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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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American Samoa, J. Geophys. Res., 102, 8781-8791, 1998.
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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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