddy F hc ppbv10•o molecules cm -2 s -1 7.10.024 0.0163.5 0.0081.7 flux measurementsthrough the hydrocarbonflux calculation. c•Fhc 10m moleculescm '2 s-1 3.4 2.1 1.3 Systematicerrors are assessedby examining the validity of the flux-gradientsimilarity methodas used at Harvard Forest F, flux; (•, standarddeviation; g, gradient;c, CO2; and hc, andthe potentialfor systematicmeasurement errors. hydrocarbon;ppbv, parts per billion by volume. 9152 GOLDSTEIN ET AL.: ETHENE, PROPENE, AND 1-BUTENE EMISSIONS

Figure l a is a plot of K derived from CO2 versusK derived CO2 flux and gradientmeasurements should not exceed20% from H20 (slope is 1.07 + 0.03 (1 standard error) and for middaysummer fluxes. R2=0.68).Figure lb is a plotof K derivedfrom CO2 versus K The largest potential for systematicerror in the hydro- derived from sensibleheat during the night (2200 to 0400 gradient is most likely associatedwith the NULL LT), when no solarradiation loading problems were apparent gradient correction. The existenceof nonzero NULL gra- (slopeis 1.12+ 0.06(1 standarderror) and R2=0.61). There dients appearsto reflect memory in the tubing, and frequent is significant statisticaluncertainty in individual exchange measurementsof the NULL gradient are crucial to correct for coefficientsderived from CO2, H20, and sensibleheat, owing both thesememory effects and for any systematicdifferences mostly to random errors inherent in measuring small between the dual analysis systems. We have tried to concentrationgradients. Outliers generally occurred when minimize systematicerrors by carefully correctingfor non- fluxes were relatively large and the gradientswere small, zero NULL gradients. Mean daytime (1000 to 1500 LT, June inducinglarge errorsin K. 1 to October 31) hydrocarbongradients were 0.045, 0.024, Values of K calculated from these three sets of measure- and 0.012 ppbv, including mean NULL gradientcorrections ments agree very well, however, within 12 + 10% (90% of-0.014, -0.005, and 0.000 ppbv, for ethene,propene, and 1- confidenceinterval from slope standarderror), when the data butene, respectively. The maximum systematicerror due to are aggregatedand averaged. Systematicerrors which effect the NULL gradientcorrections is therefore30% for midday eddy flux measurementsof H20, CO2, and sensible heat meansummer fluxes, based on the ratio of the NULL gradient equally (suchas errorsin wind measurements)would not be correctionto the correctedgradient. The total systematicerror accountedfor by this comparisonbut are expectedto be less associatedwith the mean daytimehydrocarbon fluxes should than 10% basedon closureof the energybudget [Goulden et not exceed 50%, and our analysissuggests that it may be al., 1996]. Hence the sum of systematic errors for considerablysmaller (-20%). hydrocarbonfluxes due to deviationsfrom similarity and to Results and Discussion

First, we provide evidence from several different sets of observationsfor summertimebiogenic emissionsof ethene, slope= 1.07 r2 = 0.68 .." propene,and 1-butene. Next, we examinediurnal flux cycles and evaluate which environmentalforcing factors are most O 0 0 0 important. Finally, we assessthe significanceof biogenic 0 @" 0 emissionsof theseolefins, comparingthe observedfluxes to thosereported for regionalanthropogenic sources.

Evidence of Biogenic Emissions •0 0 0 Evidence of summertime biogenic emissionsof ethene, propene,and 1-buteneis apparentin scatterplots of ambient 0 concentrationsversus acetylene (a tracer of anthropogenic O - emissions)in Januaryand July 1993 (Figure 2). Variationsof the olefin concentrationsare closelycorrelated with acetylene 50 100 150 in January,indicating their anthropogenicemission ratio. In July the correlationswith acetylenewere weak, particularly K (m2 s -l) fromCO2 for properieand 1-butene, owing to biogenic emissionsand possibly to faster loss rates in summer. Scatter plots of /

/ propeneversus acetylene (Figure 3) for all the monthsof 1993

/ slope= 1.12 r2 = 0.61 / show that significantbiogenic emissions occurred from May / to September. The impact on "background"concentrations

/ / (defined as times when acetyleneis below its 0.2 quantilein / / c5'O 30 day periods)of theseolefins is shownby comparingtheir O relativeseasonal variations with thoseof ,pentane, and hexane(Figure 4), compoundsof dominantlyanthropogenic 0 .• O0 origin. Normalized seasonalvariations of hydrocarbonswith predominantly anthropogenicsources, and with lifetimes shorter than , are nearly identical at Harvard Forest [Goldsteinet al., 1995b]. Concentrationsof pr•pene and 1- 0 0 butene are anomalously high in summer owing to the 0 - i i i i i influence of local biogenic sourcesfor these compounds. Ethene reachesits highest concentrationsin winter, but its 0 10 20 30 40 relative seasonalvariation is not as pronouncedas for butane, pentane,and hexane,also owing to the influenceof seasonal K (m2 s 'l) fromCO2 biogenicemissions.

Figure 1. Exchange coefficient (K) calculatedfrom the Diurnal and Seasonal Fluxes measuredflux and gradient of CO2 and (a) H20 and (b) sensibleheat (nighttimedata only owing to radiationloading A 2-day sequenceof data for concentrationsof hexane,1- problemson temperaturesensors during the day). butene, propene, and ethene (at 29 m), the raw gradient GOLDSTEIN ET AL.: ETHENE, PROPENE, AND 1-BUTENE EMISSIONS 9153

January 1993 July 1993 Harvard Forest 1993

Mar

0 1 2 3 0 1 2 3 Apr Jun

ß ß ß

0 1 2 3 0 1 2 3 Auq Sep eee•e ß ß

Oct Nov I I 0 1 2 3 0 1 2 3 3 4 0 2 3 4 0 1 2 3 4 acetylene (ppbv) acetylene (ppbv)

Figure 2. Concentrationsof 1-butene, propene, and ethene Figure 3. Propene versusacetylene concentration for all the versus acetylene concentration in January and July 1993. monthsof 1993. The line in each scatterplot representsthe Acetyleneis a tracer of anthropogenichydrocarbon emissions slopeof the January1993 correlation. with no significant terrestrialbiogenic source. The signature of anthropogenicemissions is evident in January. The olefins, 1-butene, propene, and ethene, all have increased concentrationsin July relative to acetyleneowing to emission active radiation(PAR) (measuredabove the canopy)than the from the forest. cycle of air temperature (Figure 6). The l-butene diurnal pattern was the least well defined, probably because its gradient was so close to the detection limit of the NMHC betweenthe levels (24-29 m), and the NULL gradient (every instrument. Fluxes of ethene, propene, and l-butene fifth run) (Figures5a-5d) revealsstriking patterns(corrected increasedlinearly with light, presentedas mean flux versus gradient is raw minus NULL). Significant excess PARin Figure7 (R2 is 0.93,0.99, and 0.96, respectively and concentrationsat the lower inlet were observedduring the day R2 for nonaggregateddata is 0.10, 0.19, and 0.09, for ethene, propene, and 1-butene, with much smaller respectively with P < 0.0001) . The correlation between gradients at night; correspondingdiurnal cycles were emissionsof olefins and incident light suggeststhat forest observed in the mean concentrations. Gradients were not vegetation was the main source of these olefins. observedfor any C2 - C6 alkanesor for acetylene,indicating Unfortunately, the observationslack the precision needed to that these species were not emitted from the forest in define the role of secondaryfactors such as humidity or observablequantity. The raw and NULL gradientsfor hexane phenology. are indistinguishableand essentially zero. The NULL Soil processeswere probably negligible sourcesof ethene, gradientsfor l-butene, propene, and ethene are measurable, propene,and l-butene. If the olefins were coming from the correlatingwith the mean concentration.As discussedabove, soil, we would expect their emissionsto be correlated with nonzero NULL gradients appear to reflect memory by the soil temperatureand to continue at night. We observe large tubing, and we have tried to minimize systematicerrors by vertical concentrationgradients above the forest at night for carefully correctingfor nonzeroNULL gradients CO2, especiallyduring stable mixing conditions,but not for Fluxes of ethene, propene, and l-butene during the the olefins. The nighttime gradient in CO2 results growing season(June 1 to October 31, 1993) more closely predominantly from soil emissions. Hence we conclude followed the diurnal pattern of incident photosynthetically olefin emissionsfrom soils were not significant. 9154 GOLDSTEIN ET AL.' ETHENE, PROPENE,AND 1-BUTENE EMISSIONS

a b _ butane - ,.; ?• ethene i•.',,Jt'• ...... pentane •' ...... propene !/.: ',.•...... hexane • • • 1-butene

! ;' ',.:

ß i • . ", ,-,:; .,, t,; , . :.-;,, ' , i ' ,',! ; i.. - :':'•:'•:,' •"'.'• , '•.. :.. . •'., • • '., ,,,.•,,'.., ...,...,,.

• ,,, • , ,'\ ..: .- \ .- , ,,;; ,, • ',:. • •.',-L."Z., ...... _ ,, ,,:.; ,,

_

i i i i i i i i i I i i i

Jan Mar May Jul Sep Nov Jan Jan Mar May Jul Sep Nov Jan

1993 Month

Figure4. Relativeseasonal variations of 1993"background" concentrations of (a) butane,pentane, hexane, and(b) ethene,propene, and 1-butene. Background is definedas the mean value when acetylene is belowits 0.2 quantilein 30-day periods.

The seasonaltrends of emissionsof ethene,propene, and Figure3. Deciduoustrees develop leaves in earlyMay, this 1-butenemay be inferredfrom mean midday gradients (1000- resultprovides evidence that coniferoustrees contribute to 1500 LT) observedover a 1-year period (Figure 8a). emissionsofthesecompounds. Emissionsbegan in April or May andended in Octoberor We maycompare observed gradients for ethene,propene, November,a slightlylonger period than was obviousfrom • and 1-butenedirectly to definerelative rates of emissionat

- c O O Concentration(29m) O Concentration(29m /• ...... Raw Gradient -J- .... NULL Gradient -I"- .... NULL Gradient

o ½% /•...... RawGradient o o 0% o¸. o o o

O ,

O

J -d

o o o

c• $ I'• o /•o o

o o

,,

,. ,,

c• • ...'

i i i i i i i i i i

221.0 221.5 222.0 222.5 223.0 221.0 221.5 222.0 222.5 223.0

1993 Julian Day 1993 Julian Day Figure5. Concentration(at 29 m), raw gradient(29-24 m), andNULL gradient(every fifth run)measured aboveHarvard Forest on August9 and 10, 1993for (a) hexane,(b) 1-butene,(c) propene,and (d) ethene. The correctedgradient equals the raw minusNULL gradients. GOLDSTEIN ET AL.: ETHENE, PROPENE,AND 1-BUTENE EMISSIONS 9155

...... Temp sløper2 PAR I• ethene x r,.o 0.00500.93 OlefinFlu•,, /k, propene0.0019 0.99 • 1-butene0.0007 0.96 /

,, 0.00070.96

CD

0 5 10 15 20

i

0 500 1000 1500

0 5 10 15 20 PAR(BE m-: s -1) Figure 7. Mean flux (+ standarderror) of ethene,propene, T and 1-butene as a function of incident PAR (June 1 to October 31, 1993). Data was parsedinto PAR windows of X /" -'7"kx "'"- Eo 300 [rE m-2 s-•. Standarderrors of the slopesof the correlation'swith PAR equal 0.0008, 0.0001, and 0.0001 for ethene,propene, and 1-butene,respectively. • ! \ "...

findthat biogenic emissions of ethene (2.63 x 10•ø molecules

.. c) cm s ) are about half the anthropogenicemission, while

i i i i biogenicemissions of propene (1.13 x 10•ø molecules cm -2 s -l) 0 5 10 15 20 are twice as large as anthropogenic emissions. This interpretationis supportedby the summertimeenhancements hour of day shown in Figure 2. Figure 6. Average diurnal cycles for fluxes of ethene, Regionalfossil fuel combustionsources for propeneand 1- propene, and 1-butene (+ standarderror) along with butene can also be scaled from the NAPAP ethene emissions temperatureand incidentphotosynthetically active radiation using emissionratios measuredduring the winter at Harvard (PAR) (June1 to October31, 1993). Data wasparsed into 2- Forest (Figure 2), 20/4/1, which agree well with those hour time windows. determinedfrom the NAPAP [1985] emission inventory for the whole United States, 19/4/1 [Middleton et al., 1990]. Using these data and our measuredratio of 4/2/1 for forest midday(4:2:1, Figure 8b) over the period June 1 to October vegetationemissions, we infer that combustionsources likely 31, 1993, eliminatinguncertainty due to the CO2 and vertical representan even smallerfraction of regionalemissions for 1- wind measurements. The relative fluxes estimated by butene than for propene or ethene. The data show integrating mean diurnal fluxes measured over the same unambiguouslythat regional biogenic emissionsof propene period(Figure 6), 2.63,1.13, and 0.41 x 10n) molecules cm-2 and 1-butene are larger than the regional anthropogenic s-• respectively, areconsistent with these values. sourcesin summer at Harvard Forest, despite proximity to a region with massiveanthropogenic sources. BiogenicVersus Anthropogenic Emissions Enhanced atmospheric concentrations of ethene and in Massachusetts propenehave previouslybeen observedin forested regions. Ethene, propene,and 1-butenehave regional anthropo- Zimmermanet al. [1988] reportedelevated levels of ethene genic emissionsdue to the combustionof fossil fuel. The and propenein the Amazon boundarylayer over a tropical 1985 National Acid Precipitation Assessment Program forestsuggesting biomass burning as a likely source,although (NAPAP) emission inventory indicates Massachusetts they noted that terrestrial or aquatic biogenic sourcescould emissionsfor etheneand propene(1-butene is not includedas have contributed. Greenberg et al. [1992] found significant a separatespecies in this inventory)to be 99% and 95% from increasesin ethene, propene, and isoprene during upslope mobile sources with total emissions of 638 and 55 million flow at Mauna Loa, Hawaii; they attributedthe isopreneto molesper year,respectively, equivalent to 5.7 x l0n) island vegetationbut the etheneand propeneto local marine moleculescm '2 s-• and 0.5 x 10•ø moleculescm '2 s-• if these emissions.Our resultssuggest that significantenhancements sourceswere evenly distributedover the state. Comparing of etheneand propeneconcentrations in theseenvironments this number to summertime emissions at Harvard Forest, we could be attributedto emissionsfrom vegetation. 9156 GOLDSTEIN ET AL.: ETHENE, PROPENE, AND 1-BUTENE EMISSIONS

winter we observed dominant contributions from

ethene anthropogenicsources, and the NAPAP [1985] United States emission inventory accurately predicted observed emission ...... propene 1-butene ratios. Our measurementssuggest that terrestrialbiogenic emissionscould provide a significantglobal sourcefor two importantreactive olefins, propene and 1-butene. Acknowledgments. This work was supportedby grants to Harvard University from the U.S. Department of Energy's (DOE) National Institute for Global Environmental Change (NIGEC) throughthe NIGEC NortheastRegional Center at Harvard University (DOE Cooperative Agreement DE-FC03-90ER61010), by the National Aeronauticsand SpaceAdministration (NAGW-3082), the National ScienceFoundation (BSR-89-19300), the Long Term Eco- logical Research program at the Harvard Forest funded by the National Science Foundation (BSR-88-11764), and by Harvard University (Harvard Forest and Division of Applied Sciences). Financialsupport for Allen Goldsteinwas providedby the Graduate FellowshipsFor Global Changeprogram of Oak Ridge Associated Universities(DOE). The authorsgratefully acknowledge helpful dis- II \ cussions with David Fitzjarrald and Katherine Moore (SUNY- ß Albany) and technical and operationalhelp from Bruce Daube Jr., Ammar Bazzaz, Robert Mendelson,Olga Itkin, and SrabaniRoi.

References

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