Jun 2, 1992 - to the details of the rhodel chemistry. One important oxidant that provides a more robust test of model performance is peroxyacetyl nitrate (PAN) ...
GEOPHYSICAL RESEARCH LETTERS,VOL. 19,NO. 11,PAGES1121-1124, JUNE2, 1992
DETEPdvIINATION OF THE RELATIVE OZONE AND PAN DEPOSITION VELOCITIES AT NIGHT
P.B.Shepson 1,J.W.Bottenheim 1,2,D. R.Hastie •, andA. Venkatram 3 Abstract.A seriesof measurements of PAN andozone
clearskyhighpressure conditions wherestableNBLs tend to form. Under such conditions,we typically observe significantlyfaster decay for PAN, relative to ozone (Shepson et al, 1992). Therehavepreviouslybeentwo studieswherePAN and
wasconducted duringsummerat threerural sitesin Canada:
Egbert andDorset,Ontario, andKejimkulik, NovaScotia. Fornightswhen a stablesurfaceinversionlayer forms,
ozoneand PAN at the surfaceare found to undergofirst orderdecay,assumedto be due only to dry deposition.
ozone depositionvelocities were measuredfor the same surface. Garlandand Penkett(1976) conductedlaboratory therelativedry deposition velocities.For all threesites,we measurements of PAN depositionvelocitiesto grassandsoil, findthatVd(O•)/Vd(PAN) = 0.42+0.19,atnight.Thisratio andobtainedaveragevaluesof ,,0.25 cm/sfor bothsurfaces. ozonedepositionvelocitiesfor grassandsoil isroughly a hctorof 5-6times-smaller thanpreviouslyCorresponding assumed. This smallerrelativedeposition velocityratiocan were0.51 and 1.6 cm/s, respectively.Studiesof PAN and
Analysis of themeasurement dataleadsto determination of
havea significantimpacton modelestimations of PAN
0 3deposition toanalfalfacanopy,conducted byHill (1971),
concentrations near the surface. We estimatethat for these
in•Iicateddepositionvelocities of 0.75 and 1.7 cm/s, respectively.Thus, thesetwo studiesindicatethatthe ratio
sites,thePAN depositionvelocityis at least0.5 cm/s,and
maybegreater duringdaytime.Thiscanhavea significant Vd(O3)/Vd(PAN) rangesfrom •, 2-6. Currentmodel formulations use, or compute,basedon diurnallyvarying impact onthetropospheric lifetimeof PAN. surfaceresistances, O3 and PAN deposition velocitiesthat Introduction
areconsistent with the•eobservedratios(e.g. Dodge, 1989; KastingandSingh, 1986; Kanakidouet al., 1991). In contrastto the assumedrelativedepositionratesfor thesetwospecies, we haveconsistently observed significantly
Thereis increasinginterestin development of computer models thatsimulatetropospheric oxidantconcentrations, in
lightof the steadyincreasein globalbackground 03
fasterapparent deposition ratesfor PAN relativeto 0 3 in a
of rural sitesin Canada. In this paperwe present concentrations, and thefact that regional episodic • number concentrations often exceed local standards. TestinganO results of ouranalysis of therelative03 andPANdeposition evaluation of these modelsrequiresmeasurement data for
velocities at these sites.
species otherthanOa, predictions for whicharenotsensitive tothedetailsof therhodelchemistry.Oneimportantoxidant thatprovidesa more robusttest of modelperformance is peroxyacetyl nitrate (PAN). PAN is producedfrom the atmospheric oxidationof hydrocarbons in the presence of
ExperimentalMethods
Measurements of PAN and0 3 weremadeduringseveral
differentmeasurement campaignsat thesurfaceat threerural NOx, andis thought to contribute significantly totheglobal sites in Canada: Egbert, and Dorset, Ontario, and Kejimkujik,Nova Scotia. Measurements at Egbertwere transport and storageof NO• (Singhand Hanst, 1981). conducted at the Centre for Atmospheric Research Measurement campaign•aimed at understanding Experiments(CARE), operatedby EnvironmentCanada, tropospheric chemistryare most often conductedat the
locatedin a rural/agricultural area ,, 60 km northwestof
surface,for obviousreasonsof convenience. However, the
Toronto. The Dorset site is an APIOS (Air Pollution in
frequent presence of a stablenocturnalboundarylayer(NBL) with shallow mixing heights can result in significant depletion of surfacelayerspecies concentrations at nightdue todrydeposition.When the nocturnalinversionbreaksup
OntarioStudy)air monitoringstation,locatedin a shallow valley,in a mixeddeciduous/coniferous (75%/25%) forested area, ~ 175 km northeastof Toronto. The Kejimkujiksite is a CAPMON (CanadianAir and PrecipitationMonitoring Network) station, located in a forest clearing (25%deciduous/75 %coniferous)inKejimkujikNationalPark, NovaScotia,ca. 150kmto the WSW of the city of Halifax,
inthemorningdueto surfaceheating,downwardmixingof undepleted pollutantsfrom aloft restoresthe surfacelayer concentrations to valuesmore representative of thosein a well mixedplanetaryboundarylayer (PBL). Thus the diurnal variationof speciesthat undergosurfacedeposition canbesignificantly influencedby theformationandbreakup oftheNBL (GarlandandPenkett,1976;Broderet al., 1981; Fehsenfeld et al., 1983). We have recentlyconducted
N.S. Measurements of PANand0.3havebeenconducted at
that site on a year-roundbasiss•nce1984 (Brice et al., 1988). The Egbertand Dorsetsiteswere participatingsites duringtheEulerianModelEvaluationField Study,conducted during1988and1990. For boththesesites,stableNBLs are simultaneousmeasurements of PAN and ozone at three surface sitesin Ontario,Canada,wherewe observe relatively knownto existduringclearhighpressureconditions,leading rapidlosses for thesespeciesat night,particularly under to significantlossesfor speciesunderthe inversion(D.R. Hastieet al., unpublished data, 1992; Hoff et al., 1989). PAN was measuredhourly at each site by gas with electroncapturedetection,for Dorset epartmentof Chemistryand Centrefor Atmospheric chromatography by Blanchardet al. (1990) andfor Egbertand Chemistry, YorkUniversity, 4700KeeleStreet, NorthYork, asdescribed Kejimkujikas describedby Brice et al. (1988). The 0rktario, Canada,M3J 1P3. of themeasurements is estimated at +_20%, andthe 'Atmospheric Environment Service,Environment Canada, accuracy precision is estimated to be 4-5%, for concentrations thatare Do_wnsview, Ontario,Canada •ENSR Consulting andEngineering, 1220 Avenida Acaso, at least3 timesthedetectionlimit of 0.010 ppb. Ozonewas measured by UV absorption usingDasibiModel AH-1008 or Cammilo,CA, USA 93010
TECO Model 049 Monitors. For all sites, measurements
weremadeusinginletssituated,, 3m abovetheground.
Copyright 1992 bytheAmerican Geophysical Union.
Data Analysis
Paper number92GL01118
0094..8534/92/92GL-01118503.00
For relatively stable conditionswhere a low lying nocturnalinversionexists, we believe that the dominantloss 1121
1122
Shepsonet al.' RelativeOzoneandPAN DepositionVelocities Thus althoughPAN loss occursvia reaction2, the PAN
Model for Dry Deposition
lifetimedoesnotdepend ontheabsolute NO concentration, butontheratio[NOg]/[NO]. The thermallifetimefor PAN
in thepresence of ND andNO2 is givenby equation (I): •
pollutant concentration profile
kl
-1
1+k_ •[NO•]
ß
At nighttheratioNO2/NOis typicallyquitelarge,i.e. >20.
For example,for the July 17, 1989 Dorsetcasediscussed
h NBL Height
below,theNO2/NOratiowasroughly25. For a median temperature for that nightof 9øC, this corresponds to an
effective lifetimeof 7 days.The[NO2]/[NO ] ratiodidnot
c,
decrease significantly duringthenightsince,asstated above,
Concentration
thesurface NO source wasverysmall. ThetotalNOx concentration decreased from ~4 to ~3 ppboverthenight
Fig. 1. AssumedNocturnalPollutantProfile.
process for both PAN and 0 3 at the surfaceis dry deposition. For a typical0 3 concentration of 30 ppb,the
time PAN decayperiod. For the casesstudiedherethe observed decaytimesfor PAN weretypicallyon theorderof 3-6 hours,andthuswe believethatthermaldecomposition of PAN is relativelyunimportant.Thusthe dominantlossterm for PAN at nightundertheNBL will alsobe dry deposition.
andlossof 0 3bythisreaction becomes unimportant, aslong
simple modelforthetimevariation of PANand0_3 , whose
lifetimeof NO with respectto reactionwith O3 is on the orderof ! minute, so that NO at nightis rapidl• removed, as thereis no significantsurfacesourceof NO. The NO..
For the conditionsdescribedabove, we can constructa
theNO titrationreactiondoesnotsignificantly depleteO3 at
verticalprofiles we assumeare as shownin Figure 1. Speciesconcentrations in the horizontallyhomogeneous boundarylayer canbe describedby:
night. F•rDorset wehave found thesoilNOfluxtob• •2t•g/m h, corresponding to a maximumemissionrateof 7
,Oc= _OF
concentrations aretypically ontheorder of 1-4ppb,andthu•
Ot
ppt/hfor a 150 m mixing height. Giventherelativelyhigh NOv concentrationat the time the NBL forms, the soil NO
fluxwouldnotcontributesignificantly.Thuswebelievethat
the0 3 + NO reaction is anunimportant lossmechanism for O3 at night at these sites. The relativelysmall rate coefficients for O3 reactionwithalkenes coupled withthe fact that their concentrations in rural areasare normally muchsmaller thanthatof 0 3 makes reaction withalkenes an unimportant lossprocess,relativeto dry deposition. The thermaldecomposition of PAN, thedominantremoval termin the daytime,alsobecomessuppressed at night,due to the large NO,/NO ratio, and lowertemperatures.In the
presence of NO,"PANthermaldecomposition results inPAN loss via reactions 1 and 2 below.
whereC is the concentration, andF is the flux of thespecies in theverticaldirectionz. IntegratingEquation(II) overthe heightof the boundarylayer yields:
_f -•.dz OC = F,-F h
o
(iii)
o
where flux atboundary the surface, and Fhi•S the flux atthe topofF• •eisthe nocturnal layer. we make the simplifyingassumption thatthe heightof theboundarylayer
is constant, Fh=0 sincethereis no mixingacross thetopof the NBL. Given the assumptionthat the concentration profileis linearwithin the boundarylayer,
CH3C(O)OONO 2 • CH3C(O)OO-+NO2 1,-1 CH3C(O)OO-4NO -. CH3C(O)O-+NO2 2 Dorset, July 22-25,
0-D
Oz
JULY
17,
1989
DORSET
t989
0.8
[P,,,.]
SLOPE = 0.46
ß [%]
0
O.e
0.125 Q. Q.
o.loo z
,
0
,
0,4.
0.075
0.050
0.2
0.025
0.0 0.000
2
0.00
0
July 23
July 24
t
I
i
[
,I
I
I
I
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
--
2.25
July 25
Fig. 2. PAN andOzoneConcentrations Measuredat Dorset, Ont., July 22-25, 1989.
Fig. 3. Plotof Equation (VII) for July17, 1989,forDorset,
Ontario.
Shepson et al.' RelativeOzoneandPAN Deposition Velocities
1123
Table1. MeasuredRelativeOzoneandPAN Deposition Velocitiesfor ThreeRuralSitesin Canada.
-
''
Dor•i'"'(1989)
"'
Date
Va{O•}/Va{PAN} r2
7/13 7/15 7/16 7/17 7/19 7/22 7/23 7/24 7/25
0.23 0.37 0.54 0.46 0.34 0.48 0.47 0.32 0.23
7/30
0.33
............ Egbert (1988') ..............
' Kejimkujik .............
DateVa{O3}/VM{PAN } r2
0.87.... 8/5.....1.1 0.97 0.84 0.95 0.93 0.81 0.96 0.97
Date
Va{,Ot}/Vd{PAN}.. ,r2 , ,
5/29/88 5/30/88 7/5/90 7/13/90 7/20/90 8/18/90 8/30/90
0.23 0.52 0.47 0.31 0.50 0.52 0.23 0.39
0.96 0.96 0.94 0.85 0.91 0.98 0.72 0.81
0.40+0.12
0.89
.... '•}'"•8 ......... 5/16/88
8/6 0.35 8/23 0.51
0.87 0.74
0.85 0.87
0.90
.Averages: 0.384-0.1.1 ......
h
0.65+0.40
(C,.+Cs)'h
0.83
In[O3o = ["•(o3 .in,[•'X•o
(IV)
fc.&=,, .......
[o3,
v•O'A•
[•,x•,
(VlB
o
andsinceFs=Vd'Cs, Equation(III) canbe writtenas:
OC,
...... 0t :
2•
h .C,
In thefollowingsection,we presentresultsof determination of theratioVd(O3)/Vd(PAN ) usingEquation(VII), for the
(V)
datafrom thesb.thi'ee/ites in the summer, between 1988 and 1990.
WetakeCi to be the concentration in the lateafternoon mixedlayer,just as the NBL forms,whenCs=Ci. Then Equation (V) canbe integratedto obtain
Results and Discussion
Fora series of measurements of O3andPANbetween 1988
and1990,we plottedtheleft vs. rightsideof Equation(VII), for thetimeperiodbetween2000 - 0600. We thenselected
[
(vi)allcasesforwhichr2wasgreaterthan0.7. Forthesethree
C, = C•exp-•---
where t is the time from the onset of formation of the
nocturnal boundarylayer. ThisEquationindicates thatunder the conditionsassumed,surface concentrations undergo exponential decay,with a first orderrateconstant equalto
(-2Vd/h). Exponential lossis oftenobserved for these compounds for all threesitesduringsummer,as shownin Figure2, for a periodin July, 1989, at Dorset. Equation (VI) thenformsthebasisfor an analysis of thedatain terms
ofdetermination of therelativedeposition velocities.From
combination of theappropriate formofEquation (VI) for0 3 andPAN, we obtainEquation(VII):
sites,a totalof 21 suchcaseswere found. A samplecaseis shownin Figure3 for July 17, 1989, for the Dorsetsite. The resultsfor all sitesare presentedin Table 1. As shown
intheTable,theaverage ratioVd(O3)/Vd(PAN) forall three siteswas 0.42 4-0.19.
The resultspresented in Table 1 indicatethat on average the03 deposition velocityat nightis 2.5 timessmallerthan that for PAN, at thesesites, at least for days when both species undergoexponentialdecay,andare reasonably well
correlated. ThustheratioVd(O3)/Vd(PAN ) atnightappears
to be approximately a factorof 5-6 smallerthanpreviously assumed, at leastfor the type of sitesstudiedhere.
It is well knownthat the 0 3 deposition velocityhasa
strongdiurnalvariability,at leastin part dueto theincrease in surfaceresistance(for soil, grass, forests) at night (GalballyandRoy, 1980). ColbeckandHarrison(1985)find
JULY 24, 1989 DORSET, ONT.
03 deposition velocities tograssto rangefromabout0.1-0.9
cm/s. For a varietyof sites,the ozonedeposition velocity seems to be2-3 timessmallerat night,mosttypically0.2-0.3 cm/s.
These observations are consistent with more recent
measurements at a forest site near Egbert during August
1988 (Pad,r•o etal.,1991), where Vd(O3) was found tobe,on
average, 0.25 cm/s at night, increasingto ~1 cm/s near
noon. Our determination of the deposition velocityratios implies thatVd(PAN) is on theorderof 0.6 cm/sat night, for these sites.
ß OZONE
For easeswherethe inversionheight, h, is stableand is
vPAN
known,we canestimate the absolute valuefor Vd(PAN ).
For this analysis,as discussed above, we assumethat the initialmeasurements (when the NBL hasformed)represent the PAN concentration at the top of the fully developed NBL, andthattheverticalprofile is represented by a linear
Slope = -2.99x10 Slope = -8.64x10
s s
•0 0
15000
I
10000
decrease to the values measured at the sunace.
I
I .........
20000
25O0O
Time, sec
Fig.4. Plotof FirstOrderDecays of Ozone andPAN, 7/24/89, Dorset,Ont.
Then the.
firstorderdecayfor PAN is givenby Equation(VIII):
in[ml, -a ............................. [PXrlo
ßt
(VIII)
1124
Shepson et al.: RelativeOzoneandPAN Deposition Velocities
Thusa plot of ln{[PAN]t/[PAN]o } vs time(sec)yieldsa slopeequalto -2Va(PAN)Yh. In Figure4 we presentthese
firstorderdexaypl•>ts forbothO3 andPAN,fortheJuly24,
1989caseat Dorset. For this casethe inversionheight,as
determined using anacoustic sounder, w• 's•table at125_+ 12 m. The s}oI:les observedwere -2.99x10-øs , and -8.64x10-øs -', for ozone and PAN, respectively, yielding
calculated dry deposition velocities for O3 andPANof 0.19 Th•s and0.54cm/s, respectively. ' ' value - for _- the 03 dry deposition velocityis consistentwith otherooservations at night,as discussed above. However,the valuefor PAN is a factorof two larger than that reportedby Garlandand Penkett(1976), and the relativedeposition velocities differ by a factorof -5-6. However,it is not clearwhetherthe Garlandand Penkett conditionsapply to day or night
conditions. Wenotethatthisabsolute valueof Vd(PAN)is
dependent on the assumedprofile in Figure 1. We wouldexpectthat the PAN deposition velocitymay varydiurnallyin a mannersimilarto thatobserved for 03At Dorset,therelativehumiditynormallyapproaches 100% for nightswhere there is a stableNBL, as 10% of the
regional surface iswater.BothO3andPANhaveverylarge surfaceresistances for deposition to water. For 0 3 the
Determinationof AtmosphericPeroxyacetylNitrate (PAN)" Atmos.Environ.,24A, 2839-2846(1990). BriceK. A., BottenheimJ. W., Anlauf K. G., andWiebeH.
A. "Long-Term Measurements of Atmospheric Peroxyacetyl Nitrate(PAN) at RuralSitesin Ontarioand
Nova Scotia;SeasonalVariationsand Long-Range
Transport" Tellu....__•s, 40B, 408-425(1988).
BroderB., D•itschH. U., and GraberW. "OzoneFluxesin
the NocturnalPlanetaryBoundary LayerOverHilly
Terrain".Atmos,.Environ., 15, 1195-1199(1981). ColbeckI. and HarrisonR. M. "Dry Depositionof Ozone: Some Measurementsof DepositionVelocity and of VerticalProfilesto 100 Metres"Atmos.Environ.,1_9.9, 1807-1818(1985). Dodge M. C. "A Comparisonof Three Photochemical OxidantMechanisms"J. Geophys.Res.....94, 5121-5136 (1989) Fehsenfeld F. C., BollingerM. J., Liu S. C., ParrishD. D., McFarlandM., TrainerM., Kley D., MurphyP. C., and Albritton D. L. "A Study of Ozone in the Colorado Mountains"J. Atm0s. Chem., i, 87-105 (1983). GalballyI. E. andRoy C. R. "Destruction of Ozoneat the
surfaceresistancefor depositionto water is -14 s/cm,
Earth's Surface"Ouart. J. R. Met. Soc., 106, 559-620 (1980).
implyingthat V d (O)< 0.07 cm/s(GalballyandRoy, 1980). .. 3. Thedepomtton velocityfor PAN to waterhasbeenestimated
GarlandJ'. A. and PenkettS. A. "Absorption of Peroxy AcetylNitrateand Ozoneby Natural Surfaces"Atmos__•.
to be ~0.008 cm/s (Kames et al., 1991). Thus if vegetation surfaces becomerelativelyhydratedat night,we mightexpectthedeposition velocityto decrease.In addition,
Hill A. C. "Vegetation:A Sink for Atmospheric Pollutants" J. Air Pollut. Control Ass..,21, 341-346 (1971).
it is well known that the increased stomatal resistance at
night significantlyimpacts on O3 depositionvelocities (Colbeck and Harrison, 1985), and may affect PAN depositionas well. Although the nature of the PANvegetationinteractionis unknown,thisimpliesthatdaytime PAN deposition velocitiesmaybe evenlargerthanthe ~0.5 cm/sfoundfor night. As thishassignificant implications for the atmosphericlifetime for PAN, it is clear that measurements of daytimePAN surfacefluxesare needed. Conclusions
This studyhas indicatedthat the deposition velocityof PAN for vegetativesurfacesin summerat night is, on
Enviroa., 10, 1127-1131 (1976). Hoff R. M., Mickle R. E., Fanaki F., FroudeF. A., Arnold J., and Markes J. "Vertical Profiles of Ozone and
Meteorologyat CARE, Egbert" EnvironmentCanada ReportARD-89-009. KamesJ'., SchweighoeferS. and SchurathU. "Henry'sLaw Constant andHydrolysisof Peroxyacetyl Nitrate(PAN)" J. Atmos..Che.m....,12, 169-180 (1991). KanakidouM., SinghH. B., Valentin K. M., and Crutzen P. J. "A Two-DimensionalStudyof EthaneandPropane Oxidationin the Troposphere"J. Geophys.Res.. 96, 15,395-15,413 (1991). KastingJ. F. and SinghH. B. "NonmethaneHydrocarbons in the Troposphere:Impact on the Odd Hydrogenand
OddNitrogenChemistry" J. Geophys.Res.,91, 13,239average,2.5 timesgreaterthanthatfor 0 3. Thisis a factor 13,256. of 5-6 greaterthanpreviouslyassumed.Thus PAN lossto Padro J., den Hartog G. and Neumann H. H. "An thesurfacemaybe moreimportant thanpreviously assumed. Investigationof the ADOM Dry DepositionModule This can have a significant impact on calculatedPAN UsingSummertime O3Measurements Abovea Deciduous concentrations for the near surfacelayers, and therefore Forest" Atmos. Environ,, 25A, 1689-1704 (1991). computermodelsmay needto incorporate relativelylarger Shepson P. B., HastieD. R., So K. W., SchiffH. I., and PAN depositionvelocities. Considering the importance of WongP. "Relationships Between PAN, PPN,and03 at PAN to the transportof tropospheric NO,, and resultant Urban and Rural Sites in Ontario" Atmos. Environ., photochemical productionof ozone, and •he scarcityof 26A, 1259-1270 (1992). informationregardingPAN deposition rates,it is clearthat SinghH. B. andHanstP. L. "Peroxyacetyl Nitrate(PAN)in additional PAN depositionvelocity measurements are theUnpolluted Atmosphere: An important Reservoir for needed. NitrogenOxides"..Geophys .....Re.s,.Lett., 8, 941-944 Acknowledgements.We thankDr. K. G. Anlauf (AES)
for providingthe Oa datafor Egbert,andA. Bates,K. W. So, S. Sharma,and-A.J. Gallantfor variousaspects of data acquisition andanalysis.We alsothanktheOntarioMinistry of theEnvironment andtheNaturalScienceandEngineering Research Councilof Canadafor financialsupport. References
BlanchardP., ShepsonP. B., So K. W., Schiff H. I., BottenheimJ. W., Gallant A. J., DrummondJ. W., and Wong P. "A Comparison of Calibration and
MeasurementTechniquesfor Gas Chromatographic
(1981).
IDepartment of Chemistry andCentre forAtmospheric
Chemistry, YorkUniversity, 4700KeeleStreet,NorthYork, Or•tario,Canada,M3J 1P3. •Atmospheric Environment Service, Environment Canada, Downsview,Ontario, Canada
3ENSR Consulting andEngineering, 1220Avenida Acaso,
Camarillo, CA, USA 93010
(Received'March 11, 1992; accepted:April 29, 1992.)
