Signatures of terminal alkene oxidation in airborne

14 Pages·2003·5.03 MB·English

JOURNAL OF GEOPHYSICAL RESEARCH, VOL.108, NO.D3,4104, doi:10.1029/2002JD002502, 2003
Signatures of terminal alkene oxidation in airborne formaldehyde
measurements during TexAQS 2000
B. P. Wert,1,2 M. Trainer,3 A. Fried,1 T. B. Ryerson,3 B. Henry,1 W. Potter,1,4
W. M. Angevine,3,5 E. Atlas,1 S. G. Donnelly,1 F. C. Fehsenfeld,3 G. J. Frost,3,5
P. D. Goldan,3 A. Hansel,6 J. S. Holloway,3,5 G. Hubler,3,5 W. C. Kuster,3
D. K. Nicks Jr.,3,5 J. A. Neuman,3,5 D. D. Parrish,3 S. Schauffler,1 J. Stutz,7
D. T. Sueper,3 C. Wiedinmyer,1 and A. Wisthaler6
Received1May2002;revised12September2002;accepted20September2002;published6February2003.
[1] Airborne formaldehyde (CH2O) measurements were made by tunable diode laser
absorption spectroscopy (TDLAS) at high time resolution (1 and 10 s) and precision
(±400and±120partspertrillionbyvolume(pptv)(2s),respectively)duringtheTexasAir
Quality Study (TexAQS) 2000. Measurement accuracy was corroborated by in-flight
calibrations and zeros and by overflight comparison with a ground-based differential
optical absorption spectroscopy (DOAS) system. Throughout the campaign, the highest
levelsofCH Oprecursorsandvolatileorganiccompound(VOC)reactivityweremeasured
2
in petrochemical plumes. Correspondingly, CH O and ozone production was greatly
2
enhanced in petrochemical plumes compared with plumes dominated by power plant and
mobile source emissions. The photochemistry of several isolated petrochemical facility
plumes was accurately modeled using three nonmethane hydrocarbons (NMHCs) (ethene
(C H ),propene(C H )(bothanthropogenic),andisoprene(C H )(biogenic))andwasin
2 4 3 6 5 8
accord with standard hydroxyl radical (OH)-initiated chemistry. Measurement-inferred
facility emissions of ethene and propene were far larger than reported by inventories.
Substantial direct CH O emissions were not detected from petrochemical facilities. The
2
rapid production of CH O and ozone observed in a highly polluted plume (30+ parts per
2
billionbyvolume(ppbv)CH Oand200+ppbvozone)originatingoverHoustonwaswell
2
replicated by a model employing only two NMHCs, ethene and propene. INDEXTERMS:
0345AtmosphericCompositionandStructure:Pollution—urbanandregional(0305);0365Atmospheric
CompositionandStructure:Troposphere—compositionandchemistry;KEYWORDS:formaldehyde,
troposphericoxidation,urbanpollution,alkeneoxidation,petrochemicalfacilityemissions
Citation: Wert,B.P.,etal.,SignaturesofterminalalkeneoxidationinairborneformaldehydemeasurementsduringTexAQS2000,
J.Geophys. Res.,108(D3),4104,doi:10.1029/2002JD002502,2003.
1. Introduction in the oxidation of a large array of volatile organic com-
pounds(VOCs).However,onlyasmallnumberofVOCsare
[2] Gas-phaseformaldehyde(CH2O)isaubiquitouscom- bothabundantandefficientCH Oproducers.Inthepolluted
ponentofthetroposphereandistypicallythemostabundant 2
boundary layer the terminal alkenes are generally the most
carbonyl compound found in the boundary layer atmos-
important CH O precursors, particularly isoprene (C H ),
phere. CH O is largely produced in situ, as an intermediate 2 5 8
2 ethene(C H ),andpropene(C H )[Dodge,1990;Leeetal.,
2 4 3 6
1998b; Goldan et al., 2000]. In many cases, the important
1Atmospheric Chemistry Division, National Center for Atmospheric CH Oprecursorsalsocontributesubstantiallytototalhydro-
Research,Boulder,Colorado,USA. 2
carbon hydroxyl radical (OH) reactivity ((cid:1)[k OH *
2Also at Department of Chemistry and Biochemistry, University of x
Colorado,Boulder,Colorado,USA. [VOCx]), and thereby total VOC reactivity. Here kx is the
3Aeronomy Laboratory, National Oceanic and Atmospheric Adminis- bimolecularrateconstantforreactionofOHwitheachVOC.
tration,Boulder,Colorado,USA. Due to the heterogeneous spatial distribution of CH O
4Also at Department of Chemistry and Biochemistry, University of 2
precursors, and the relatively short lifetime of CH O (2–4
Tulsa,Tulsa,Oklahoma,USA. 2
5AlsoatCooperativeInstituteforResearchinEnvironmentalSciences, hours in the sunlit troposphere), continental boundary layer
UniversityofColorado,Boulder,Colorado,USA. CH2O concentrations span nearly 2 orders of magnitude
6InstituteforIonphysics,UniversityofInnsbruck,Innsbruck,Austria. [Grosjean, 1991; Fried et al, 1997; Lee et al., 1998b],
7Department of Atmospheric Sciences, University of California, Los providing asensitive indicator of recentVOC oxidation.
Angeles,LosAngeles,California,USA.
[3] CH2O is also emitted directly into the atmosphere by
incomplete fossil fuel combustion [Sigsby et al., 1987;
Copyright2003bytheAmericanGeophysicalUnion.
0148-0227/03/2002JD002502$09.00 Altshuller, 1993; Anderson et al., 1996], biomass burning
ACH 8 - 1
ACH 8- 2 WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS
2
[Lee et al., 1998a; Yokelson et al., 1999], industrial pro-
cesses, and vegetative emissions [Carlier et al., 1986].
However, primary CH O emissions are typically small
2
relative to photochemical production. Using ambient meas-
urements,Lietal.[1994]haveestimatedthattheamountof
CH O directly emitted from eastern North America is 16
2
times less than that produced by the oxidation of emitted
anthropogenic and biogenic VOCs.
[4] Since VOC oxidation also drives tropospheric ozone
production in the polluted boundary layer [Haagen-Smit,
1952; Chameides et al., 1992; Sillman, 1999], CH O
2
measurements are useful to deconvolving sources of ozone
pollution, an issue central to urban and regional air quality.
For instance, CH O measurements made during the 1995
2
SouthernOxidantsStudy(SOS95),basedinNashville,TN,
were used to corroborate the contribution of biogenic
isoprene to regional ozone production [Lee et al., 1998b];
and satellite measurements of CH O have been used to
2
evaluate biogenic isoprene emissions over North America
[Chance et al.,2000; Palmeret al.,2001]. Tosimilar ends,
the present study utilizes high time resolution airborne
CH OmeasurementscollectedduringtheTexasAirQuality Figure 1. Regional distribution of CH O maximums
2 2
Study (TexAQS) 2000 to examine the role of various measured below 1000 m flight altitude for 0.2(cid:2) latitude by
hydrocarbons in the photochemistry of the Houston, Texas 0.2(cid:2)longitudegridcells.Thesolidcirclesshowthelocation
area. of NO emitters and are sized according to reported NO
x x
[5] During the 1990s, ozone levels in exceedance of emissions. Labels are provided for cities (Houston and
National Ambient Air Quality Standards (NAAQS) were Dallas–Fort Worth), utility power plants (W.A. Parish,
regularlyreportedforthegreaterHouston,TXmetropolitan Monticello, and Welsh), isolated petrochemical facilities
area. On average, only Los Angeles reported more intense (Sweeny, Freeport, and Chocolate Bayou), and areas of
episodes of ozone pollution during this period [Lin et al., concentrated petrochemical production (Ship Channel and
2001]. Over 4 million people live in the greater Houston Texas City) discussed in the text.
metropolitan area [U.S. Census Bureau, 2000], resulting in
largemobile source emissions of VOCs andNO [TNRCC,
x CH O precursors) in the Houston atmosphere? (2) Can the
2
1996]. Oneofthe largest electric utility power plantsinthe
fast CH O measurements reported here be used to accu-
2
nation, the W.A. Parish facility, is located just outside of
rately extrapolate intermittent, lower resolution CH O pre-
2
Houston (see Figure 1) (D. K. Nicks et al., Fossil-fueled
cursor measurements made on the same aircraft? (3) To
power plants as a source of atmospheric carbon monoxide,
what extent did the important CH O precursors contribute
2
submitted to Atmospheric Environment, 2002). In addition,
torapidozoneproductionintheHoustonarea?and(4)Were
a large fraction of the U.S. olefin production capacity is
emission inventories accurate with respect to the important
found in the Houston–Galveston–Brazoria area, with an
CH O precursors?
2
especially high density located along the Ship Channel
inside Houston city limits (see Figure 1). Biogenic emis-
2. Experimental
sions are also a source of VOCs in and around Houston,
although detailed vegetation surveys [Wiedinmyer et al., 2.1. Airborne Sampling
2001] indicate that these emissions should be substantially [8] The measurements considered here were made from
less than in areas such as Tennessee [Geron et al., 1994; the NCAR L-188C Electra aircraft. CH O measurements
2
Goldan et al., 2000]. were made by the NCAR tunable diode laser absorption
[6] TheTexAQS2000missionwasconductedtoelucidate spectrometer (TDLAS). Supporting measurements included
the role of such emissions, along with meteorology, in 1 Hz ozone, CO [Holloway et al., 2000], CO , NO, NO ,
2 2
Houston’s intense photochemical pollution [Kleinman et NO [Ryerson et al., 2000], HNO [Huey et al., 1998;
y 3
al., 2002]. The study was conducted from mid-August to Neuman et al., 2002], SO , size-resolved aerosol number
2
mid-September 2000 and involved the combined efforts of [Brock et al., 2000], and spectrally resolved actinic flux.
the Texas Natural Resource Conservation Commission Peroxyacyl nitrate compounds were measured once every
(TNRCC), the National Oceanic and Atmospheric Admin- 3.5 min by gas chromatography (GC) (F. Flocke et al., The
istration (NOAA), the National Center for Atmospheric behavior of PANs and the budget of total reactive nitrogen
Research (NCAR), Brookhaven National Laboratories (NO ) during the TOPSE campaign, manuscript in prepa-
y
(BNL),theUniversityofTexas,BaylorUniversity,andmany ration, 2002). Acetaldehyde was measured by proton trans-
other laboratories and universities. Multiple well-instru- fer mass spectrometry (PTRMS) [Lindinger et al., 1998].
mentedaircraft andgroundsiteswereemployed. Thirty-nine whole air canister samples, typically integrated
[7] The present study addresses four primary questions over8s,wereacquiredoneachflightatthediscretionofthe
utilizing airborne CH O observations collected during Tex- flight scientist; subsequent GC analysis provided an exten-
2
AQS2000:(1)WhatwerethemajorsourcesofCH O(and sivesuiteofspeciatedVOCs(seeTable1)[Schauffleretal.,
2
WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS ACH 8 - 3
2
Table 1. VOCs Measured by the NCAR Whole Air Sampler determined to be ±6% [Gilpin et al., 1997; Fried et al.,
(WAS) 1998, 2002]. During the first portion of the TexAQS
intensive, ambient CH O measurements were made at 10 s
Alkanes Alkenes Aromatics Alkynes 2
resolution.Dataintegrationwasreducedto1sbeginningon
methane ethene benzene ethyne
the 1 September flight, and this resolution was maintained
ethane propene toluene
propane 1-butene ethylbenzene for the remainder of the mission. Estimated TDLAS pre-
isobutene isobutene m-xylene,p-xylene cisionduringtheTexAQSmissionwas120partspertrillion
n-butane t-2-butene o-xylene by volume (pptv) for 10 s measurements, and 400 pptv for
isopentane c-2-butene styrene
1 s measurements (reported at the 2s level) (Wert et al.,
n-pentane 1,3-butadiene isopropylbenzene
n-hexane 3-methyl- n-propylbenzene submitted manuscript, 2002). Ambient measurements
1-butene covered roughly 50% of the total flight time, excluding
2,2-dimethylbutane 1-pentene 1,2,3-trimethyl takeoff and landing.
benzene
2,3-dimethylbutane isoprene 1,2,4-trimethyl [11] Extensive tests have found that only methanol
(CH OH) interferes with this technique. Laboratory studies
benzene 3
3-methylpentane t-2-pentene 1,3,5-trimethyl using the same TDLAS arrangement as in TexAQS have
benzene shown the methanol interference to be +3.8% at equal
2-methylhexane c-2-pentene methanol and CH O concentrations [Fried et al., 2002]
3-methylhexane 2-methyl- 2
(Wert et al., submitted manuscript, 2002), consistent with
2-butene
2,2,4-trimethyl cyclopentene independent measurements made at Rice University
pentane [Richter, 2001]. Thus ambient methanol levels of 10 parts
n-heptane a-pinene per billion by volume (ppbv) would cause the TDLAS
n-octane
CH O measurement to be too high by 380 pptv. TDLAS
n-nonane 2
n-decane CH2Odata werecorrected usinganalgorithm derivedfrom
cyclopentane 144 coincident measurements of methanol (in situ GC) and
cyclohexane CH O made on the Electra. Given the 1–3 ppbv CH O
2 2
dimethylcyclopentane
boundary layer background in the Houston area, and the
dimethylcyclohexane
emphasis of the present study on plume chemistry, the
methanol bias and associated correction were largely negli-
gible for the data of interest (<10%). All analyses in this
1999]. In situ GC samples were acquired once every 15
paper utilize methanol corrected data.
min, measuring a different subset of VOCs, including a
limited suite of oxygenates [Goldan et al., 2000]. [12] Further evidence for the accuracy of the TDLAS
CH O measurement was provided by comparison with
[9] Flights surveyed the greater Houston metropolitan 2
UCLA’s ground-based differential optical absorption spec-
area on 8 different days. Two days each were dedicated to
trometer(DOAS)[StutzandPlatt,1997;Harderetal.,1997]
exploring the Dallas–Fort Worth area, and power plant
during TexAQS; these two systems were also successfully
plumes throughout eastern Texas. Flight tracks usually
compared throughout the SOS 99 campaign (Wert et al.,
involved multiple crosswind intercepts of one or several
submitted manuscript, 2002). During TexAQS the Electra
plumes at a number of distances downwind, in order to
aircraftconductedoverflightsoftheLaPorteairport,thesite
characterizethe temporal evolution of the plume chemistry.
of extensive chemical instrumentation including the UCLA
The bulk of sampled plumes were either of mobile source
DOAS. TDLAS data was averaged for a 5–10 s period of
(urban), petrochemical facility, or power plant origin, with
closestapproachinallbutonecase,correspondingtoapath
many plumes representing some combination of these
comparableinlength(0.5–1km)andheight(40–60mflight
emission types. In order to capture midday photochemistry
altitude) to that of the DOAS (1.9 km path length, 2–50 m
and the daily ozone maximum, flights typically took off
height above ground). Ground-based O and CO measure-
between 1000 and 1100 CST, and lasted for about 6 hours. 3
ment variability was acceptably small during the DOAS
2.2. TDLAS System integration periods (20 min to 1 hour). Bivariate regression
[10] The TDLAS and its forerunner systems have been oftheTDLASandDOASCH2Odataproducedtherelation-
described [Fried et al., 1998, 2002] (B. P. Wert et al., ship:(TDLASCH2O)=0.95*(DOASCH2O)(cid:1)0.70ppbv
Designandperformanceofatunablediodelaserabsorption (n = 6, c2 = 5.5), with slope and intercept uncertainties of
spectrometerforairborneformaldehydemeasurements,sub- 0.15 and 0.65, respectively (see Figure 2). CH2O measure-
mitted to Journal of Geophysical Research, 2002, herein- mentsmadebythreeseparateDOASpathsatLaPorte(0.75,
afterreferredtoasWertetal.,submittedmanuscript,2002), 1.9, and 5.5 km path lengths) agreed well during all over-
thus only a short summary is included here. Gas-phase flight periods but one, indicating good mixing. The over-
CH O was measured by probing the 2831.6417 cm(cid:1)1 flight associated with conditions of poor mixing was that
2
rovibrational line with a tunable diode laser coupled to a whichyieldedthehighconcentrationpoint.Removalofthis
100 m path length astigmatic Herriott absorption cell. pointproducesaslopeof1.23(±0.29).Theseresultssuggest
Extensive field and laboratory analysis has shown that the that systematic measurement errors associated with the two
TDLAS inlet and Herriott cell produce only minimal different techniques are small(< (cid:2)10%).
sampling artifacts under the conditions encountered during
TexAQS[Wertetal.,2002].Frequentinstrumentzeroswere 2.3. Photochemical Model
acquired, and flowing gas-phase standards were generated [13] Thephotochemicalmodelusedinthecurrentstudyis
via a permeation oven, the accuracy of which has been similar to earlier plume simulations by, for example, Hov
ACH 8- 4 WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS
2
the boundary layer (0.5–2.5 km) (C. Senff, private com-
munication, 2001), indicative of surface CH O precursor
2
sources and the short lifetime of CH O under summer
2
conditions. The regional distribution of CH O maximums
2
observedbelow1000mflightaltitudeisshowninFigure1,
atagridcellresolutionof0.2(cid:2)latitudeby0.2(cid:2)longitude(22
(cid:3) 19 km, respectively). Of primary interest to the present
studyarethehighCH OlevelsfoundinthegreaterHouston
2
metropolitan area on many days. These large mixing ratios
of between 10 and 32 ppbv were observed in plumes
substantially smaller in spatial extent than the Houston
metropolitan area. In contrast to Houston, Dallas–Fort
Worth had CH O levels that were much lower (2–6 ppbv)
2
and more spatially uniform. Measurements of the Dallas–
Fort Worth plume are compared with those from Houston,
as the populations of the cities are similar (Dallas–Fort
Worth: Dallas and Tarrant counties, 3.6 million people;
Houston: Harris, Fort Bend, Brazoria, and Galveston coun-
ties, 4.2 million people) [U.S. Census Bureau, 2000]. The
only power plant plumes with CH O levels significantly
2
above background were those from the Welsh and Mon-
ticello plants (up to 17 ppbv CH O over a very limited
2
area), which encountered strong biogenic isoprene emis-
Figure 2. Bivariate regression of CH O measurements sions (see Figure 1).
2
made by the NCARTDLAS on the Electra aircraft and the
UCLA DOAS at the La Porte site during close overflights. 3.2. Houston Area CH O Precursor Distributions
2
[15] To provide an assessment of the relative CH2O
production expected from measured VOCs, the present
analysis calculates the ‘‘CH O production potential’’ of
andIsaksen[1981]andSillman[2000],wheretheplumeofa 2
individual VOC samples [Lee et al., 1998b]. For a given
powerplantwasfollowedinanidealizedLagrangianfashion
VOC mixture, the CH O production potential is here
and the vertical and horizontal spreading were explicitly 2
expressed as (cid:1)(k OH * [VOC ] * (gCH O )). Note
described.Inthepresentmodeltheverticaldispersionofthe x x 2 (VOCx)
that in this analysis the CH O yield term (gCH O) reflects
plume was controlled by the growth of the planetary boun- 2 2
only the first generation products of OH-initiated VOC
darylayer(PBL),anddiffusiveverticaltransportwascalcu-
oxidation; production from moderately lived intermediates
lated as described by Trainer et al. [1991]. The maximum
such as acetaldehyde (CH CHO) is not considered. Molar
height of the PBL was scaled to approximate observed 3
CH O yields used for the most abundant reactive CH O
conditions.Horizontalspreadingoftheplumewasdescribed 2 2
precursors were ethene (1.6) [Niki et al., 1981], propene
by a diffusion coefficient chosen to approximate the
(1.0) [Niki et al., 1978], and isoprene (0.6) [Carter and
observed growth in plume width as a function of distance.
Atkinson, 1996]. For the alkanes yields of 0.1 to 0.3 were
Theadvectivewindwasafreeparameterthatcontrolledthe
typically used (exceptions include methane (1) and isobu-
peakconcentrationintheplume,andwaschosenaccording
tane (0.8)); the yields of alkynes and most aromatics were
to measurements aboard the aircraft, or radar profile meas-
set to zero [Lee et al., 1998b]. OH reaction rate constants
urements made from the ground. Horizontal grid resolution
were given by Atkinson [1990, 1997] and DeMore et al.
wasfixedat500mandtheplumewasemittedintoa100(cid:3)
[1997].
500(cid:3)500mgridcell.Plumespreadingandphotochemistry
were concurrently calculated within this grid framework. [16] CH2O production potentials were calculated for 390
VOC samples collected in canisters aboard the Electra at
The chemistry was based on the studies of Trainer et al.
flightaltitudeslessthan1000mduringTexAQS,producing
[1987, 1991] where the reaction rates and products were
theHouston–GalvestonareadistributiondepictedinFigure
updatedaccordingtoJPL97[DeMoreetal.,1997]andtothe
4. A distinct maximum in CH O production potential is
studyofAtkinson[1997].Photolysisrateswerecalculatedby 2
clearlyevidentinthevicinityoftheHoustonShipChannel.
the TUVradiative transfer model of Madronich [1987] and
Of the VOC samples taken in this area, six yielded pro-
validated by comparison with actinic flux measurements
duction potentials between 13 and 63 s(cid:1)1. Only two other
made on the Electra. The TUV model is based upon the
samples acquired during TexAQS had CH O production
discrete ordinates model modified to solve the radiative 2
potentials greater than 10 s(cid:1)1. One of these samples was
transfer equation in pseudospherical coordinates [Dahlback
taken in a plume downwind of Texas City, an area of
and Stamnes,1991].
concentrated petrochemical industry south of Houston (see
Figure 1). The other was measured in a plume intercepted
3. Results
over eastern Texas, from a petrochemical facility that
3.1. TexAQS CH2O Distributions reported the largest ethene emissions in Texas (Eastman
[14] As depicted in Figure 3, the vertical distribution of Chemical) [TNRCC, 1999]. On average, the terminal
CH OmeasuredduringTexAQSshowsarapiddecayabove alkenes,largelyetheneandpropene,composed95%oftotal
2
WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS ACH 8 - 5
2
Figure3. TheverticaldistributionofCH OmeasurementsmadeduringTexAQS2000.MedianCH O
2 2
values for 1000 m flight altitude bins are denoted by the solid circle symbols.
CH O production potential and 91% of total hydrocarbon with ethene contributing 30%, propene 22%, and isoprene
2
OH reactivity in these eight highlypolluted samples. Addi- 14% (1-butene, isobutene, 1, 3 butadiene, 3-methyl-1-
tional sources of substantial CH O production potential butene, and 1-pentene also contributed to a lesser degree).
2
were the Sweeny, Freeport, and Chocolate Bayou petro- Terminalalkeneswerealsoresponsibleforamajority(71%)
chemical facilities south of Houston (2–9 s(cid:1)1); these three of the total OH hydrocarbon reactivity of these samples.
facilities will be collectively referred to as the ‘‘isolated [18] Seven VOC samples, acquired on 5 different days
petrochemical facilities.’’ The forested region northeast of between1130and1600CSTwereselectedasrepresentative
HoustonalsofeaturedsomewhatelevatedCH Oproduction of Houston mobile source emissions based upon chemical
2
potentials (1–3 s(cid:1)1) due largely to isoprene. In all such ratios (ethene/ethyne < 1.7 and propene/ethyne < 0.5), CO
cases, the bulk of the CH O precursors must have been mixingratios(>150ppbv),CO mixingratios(<375ppmv),
2 2
emitted within about an hour prior to sampling, given the location over the Houston urban core, and wind direction.
rapid midday summertime photochemistry. MedianCH Oproductionpotential(0.6s(cid:1)1)andOHhydro-
2
[17] The industrial origin of the eight highest CH2O carbon reactivity (0.7 s(cid:1)1) for these samples were substan-
production potential samples (>10 s(cid:1)1) was confirmed by tiallylowerthanseenintheindustriallyinfluencedsamples.
ethene/ethyne and propene/ethyne ratios (T. B. Ryerson et All nine canister samples acquired over the Dallas–Fort
al., Effect of co-located petrochemical industrial emissions Worth metropolitan area yielded similar signatures, consis-
of reactive alkenes and NO on tropospheric ozone forma- tent with the lower, more spatially uniform CH O levels
x 2
tion,inpreparation,2002,hereinafterreferredtoasRyerson measured throughout the Dallas–Fort Worth plume (see
et al., in preparation, 2002). Strong industrial signatures Figure1).TheseobservationssuggestthattheCH Ospecies
2
(ethene/ethyne (cid:4) 3 and/or propene/ethyne (cid:4) 1) were iden- should represent an excellent proxy for the important reac-
tified in 98 canister samples acquired over the greater tiveVOCsintheHoustonatmosphere,theutilityofwhichis
Houston metropolitan area. These samples had a median explored in the remainder of this paper.
ethene/ethyne ratio of 4.9, propene/ethyne ratio of 1.6,
CH Oproductionpotentialof2.9s(cid:1)1,andOHhydrocarbon 3.3. Houston Area Plumes
2
reactivityof3.1s(cid:1)1.TheCH2Oproductionpotentialofthese [19] MeasurementsofCH2OintheHoustonareaarenow
98 samples was due largely to the terminal alkenes (78%), examined for consistency with the precursor distributions
ACH 8- 6 WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS
2
Figure 4. Measured Houston–Galveston VOC distribution coded by size for CH O production
2
potential and by color for the percentage of terminal alkene contribution to total OH hydrocarbon
reactivity.
discussed above. Complicating this however, lifetimes of 13ppbv)thanthoseofthemobilesource(upto4.5ppbv)and
ethene (3–8 hours) and propene (1–2.5 hours) are short powerplantplumes(upto3ppbv).
(lifetimerangescalculatedforOH=1(cid:3)107molcm(cid:1)3,O3 [21] Within individual plumes transects, tight positive
=120ppbv,andOH=4.5(cid:3)106molcm(cid:1)3,O =60ppbv, correlations were found between CH O and directly emit-
3 2
respectively) [Atkinson, 1997]. Therefore, regressions of ted species (CO, CO , and NO ), as well as secondary
2 y
CH O versus CH O production potentials did not yield photoproducts (ozone HNO , and acetaldehyde), as listed
2 2 , 3
strong correlations; this lack of correlation was accentuated in Table 2 for two petrochemically dominated plumes.
by variable meteorology and limited VOC measurements. Figure 7 depicts such relationships, showing CH O and
2
[20] Instead, levels of CH2O in Houston area plumes are coincidentally measured NOy and ozone from a Ship
comparedfortransectsofroughlyequivalentphotochemical Channel dominated plume transect. Associated correlation
age.Plumesourcesweredeterminedusingtracersincluding coefficient(r2)valuesweregreaterthan0.95forthe20-km-
SO , CO, and CO [Brock et al., 2003; Ryerson et al., wideplumecrosssection,indicatingthattherelationshipof
2 2
manuscript in preparation, 2002]. Based on these tracers, CH O, NO , and ozone was very uniform throughout the
2 y
distinctpower plant(W.A.Parish), mobilesource(Houston plume (see Table 2). Note that these good correlations are
urban),andpetrochemical(ShipChannel)dominatedplumes largely the result of mixing/dilution, as will be discussed
were identified in the 28 August, 30.1(cid:2) latitude E-W flight below.
transect appearing in Figures 5 and 6. These plumes had [22] When comparing plumes of different sources, large
passedoverthecoreoftheHoustonmetropolitanarea2.5–3 differences were observed in such CH O/‘‘tracer’’ relation-
2
hours (40 km) previously. Peak NO levels in each of the ships.Thisisclearlyevidentinthesurveyofpetrochemical,
y
three plumes were roughly comparable (9–14 ppbv), and mobilesource(urban),andpowerplantplumespresentedin
consistent NO /NO ratios (0.21–0.27) indicated that the Table 3. CH O/NO slopes for petrochemical plumes,
x y 2 y
three plumes were well processed and of similar photo- including the Ship Channel plume (see Figure 7b), were
chemical age. As shown in Figure 6, levels of CH O in the between 0.8 and 3.1. In contrast, CH O/NO slopes ranged
2 2 y
petrochemically dominated plume weremuch higher (up to from 0.2 to 0.4 for the Houston and Dallas–Fort Worth
WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS ACH 8 - 7
2
3.4. Isolated Petrochemical Facility Plumes
[24] Using the model discussed above, we now analyze
several petrochemical plumes to determine if measured
ethene and propene emissions can account for the majority
of observed CH O and ozone, and if measurements are
2
consistentwithNO andVOCinventories.Forthisexercise,
x
plumemeasurementsfromtheisolatedpetrochemicalfacili-
ties, Freeport, Sweeny, and Chocolate Bayou are used
[Brock et al., 2003; Ryerson et al., manuscript in prepara-
tion, 2002]. These facilities primarily conducted olefin-
producing processes, and are analogous to many of the
larger petrochemical facilities located along the Houston
Ship Channel (C. Wiedinmyer and G. Frost, personal
communication, 2001).On27and28August,plumes from
thesefacilitiesweresuperimposedoncleanbackgroundair,
theresultofprevailingwindsfromtheGulfofMexico(see
Figure5).Thiskepttheplumeslargelyunperturbedbyother
emission sources for several hours. Plume model results
presented here are for 28 August, as intermittent CH O
2
instrument problems were encountered on 27 August.
Sufficient CH O data were collected on 27 August to
2
demonstratethattheFreeportandSweenyplumesexhibited
Figure5. Measurements of CH Oin the Houston areaon
2 consistent behavior on both days. Emission estimates
28 August below 1000 m flight altitude. The solid circles
inferredfromaircraft observationsarereported forFreeport
show the location of NO emitters and are sized according
x and Sweeny, but not Chocolate Bayou, as VOC data were
toreportedNO emissions.Generalwinddirectionisshown
x collected only on the very edge of the Chocolate Bayou
by the arrow in the bottom left corner.
plume.MeasurementsofacetaldehydeandCH Owereused
2
toapportionemissionsofetheneandpropene,sincepropene
oxidation produces acetaldehyde while ethene oxidation
plumes (mobile source dominated), and the W. A. Parish does not [Niki et al., 1978, 1981; Grosjean and Grosjean,
plume (power plant). Reported CH O/tracer slopes were 1996;Grosjeanetal.,1996;Calvert etal.,2000;Bacheret
2
determined from individual plume transects where NO / al., 2001].
x
NOy ratios were generally between 0.2 and 0.3. Some [25] Model NOx and VOC inputs were derived from
uncertainty in this comparison arises due to HNO loss observations made1–2hoursdownwind of the petrochem-
3
[Trainer et al., 2000], and the relatively short lifetime of ical complexes on both 27 and 28 August, between about
CH O that will cause such ratios to change significantly 1200 and 1300 CST. It was assumed that by this time the
2
with the photochemical age of the air mass (see Table 2). boundary layer was well mixed vertically and the measure-
Nonetheless, the striking and consistent CH O/NO slope ments at the flight altitude of 500–600 m were representa-
2 y
differences found between plumes of different source types tive of the whole boundary layer (species ratios were
on multiple days provides further evidence that petrochem- constant). Uncertainty in estimated NO emission rates for
x
ical emissions are indeed the dominant source of extreme the isolated petrochemical facilities was largely due to
CH O concentrations in Houston. model parameters such as wind speed (±1 m s(cid:1)1), and
2
[23] The tightly positively correlated CH2O/O3 slopes boundarylayerheights.Consideringtheseuncertainties,the
repeatedly measured in mobile source, power plant, and adopted and reported NO emission rates for the isolated
x
petrochemical plumes (see Tables 2 and 3 and Figure 7c), petrochemicalcomplexescomparedfavorably(seeTable4).
suggest that oxidation of CH O precursors (including Previous plume studies provide precedent for this good
2
photolysis of CH O itself) played a major role in ozone agreement [Ryerson et al., 1998].
2
production. This is consistent with the observation that [26] Estimates of VOC emissions were derived using
CH Oprecursorswereresponsibleforasubstantialfraction measured plume VOC/NO ratios, with corrections applied
2 x
of total OH hydrocarbon reactivity, especially in the most for VOC and NO losses occurring between the point of
x
pollutedVOCsamples.AsdepictedinFigure6,CH Oand emission and measurement. For the Freeport plume, six
2
ozone levels observed the same basic trends in the 28 separate VOC measurements were made at a variety of
August Houston plumes. Measurements from 1 September, downwind distances. One VOC sample apiece was col-
ahighlypollutedday(seeFigures7aand7cand8aand8b), lected from the Sweeny and Chocolate Bayou plumes. Due
also show CH O and ozone tracking one another. It should to the dominance of ethene and propene in these samples,
2
be noted that a spatial (temporal) offset was observed the model assumed that VOC emissions for each facility
between the CH O and ozone gradients, as expected, due were composed exclusively of these two terminal alkenes.
2
to the longer lifetime and slower relative net production Isoprene was also measured in all plumes, the majority of
rate of ozone. Throughout the TexAQS campaign, high which was attributed to biogenic emissions from the dis-
CH O levels, often 10–20+ ppbv above background, were continuous forests of the region, based upon VOC samples
2
always found coincident with exceedance levels of ozone taken outside of the plumes and biogenic inventories
(>120 ppbv). [Wiedinmyer et al., 2001]. A background isoprene flux
ACH 8- 8 WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS
2
Figure6. MixingratiosofCH OandotherspeciesinacrosssectionoftheHoustonplumeacquiredat
2
30.1(cid:2)latitudeon28August,showingdistinctpowerplant(W.A.Parish),mobilesource(Houstonurban),
and petrochemical (Ship Channel) dominated plumes.
common to all of the plumes was therefore included in the [28] The Freeport and Sweeny plumes were also mod-
model (5.2 (cid:3) 1015 mol m(cid:1)2 s(cid:1)1). eled using NO and VOC emissions taken directly from
x
[27] Using these observationally derived NOx and VOC TNRCC inventories, along with the measurement-inferred
emission values, the model accurately replicated measured biogenic isoprene background. Note that the only real
ethene,propene,CH O,acetaldehyde, (NO (cid:1)NO ),(NO difference between this and the previously discussed
2 y x y
(cid:1) (NO + HNO )), and ozone concentrations. As noted model case was the magnitude of the industrial VOC
x 3
above,observationallyderivedNO valueswereeffectively inputs (see Table 4). When using inventory inputs, less
x
equivalent to inventory values. Figure 9 provides an over- than 10% of the measured CH O, and 40% of the
2
viewofresultsforthe29.3(cid:2)latitudeinterceptoftheSweeny measured ozoneenhancement over background levels were
and Freeport plumes. Not only did the model successfully predicted (see Figures 9b and 9d, dotted gray lines). In this
simulate the rapid CH O enhancement in the plume, it also case, modeled CH O and ozone production was largely
2 2
accurately captured the CH O background due to the due to the presence of the isoprene background, with
2
area-wide isoprene emissions (see Figure 9b). Ethene and TNRCC inventory VOCs contributing minimally. On a
propene emission apportionment was corroborated by related note, inclusion of a large measured isobutane
agreement with both CH O and acetaldehyde (Figures 9b emission to the ethene, propene, and isoprene mix used
2
and 9c). It is important to note that in all cases the model to model the Chocolate Bayou plume (not depicted)
needed only standard OH-initiated VOC oxidation to produced very little additional CH O and ozone, due to
2
achieve good agreement with observations. The observed the much lower reactivity of alkanes.
‘‘shoulder’’ feature seen on the east side of the Freeport [29] One potential weakness of the above analysis was
plume, particularly apparent in the ozone measurement its reliance on just a handful of canister measurements to
(Figure9d),wasduetoincursionoftheFreeportAcomplex characterize the VOC emissions of the isolated petrochem-
plume into the Freeport Bcomplex plume. The model only ical facilities. However, since CH O is efficiently pro-
2
attempted to replicate the larger Freeport B plume (see duced by the dominant reactive VOCs in these plumes, it
Figure 5). represents a good surrogate for plume VOCs. The very
WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS ACH 8 - 9
2
Table2. SlopesofCH OVersusSeveralOtherSpeciesMeasuredinthe1SeptemberShipChannelDominatedPlumeand
2
the 28AugustFreeportPlume
Distance(km) Age(hours) NO/NO CHO/NO r2 CHO/CO r2 CHO/O r2 CHO/HNO r2
x y 2 y 2 2 3 2 3
1SeptemberShipChannelDominatedPlume
10 0.8 0.69 0.47 0.73 0.21 0.89 0.68 0.87 7.7 0.87
25 2 0.49 0.79 0.99 0.18 0.95 0.41 0.96 5.0 0.94
50 4 0.29 0.73 0.84 0.13 0.92 0.16 0.86 2.5 0.80
75 6 0.24 0.87 0.98 0.09 0.97 0.15 0.98 2.8 0.96
100 8 0.22 0.74 0.89 0.05 0.91 0.12 0.86 1.9 0.77
28AugustFreeportPlume
8 0.6 0.77 0.14 0.90 0.07 0.50 na na na na
33 2.3 0.36 1.35 0.95 0.31 0.94 0.25 0.99 3.7 0.92
60 4.2 0.29 0.87 0.93 0.10 0.84 0.15 0.95 2.2 0.85
robust positive CH O/tracer correlations measured in the vertically homogeneous species ratios as well. Therefore,
2
plumes (see Tables 2 and 3) show that facility emissions plume VOC/NO ratios measured at flight altitudes of
x
were sufficiently collocated, and atmospheric mixing 500–600 m by the Electra should be representative of the
adequately strong, as to rapidly (0.5–1 hour) produce entire boundary layer vertical profile, except for very
plumes of constant species ratios, despite the fact that the fresh plumes (<0.5 hour under moderate winds) and
facilities were not perfect point sources. The strong perhaps for plumes affected by special meteorological
boundary layer mixing necessary to create this measured conditions such as coastal effects. Support for this con-
horizontal uniformity would also be expected to produce clusion is provided by airborne LIDAR ozone measure-
Figure7. MeasurementsofCH O,NO ,andozoneinacrosssectionofthe1SeptemberShipChannel
2 y
dominatedplume,2hoursdownwindoftheShipChannel(a).RegressionsofCH OversusNO (b)and
2 y
ozone (c) (see Table 2 for slopes and correlation coefficients).
ACH 8- 10 WERTETAL.:TERMINALALKENEOXIDATIONINAIRBORNECH OMEASUREMENTS
2
Table 3. Slopes of CH O Versus NO and Ozone Measured in increased downwind of these initial intercepts despite
2 y
PlumesofPetrochemical,MobileSource(Urban),andPowerPlant dilution, consistent with rapid photochemical production
Origin of CH O from emitted VOCs, rather than primary emis-
2
sion. In addition, the model achieved good agreement with
PlumeSource Date NO/NO CHO/NO r2 CHO/O r2
x y 2 y 2 3 observations invoking only photochemistry. The domi-
Petrochemical
nance of in situ CH O production was also supported by
Freeport 27August 0.37 0.85 0.93 0.19 0.97 2
extensive, continuous PTRMS measurements of propene,
Freeport 28August 0.36 1.35 0.95 0.25 0.99
Freeport 6September 0.31 1.30 0.78 0.18 0.84 acetaldehyde, and CH O made at the La Porte ground site
2
Sweeny 27August 0.27 1.85 0.92 0.11 0.85 near the Ship Channel (T. Karl et al., The use of proton-
Sweeny 28August 0.32 1.66 0.99 0.12 0.97 transfer mass spectrometry to characterize VOC sources at
ChocolateBayou 28August 0.29 3.13 0.98 0.33 0.99
the La Porte super site during the Texas Air Quality Study
ShipChannel 27August 0.24 1.28 0.96 0.15 0.91
ShipChannel 28August 0.29 0.92 0.82 0.13 0.88 2000, manuscript in preparation, 2002). Direct CH2O
ShipChannel 1September 0.24 0.87 0.98 0.15 0.98 emissions from mobile sources are not expected to have
contributed importantly to peak daytime Houston CH O
MobileSource(Urban) 2
abundances either, based upon work by Zweidinger et al.
Houston 28August 0.21 0.36 0.81 0.11 0.53
Dallas–Ft. 7September 0.28 0.25 0.86 0.095 0.80 [1988], Altshuller [1993], and others, and the much lower
Worth measured CH O levels in the Houston and Dallas–Fort
2
Worth mobile source plumes.
PowerPlant
W.A.Parish 28August 0.21 0.24 0.83 0.08 0.94 3.6. Photochemistry of a Highly Polluted Plume
[32] OzoneexceedanceeventsmeasuredoverHoustonby
the Electra typically involved ozone enhancements of 60–
ments made during TexAQS that showed strong vertical
150+ ppbv above background. In comparison, ozone
mixing (C. Senff, private communication, 2001) as well
as limited in situ measurement profiles made by the
Electra.
[30] Measurement derived VOC emissions for the Free-
port and Sweeny plumes used in this model exercise were
estimated to be accurate within at least a factor of 2. This
was based upon the high degree of agreement achieved
between predicted and measured secondary photoproducts,
particularly CH Oandacetaldehyde,andtheplumespecies
2
ratiouniformitydemonstratedbyCH O/tracerrelationships.
2
Also accounted for were the uncertainties associated with
NO emissions, wind speeds (plume ages), and boundary
x
layerheights.ConsistentCH O/tracerslopeswerefoundon
2
eachdaytheFreeport(3days)andSweenyplumes(2days)
were intercepted, as listed in Table 2 for intercepts of
comparablephotochemicalages(NO /NO ratios),suggest-
x y
ing that the VOC emissions deduced here were the con-
sequences of standard operations rather than unreported
unusual emission events (‘‘upsets’’). As seen in Table 4,
derived ethene and propene emissions for the Freeport and
Sweeny facilities are 50–100 times larger than reported
(1999TNRCCVOCinventory),andinsumabout20times
larger than total reported VOC emissions. Analysis of the
Chocolate Bayou plume produced similar disparities.
3.5. Primary CH O Emissions
2
[31] The isolated petrochemical facility plumes were
also used to assess the importance of primary petrochem-
ical CH O emissions. If primary emissions were large,
2
they would need to be considered in assessments of
emissions and photochemistry made using CH O data.
2
However, only small enhancements of CH O were found
2
in close proximity intercepts of the isolated petrochemical
facility plumes, providing an upper limit on direct emis-
sions. In the Freeport plume, CH O levels were 1–2 ppbv
2
above background about 20 min downwind (8 km), on
both 27 and 28 August (see the 29.1(cid:2) latitude pass in Figure8. MeasuredCH O(a)andozone(b)mixingratios
2
Figure 5). A similar enhancement was also measured 30 in the Houston–Galveston area on 1 September at flight
min (7 km) downwind of the Chocolate Bayou facility on altitudeslessthan1000m.Generalwinddirectionisshown
27 August. In all cases, plume CH O levels dramatically by the arrow in the upper left corner of (a).
2