Ozone in the Pacific Tropical Troposphere From Ozonesonde Observations

S.J. Oltmans I, B.J. Johnson I, J.M. Harris I, H. V6mel t'2, K. Koshy 3, P. Simon, 4, R.

Bendura s, A.M. Thompson °, J.A. Logan 7, F. Hasebe s, M. Shiotani 9, M. Maata 3, G. Sami 3,

A. Samad 3, J. Tabuadra'v_ -_. H. En_quez t°, M. Agama I l, j. Cornejo I i, and F. Paredes II

NOAA, Climate Monitoring and Diagnostics Laboratory (CMDL), Boulder, CO 80305,

USA

z Cooperative Institute for Research in the Environmental Sciences (CIRES), University

of Colorado, Boulder, CO, 80309, USA

3 University of the South Pacific, Suva, Fiji

4 MeteoFrance, Papeete, Tahiti, French Polynesia

s NASA, Langley Research Center, Hampton, VA, USA

6 NASA, Goddard Space Flight Center, Greenbelt, MD, USA

7 Harvard University, Cambridge, MA, USA

s lbaraki University, Mito, Japan

9 Hokkaido University, Sapporo, Japan

l0 INAMHI, Quito, Ecuador

_1INAMHI, San Cristobal, Galapagos, Ecuador

ABSTRACT: Ozone vertical profile measurements obtained from ozonesondes flown at

Fiji, Samoa, Tahiti and the Galapagos are used to characterize ozone in the troposphere

over the tropical Pacific. There is a significant seasonal variation at each of these sites. At

sites in both the eastern and westem Pacific, ozone is hi_est at almost all levels in the

troposphere during the September-November season and lowest during March-May.

There is a relative maximum at all of the sites in the mid-troposphere during all seasons

of the year (the largest amounts are usually found near the tropopause). This maximum is

particularly pronounced during the September-November season. On average, throughout

the troposphere at all seasons, the Galapagos has larger ozone amounts than the western

Pacific sites.A trajectoryclimatologyis usedto identifythemajorflow regimesthat are

associatedwith thecharacteristicozone behavior at various altitudes and seasons. The

enhanced ozone seen in the mid-troposphere during September-November is associated

with flow from the continents. In the western Pacific this flow is usually from southern

Africa (although 10-day trajectories do not always reach the continent), but also may

come from Australia and Indonesia. In the Galapagos the ozone peak in the mid-

troposphere is seen in flow from the South American continent and particularly fi'om

northern Brazil. The time of year and flow characteristics associated with the ozone

mixing ratio peaks seen in both the western and eastern Pacific suggest that these

enhanced ozone values result from biomass burning. In the upper troposphere low ozone

amounts are seen with flow that ori_nates in the convective western Pacific.

Introduction

The tropical Pacific is often considered to be a region remote from major polluting

influences because of its isolation from heavily industrialized landmasses. Recent fie!d

campaigns (Hoell et al., 1999) have emphasized that though this is often the case, the

signature of pollution, particularly from biomass burning, makes a significant imprint on

the air chemistry of the region. In a number of instances layers of enhanced ozone

(mixing ratios >80 ppbv) were found in the mid-troposphere in the remote western

Pacific (Stoller, et al., 1999). These layers in addition to having enhanced ozone were

replete with markers of biomass burning (Gregory et aI., 1999).

Beginningin August1995aspart of thePacificExploratoryMission(PEM)TropicsA,

ozoneverticalprofile measurementswerestartedat PagoPago,AmericanSamoa(14.3S,

170.6W)andPapeete,Tahiti (18.0S,149.0W).At Samoa profiles were also obtained as

part of an earlier measurement program from 1986-1989. These earlier profiles provide

an opportunity for comparison with measurements during the more recent period. Profile

measurements were continued at Tahiti and Samoa through PEM Tropics B with the

program at Tahiti completed in December 1999. At Samoa weekly soundings continue as

part of the Southern Hemisphere Additional Ozonesondes (SHADOZ) project. During

most of the measurement period soundings were done weekly. During the aircraft field

campaigns in September-October 1996 (PEM Tropics A) and March-April 1999 (PEM

Tropics B) soundings were done twice a week. In January 1997 weekly soundings were

begun at Suva, Fiji (18.1S, 178.2E) in anticipation of PEM Tropics B. As part of the

Soundings of Ozone and Water Vapor in the Equatorial Region (SOWER) project ozone

profile measurements were started on a campaign basis in March 1998 at San Cristobal,

Galapgos (0.9S, 89.6W), were increased to bi-weekly soundings in September 1998, and

to weekly soundings as part of SHADOZ early in 1999. Soundings continue at Fiji and

the Galapagos in 2000 as part of SHADOZ and SOWER.

This set of ozone profiles obtained using balloon-borne ozonesondes provides

information on the distribution of ozone throughout the troposphere of the tropical Pacific

that has not been available in the past. In particular, new insights on the short-term and

seasonal variability of ozone in this region as well as differences between the eastern and

western Pacific can be gleaned from these data. In addition, isentropic trajectories are

usedto look atthetransportassociatedwith particularfeaturesof individualprofiles,and

alsoat the influenceof climatologicaltransportpatternson theprinciplefeaturesof the

ozonedistribution in thetropical tropospherein thisregion.

Methods

Ozonesondes

The ozone vertical profiles were obtained using the electrochemical concentration cell

(ECC) ozonesonde (Komhyr et al., 1995). This has become a standard technique for

obtaining ozone profiles with high vertical resolution in both the troposphere and

stratosphere to altitudes of approximately 35 kin. The measurements of ozone have an

accuracy of+5% through most of the troposphere with somewhat poorer performance

(+10%) for very low mixing ratios (<10 ppbv) encountered occasionally in the ta'opics_

The only imporiant interferent in the measurement technique, which is based on the

oxidation reaction of ozone with potassium iodide in solution, is sulfur dioxide that is not

encountered at these sites at sufficient concentrations to be of significance. The data were

obtained with an altitude resolution of about 50 m but for the analysis performed here

were averaged into 250 m layers. Only at Samoa are total column ozone measurements

from a collocated Dobson spectrophotometer available for comparison with the integrated

total ozone from the ozonesonde. These comparisons give an average ratio of 1.03 + 0.05

between the Dobson total ozone measurement and the integrated total ozone from the

ozonesonde, giving confidence that the ozonesonde measured ozone amounts from all of

the sites can be compared with each other. During the course of the measurement

program a change was made in early 1998 in the sensing solution recipe (Johnson et al.,

2000)atall of thesitesexcepttheGalapagoswherethenewrecipewasusedfrom the

beginning.This changeprimarily affectsthemeasurementsin thestratosphere.From

comparisonflights madeat thesesites,aswell aslaboratorytests,anempiricalcorrection

has been derived, and the earlier data have been corrected using this relationship. Data

archived in the Global Troposhere Experiment (GTE) archive for PEM Tropics at NASA

Langley have this correction applied.

4-

Trajectories

For the purposes of characterizing the tropospheric air-flow patterns influencing transport

to the tropical sites, insentropic trajectories have been calculated. The trajectories are

computed from the ECMWF analyses using the model described in Harris and Kahl

(1994). The limitations in such trajectories must be recognized in interpreting the results

that are obtained, but they do provide a useful tool for obtaining a picture of the flow

patterns that may influence ozone behavior at these sites. The computed trajectories are

used both to investigate individual cases, and by grouping the trajectories using an

objective clustering technique (Moody, 1986; Harris and Kahl, 1990) that gives an

indication of the primary flow regimes influencing a particular location at a given

altitude. The trajectories are computed twice daily (00 and 12 UT) for 10 days backward

in time. Air parcels reaching a site after 10 days of travel may have undergone diabatic

processes that are not accounted for in the model used here, and these are an important

contribution to the uncertainty in the computed pathway of the air parcel (Merrill, 1996).

Transport from sources or sinks more than 10 days travel time from a particular site could

also have a significant influence on ozone levels measured at the site.

Day-to-day variations

Variationson theorderof severaldaysare a large source of the variability seen in

tropospheric ozone at the Pacific tropical sites studied here as will be shown in the

analysis in this section. These variations have been studied from surface observations at

Samoa (Harris and Oltmans, 1998). The surface variations were found to result from

changes in airflow to the site that tapped different sources and sinks. During all seasons

air coming from higher latitudes and altitudes has about 50% more ozone than air with a

tropical origin (Harris and Oltmans, 1998). Although the ozone soundings are done on an

approximately weekly schedule compared to the continuous observations at the surface,

the variability from sounding-to-sounding captured in the time-height cross-section of

ozone mixing ratio for 1996 at Samoa (Plate 1) is also apparent. During 1996 at Samoa

the soundings were done twice a week during September and October so that the

variability is well represented during this time of the year. This can also be seen in the

plot of the average profiles at 0.25 km increments for individual seasons at Tahiti (Figure

1). The median is the horizontal line inside of the box, and the box represents the inner

50 thpercentile of the data. The whiskers represent the inner 90 thpercentile of the data.

The solid diamond is the mean. The September-November period has greatly enhanced

variability especially in the 2-1 Okra layer when compared to the March-May period.

These seasonal profiles are based on several years of data, and represent the variability

that is primarily contributed by the short-term fluctuations within a season.

In theGalapagosthetime-heightcross-sectionfor 1999(Plate2), theonly completeyear

availablefor this site,showsmanyof thesamefeaturesseenatSamoa.Theseasonal

profiles for theGalapagos(figure 2) showsimilarbehaviorto Tahitiwith amaximumin

thevar/ability in September-Novemberandaminimumin theMarch-Mayseason.The

variability is greater at Tahiti in September-November than it is in the Galapagos. At

Samoa (Figure 3) the variability in this season is also larger than at the Galapagos but not

quite as large as at Tahiti. Since Fiji (Figure 4) also shows greater variability during this

season, the larger variability is likely a real difference between the eastern and western

Pacific.

Seasonal variation

As can be seen in figures 1 - 4, not only is the variability of ozone in the troposphere

greater in September-November than in March-May, but also the mean (and the median)

values are greater as well. Time-height cross-sections based on 14-day averages over the

entire record of measurements (which varies somewhat for each site) are shown in plate

3. Although there are unique features at each site, a pattern is discernable that is reflected

at all sites. In the western Pacific there is a prominent layer of enhanced ozone in the

mid-troposphere in September and October. There also seems to be a distinct pulse in

June and July of somewhat smaller magnitude with a relative minimum in late July and

August. Associated with this June-July enhancement in the mid-troposphere, ozone in the

boundary layer also increases and this produces a seasonal maximum near the surface

that occurs earlier than the peak in the mid-troposphere. During austral winter and spring

higher ozone amounts descend to about 10-12 km giving higher ozone in the upper

troposphere.Thisseemsto be associated with ozone in the lower stratosphere. In

Januarv.'-May ozone at all latitudes is almost always lower than at a corresponding

altitude during the rest of the year. Also at this time of year ozone in the upper

troposphere is often very low with amounts approaching those seen at the surface which

are almost always quite low (<15 ppbv) in this season.

An earlier set of profiles obtained at Samoa from August 1986 - January 1990 (Figure 5)

show similar characteristics to those seen in the more recent 5-year data set. For March-

May the earlier period has somewhat more ozone that appears to be driven by several

profiles with greater ozone amounts, particularly in the low and mid-troposphere (Figure

5a). In September-November, however, the amounts and variability are very similar

(Figure 5b).

In the Galapagos the general picture is similar to the westem Pacific sites, but with some

important differences (Figure 6). The mid-tropospheric ozone maximum occurs earlier in

the year and diminishes earlier as well. Near the surface the seasonal variation is smaller.

The most noticeable difference is the lack of very low mixing ratios in the middle and

upper troposphere especially during the January-May time of year. This is consistent with

the fact that the western Pacific is a more convective region where boundary layer air can

be mixed into the upper troposphere. Other than this noticeable lack of low ozone

amounts the differences among the western Pacific sites is similar to the difference

between the eastem and westem Pacific. It is also clear that in the Galapagos ozone

amountsaregreaterthanin the western Pacific above 4 km in September-November and

above 5 km in March-May.

6@

Flow characteristics

Although the seasonal ozone behavior at all of the sites has some features in common

such as the maximum during the austral spring (September-November), this does not by

itself imply that the sources and sinks influencing the measured ozone at the sites are the

same. For example the proximity of the Galapagos to the South American continent

suggests that this continent is more likely to have a greater influence on this site than the

western Pacific. The following analysis shows that while the airflow patterns at the

western Pacific sites are quite similar, they are much different from those in the

Galapagos. The differences in airflow direction with season are greatest in the boundary

layer and somewhat greater at all levels in the Galapagos than in the western Pacific. To

carry out tiffs analysis a 10-year climatology of 10-day back trajectories was computed at

three levels (1, 6, and 13 krn) for all four of the sites for each of four seasons. These

trajectories were grouped into six clusters at each site for each altitude. Examples

representative of various regimes are discussed here (Figures 7-12).

Because of the similarity in flow patterns at the western Pacific sites the description of

the behavior at Tahiti is used to indicate the overall pattern with some differences fi-om

the other sites noted. At 1 km, a level in the marine boundary layer, the largest seasonal

contrast is between December-February (Figure 7a) and June-August (Figure 7b). These

are the seasons of minimum and maximum surface ozone at Samoa (Harris and Oltmans,

1997).During theaustralsummer(Dec.-Feb.)flow atthis level ispredominantlyfrom the

tropicalPacificfor all of thesitesin thewesternPacific.In theotherseasons15-65%of

theflow is from thewestandmoresoutherlylatitudeswith thelargestpercentageof flow

from thesouthoccurringin australwinter.Fiji showsthis mostprominentlywith 65% of

thetrajectoriescomingfrom thesouthandwestduringJune-Augustwhile atTahiti and

Samoathepercentageof southerlyflow iscloserto 30%(Figure7b).

At 6km atTahiti flow (Figure8) is moreuniformly fromthewestwith only about20%

comingfrom theeastonanannualbasis.Thereis asoutherlycomponentto theairflow

but it usuallydoesnotextendsouthof 40Swithin 10days,in contrastto the low level

flow thatreachesmuchhigherlatitudes.Throughouttheaustralwinter andspring(June-

November)at least25%of thetrajectoriesarrivingatTahiti comefrom asfar westasthe

mid IndianOceanandabout10%reachsouthernAfrica (Figure8b)within 10days.To

Fiji theflow is evenmorevigorousfromthewestandabout5%of thetrajectoriescome

from SouthAmerica.A numberof trajectoriesalsohavetheir 10-dayoriginsover

northernAustralia.In thesummerflow is lessvigorousandsometrajectorieshavea

northerlycomponent.

In theuppertroposphere(13 lan) theflow is fromthewestbut amajority of the

trajectoriesalsohaveanortherlycomponent(Figure9).This northerlyflow bringsair

from nearor evennorthof theequatorandis strongestin September-Novemberbut is

presentin otherseasonsaswell. As in themid-troposphere,a significantnumberof the

trajectoriescomefromthe IndianOceanandAfrica.

l©

The flow patterns in the Galapagos differ significantly from those in the westem Pacific.

At the lowest level (Figure 10) the flow is overwhelmingly from the south and over the

ocean in all seasons. There is some variation with season with December-May showing

10-15% of the trajectories coming from the Atlantic. In June-November the flow is

exclusively from the south, often paralleling the South American coast. The flow at 6 km

is in strong contrast to that in the boundary layer. On an annual basis 75% of the

trajectories arrive at San Cristobal from the east and have passed over the South

American continent in the previous 10 days. In December-February (Figure I la) about

10% of trajectories arrive from the tropical north Pacific, another 10% from the tropical

south Pacific, and the remainder from the east off continental South America. During

June-November (June-August shown in Figure 1 lb) about 10-15% of the flow is from the

tropical south Pacific with the remainder from the east or with little movement (cluster 3

in figure 1 lb). At 13 km (Figure 12) the flow is about equally divided into weak flow

from the east or northeast and vigorous flow from the west. There is a fairly strong

seasonality with December-May dominated by trajectories arriving from the west, but

about half the trajectories during June-November come from the east and northeast.

Discussion

In the previous sections the day-to-day and seasonal variations have been described as

well as the major air flow characteristics affecting the ozonesonde sites in the western

and eastern tropical Pacific. In this section important features of the ozone profile are

linked to the trajectories to show how important source and sink regions for ozone may

contributeto both theshortertermandseasonalvariations.Sincethetropicsareknownto

beasignificantareaof biomassburning(seeTRACE-A andSAFARI SpecialIssueof the

Journal of Geophysical Research, 101, 1996) particular attention is paid to characterizing

the possible influence of burning on ozone at these sites by linking enhanced ozone layers

with flow from potential source regions. It appears that several burning reNons contribute

to what is seen at these sites. The low ozone mixing ratios seen in the upper troposphere

at the western Pacific sites are also briefly discussed.

It is clearly seen from Plate 3 that there is a persistent layer of enhanced ozone in the

mid-troposphere in the June-November season. The climitological trajectory analysis also

shows this season to be one of regular occurrences of flow from potential burning related

source regions in southern Africa and Australia for the western locations and South

America for San Cristobal. Several individual profiles are examined that contain

enhanced mid-tropospheric ozone along with the trajectories calculated for these cases.

Profiles that show little or no ozone enhancement are also examined in relation to flow

characteristics. An event of very high ozone seen at Fiji and Samoa in November 1997 is

investigated because the flow path suggests an Indonesian source for the enhanced ozone.

A profile with an enhanced mid-tropospheric layer characteristic of those seen during the

September and October period in the western Pacific is shown in figure 13 for a sounding

done at Samoa on October 30, 1998. The peak ozone mixing ratio of 105 ppbv at -6 km

has a trajectory (Figure 14) that reaches back to southern Africa I0 days prior to the

sounding. Ozone profiles with peaks greater than about 70 ppbv (Figure 15) do not

alwayshavetrajectories(Figure 16)thatextendbackto Africa in 10days,but they

alwayshavepathsthathavea strongwesterlycomponentthatgoesto thewestof

Australia into theIndianOcean.OnmanyoccasionsthetrajectoriespassoverAustralia,

usuallythroughthemiddleor southernpartof thecontinent.Themarkedcontrastthatcan

occurduringamonth(seefigure 13)illustratesthedominantrole thattheairflow (Figure

16)to aparticular siteplaysin theozoneprofile,particularlyduringthisseason.For the

profile obtainedonOctober30, 1997the 10-daytrajectory(Figure16b)in themid-

troposphereshowslittle air movementfrom thevicinity of Samoaandmixing ratiosare

20-25ppmvor lessthroughoutthetroposphere.

From figure 8 it is clearthata largefractionof theair parcelsreachingTahiti (andalso

SamoaandFiji) passoverAustralia.Fromthedatathatareavailablefrom the

ozonesondesandtrajectoriesit is difficult to determineif trajectoriesthatpassover

Australiamaybe influencedby burningin AustraliaratherthansouthernAfrica (Olsonet

al., 1999).In somecasesthetrajectoriesshowthattheairmovesmoreslowly (for

examplecluster3 in figure 8b).An exampleof suchacaseis shownin theprofile of

October31, 1995atTahiti (Figure17).Thecorrespondingtrajectoryjust reachesthe

York Peninsulaof Australia(Figure 18)in 10days.Therelativelyslowtransport,the

largepeakmixing ratio,andthethin verticalextentof thelayersuggestthat thesourceof

theelevatedozoneseenat Tahiti mayhavebeenrelativelynearby(i.e.,Australiarather

thansouthernAfrica).

In 1997extensiveburningtookplacein Indonesiaassociatedwith droughtconditionsthat

,,,,'erea consequenceof thestrongE1Nino. Ozoneprofile measurementsover the

Indonesianregionhaveshownhighozonewasfoundin IndonesiaandMalaysiain

connectionwith theburningin theregion(Fujiwaraet al., 1999).OnNovember19and

20 atFiji andSamoa(Figures19)someof thehighesttroposphericozoneamountswere

seenfor anyeventrecordedatthesesites.These enhancements, which encompassed the

entire troposphere above the two sites, increased the total tropospheric column by about

20 DU, or more than 70% over average values for the month. The trajectories show that

at both sites air was coming from Indonesia (Figure 20a-d) throughout the depth of the

tropospheric column. Although these profiles were the only ones measured during the

event, trajectories show that a flow pattern nearly identical with the one during this large

ozone enhancement persisted for about 5 days around the time that the profiles were

obtained (Figure 20d).

As was noted earlier, during the austral summer ozone is generally quite low throughout

the troposphere in the western Pacific. Low values less than 10 ppbv are seen as in the

profile for February 20, 1999 at Samoa (Figure 21) where in the 13-15 km layer ozone is

about 5 ppbv. The trajectory at 13 km (Figure 22) shows that air came from northeast of

Australia in a region associated with extensive convection during this time of year. The

low ozone mixing ratios are not usually a result of mixing directly from the surface near

the site, although surface mixing ratios are low enough. This can be seen in Plate 3a, 3c,

and 3d where the minima in the upper troposphere are not connected to the low values at

the surface as illustrated by the individual profile shown in figure 21. Instead there is

normallyarelativemaximumin themixing ratio in themid-troposphere.It ismore likely

thatair with very low ozoneismixedverticallyby convectionin theregionof northern

AustraliaandeasternIndonesiaandtransportedto thesesitesin theuppertroposphere

(Kley etal., 1997).

In theGalapagosmanyof theprofilesduringAugust-Octoberalsoshowpronouncedmid-

troposphericpeaks(Figure23)asseenonOctober9, 1999.Thetrajectoryfor this event

(Figure24)crossesBrazil in lessthan10daysovera regionthatis heavilyinfluencedby

biomassburningduringthis time of theyear(Fishmanetal., 1996).Investigationof each

profile with anenhancedozonelayerin themid-troposphereduringthis timeof the year

showedatrajectorythatpassedoverBrazil.At timesof theyearwhenburningis not

happeningin Brazil, profilesat theGalapagosdonot showpronouncedtropospheric

peaks(Figure25).Profileswith mixing ratiosnear50ppbvin themid-troposphereare

occasionallyseen(Figure25a).Thesehigheramountsaregenerallyassociatedwith flow

from continentalSouthAmerica(Figure26a).However,flow from thecontinentmay

alsohaverelativelylow ozoneamounts(Figures25band26b).It appearsthateven

thoughtransportto thesite is similar,thatunlessthis flow tapsasourceregionwith

higherozone,thatpassingoveracontinentis not sufficient(althoughit doesappear

necessary)to give enhancementsin theozoneamount.Thiswasalsoneartheendof the

1997-98ENSOwarm phasewith well abovenormalprecipitationboth in theGalapagos

andwesternequatorialSouthAmerica.Theprofile of April 4 suggeststhatconvection

hasinfluencedtheprofile sincehumidity ishigh up to about9km (thedepression

betweentheair temperatureandfrost-pointtemperatureshownin figure 22bis small).

Conclusions

Fromanextensivesetof ozoneprofilesobtainedoverthepastseveralyearsin the

tropicalPacific,apictureemergesof aregionof bothvery low troposphericozone

amounts(-10 ppbv)andsurprisinglylarge(-100 ppbv) concentrationsthatarefoundat

all of thesitesstudied.The low concentrationsareexpectedin light of thestrong

photochemicalsinkin this region(Kley, et al., 1997),andtheimportantroleof

convection,particularlyduring theaustralsummer.Ontheotherhandtheubiquitous

presenceof mid-troposphericlayersof enhancedozonedemonstratesthatthe influenceof

biomassburningis widespreadthroughoutthetropicalzoneduringAugust-November.

On averageozoneattheequatorialGalapagossite in theeasternPacific is greaterthanat

thethreewesterntropicalPacific sites.However,for individual profiles,themid-

troposphericenhancementsseenin thewesternPacificattainlargermixing ratiosthan

seenin theGalapagos.This is in spiteof thefact thattheGalapagosarelocatedmuch

closerto thepotentialsourceof burning-producedozonein Brazil thanthewestern

Pacificsitesareto thesouthAfricanburningsources.Therelativeproximity of sourcesin

AustraliaandIndonesiamay,however,partiallyaccountfor this. Alternatively,themuch

greateramountofbiomassburnedin southernAfrica mayproducelargerozone

enhancements(Olsonet al., 1999).Theextensiveburningthat tookplacein Indonesia

duringthelatterhalf of 1997appearsto havebeenthesourceof very largeozone

amountsseenin SamoaandFiji in November1997.

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/g5

Plate Captions:

Plate 1. Time-height cross-section of ozone mixing ratio at Pago Pago, American Samoa

for 1996. Soundings are weekly except during the PEM Tropics A intensive measurement

period from August-September when soundings were done twice weekly

Plate 2. Time-height cross-section of ozone mixing ratio fi-om weekly soundings at San

Cristobal, Galapagos for 1999.

Plate 3. Time-height cross-sections of ozone mixing ratio based on 14-day averages over

the entire record of measurements at Samoa, Galapagos, Fiji, and Tahiti.

Figure Captions:

Figure 1. The average ozone profile and variability for 0.25 km layers at Tahiti for two

seasons (March-May and September-November) for the period 1995-1999. The median is

the horizontal line inside the box, and the box represents the inner 50 th percentile of the

data. The "whiskers" represent the ilmer 90 th percentile of the data. The solid diamond is

the mean.

Figure 2. Average ozone profile and variability for 0.25 km layers at Galapagos for two

seasons (March-May and September-November) for the period 1998-2000.

Figure 3. Average ozone profile and variability for 0.25 km layers at Samoa for two

seasons (March-May and September-November) for the period 1995-2000.

Figure 4. Average ozone profile and variability for 0.25 km layers at Fiji for two seasons

(March-May and September-November) for the period 1997-2000.

Figure5. Comparisonof ozonemixing ratioprofilesat Samoafrom twotime periods;

1986-1989and 1995-2000for a)March-Mayandb) September-November.

Figure6. Comparisonof theozonemixing ratioprofilesat SamoaandtheGalapagosfor

March-MayandSeptember-November.

Figure7. Clustersof trajectoriesarrivingatTahiti at 1km for a)December-Februaryand

b) September-Novemberfor theperiod1990-1999.Theclustersarenumbered1-6and

thepercentageof thetotalnumberof trajectoriesin eachclusteris alsoshown.

Figure8. Clustersof trajectoriesarrivingatTahiti at6 km andfor a)March-Mayand

b) September-November.

Figure9. Clustersof trajectoriesarrivingatTahiti at 13km andfor all seasons.

Figure10.Clustersof trajectoriesarrivingattheGalapagosat 1km andfor all seasons.

Figure11.Clustersof trajectoriesarrivingattheGalapagosat6 km andfor

a)December-Februaryandb) June-August.

Figure12.Clustersof trajectoriesarrivingattheGalapagosat 13km andfor all seasons.

Figure13.Ozonemixingratio profile at Samoaon October30, 1998.Thethicker solid

line is theozonemixing ratio.Thethinnerline to theright is theair temperatureandthe

thinnerline to the left is thefrost-pointtemperature.

Figure14.Ten-daybacktrajectoryarrivingat Samoaa 5kmat 00ZonOctober30, 1998.

Thenumbersalongthetrajectorypathmarkthenumberof daysbackin time theair

parcelwaslocated.Theelevationchangeof theair parcelis shownin thelower panel.

Figure15.Ozonemixing ratio profilesatSamoaona)October2, 1997and

2O

b) October 30, 1997.

Figure 16. Trajectories to Samoa at 5 km on a) October 2, 1997 and b) October 30, 1997.

Figure 17. Ozone mixing ratio profile at Tahiti on October 30, 1998.

Figure 18. Trajectory to Tahiti at 5 km on October 31 1998 at 12Z.

Figure 19. Ozone mixing ratio profile at a) Fiji on November 19, 1997 and b) Samoa on

November 20, 1997.

Figure 20. Trajectories to Fiji at 8 km on a) November 20, 1997 and b) November 23,

1997, and to Samoa c) at 6 km on November 20, 1997 and d) at 10 km on

November 21, 1997.

Figure 21. Ozone mixing ratio profile at Samoa on February 20, 1999.

Figure 22. Trajectory to Samoa at 6 krn on February 21, 1999 at O0Z.

Figure 23. Ozone mixing ratio profile at the Galapagos on October 9, 1999. The thicker

solid line is the ozone mixing ratio. The thinner line to the right is the air temperature and

the thinner line to the left is the frost-point temperature.

Figure 24. Trajectory to the Galapagos at 6 km on October 9, 1999 at 00Z and 12Z.

Figure 25. Ozone mixing ratio profiles at the Galapagos on a) March 25, 1999 and

b) April 4, 1998.

Figure 26. Trajectories to the Galapagos at 6 km on a) March 25, 1999 and

b) April 4, 1998.

Ozone in the Pacific Tropical Troposphere From Ozonesonde Observations

S.J. Oltmans I, B.J. Johnson t, J.M. Harris l, H. V6mel I'2, K. Kosh_, P. Simon, 4, R.

Bendura s, A.M. Thompson 6, J.A. Logan 7, F. Hasebe 8, M. Shiotani 9, M. Maata 3, G. Sami 3,

A. Samad 3, J. Tabuadravu _, H. Enriquez l°, M. Agama I _, J. Comejo 11, and F. Paredes l_

i NO.%A., Climate Monitoring and Diagnostics Laboratory (CMDL), Boulder, CO 80305,

USA

2 Cooperative Institute for Research in the Environmental Sciences (CIRES), University

of Colorado, Boulder, CO, 80309, USA

3 University of the South Pacific, Suva, Fiji

4 MeteoFrance, Papeete, Tahiti, French Polynesia

s NASA, Langley Research Center, Hampton, VA, USA

6NASA, Goddard Space Flight Center, Greenbelt, MD, USA

7 Harvard University, Cambridge, MA, USA

8 Ibaraki University, Mito, Japan

9 Hokkaido University, Sapporo, Japan

l0 INAMHI, Quito, Ecuador

l_ INAMHI, San Cristobal, Galapagos, Ecuador

ABSTRACT: Ozone vertical profile measurements obtained from ozonesondes flown at

Fiji, Samoa, Tahiti and the Galapagos are used to characterize ozone in the troposphere

over the tropical Pacific. There is a significant seasonal variation at each of these sites. At

sites in both the eastem and western Pacific, ozone is highest at almost all levels in the

troposphere during the September-November season and lowest during March-May.

There is a relative maximum at all of the sites in the mid-troposphere during all seasons

of the year (the largest amounts are usually found near the tropopause). This maximum is

particularly pronounced during the September-November season. On average, throughout

the troposphere at all seasons, the Galapagos has larger ozone amounts than the western

Pacific sites.A trajectoryclimatologyisusedto identifythemajorflow regimesthat are

associatedwith thecharacteristicozonebehaviorat variousaltitudesandseasons.The

enhancedozoneseenin themid-troposphereduringSeptember-Novemberis associated

with flow fromthe continents.In thewesternPacific this flow is usuallyfrom southern

Afiica (although10-daytrajectoriesdonotalwaysreachthecontinent),but alsomay

comefrom AustraliaandIndonesia.In theGalapagostheozonepeakin themid-

troposphereis seenin flow from theSouthAmericancontinentandparticularlyfrom

northernBrazil. Thetimeof yearandflow characteristicsassociatedwith theozone

mixing ratiopeaksseenin both thewesternandeasternPacificsuggestthatthese

enhancedozonevaluesresultfrom biomassburning.In theuppertropospherelow ozone

amountsareseenwith flow thatoriginatesin theconvectivewesternPacific.

Introduction

The tropical Pacific is often considered to be a region remote from major polluting

influences because of its isolation from heavily industrialized landmasses. Recent field

campaigns (Hoell et al., 1999) have emphasized that thou_ this is often the case, the

signature of pollution, particularly from biomass burning, makes a significant imprint on

the air chemistry of the region. In a number of instances layers of enhanced ozone

(mixing ratios >80 ppbv) were found inthe mid-troposphere in the remote western

Pacific (Stoller, et al., 1999). These layers in addition to having enhanced ozone were

replete with markers of biomass burning (Gregory et al., 1999).

Beginningin August 1995aspartof thePacificExploratoryMission(PEM)TropicsA,

ozoneverticalprofile measurementswerestartedat PagoPago,AmericanSamoa(14.3S,

170.6W)andPapeete,Tahiti (18.0S,149.0W).At Samoaprofileswerealsoobtainedas

partof anearliermeasurementprogramfrom 1986-1989.Theseearlierprofilesprovide

anopportunityfor comparisonwith measurementsduringthemorerecentperiod.Profile

measurementswerecontinuedat Tahiti andSamoathroughPEMTropicsB with the

programatTahiti completedin December1999.At Samoaweeklysoundingscontinueas

partof the SouthernHemisphereAdditionalOzonesondes(SHADOZ)project.During

mostof themeasurementperiodsoundingsweredoneweekly.During theaircraft field

campaignsin September-October1996(PEMTropicsA) andMarch-April 1999(PEM

TropicsB) soundingsweredonetwiceaweek.In January1997weeklysoundingswere

begunat Suva,Fiji (18.1S,178.2E)in anticipationof PEMTropicsB. As partof the

Soundingsof OzoneandWater Vaporin theEquatorialRegion(SOWER)projectozone

profile measurementswerestartedonacampaignbasisin March1998atSanCristobal,

Galapgos(0.9S,89.6W),wereincreasedto bi-weeklysoundingsin September1998,and

to weeklysoundingsaspartof SHADOZearlyin 1999.SoundingscontinueatFiji and

theGalapagosin 2000aspart of SHADOZandSOWER.

This setof ozoneprofilesobtainedusingballoon-borneozonesondesprovides

informationon the distributionof ozonethroughoutthetroposphereof thetropicalPacific

thathasnotbeenavailablein thepast.In particular,newinsightson theshort-termand

seasonalvariability of ozonein thisregionaswell asdifferencesbetweentheeasternand

westernPacific canbegleanedfrom thesedata.In addition,isentropictrajectoriesaree

used to look at the transport associated with particular features of individual profiles, and

also at the influence of climatological transport patterns on the principle features of the

ozone distribution in the tropical troposphere in this region.

Methods

Ozonesondes

The ozone vertical profiles were obtained using the electrochemical concentration cell

(ECC) ozonesonde (Komhyr et al., 1995). This has become a standard technique for

obtaining ozone profiles with high vertical resolution in both the troposphere and

stratosphere to altitudes of approximately 35 km. The measurements of ozone have an

accuracy of+5% through most of the troposphere with somewhat poorer performance

(4-10%) for very low mixing ratios (< 10 ppbv) encountered occasionally in the tropics.

The only important interferent in the measurement technique, which is based on the

oxidation reaction of ozone with potassium iodide in solution, is sulfur dioxide that is not

encountered at these sites at sufficient concentrations io be of significance. The data were

obtained with an altitude resolution of about 50 m but for the analysis performed here

were averaged into 250 m layers. Only at Samoa are total column ozone measurements

from a collocated Dobson spectrophotometer available for comparison with the integrated

total ozone from the ozonesonde. These comparisons give an average ratio of 1.03 4- 0.05

between the Dobson total ozone measurement and the integrated total ozone from the

ozonesonde, giving confidence that the ozonesonde measured ozone amounts from all of

the sites can be compared with each other. During the course of the measurement

program a change was made in early 1998 in the sensing solution recipe (Johnson et al.,

2000)at all of thesitesexcepttheGalapagoswherethenewrecipewasusedfromthe

beginning.This changeprimarily affectsthemeasurementsin thestratosphere.From

comparisonflights madeatthesesites,aswell aslaboratorytests,anempiricalcorrection

hasbeenderived,andtheearlierdatahavebeencorrectedusingthis relationship.Data

archivedin theGlobalTroposhereExperiment(GTE)archivefor PEMTropicsatNASA

Langleyhavethis correctionapplied.

Trajectories

For the purposes of characterizing the tropospheric air-flow pattems influencing transport

to the tropical sites, insentropic trajectories have been calculated. The trajectories are

computed from the ECMWF analyses using the model described in Harris and Kahl

(1994). The limitations in such trajectories must be recognized in interpreting the results

that are obtained, but they do provide a useful tool for obtaining a picture of the flow

patterns that may influence ozone behavior at these sites. The computed trajectories are

used both to investigate individual cases, and by grouping the trajectories using an

objective clustering technique (Moody, 1986; Harris and Kahl, 1990) that gives an

indication of the primary flow regimes influencing a particular location at a given

altitude. The trajectories are computed twice daily (00 and 12 UT) for 10 days backward

in time. Air parcels reaching a site after 10 days of travel may have undergone diabatic

processes that are not accounted for in the model used here, and these are an important

contribution to the uncertainty in the computed pathway of the air parcel (Merrill, 1996).

Transport from sources or sinks more than 10 days travel time from a particular site could

also have a significant influence on ozone levels measured at the site.

Day-to-day variations

Variationson theorderof severaldaysarealargesourceof thevariability seenin

troposphericozone at the Pacific tropical sites studied here as will be shown in the

analysis in this section. These variations have been studied from surface observations at

Samoa (Harris and Oltmans, 1998). The surface variations were found to result from

changes in airflow to the site that tapped different sources and sinks. During all seasons

air coming from higher latitudes and altitudes has about 50% more ozone than air with a

tropical origin (Harris and Oltmans, 1998). Although the ozone soundings are done on an

approximately weekly schedule compared to the continuous observations at the surface,

the variability from sounding-to-sounding captured in the time-height cross-section of

ozone mixing ratio for 1996 at Samoa (Plate 1) is aiso apparent. During 1996 at Samoa

the soundings were done twice a week during September and October so that the

variability is well represented during this time of the year. This can also be seen in the

plot of the average profiles at 0.25 km increments for individual seasons at Tahiti (Figure

1). The median is the horizontal line inside of the box, and the box represents the inner

50 th percentile of the data. The whiskers represent the inner 90 th percentile of the data.

The solid diamond is the mean. The September-November period has greatly enhanced

variability especially in the 2-10km layer when compared to the March-May period.

These seasonal profiles are based on several years of data, and represent the variability

that is primarily contributed by the short-term fluctuations within a season.

In the Galapagosthetime-heightcross-sectionfor 1999(Plate2), theonly completeyear

availablefor this site,showsmanyof thesamefeaturesseenat Samoa.Theseasonal

profiles for theGalapagos(figure2) showsimilarbehaviorto Tahiti with amaximumin

thevariability in September-Novemberandaminimumin theMarch-Mayseason.The

variability is greateratTahiti in September-Novemberthanit is in theGalapagos.At

Samoa(Figure3) thevariability in this seasonis alsolargerthanattheGalapagosbutnot

quite aslargeasat Tahiti. SinceFiji (Figure4) alsoshowsgreatervariability duringthis

season,the largervariability is likely arealdifferencebetweentheeasternandwestern

Pacific.

Seasonalvariation

As canbe seenin figures 1 - 4, not only is the variability of ozone in the troposphere

greater in September-November than in March-May, but also the mean (and the median)

values are greater as well. Time-height cross-sections based on 14-day averages over the

entire record of measurements (which varies somewhat for each site) are shown in plate

3. Although there are unique features at each site, a pattern is discernable that is reflected

at all sites. In the western Pacific there is a prominent layer of enhanced ozone in the

mid-troposphere in September and October. There also seems to be a distinct pulse in

June and July of somewhat smaller magnitude with a relative minimum in late July and

August. Associated with this June-July enhancement in the mid-troposphere, ozone in the

boundary layer also increases and this produces a seasonal maximum near the surface

that occurs earlier than the peak in the mid-troposphere. During austral winter and spring

higher ozone amounts descend to about 10-12 km giving higher ozone in the upper

troposphere.This seems to be associated with ozone in the lower stratosphere. In

January-May ozone at all latitudes is almost always lower than at a corresponding

altitude during the rest of the year. Also at this time of year ozone in the upper

troposphere is often very low with amounts approaching those seen at the surface which

are almost always quite low (<15 ppbv) in this season.

An earlier set of profiles obtained at Samoa from August 1986 -January 1990 (Figure 5)

show similar characteristics to those seen in the more recent 5-year data set. For March-

May the earlier period has somewhat more ozone that appears to be driven by several

profiles with greater ozone amounts, particularly in the low and mid-troposphere (Figure

5a). In September-November, however, the amounts and variability are very similar

(Figure 5b).

In the Galapagos the general picture is similar to the western Pacific sites, but with some

important differences (Figure 6). The mid-tropospheric ozone maximum occurs earlier in

the year and diminishes earlier as well. Near the surface the seasonal variation is smaller.

The most noticeable difference is the lack of very low mixing ratios in the middle and

upper troposphere especially during the January-May time of year. This is consistent with

the fact that the western Pacifc is a more convective region where boundary layer air can

be mixed into the upper troposphere. Other than this noticeable lack of low ozone

amounts the differences among the western Pacific sites is similar to the difference

betnveen the eastern and western Pacific. It is also clear that in the Galapagos ozone

amountsaregreaterthanin thewesternPacificabove4 km in September-Novemberand

above5km in March-May.

Flow characteristics

Although the seasonal ozone behavior at all of the sites has some features in common

such as the maximum during the austral spring (September-November), this does not by

itself imply that the sources and sinks influencing the measured ozone at the sites are the

same. For example the proximity of the Galapagos to the South American continent

suggests that this continent is more likely to have a greater influence on this site than the

western Pacific. The following analysis shows that while the airflow patterns at the

western Pacific sites are quite similar, they are much different from those in the

Galapagos. The differences in airflow direction with season are greatest in the boundary

layer and somewhat greater at all levels in the Galapagos than in the western Pacific. To

carry out this analysis a 10-year climatology of 10-day back trajectories was computed at

three levels (1, 6, and 13 kin) for all four of the sites for each of four seasons. These

trajectories were grouped into six clusters at each site for each altitude. Examples

representative of various regimes are discussed here (Figures 7-12).

Because of the similarity in flow patterns at the westem Pacific sites the description of

the behavior at Tahiti is used to indicate the overall pattern with some differences from

the other sites noted. At 1 km, a level in the marine boundary layer, the largest seasonal

contrast is between December-February (Figure 7a) and June-August (Figure 7b). These

are the seasons of minimum and maximum surface ozone at Samoa (Harris and Oltmans,

1997).During theaustralsummer(Dec:Feb.) flow atthis level is predominantlyfrom the

tropicalPacific for all of thesitesin thewesternPacific.In theotherseasons15-65%of

theflow is from thewestandmoresoutherlylatitudeswith the largestpercentageof flow

from thesouthoccurringin australwinter.Fiji showsthismostprominentlywith 65%of

thetrajectoriescomingfrom thesouthandwestduringJune-Augustwhile at Tahiti and

Samoathepercentageof southerlyflow is closerto 30%(Figure7b).

At 6 km atTahiti flow (Figure8) is moreuniformly from thewestwith only about20%

comingfrom theeastonanannualbasis.Thereis asoutherlycomponentto theairflow

but it usuallydoesnotextendsouthof 40Swithin 10days,in contrastto the low level

flow thatreachesmuchhigherlatitudes.Throughouttheaustralwinterandspring(June-

November)at least25%of thetrajectoriesarrivingatTahiti comefrom asfar westasthe

mid IndianOceanandabout10%reachsouthemAfrica (Figure8b)within 10days.To

Fiji theflow is evenmorevigorousfrom thewestandabout5%of thetrajectoriescome

from SouthAmerica.A numberof trajectoriesalsohavetheir 10-dayorigins over

northernAustralia.In thesummerflow is lessvigorousandsometrajectorieshavea

northerlycomponent.

In theuppertroposphere (13 km) the flow is from the west but a majority of the

trajectories also have a northerly component (Figure 9). This northerly flow brings air

from near or even north of the equator and is strongest in September-November but is

present in other seasons as well. As in the mid-troposphere, a significant number of the

trajectories come from the Indian Ocean and Africa.

Theflow patternsin theGalapagosdiffer significantlyfrom thosein thewestemPacific.

At _helowestlevel (Figure 10)theflow is overwhelminglyfrom thesouthandover the

oceanin all seasons.Thereis somevariationwith seasonwith December-Mayshowing

10-15°';of thetrajectoriescomingfrom theAtlantic. In June-Novembertheflow is

exclusivelyfrom thesouth,oftenparallelingtheSouthAmericancoast.Theflow at 6 km

is in strongcontrastto that in theboundarylayer.On anannualbasis75%of the

trajectoriesarriveat SanCristobalfrom theeastandhavepassedover theSouth

Americancontinentin theprevious10days.In December-February(FigureI 1a)about

10%of trajectoriesarrivefrom thetropicalnorthPacific,another10%from thetropical

southPacific,andtheremainderfrom theeastoff continentalSouthAmerica.During

June-November(June-Augustshownin Figure1lb) about10-15%of theflow is from the

tropicalsouthPacificwith theremainderfrom theeastorwith little movement(cluster3

in figure 1lb). At 13km (Figure 12)theflow is aboutequallydividedintoweakflow

fromthe eastor northeastandvigorousflow from thewest.Thereis afairly strong

seasonalitywith December-Maydominatedby trajectoriesarriving fromthewest,but

abouthalf thetrajectoriesduringJune-Novembercomefrom theeastandnortheast.

Discussion

In theprevioussectionstheday-to-dayandseasonalvariationshavebeendescribedas

well asthemajor air flow characteristicsaffectingtheozonesondesitesin thewestern

andeasterntropicalPacific. In thissectionimportantfeaturesof theozoneprofile are

linked to thetrajectoriesto showhow importantsourceandsink regionsfor ozonemay

contributeto boththeshortertermandseasonalvariations.Sincethetropicsareknownto

beasignificantareaofbiomassburning(seeTRACE-A andSAFARISpecialIssueof the

Journal of Geophysical Research, 101, 1996) particular attention is paid to characterizing

the possible influence of burning on ozone at these sites by linking enhanced ozone layers

with flow from potential source regions. It appears that several burning regions contribute

to what is seen at these sites. The low ozone mixing ratios seen in the upper troposphere

at the western Pacific sites are also briefly discussed.

It is clearly seen from Plate 3 that there is a persistent layer of enhanced ozone in the

mid-troposphere in the June-November season. The climitological trajectory analysis also

shows this season to be one of regular occurrences of flow from potential burning related

source regions in southern Africa and Australia for the western locations and South

America for San Cristobal. Several individual profiles are examined that contain

enhanced mid-tropospheric ozone along with the trajectories calculated for these cases.

Profiles that show little or no ozone enhancement are also examined in relation to flow

characteristics. An event of very high ozone seen at Fiji and Samoa in November 1997 is

investigated because the flow path suggests an Indonesian source for the enhanced ozone.

A profile with an enhanced mid-tropospheric layer characteristic of those seen during the

September and October period in the western Pacific is shown in figure 13 for a sounding

done at Samoa on October 30, 1998. The peak ozone mixing ratio of 105 ppbv at -6 km

has a trajectory (Figure 14) that reaches back to southern Africa 10 days prior to the

sounding. Ozone profiles with peaks greater than about 70 ppbv (Figure 15) do not

In 1997extensiveburningtookplacein Indonesiaassociatedwith droughtconditionsthat

were a consequence of the strong E1 Nino. Ozone profile measurements over the

Indonesian region have shown high ozone was found in Indonesia and Malaysia in

connection with the burning in the region (Fujiwara et aI., 1999). On November 19 and

20 at Fiji and Samoa (Figures 19) some of the highest tropospheric ozone amounts were

seen for any event recorded at these sites. These enhancements, which encompassed the

entire troposphere above the two sites, increased the total tropospheric column by about

20 DU, or more than 70% over average values for the month. The trajectories show that

at both sites air was coming from Indonesia (Figure 20a-d) throughout the depth of the

tropospheric column. Although these profiles were the only ones measured during the

event, trajectories show that a flow pattern nearly identical with the one during this large

ozone enhancement persisted for about 5 days around the time that the profiles were

obtained (Figure 20d).

As was noted earlier, during the austral summer ozone is generally quite low throughout

the troposphere in the western Pacific. Low values less than 10 ppbv are seen as in the

profile for February 20, 1999 at Samoa (Figure 21) where in the 13-15 krn layer ozone is

about 5 ppbv. The trajectory at 13 km (Figure 22) shows that air came from northeast of

Australia in a region associated with extensive convection during this time of year. The

low ozone mixing ratios are not usually a result of mixing directly from the surface near

the site, although surface mixing ratios are low enough. This can be seen in Plate 3a, 3c,

and 3d where the minima in the upper troposphere are not connected to the low values at

the surface as illustrated by the individual profile shown in figure 21. Instead there is

normallyarelativemaximumin themixing ratio in themid-troposphere.It is morelikely

that airwith very low ozoneis mixedverticallyby convectionin theregionof northern

AustraliaandeasternIndonesiaandtransportedto thesesitesin theuppertroposphere

(Kley et al., 1997).

In theGalapagosmanyof theprofilesduringAugust-Octoberalsoshowpronouncedmid-

troposphericpeaks(Figure23) asseenonOctober9, 1999.Thetrajectoryfor this event

(Figure24) crossesBrazil in lessthanl0 daysoveraregionthatis heavilyinfluencedby

biomassburningduringthis time of theyear(Fishmanet al., 1996).Investigationof each

profile with anenhancedozonelayerin themid-troposphereduringthis timeof the year

showed a trajectory that passed over Brazil. At times of the year when buming is not

happening in Brazil, profiles at the Galapagos do not show pronounced tropospheric

peaks (Figure 25). Profiles with mixing ratios near 50 ppbv in the mid-troposphere are

occasionally seen (Figure 25a). These higher amounts are generally associated with flow

fi-om continental South America (Figure 26a). However, flow from the continent may

also have relatively low ozone amounts (Figures 25b and 26b). It appears that even

though transport to the site is similar, that unless this flow taps a source region with

higher ozone, that passing over a continent is not sufficient (although it does appear

necessary) to give enhancements in the ozone amount. This was also near the end of the

1997-98 ENSO warm phase with well above normal precipitation both in the Galapagos

and western equatorial South America. The profile of April 4 suggests that convection

has influenced the profile since humidity is high up to about 9 km (the depression

between the air temperature and frost-point temperature shown in figure 22b is small).

Conclusions

From anextensivesetof ozoneprofilesobtainedover thepastseveralyearsin the

tropicalPacific,apictureemergesof aregionof bothverylow troposphericozone

amounts(~'10ppbv)andsurprisinglylarge(~100ppbv)concentrationsthatarefound at

all of thesitesstudied.Thelow concentrationsareexpectedin light of thestrong

photochemicalsink in thisregion (Kley,et al., 1997),andtheimportantrole of

convection,particularlyduringtheaustralsummer.On theotherhandtheubiquitous

presenceof mid-troposphericlayersof enhancedozonedemonstratesthatthe influenceof

biomassburningis widespreadthroughoutthetropicalzoneduringAugust-November.

On averageozoneat theequatorialGalapagossite in theeasternPacificis greaterthanat

thethreewesterntropicalPacific sites.However,for individualprofiles,themid-

troposphericenhancementsseenin thewesternPacificattainlargermixing ratiosthan

seenin theGalapagos.This is in spiteof thefactthat theGalapagosarelocatedmuch

closerto thepotentialsourceof burning-producedozonein Brazil thanthewestern

Pacific sitesareto thesouthAfricanburningsources.Therelativeproximity of sourcesin

AustraliaandIndonesiamay,however,partiallyaccountfor this. Alternatively,themuch

greateramountofbiomass burned in southem Africa may produce larger ozone

enhancements (Olson et al., 1999). The extensive burning that took place in Indonesia

during the latter half of 1997 appears to have been the source of very large ozone

amounts seen in Samoa and Fiji in November 1997.

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Hoell, J.M, D.D. Davis, D.J. Jacob, M.O. Rogers, R.E. Newell, H.E. Fuelberg, R.J.

McNeal, J.L. Raper, and R.J. Bendura, Pacific Exploratory Mission in the tropical

Pacific: PEM-Tropics A, August-September 1996, J. Geophys. Res., 104, 5567-5583,

1999.

Johnson,B.J.,S.J.Oltmans,H. V6mel, andT. Deshler,Characterizationof theECC

ozonesonde,J. Geophys. Res., in preparation, 2000.

Kley, D., H.G.J. Smit, H. Vrmel, H. Grassl, V. Ramanathan, P.J Crutzen, S. Williams, J.

Meywerk and S.J. Oltmans, Tropospheric water-vapour and ozone cross-sections in a

zonal plane over the central equatorial Pacific, Q. J. R. Meteor. Soc., 123, 2009-2040,

1997.

Kornhyr, W.D., R.A. Barnes, G.B. Brothers, J.A. Lathrop, and D.P. Opperman,

Electrochemical concentration cell ozonesonde performance evaluation during STOIC

1989, J. Geophys. Res., 100, 9231-9244, 1995.

Merrill, J.T., Trajectory results and interpretation for PEM-West A, J. Geophys. Res.,

101, 1679-1690, 1996.

Moody, J. L., The Influence of Meteorology on Precipitation Chemistry at Selected Sites

in the Eastern United States, Ph.D. Thesis, 176pp, University of Michigan, Ann Arbor,

Michigan, 1986.

Olson, J.R., B.A. Baum, D.R. Cahoon, J.H. Crawford, Frequency and distribution of

forest, savanna and crop fires over tropical regions during PEM Tropics A, J. Geophys.

Res., 104, 5865-5876, 1999.

Stoller, P., J.Y.N. Cho, R.E. Newell, V. Thomas, Y. Zhu, M.A. Carroll, G.M. Albercook,

B.E. Anderson, J.D.W. Barrick, E.V. Browell, G.L. Gregory, G.W. Sachse, S. Vay, J.D.

Bradshaw, and S. Sandholm, Measurement of atmospheric layers from the NASA DC-8

and P3-B aircraft during PEM-Tropics A, J. Geophys. Res., 104, 5745-5764, 1999.

Plate Captions:

Plate 1. Time-height cross-section of ozone mixing ratio at Pago Pago, .American Samoa

for 1996. Soundings are weekly except during the PEM Tropics A intensive measurement

period from August-September when soundings were done twice weekly

Plate 2. Time-height cross-section of ozone mixing ratio from weekly soundings at San

Cristobal, Galapagos for 1999.

Plate 3. Time-height cross-sections of ozone mixing ratio based on 14-day averages over

the entire record of measurements at Samoa, Galapagos, Fiji, and Tahiti.

Figure Captions:

Figure I. The average ozone profile and variability for 0.25 km layers at Tahiti for two

seasons (March-May and September-November) for the period 1995-1999. The median is

the horizontal line inside the box, and the box represents the inner 50 th percentile of the

data. The "whiskers" represent the inner 90 th percentile of the data. The solid diamond is

the mean.

Figure 2. Average ozone profile and variability for 0.25 km layers at Galapagos for two

seasons (March-May and September-November) for the period 1998-2000.

Figure 3. Average ozone profile and variability for 0.25 km layers at Samoa for two

seasons (March-May and September-November) for the period 1995-2000.

Figure 4. Average ozone profile and variability for 0.25 km layers at Fiji for two seasons

(March-May and September-November) for the period 1997-2000.

Figure5. Comparisonof ozonemixing ratioprofilesatSamoafrom two timeperiods;

1986-1989and 1995-2000for a)March-Mayandb) September-November.

Figure6. Comparisonof theozonemixingratioprofilesat SamoaandtheGalapagosfor

March-MayandSeptember-November.

Figure7. Clustersof trajectoriesarrivingatTahiti at 1km for a)December-Februaryand

b) September-Novemberfor theperiod 1990-1999. The clusters are numbered 1-6 and

the percentage of the total number of trajectories in each cluster is also shown.

Figure 8. Clusters of trajectories arriving at Tahiti at 6 km and for a) March-May and

b) September-November.

Figure 9. Clusters of trajectories arriving at Tahiti at 13 km and for all seasons.

Figure 10. Clusters of trajectories arriving at the Galapagos at 1 km and for all seasons.

Figure 11. Clusters of trajectories arriving at the Galapagos at 6 km and for

a) December -February and b) June-August.

Figure 12. Clusters of trajectories arriving at the Galapagos at 13 km and for all seasons.

Figure 13. Ozone mixing ratio profile at Samoa on October 30, 1998. The thicker solid

line is the ozone mixing ratio. The thinner line to the right is the air temperature and the

thinner line to the left is the frost-point temperature.

Figure 14. Ten- day back trajectory arriving at Samoa a 5kin at 00Z on October 30, 1998.

The numbers along the trajectory path mark the number of days back in time the air

parcel was located. The elevation change of the air parcel is shown in the lower panel.

Figure 15. Ozone mixing ratio profiles at Samoa on a) October 2, 1997 and

b) October30, 1997.

Figure 16.Trajectoriesto Samoaat5km ona)October2, 1997andb) October30, 1997.

Figure 17.Ozonemixing ratioprofileatTahiti onOctober30,1998.

Figure 18.Trajectoryto Tahiti at 5km onOctober311998at 12Z.

Figure 19.Ozonemixing ratio profileat a)Fiji onNovember19,1997andb) Samoaon

November20, 1997.

Figure20.Trajectoriesto Fiji at 8 km on a) November 20, 1997 and b) November 23,

1997, and to Samoa c) at 6 km on November 20, 1997 and d) at 10 km on

November 21, 1997.

Figure 21. Ozone mixing ratio profile at Samoa on February 20, 1999.

Figure 22. Trajectory to Samoa at 6 km on February 21, 1999 at 00Z.

Figure 23. Ozone mixing ratio profile at the Galapagos on October 9, 1999. The thicker

solid line is the ozone mixing ratio. The thinner line to the right is the air temperature and

the thinner line to the left is the frost-point temperature.

Figure 24. Trajectory to the Galapagos at 6 km on October 9, 1999 at 00Z and 12Z.

Figure 25. Ozone mixing ratio profiles at the Galapagos on a) March 25, 1999 and

b) April 4, 1998.

Figure 26. Trajectories to the Galapagos at 6 km on a) March 25, 1999 and

b) April 4, 1998.

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