JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102,NO. D15, PAGES 18,981-18,995,
AUGUST 20, 1997
Composition of the troposphere over the Indian Ocean
during the monsoonal transition
KevinP. Rhoads,
1PaulKelley,
2Russell
R. Dickerson,
2Thomas
P. Carsey,
3Michael
Farmer,
3DennisL. Savoie,
4andJoseph
M. Prospero
4
Abstract. The atmosphere
overthe equatorialIndianOceanis a uniqueenvironmentin whichto
studythechemicalandradiativeeffectsof anintensesourceof anthropogenic
emissions
fromthe
northern
hemisphere
directlycoupledto therelativelypristinebackground
conditions
presentin the
southern
hemisphere.As aninitialinvestigation
intotheroleof theintertropical
convergence
zone(ITCZ) oninterhemispheric
transport
of pollutants,
a numberof traceatmospheric
species
weremeasured
aboardtheNationalOceanicandAtmospheric
Administration
(NOAA) R/V MalcolmBaldrige betweenDurban,SouthAfrica,andColombo,Sri Lanka,fromMarch 12 to April
22, 1995. Sharpincreases
in theconcentrations
of carbonmonoxide(CO), carbondioxide(CO2),
andaerosols
wereassociated
withfourdistinctmeteorological
regimestransected
by thecruise
trackfrom 33øSto 9øN. Acrossthe ITCZ, aerosolconcentrations,
includingnon-sea-salt
sulfate,
nitrateand ammonium,increasedby a factorof 4. Surfaceozonemeasurements
showeda latitudinalgradientwith a minimumneartheequatoranda strongdiurnalvariationin theequatorial
regions.The latitudinalprofileof gas-phase
reactivenitrogenparalleledozoneandwashigherin the
remotesouthernhemisphere
thanin the remotenorthernhemisphere.Evidenceof directanthropogenicimpacton theregionwasobserved
morethan1500km fromthesouthern
tip of India.
Backtrajectories,
calculated
withNOAA'smediumrangeforecast
datausingtheHybridSingleParticleLagrangian
Integrated
Trajectory(HY-SPLIT) program,identifiedtheoriginof theair
massregimescharacterized
by the tracegasandaerosoldata. Continentalemissionsin the northernhemisphere
wereshownto havea majorimpactontheradiativeproperties
andoxidizingcapacity of the marine atmosphere.
Introduction
It is often difficult to discern the role of anthropogenic
emissionson the chemistryof the remote atmospherebecause
of our inability to define adequatelynatural backgroundconditions. The Indian Ocean, borderedto the south by one of the
most sparselypopulatedareason Earth and to the north by the
denselypopulatedand rapidly developingIndian subcontinent,
is a uniqueenvironment in which to study the large-scale impact of air pollutants on marine tropospheric chemistry and
cloudproperties. The large-scaleatmosphericcirculation in
this region is dominatedby a completebiannualreversalin the
low-level
winds
associated
with
the Indian
monsoon.
This
flow pattem is largely driven by the heat gradient betweenthe
Indian
subcontinent
and the ocean as a result of the seasonal
change in solar zenith angle [Cadet, 1979]. In the winter
monthsthe atmosphereover the continentalareasin the north
cools relative to the ocean, forming a region of general subsidence. This high-pressuresystem gives rise to the Indian dry
•Department
of Chemistry,
University
of Maryland,CollegePark.
seasonandgeneratesa persistentnortheasterlyoffshoreflow.
Under the intenseinsolationof the summer months, heat lows
developover the landmassareasin the north, reversingthe
circulationand producinga moisturerich southwesterlyonshoreflow that brings torrentialrains over India. Duringthe
transition period between the northeast monsoon (December-
February)and the southwestmonsoon (June-August),zonal
temperature
gradientsandeasterliesin the IndianOceantropics
areweakwith the intertropicalconvergence
zone (ITCZ) typically foundat about 10øS. At this time the cross-equatorial
monsoonalflow may provide a mechanismby which the
northern hemisphere air, laden with continental aerosols, an-
thropogenic
sulfates,andreactivegases,is coupledto the relatively pristineair of the southernhemisphere
at the confluxof
the two air masses into the ITCZ.
Therearevery fewmeasurements
of tracechemicalspecies
from the Indian Ocean. The 1987 Soviet American Gases and
Aerosols(SAGA) cruisetraversed
the easternportionof •theIndian Oceanat the end of Junealong 90øEfrom 30øSto about
5øN [Johnsonet al., 1990]. The surfacedata collected showed
a strong latitudinal gradient in ozone concentrationfrom about
2Department
of Meteorology
andtheCenterfor Clouds,
Chemistry 30 partsper billion by volume(ppbv)at 30øSto 10 ppbv at
3Atlantic
Oceanographic
andMeteorological
Laboratory,
NOAA, 10øSandthen remainedrelativelyconstantthroughthe ITCZ
andClimate,Universityof Maryland,CollegePark.
Miami, Florida.
to 5øN. Similar gradientswere observedin the Pacific Ocean
4Rosenstiel
School
of MarineandAtmospheric
Science,
University with a minimum of less than 5 ppbv found near the ITCZ.
of Miami, Florida.
Ozoneprofiles from a year of soundingsmadeat ReunionIsland(56.6øE,22.1øS)[Baldyet al., 1996]showeda strongseasonal variation in total troposphericozone that approxiCopyright1997 by the AmericanGeophysicalUnion.
mately follows the seasonalbiomass burning activities in
Papernumber97JD01078.
southeastern
Africa andMadagascar.Duringthe burningsea0148-0227/97/97JD-01078509.00
son from Septemberto November(australspring), layers of
18,981
18,982
RHOADSET AL.: TROPOSPHERE
OVERTHE INDIAN OCEAN
high ozone were observedin strata influencedby westerlies [Malingreau et al., 1985], the end of the dry season.In southabove the inversion level that could be traced back to the Afriern Africa, burning occursmostly in July-September[Andreae
can continent. From January to March, biomass burning is et al., 1994], consistent with the prominent plumes observed
greatly reduced.The soundingdata from this period suggest in AVHRR [Husar et al., 1997].
that convective
cells near the 1TCZ enhance the autumn tro-
As an exploratory experiment to investigate interhemi-
posphericozoneminimumby lofting marineboundarylayer spheric differencesin Indian Ocean troposphericchemistry,
air containinglittle ozoneand high relativehumidity.
Strongseasonalandlatitudinal variationshave also been
observedin the aerosol and precipitation composition in the
region. Chesteret al. [1991] reportedmineraldustconcentra-
instruments
were installed
aboard the National
Oceanic and
Atmospheric Administration (NOAA) R/V Malcolm Baldrige
duringthe World OceanCirculation Experiment (WOCE) I5/I7
repeatleg from SouthAfrica to Sri Lanka. This paperwill contionsdecreasing
from15-20gg m'3in thenorthto 0.01-0.25 centrateon the meteorologicalobservationsduringthe cruise,
I.tg m'3below35øSin thefar southern
ocean.Theauthors
noted thesurfaceCO, NO, NO2,NOv andO.•measurements,
as well as
that the seasonalreversal of the predominatewind flow that bulk aerosoldata. In addition,the resultsof NO2 photolysis
accompaniesthe monsoonalinversion in the northernocean rate coefficient and aerosoloptical depth determinationswill
doesnot affect southeasterlywinds in the southern ocean re- be presented.The resultsfrom 20 ozonesondesand a discusgion (southof about 10øS). Savoie eta!. (1987) have shown sion of the diurnal variation in boundarylayer ozone will be
that there are also strong gradientsin the concentrationsof presentedseparately.
particulatenitrateand sulfatewith the highest valuesfor both
speciesfoundin the north. Precipitationsamplescollected
from Amsterdam Island (77.5øE, 37.7øS) [Galloway and
Gaudry,1984; Moody et al., 1991] indicate that the central
southernIndian Oceanis generally representativeof pristine
marine conditions comparableto that of the remote Pacific
[Prosperoand Savoie,1989]. Occasionally,air massesshowing evidenceof stronganthropogenicimpact wereidentified
which significantly contributedto both the averageconcentrations and total depositionobserved[Moody et al., 1991].
Theseepisodeswerecharacterized
by particularlyhigh nitrate
(NO3')andnon-sea-salt
sulfate
(SO42')
levelsfromAfricaand
Experimental Techniques
The NOAA R/V Malcolm Baldrige departedDurban, South
Africa (29.8øS, 26.1øE) on March 21, 1995 (Julian day (J.D.)
80) and arrived in Colombo, Sri Lanka (6.7øN, 79.6øE), on
April 22 (J.D. 112) travelling on the cruise track shown in
Figure 1. The ship's primary mission was to conductoceanographic researchwhich requiredthe ship to remain stationary
for considerabletime eachday (up to 15 hours). These stationary periodsinterruptedsamplingwhen ship dieselexhaustwas
carriedpastthe intakes,as discussedbelow.
Australia.
Gradientsin aerosol optical thickness (AOT) determined
Aerosol Sampling and Aerosol Optical Depth
from advancedvery high resolution radiometer (AVHRR)
measurements
suggestthat there are two dominantsourcesfor
Bulk aerosolsampleswere collectedfor major ion analysis
aerosolsover the Arabian Sea. High values of AOT are seen on top of a 10 m towermounted5 m aft of the ship'sbow. Toover the northern end of the Arabian Sea. This is believed to
tal heightabovethe seasurfacewas -15 m. Aerosolparticles
be due to soil dust derived from sources in the Arabian Peninwere collectedby drawingambient air througha 20 x 25 cm
sula and from soils in the Tigris and Euphrates basin Whatman-41 filter at a flow rate of about 1 m3 min'•. These fil[Ackermanand Cox, 1989; Prospero, 1981], althoughsoils in ters have total masscollectionefficienciesof greaterthan 90%
the arid regionsof NW India andEastAfrica couldalso contrib- for non-sea
salt(NSS)sulfate(8042), 95%fornitrate(NO.•')
ute [Ackermanand Cox, 1989]. The seasonalityof dust storms andsea salt, and 99% for mineral dust[Savoie et al., 1989a].
and haze conditionsin this region [Ackermanand Cox, 1989] Because Whatman-41 and other cellulose filters that are coated
is consistentwith the seasonalityof the AOT distributionsob- with NaC1are known to collect HNO3 vapor with high effiservedin AVHRR [Husar et al., 1997]. The dust concentrations ciency, we consider our measuredNO.•' concentrationsto be
over the Arabian Sea are comparableto those measuredin the thoseof total inorganicNO3',i.e., particulateNO3'plusgaseMediterranean and off the west coast of Africa [Savoie et al.,
ousHNO3 [Savoieet al., 1989b]. However,the concentration
1987; Prodi et al., 1983], and the seasonality is consistent of gaseousHNO3 in the near-surface
marineboundarylayer is
with the dust transport distributions shown in AVHRR and typically around20% or less of the total NO3' [Savoie and
with the monsoon circulation [Ackerman and Cox, 1989].
Prospero,1982]. The concentrationof ammonium(NH4*)reDust concentrationsdrop sharply as one moves from the Ara- portedhereshouldbe regardedasthe totalof aerosolNH4*plus
bian Sea southwardinto the southern hemisphere circulation any gas-phaseammonia(NH 3) that might react with acid aero[Savoie et al., 1987; Prodi et al., 1983]. A second major solspecies
onthefilter(e.g.,NSSSO42'
andNO3').
source of Arabian
Sea aerosols is located in southern India as
suggested
by a prominentbulgein the AOT distributions
along
the SW coast of India from about 15øN to the southerntip.
This aerosol plume appearsto be attributable to pollution
sourcesnot soil dust. Dust storm statistics suggestthat there
are major sourcesof mineral dustin northernIndia [Ackerman
and Cox, 1989] but that there is very little duststorm activity
To avoid contaminationfrom the ship itself, a weather vane
and an anemometeron top of the 10 m bow tower were connectedto circuitrythat limited samplingto flow for at least 10
s at a minimum
speed
of 2 m s'l froma 270ø sectorcentered
on
bow. Filters were changedtwice a day and transferredto a
freezerfor storage.A blank samplewas createdfor every four
exposedfilters by manipulating an unexposedfilter through
in southern India below 15øN at any time of year. AVHRR the samplingprotocolwithoutactuallydrawing air throughit.
suggests that during the spring there are very few major Upon arrival at Colombo,Sri Lanka, the sampleswerepacked
sources of aerosols evident in the Indian Ocean south of the
in dry ice and shippedto the Rosenstiel School of Marine and
equator.Biomassburningis not active at this time of year. In AtmosphericSciences,Miami, Florida. After extracting the
Oceanea, burning appears to peak in September-October filter paperswith ultapurewater, the extractswere analyzedfor
RHOADSET AL.: TROPOSPHERE
OVER THE INDIAN OCEAN
18,983
30
__ end
112
15
Colombo,
---- 10'7
Sri
._
Lanka
INmEI
---
101.6
-
Intertropical
IS'HmEI
--
.97 Convergence
Zo.n..e..,.n.
......
.,
.• ....
91.5
•,-'..,•,++'
ismm'X
!S
Durban,
ß Africa
i
15
start
s9
Cruise track
7
81
30
----*--- 10 Day backtrajectory
-30
+ Juliandaymarker
45 ..... 60
75
EastLongitude
90
105
Figure 1. The cruisetrack(solidline) andrepresentative
10 daybacktrajectories(dashed
lines) for the four
flow regimesencountered
calculated
fromtheNationalOceanicandAtmospheric
Administration's
(NOAA)MediumRangeForecast(MRF) database
usingtheHybridSingle-Particle
Langrangian
Integrated
Trajectories
(HYSPLIT) model. The trajectories
havediamondsindicatinga 1 day time stepandhaveendpointaltitudesof 1 km.
Thefourflow regimesarecodedas SHmX,southernhemisphere
marineextratropical;SHmE,southernhemispheremarineequatorial;
NHmT,northern
hemisphere
marineequatorial;
andNHcT,northernhemisphere
continental tropical.
min intervalsby switchingthe sampleflow througha Pd/A10
2
3
catalyst (DegussaCorp., Plainfield, New Jersey, type E 221
and$O42'within+5% by ion chromatography.
Non-seasalt P/D) heatedto 250øC [Dickersonand Delany, 1988' Doddridge
sulfatewascalculated
astotal SO4
2' minusNa* multipliedby et al., 1994]. Weekly calibrationswere performedusing a sec0.2517,themassratioof SO42':
Na*[Savoieet al., 1992]. The ondary standardof 0.97 parts per million by volume (ppmv)
mineral dust collectedwas quantifiedwithin +10% by ashing (Scott-Marrin, Inc., Riverside, California) previously referencedto a NationalInstituteof Standards
andTechnology
the extractedfilter paperat 500øCand weighingthe residue.
Aerosolopticalthicknessmeasurements
were madeusingan (NIST) standard. A small changein calibration(-4%) over the
EKO sunphotometer(Model MS-120, EKO Instruments, To- period of the cruise was attributed to degradation of the
kyo, Japan)operatingat peak wavelengthsof 500, 675, 875, Pd/A1203
catalyst.A background
timeseries
wascreated
by
945 nm. This instrument was calibrated 3 months before the
linearly interpolating betweenzero modes. After subtracting
voyageat the World Radiation Center (Davos, Switzerland), the backgrounddata the 30 s values were used to calculate
and the linearity was reportedto be 0.3%. The estimatedwave- hourly averagesthat show an uncertaintyof about +5 ppbv
lengthaccuracyof the dielectricthin film interferencefilters is (_+1o).
sodium(Na*)within +2% by flame atomic absorption,NH4*
within +5% by automatedcolorimetry,and chloride(CI'), NO3',
+2 nm with a half bandwidth of 5-6 nm [Pinker et al., 1994].
Becauseof the movement of the ship the instrument was
Measurementsof atmospheric CO2 concentrations were
madeusing a nondispersive infrared CO2 analyzer (Model LIalignedby hand, and the measurements
were recordedas the 6251, Li-COR, Lincoln, Nebraska). This instrumentcompares
maximumsignal obtainedusing the peak-picking function of the absorption dueto ambient CO: to that dueto a CO2 referthe device. Only a few usefulobservationswerecollecteddur- encegas. The analyzer operatedin a differential mode, with a
ing the voyagebecauseof excessive
shiproll andcloudiness. "zero"
reference
gasof CO:at about10ppmvbelowambient
CO and CO2 Measurements
levels flowing continuouslythrough the referencecell of the
instrument. Ambient air was drawn from the bow of the ship
Decorontubingat about5 L min'•. Beforeentering
Sampleair for continuouscarbonmonoxide(CO)measure- through
mentswasdrawnthrougha 0.5 gm Teflon inlet filter mounted the analyzer the sample streampassedthrough a water vapo.r
trap, a particle filter, and a flowmeter. The instrumentwas
at the 7 m level on the bow tower. A modified commercial nondispersiveinfraredgas filter correlationCO analyzer(Model calibratedat 1 hour intervals using three CO2 referencegases.
48, Thermo Environmental Instruments, Franklin, Massachu- This techniquehasan absoluteaccuracyof _+0.5ppmv with the
setts)located30 m awaywasusedwith a flow rateof 1.0 L precisionestimatedat +0.15 ppmv CO2 [Wanninkhofand
min'•. Chemicalzeroingof theinstrument
wasperformed
at 20 Thoning, 1993].
18,984
Ozone
RHOADS ET AL.: TROPOSPHEREOVER THE INDIAN OCEAN
Measurements
Continuous ozone measurementswere made using a commercial UV absorption instrument (Model 300, Environics,
Inc., West Willington, Connecticut) as described by Piotrowicz et al. [1990]. Sample air was drawn through Teflon
tubing with an in-line 0.5 gm Teflon filter mountedon the top
deck of the ship approximately14 m above the surface. Intake
line lossesof ozone were determinedto be lessthan 5%. Daily
checks of instrument backgroundwere made by passing the
sample flow through a small volume of Hopcalite catalyst
(Mine Safety Appliances Co., Pittsburgh, Pennsylvania).
Zero mode detectionlimit was 0.45 ppbv (S:N=I: 1, ___2•)using
30 s averaging. Over the courseof the cruise,zero mode measurements varied by less than 2 ppbv and showed no diurnal
variation.
NO, NO2, and NOy Measurements
Nitric oxide (NO)measurements were made with an ozone
chemiluminescence instrument built by NOAA's Atlantic
OceanMarine Laboratory(AOML) [Carseyet al., 1997] on the
basisof the designdevelopedby Ridley et al. [1988]. This instrumentused a photon counter to collect emissionevents over
10 s intervals and showed a sensitivity to NO of approximately 4 counts/partsper trillion by volume (pptv) with a zero
mode detection limit of about 18 pptv (10 s, S:N=i:I, +2q).
The detectorsampled1.0 L min'• at the 7 m level of the bow
tower from one of two inlets: a combined NO/NO2 inlet or a
NOyinlet leadingdirectlyto a tower-mounted
Mo converter
maintainedat 380øC [Luke and Dickerson,1987; Fehsenfeldet
subtractedfrom the NO2 time seriesby linear interpolation.
Although instrumentnoise was typically of order of 5-10
pptv, uncertainty in the artifact level increases the detection
limit of hour NO2 averagesto about 20 pptv. Since the concentrationof NOvin the"zeroair" usedduringcalibrationswas
not significantly lower than the levels observed in ambient
air,theNOydatawascorrected
for theNO artifactonly.
A tower-mounted
massflow controllerreducedthe pressure
to about200 torr in the 30 m of black Teflon tubing from the
inlets to the analyzer and limited the residencetime to about
10 s. A wind-sectorsignal from a tower-mountedanemometer
was usedto control a pump which suppliedan excessflow of
scrubbedair to both inlets during periodswhen ship exhaust
couldbe aspiratedby the instrument. The wind-sectorsignal
was digitally recordedat 30 s intervals and was later used to
remove periods of contaminationfrom the CO, ozone, and reactive nitrogendata. Even with this protection, ship exhaust
signal spikesinterfered with instrument performanceand limited the numberof reactivenitrogen 10 s data pointsto 34,953
(NO, 11,894; NOy,15,072; NO2,7,987) for the periodfrom
J.D. 82 to J.D. 112. Althoughthesedataonly cover approximately 15% of the total time at sea, a considerableportion of
the dataloss occurredduringperiodswhen the ship was stationary. Thereforethe datapresentedhereprovidean adequate
representationof the compositionair over the entire region
traversedduring the voyage.
Measurements of j(NO2)
Measurements
of the photolysisratecoefficientof NO2,
Ambient air was drawn
j(NO2), weremadeusingan upwardlooking 2• steradian(sr)
throughtheseinlets with in-line Teflon filter holdershousing
0.5 gm Teflon filters that were changedat least once each day.
The concentration of NO2 was determinedby reductionto NO
using a broadband UV photolytic converter [Kley and
McFarland, 1980; Carroll et al., 1992, Poulida et al., 1994].
The instrumentwas typically cycled throughfour 5 min modes:
chemicalactinometerpositionedon top of the 10 m bow tower
suchthatthe sky wasunobstructed
[Kelley et al., 1995]. The
al., 1987; Nunnermacker, 1990].
NOy,NO2,NO, andzero(prereactor).
Calibrations were performedtwice daily at the tower inlets
by standard addition and dynamic dilution of a NO standard
(52.1 ppbv, Scott-Marrin, Inc., Riverside, California) and a
tower-mountedNO2 permeation tube maintained at 35øC. The
NO 2 permeationrate was determinedby conversionto NO on a
hot Mo converter with a known efficiency of 99.9 + 0.5%.
This converterwas characterizedbeforeand after the projectby
gas-phasetitration. The average conversion efficiency of the
photolytic converterwas found to be 47 + 11(10)%. Artifacts
were observed in both the NO and NO2 signals during the
courseof the experiment. The NO artifact was determinedby
averagingthe NO signal during the 2 or 3 hours before sunrise
and after sunseteach day and was found to range from 5 to 60
pptv with a typical value of about25 pptv. Thesepoints were
usedto calculatea linearly interpolated NO backgroundwhich
was subtracted from each of the 10 s observations
in the time
series. The uncertainty in the 1-hour-averagedNO mixing ratios is estimatedat +2 pptv (95% confidencelevel). The NO2
artifact (17 + 7(l•)pptv) was found when passing sample air
through the Mo converter followed by the photolytic converter. An increase in signal was observedwhen the photolysis cell was illuminated. No increase was found when a
secondMo converter was installed downstreamof the photolyric converter when the cell was not illuminated. This artifact was quantified during the calibrations each day and was
actinometer
wassupplied
with the outputof a NO2permeation
tube diluted with ambient air filtered through a charcoal/calcium
sulfatescrubber.The resultingmixtureshoweda
NO2mixingratio of 6180 + 25(1•) ppbv. Assuming
plug
flow, the calculated
residence
time in the exposedsectionof
the tubewasabout0.8 s andwascorrected
for atmospheric
temperatureand actinometerpressure. A modified commercial
chemiluminescenceanalyzer (Model 42, Thermal Environmental INstruments, Franklin, Massachusetts) measuredNO
generated
by the photolyticdissociationof NO2 in the actinometertube. Twice everyhour the backgroundconcentration
of NO in theNO2/dryairmixturewasquantified
by switching
to
a darkpathwhichbypassed
the quartzphotolysis
tube.
Weeklyanalyzercalibrations
weremadeusinga 52.1 ppbv
NO standard(Scott-Marrin,Inc., Riverside,California). The
detectionlimit determined
duringbackground
cycles(30 s, 1:1
S:N,+2•) wason theorderof 10'4S'•. Sincethephotolysis
ratecoefficientis calculated
usingthe ratio of the changein
NO concentration
to the concentration
NO2,
j(NO2) = (A[NO]/[NO2])(1 / At),
(1)
the absolutesensitivityof the analyzerto NO is not important
to the result. Periodicmeasurements
of the NO2permeation
tubeoutputwere madeusinga Mo converterat 380øC. Its con-
versionefficiencywas determined
to be 99.9 + 0.5% using
gas-phase titration of NO with ozone before and after the
cruise. Data reduction and corrections were similar to that de-
scribedpreviously [Kelley et al., 1995] with an additional cor-
rectionfor titrationof NO by the incidentalO3 in the 35 m re-
RHOADSET AL.: TROPOSPHEREOVER THE INDIAN OCEAN
turn path from the actinometerto the NO detector. The largest
correctionswere found at the highestphotolysisrates and were
about 10%. The total uncertainty in the absolute value of
j(NO2) is 9% at the 95% confidencelevel.
Meteorology
18,985
and Surface
Back Trajectories
Surfacewinds (on the basis of shipboard observations) in
theSHmXregimewerethestrongest
of thecruise(7 m s'1 average)with winds starting from the north andbacking continuously for the 10 day period. A low centered at about 50øS
Back
Trajectories
Back trajectories were calculated from NOAA's Medium
RangeForecast(MRF) databaseusing the Hybrid Single-Particle Langrangian Integrated Trajectories (HY-SPLIT) program
[Draxler et al., 1991]. This modelwas usedto calculateanalyses for 1000 and 2000 m above the ship's position at 0000
and 1200 UTC for each day of the cruise. Since the marine
boundarylayer varied from 500 to 2000 m, it shouldbe noted
that these calculated trajectories indicate the synoptic situation and generalorigin of the air mass sampledand shouldnot
be interpretedas the literal path traversedby an air parcel.
Results and Discussion
Four distinct air masses, or meteorological regimes, were
encounteredduring the cruise(Figure 1). These have been labeled southern hemisphere maritime extratropical (SHmX),
southern hemisphere maritime equatorial (SHmE), northern
hemispheremaritime equatorial (NHmE), and northern hemispherecontinentaltropical(NHcT). The trace gas and aerosol
data mirror the changesobservedin the meteorology (Figure
2). For purposesof comparisonthe data have been partitioned
into thesefour groupsand are presentedin that context (Table
1).
trackedeastwardduringthe periodwith a troughextendingto
near the ship'spositionat 28øS. Weak high pressurefilled in
from the west startingJ.D. 88. Back trajectoriesfrom the period typically show quite long traces acrossthe south Indian
and Atlantic Oceanswith the air parcelsspendingmuchof their
time in the zonal flow of the remotemarine free troposphere
(SHmX in Figure 1). However, towardthe end of this period
severaltrajectoriespassover SouthAfrica. From 18øS to 22øS
the back trajectoriesshowthat parcelspassedover the nearby
island of Madagascar. This 3 day period was probably contaminatedby local emissionsand thereforeis unrepresentative
of the large-scaleIndian Ocean troposphere.
A largeincreasein averageair and sea surfacetemperatures
was seenin the SHmE while the averagewind speeddecreased.
Winds startedfrom the east but then clocked slowly aroundto
the southwestas an extendedtrough of low pressureformed to
the eastof the ship. Most of the back trajectoriesfor this period are influenced by the easterly trade winds and remain entirely over the southernIndian Ocean (SHmE in Figure 2).
There was heavy rain for much of the day at 9øS (near the
ITCZ), but this was not accompaniedby a shift in wind direction or speed.
The ITCZ
was entered at about 6øS on J.D.
t< IsHmx
] 71< SHm[••••NHm•
160I II.........
Carbon
Dioxide
II
[
/
120
F
II•
101.
Carbon
Monoxide
II
]l Nitrate II
II• NSS
SMhte II
II = Ao,m
II
!•,;! I
,. .
.[
6
17]
s
[• $t•!• I'• H
•[..:,•.,["'•
/!l[• •.• 4
•' '• •
CO
(ppbv) m
CO2
(ppm•3•)
3 NSS Sulhte,
Nitrate,
and
2 A•onium
/ m3)
I .....
82
I ....
•
I ....
•
V I , , i [ I , •, , , I 'V ' ' '
•
102
107
-!
112
JulianDay
Figure 2. Time seriesshowingstrongcorrelationbetweenCO (ppbv), CO2(ppmv - 358 ppmv) and bulk
aerosols
including
ammonium,
non-sea-salt
(NSS)sulfate
andnitrate(ggm'3)for thecruise.COandCO2data
havebeenaveragedinto 1 hourperiods,while aerosoldatarepresents
samplestypicallycollectedover 12 hour
periods.Increased
CO andaerosols
duringJulianDays(J.D.) 91.5-94.3coincides
with closepassageto Madagascar,andthe islandsof Reunionand Maritius. Prior to J.D. 92 the ship wasin the SHmX regimeandafter
J.D.93 theSHmEwasentered.The sharpincreaseat J.D. 101.6 marksthe transitionacrossthe intertropical
convergencezone (ITCZ), while the increaseat J.D. 107.0 is the transitionfrom NHmE to NHcT air mass. The
AOT markerson the x axis indicate when aerosol optical thickness determinations cited in Table 3 were collected.
Surface
winds, as observedfrom the ship, were westerly at the 1TCZ
and slowly backedto the east before becominglight and vari-
18,986
RHOADS ET AL.: TROPOSPHEREOVER 'ITIE INDIAN OCEAN
Table 1. Surface Observationsby Flow Regime
Designation
SHmX
SHmE
NHmE
NHcT
Latitude
34øS to 22øS
18øS to 6øS
6øS to 6øN
4.5øN to 9øN
Julianday
81.0 to 91.5
94.3 to 101.6
101.6 to 107.0
107.0 to 112.0
25.5 + 2.4
30.5 + 1.2
31.5 + 1.3
31.3 + 0.8
21.6 + 1.6
27.3 + 0.8
28.7 + 0.2
28.9 + 0.4
23.7 + 1.7
29.1 + 0.6
29.8 + 0.3
29.9 + 0.4
6.6+ 3.0
3.3+ 9.6
1.9+ 0.9
3.8+ 1.5
54.1 + 4.0
17.0+ 1.8
358 + 0.2
13.6+ 13
'"
165+ 49
59.3 + 3.7
11.5+2.5
358 + 0.4
4.8 + 8.0
44.3+ 12
141+ 34
89.0 + 3.3
7.5 + 1.2
362+ 0.5
4.63 + 3.15
18.9+ 5.9
104+ 16
120.4+ 9.8
15.7+2.9
363+ 1.2
4.66 + 4.0
38.8+ 34
178+ 71
Meteorological Data
AverageMaximum Air
Temperature,øC
AverageMinimum Air
Temperature,øC
Sea Surface
Temperature,øC
WindSpeed,m s'•
Trace
Gas Data
[CO], ppbv
[O3],ppbv
[CO2],ppmv
[NO], pptv *
[NO2],pptv
[NOy], pptv
Aerosol
Data
Number of Samples
17
13
12
9
Na+,gg m'3
CI', gg m-3
SO42',
gg m'3
NSSSO42',
gg m'3
NH4+,gg m'3
NO3-,gg m'3
Mineraldust,gg m'3
2.73+2.17
4.91+4.12
0.82+ 0.49
0.22+ 0.15
-0.01+ 0.02
0.01+ 0.04
< 0.1
3.30+0.68
5.83+1.18
1.19+ 0.18
0.36+ 0.12
0.08+ 0.03
0.22+ 0.07
< 0.1
0.47+0.18
0.80+0.33
1.52+ 0.34
1.40+ 0.36
0.24+ 0.05
1.01+ 0.25
< 0.1
1.11+0.44
1.58+0.74
2.96+ 1.04
2.68+ 0.95
0.71+ 0.33
1.71+ 0.62
6.2+ 4.4
All averagevaluesareshownwith + 1 standarddeviation.
SHmX, southernhemispheremarineextratropical;SHmE, southernhemispheremarine equatorial; NHmE, northernhemispheremarineequatorial;NHcT, northernhemispherecontinental
tropical;ppbv,partsper billionby volume;ppmv,partsper millionby volume;pptv, partsper
trillion by volume;NSS, non-sea-salt.
* Values for NO are calculatedfrom daytimeobservations
only.
able at the equator. Weak high pressurein the Arabian Sea
dominatedthe flow in this region. Back trajectoriesstartingat
are suspected
to be the sourceof these increases,since a number of back trajectoriesindicate that the ship interceptedair
parcelswhich crossedover the island in the boundarylayer.
However, the ship also madetwo closepasses(<15 km) to the
populatedislands of Mauritius and Reunion and are likely to
1000 m werequiteshortand reflect the extremelylight easterlies of the region (NHmE in Figure 1). Evidenceof the calm
winds in this regime is clearly seenin the large decreasein seahave contributed to the local contamination.
salt aerosolloading(Na* and CI', Table 1).
In the SHmE regime the averageCO mixing ratio increased
The NHcT regimewasusheredin on the morningof J.D. 107
with heavy rain and funnel cloud sighting. Surfacewinds by almost 10% to 59.3 + 3.7 ppbv, even thoughthe sourcerequicklywentfrom the westto the northeastandremainedthere gion determinedfrom back trajectories, the remote Indian
for the next three days with surfacehigh pressureover the Ocean, did not change. This rise was accompaniedby a sigin theconcentrations
of NSS SO42'andNH4+.
southerntip of India. Starting with J.D. 108 andcontinuing nificantincrease
for the remainderof the cruise,back trajectoriescrosssouthern It has been proposedthat this is the result of cyclonic flow
India andSri Lanka (NHcT in Figure 1). Closerto the southern aroundlocal disturbancesalong the ITCZ which allows SHmE
air to cross along the western edge of the disturbancesand
tip of India windsveeredto the northwest.
bring NHmE air down along the easternedge, causingan increasein CO and aerosolsto the south of the ITCZ (Figure 3a)
Carbon
Monoxide,
Carbon
Dioxide,
and
Aerosols
(T. N. Krishnamurti, personal communication, 1995). Traditionally, the most simple model of the ITCZ consists of congimescanbe foundin the CO and aerosoltime seriesplottedi n verging flow from both hemispheresto a continuousarea of
Figure 2. The lowest CO and CO2 levels of the cruisewere uniform rising motion that acts as a barrier to cross-hemisphericmixing. More realistically, the ITCZ is an area of disfoundin the SHmX regime(54.1 + 4.0 ppbv and 358 + 0.2
ppmv). This is expectedfrom the remotemarineback trajecto- crete cloud clustersseparatedby regionsof relatively clear sky
ries and is consistentwith recentdata from Cape Grim, Austra- and little convergence. During 3 days in this regime, back tralia [Novelli et al., 1994].
jectoriesshow cross-ITCZ flow (e.g. Figure 3b).
Elevated CO is found between J.D. 91 and J.D. 94 (22øS and
The ITCZ wasencountered
at 6øSwhereCO and CO2increase
18øS).A localmaximum
in theNOyandaerosol
concentrationssharply and mark the start of the third regime on J.D. 102.
is also evidentduring this period. Emissionsfrom Madagascar Showinga gain of more than 50% over the SHmE regime, the
Clear indications of the four distinct meteorological re-
RHOADSET AL.:TROPOSPHERE
OVERTHEINDIANOCEAN
18,987
averageCO in the NHmE regimeis 89.0 + 3.3 ppbv, while the
CO2 climbedfrom 4 ppmvto 362 ppmv. The crossingtransition is also clearly observed in the aerosol concentrations.
Theaverage
levelsof NO3',NSS8042',andNH4*arenearly4
times higher than in the southernhemisphere. The back trajectories for this period are contained entirely within the
northernIndian Oceanand are quiteshortbecauseof very weak
winds.
The transitioninto the fourth regimeoccurredduringthe 3
hoursafterthe passageof a squallline on the morning of J.D.
107. During this period, CO increasedfrom 92 ppbv to 122
ppbv concurrentlywith dramaticrises in the CO2 and aerosol
concentrations.Filters collectedduring this episode were dark
DirectImpac
grey in color and showedsignificantquantitiesof mineral dust,
suggesting a direct continental impact on the air mass. The
back trajectory (1000 m)analyses indicate that the influence
of the continentwas seenafter 2-6 days over the Indian Ocean
at a distanceof over 1500 km. AVHRR data show high AOT
valuesover the northernregionsof the Bay of Bengal;the gradientssuggestthat the sourceslie mostly in India [Husar et al.,
Indi
mpact
1997]. The trajectoriesobtainedin this work suggestthat
someof the aerosols(andgases)couldhavebeentransported
Clean
Air
acrossIndia from this region. In addition, AOT distributions
also suggestthat thereare substantialsourcesin Bangladesh
and northernBurma,possibly causedby biomassburning
whichpeaksduringMarch throughMay [Hao and Liu, 1994].
The relativemagnitudeof the abruptchangesobservedduring the transition from the NHmEto the NHcT regimereflect
SubtropicalHigh
H
the chemicalsignatureof the emissions from India and can be
Figure 3a.
Counterclockwisecirculation aroundcyclonic
disturbancesalong the ITCZ that has been proposed as a
mechanismfor interhemisphericmixing within the lower troposphere. It appearsas air from the northern hemisphere
sweepsaroundto the east of numerouseddiesthat are present
nearthe ITCZ and mixes with cleanerair approachingfrom the
south (T. N. Krishnamurti, personal communication, 1995).
The consequences
of this mixing are visible in the trace gas
and aerosoldata collectedin the SHINE regime.
usedto characterize
the plumeandestimatethe degreeof photochemicalaging. The backtrajectoriesfor the NHcT period
show2-6 day transporttimes at 1000 m altitude, approximatelythetop of theboundarylayer. The lifetimesof CO2 (15 years)and CO (~ 30 days)[Khaliland Rasmussen,1990] are
long comparedto the travel times, and we can use ratios of
changesin concentrationsto account for dilution in the manner of Crutzen et al. [1979]. We assumethat in India the ratio
of anthropogenic
emissions
of CO, NOx,and SO2to emissions
April7 (J.D.97) BackTrajectory
:2kin
lkm
250
750
Figure 3b. Ten daybacktrajectories
at 1000and2000 m endingat J.D. 97 showan air parceloriginatingin
thenorthernhemisphere
andcrossing
the ITCZ (6øS). The lowersectionshowsthe verticalpath of the trajectories as a functionof longitudeand pressure.
18,988
RHOADS
ETAL.:TROPOSPHERE
OVER
THEINDIANOCEAN
Table
2.
Emission
Ratios
Species
U.S. emissions1994a
U.S. emissions1960a
U.S. molarratios 19947 %
U.S. molarratios 19607 %
CO2
CO
NO3'(N)
SO4
2-(S)
5150
2900
89
100
2.7
5.4
6.5b
3.9b
0.40
0.42
9.6c
10c
0.26
0.47
10
0.11
0.03
1700
30
0.43
7.8
1.43
18
0.44
8.7
1.47
17
Biomass
burningratios? %
Observedconcentration
change,ppbv
U.S. molar ratios 1960,f %
Plumeratios,f %
FractionRemaining,
g%
Emissions
in teragrams
(Tg= 1012
g).
Tg nitrogen.
Tg sulfur.
Moles of tracespeciesper moleCO2.
Biomass ratios from Crutzenand Andreae [1990].
Molesof tracespecies
permoleof CO.
Plumeratio dividedby U.S. molarratio 1960
of CO2aresimilarto thoseof theU.S. priorextensive
controls
(Table2). For example,in 1994, U.S. emissionsof CO were
2.7% of CO2,butin 1960 they were5.4%. In India, wood,
dung,andcropresidueareimportantfuels[CrutzenandAndreae,1990],butthe NOx/COandSOx/COemissions
ratiosare
lower than emission ratios from fossil fuel combustion.
Biomassburningresultsin muchhigheremissions
of nitrogen
ductionsin these reactive species;both nitrate and sulfate are
reduced to about 18% of their calculated
initial concentrations.
This is equivalentto almosttwo e-foldinglifetimesfor a travel
time of 2-6 days. A lifetime of 1-3 days seemsreasonablefor
conversionof NO• to nitrateand SO2to sulfateand subsequent
removal in the absenceof precipitationwhen depositionto the
ocean is the main sink.
oxides than sulfur oxides; the observedconcentrations(Table
2) suggestthat suchfiresdid not contributesignificantlyto
the pollutantsobservedin this study.
The 1960CO2emissions
for NorthAmericais estimated
to
be 2900 teragrams(Tg)[EnvironmentalProtectionAgency,
1995]. The ratio of molesof CO to molesof CO2emittedis
calculatedas the ratio of the teragramsemitted multiplied by
the inverseratio of the gases'molecularmasswhich gives
Surface
Ozone
Up to J.D. 107, surface03 showsa nearly continuoustrend
of slowly decreasingconcentrationwith time (Figure4). Superimposed
on this trendis a diurnalcycleof ozonedestruction
that starts at sunrise, continues until midafternoon, and then is
replenished
duringthe night.In the SHmXregimethe average
03 is 17.0+ 1.8 ppbvanddropsto 11.5 + 2.5 ppbv in SHmE.
5.4%. Similarly,the molarratiosfor the differences
between The lowestlevels of 03 foundon the cruisewerein the NHmE
the NHmE air andNHcT air werefound. The changein the CO2
regime(7.5 + 1.2 ppbv). In the same3 hoursof J.D. 107 that
concentrationbetweenthe two regimesis about 1700 ppbv.
had large CO increases,hourly averagedozonealso increased
Thechange
in CO concentration
is about30 ppbv. Sincethe from 10.4 ppbv to 15.2 ppbv. In the NHcT region ozone inconcentrations
are expressed
as volumetricmixing ratios,the creasedto 15.7 + 2.9 ppbv as continental influence was deratio of the differencescan be found directly giving 1.8%. The
tected. Figure 5a showsa scatterplotof hourly averagedCO
observed
plumeratiois muchlessthanthe emissions
ratio(versus03 for the wholecruiseseparated
by region. In the first
5.4%) andfar lowerthancanbe accounted
for by OH attackin three regimes there is no discernible correlation between
the shorttime indicatedby the back trajectoryanalysis. This ozoneand CO. In the continentallyimpactedarea,NHcT, there
suggests
thatnoncombustion
sources
of CO2areimportant.In is positivecorrelationsuggestinga commonsourceof the CO
the dryseason,Indiansoilsemitsubstantial
amounts
of CO2 andozoneprecursors
but the correlationcoefficientis statisti(Raich and Potter, 1995). Our observationsare consistent cally insignificant(r = 0.53) for a few degreesof freedom.
with 3 timesasmuchCO2beingemittedby soil andplant res-
Significantdestructionof ozonewas seenin the diurnalcy-
pirationasby combustion.
Theabruptchange
in COcanpri- cle for eachregime with the lowest middayminima occurring
marilybe attributed
to anthropogenic
emissions,andcalcu- in the two equatorialregions. This cycle is clearly visible in
lation of the ratios for nitrate and sulfate relative to CO prothe hour-averaged
data(Figure4), particularlyin the equatorial
vides a frameworkin which to comparethe plume concentraregions. Similar daily oscillations in the ozone mixing ratio
tions with the emission inventories. The fraction calculated
were observedby Johnson et al. [ 1990] in the westernIndian
fromtheplumeratio dividedby the NorthAmerican
emission Oceanduringthe 1987 SAGA experiment.Near the endof June
ratio,
they observeda diurnalvariationof 4 ppbvfrom 30øS to 10øS,
( A[X]/A[CO]
)Observed (2) wherethe ozone mixing ratio was slightly over 20 ppbv. In
the presentdataset, lossesof 37% and 30% of the predawn
maximum ozone occurredin the SHmE and NHmE regimes, re-
fraction
remaining
=( /X[X]//X[CO]
)Emission
gives an estimateof materialremainingin the plumeat the
time of sampling.The ratio of changesin nitrateandsulfateto
changesin carbonmonoxide(Table2) showssubstantialre-
spectively. The lowestlevels of 03 werefoundin the NHmE
regime; ozone peakedat sunriseandreacheda minimumjust
after solarnoon. This strong diurnalvariation may be caused
RHOADS ET AL.: TROPOSPHE• OVER THE INDIAN OCEAN
Isi•mEI••.qNHmENHCTI•
_< IsH.
mXI
25
18,989
20
[O•]
ppbv
15
10
82
87
92
97
102
107
112
Julian Day
Figure 4. Ozone (partsper billion by volume) time seriesfor the cruise. Data have been averagedinto 1 hour
periods. Note the strongdiurnalvariationin the SHmEandNHmE regimesfrom J.D. 94 to 107 and the rapid
rise due to photochemicalozoneproductionenteringthe NHcT regime on J.D. 107.
25
&
&
20
[O3]
&&
15
ppbv
+&
10
o SHmX
i
5
i
i.
I
!
i
i
,If••iI •••,
I
80
x+
,•
i
SHINE
NHmE
NHcT
i
120
140
160
[CO] ppbv
25
o
x
+
•
SHmX
SHmE
NHmE
NHcT
b
20
o "o ß o
ppbv15
x
••,
, 60 ,,
_
ß
• • o
•++x
110
*, •,•
x
T,,,
160
,
210
260
[NO
x] pptv
•i•re 5. Scatterplots
of houraveraged
datapairsof (a) CO .d O• .d (b) •O, .d O•. •e contributions
fromthefourmeteorological
regimesarerepresented
by differentsymbols.Ozonea•d •O• maximawereobservedi• the most southernand northernregionssampled. I• the SH•
of ozoneswererecorded
withrelativelyhigh•O, a•d lo. CO .ixi•
regime (ope• circles), modestlevels
ratiosi•dicati•g a re•io• of generals•b-
siae•ce of free troposphericair. At the •orthem e•d of the cruisei• the •cT
regime (solid triangles), ozone
.as positively
correlated
withNO, (r = 0.72,slope= 0.042),i•dicati•ga regio• of photoche.icalozoneproduction.
18,990
RHOADSET AL.:TROPOSPHE•OVERTIlE INDIANOCEAN
will be presentedseparately.
tainedduringthe four Mauna Loa ObservatoryPhotochemistry
(MLOPEX) 2 intensives
andMLOPEX 1. During theseexperi-
NO, NO:, and NOy
by correlationcoefficientsin the range of 0.7-0.8 and slopes
ranging from 0.06-0.14 ppbv/pptv [Brasseuret al., 1996].
by reactionsinvolving halogens[e.g.Vogt et al., 1996] and
mentsthe relationshipbetween03 andNOywascharacterized
NO•/O3ratiosfor eachregimerangefrom
The gas-phase
reactivenitrogen
data(Figure6c), NOy,show The averageobserved
a latitudinalprofile similar to surfaceozone (Figure4). The
about 10 to 14 (pptv/ppbv). Thesevaluesare on the upperend
risein NOyconcentration
around
J.D.92 (Figure6c)is believed of ratiosobservedduringthe MLOPEX projects. Murphy et al.
to be the resulto• local emissionsfrom nearbyislands. As
with surface03, theregimemaximaof 165 _+49 and178 _+71
pptv occurin the SHmXandNHcT,respectively.Figure5b
displaysa scatterplotof hourlyaveraged
NOr plottedagainst
03. DatafromtheSHmX (r = 0.21, slope= 0.014) andNHcT(r
= 0.72, slope = 0.042) regionsshow a positive correlation;
however, only the correlation in the NHcT regime is significant. This relationship is consistentwith measurements
oh-
[1993] found that this ratio rangedfrom 5 to 25 (pptv/ppbv)in
the tropical upper tropospherenear Darwin, Australia. Viewed
in the context of the low aerosol loading and the low concen-
trations
of CO, thecoincident
highlevelsof NOyandozonein
the SHmX region suggestthat downwardtransport from the
upper troposphereor stratospheremay be the primary source
of the air mass. The climatology of the region supportsthis
conclusion,since this period is the rainy season in the south-
NO Data
[NO] 4o
pptv 30
20
10
0
JulianDay
lNO2]80
NO2nl•ata
or
/X _.
pptv/1
20
0
82
86
90
94
98
102
106
110
JulianDay
[NOy ]
pptv
8•' ' '8•' ' '9•)' ' '9•' ' '9•' ' '1(•2'' '1•6' ' "li0'
JulianDay
Figure6. (a) NO, (b) NO2,and(c) NOyminute-averaged
mixingratiodistributions
for eachdayof the cruise.
The box plotswithin eachfigureshowdatadistribution;
bottom(25%), middle(50%), andtop (75%) horizontal box lines indicatethe quartiles. The horizontallines at the bottomand top of the vertical whiskersemanating from eachbox indicatethe 10% and90% percentiles.Data pointsbeyondthe 10% and 90% percentilesare
displayedas dots. Aerosolnitratedata(equivalentpptv/10)are shownin Figure6b for comparison.
RHOADS ET AL.: TROPOSPHEREOVER TIlE INDIAN OCE•
ern hemisphere
and lightningassociated
with deepconvective
activityis common.In contrast,the increasein the NOvmixing ratio in the NHcT regimewas accompanied
by increased
levelsof ozone,CO andaerosols.Thusthe NOyincreasein
thisregioncanbe attributedto emissionsfrom India followed
by photochemistry
in the air massduringpassageoverthe In-
18,991
Minute [NO] J.D. 82-111.7
42
28
14
120
8O
4O
dian Ocean.
120
8O
[NO] J.D. 82-111.7
4O
1o
120
8
4O
8O
.......•
.............................
NHcT •
IIIIII
-8
6
0
8
16
24
32
40
48
[NO] bin- 2 pptv
[NOl
pptv4
2
Minute
[NO2]
J.D.
101.8-111.7
b
0
100 .........
•
.....................................................
NHmE
5oE• :i
-2
'U'•':i
'""t'!
!•"i"•!'l
i i I i i •l
i
0
4
8
12
16
20
24
Time (UTC)
!
I
!
60
90
E
...::::.:::1:-:i-.:1::::....111..
...............................................
--==•
:::.::--•.':::1:::::
...............................
.....
:1:
1:::-.'11.'::1.::
NlteT
1::::::.::::1
l
3o,[i........
,'•i............
[NO2
] J.D.101.8-111.7
35
I
0
10
: b
20
"•"111
30
40
50
60
70
[NO2]bin- 2.5pptv
30
25
Minute
[NOy]
J.D.82-111.7
90
[NO2]
60
pptv 20
30
120
80
15
40
210
10
140
0
4
8
12
16
20
24
Time (UTC)
70
75
50
25
[NOy]
J.D.82-111.7
40
160
80
120
160
200
240
280
320
360
[NOy]
bin-8 pptv
150
Figure 8. (a) NO, (b) NO2,and(c) NOvminuteaveraged
data
[NOy]
140
displayedas histograms. Bars for eachmixing ratio interval
indicate the total number of observations
pptv 130
and are divided to
show the contribution from each region.
ll0
100
':':"'
.,NOyl
, .....
,,,,,
!20
•"'"'}
.....
i:;'i'iø"i
i"'•;'I'•:
..••11.,•i
,.!i•7!,,.17
'
0
4
8
12
16
20
24
Time (UTC)
Figure7. (a) NO, (b) NO2,and(c) NOyaveragediurnalvariations calculated from the median values of minute data for each
hour.Adjustments
weremadeto observationtimes so that for
eachday sunrise,solarnoon, andsunsetwouldoccurat 0230,
0930, and ]430 UTC, respectively (indicatedon plots as SR,
sunriseand SS, sunset). Error bars shown indicate 95% confidence intervals.
The daytime NO mixing ratios shown in Figure 6a do not
show clear distinctions among the four regimes. The mean
valuesfor eachregime shownin Table 1 were calculatedby averaging observationsmadeeach day betweensunriseand sunset. From the total of 11,894 ten second NO measurements,
6919 daytime values were usedto calculate 1952 one minute
averageswhich were combinedto generatestatistics for each
day andregion. Taken as a single population, the minute averagesgive a NO concentration
of 6.78 + 0.4 ppt (95% confidencelevel in the mean) for the Indian Ocean. The highest NO
concentrationswere encounteredon J.D. 83 shortly after de-
18,992
RHOADS ET AL.: TROPOSPHEREOVER THE INDIAN OCEAN
parturefrom Durban. As the ship moved away from the continent to the southeast,the NO levels fell off sharply. Back trajectoriesfor this period (J.D. 84-86) show that the air encountered passedto the south of Africa and originated from the remote southernAtlantic Ocean. On J.D. 86 the ship turnedto
the northeastand the impact of the continental outflow on the
NO concentrationscan once again be seen in the data over the
courseof the following few days. This period coincides with
the transition from winds originating in the southernAtlantic
to flow from the easternIndian Ocean. By J.D. 95, the midday
NO mixing ratio decreasedto 5 pptv and little variation in the
daily averageswas observedduringthe remainderof the trip
exceptfor a slight increaseobservedaroundthe ITCZ.
The diurnal variations in the median reactive nitrogen mixing ratios are shown in Figure 7. The mediansfor each hour
were determinedby separatingthe minute data for each gas into
24 one hour bins. Before binning, the time stamp associated
with eachaveragewas normalizedto a longitude of 55øE such
that daylight hours are synchronous. At this latitude, sunrise
occurred at about 0230
UTC
and sunset at 1430
UTC.
The
magnitudeof the diurnal variationof NO is similar to observationsmadein the remotePacific [e.g., McFarland et al., 1979;
Torres and Thompson,1993] and in the North Atlantic for marine air masses[Dickerson et al., 1995]. The daily cycle of
NOywith the sumof NO andNO2(NOx)subtracted
(Figure7c)
confirm the assertionthat this region was dominatedby a mixture of air masseswith vastlydifferenthistories.The NO2 plot
(Figure8b)clearly showstwo distinct populations associated
with the NHmE and NHcT regimes. As a crudecheck of the
consistencybetweenthe NO•, ozone and NO2 photolysis rate
measurements,the steadystate concentrationof NO2was calculatedfor eachregimeusingan equationderivedfrom the three
step Leighton photostationary state mechanism[Leighton,
1961]. The observedNO2 valuesexceedthe calculatedconcentrationsby about 30 pptv confirmingthe uncertaintyestimate
of +20 pptv in the NO2observations(as notedin the Experimental Techniquessection).
NO2 Photolysis Rate and Aerosol Optical Depth
Therewasa significantreductionin the measured
peak NO2
photolysis rate in the NHcT region relative to the other regimes. Figure 9 showsthe photolysisrateson J.D. 96 (SHmE)
andJ.D. 108 (NHcT),two of the most cloud-freedaysduring
the cruise. During the 2 hourssurroundingsolar noon the av-
eragej(NO:) for J.D. 96 was8.4 x 10'3s'l, whilefor J.D. 108 i t
was7.5x 10'3s-1,a difference
of 11%. Asnotedin thehourly
decklog, cloudcoverwasonly oneeighthto onequarterof the
sky duringboth periods. The Sunwas morenearlyoverheadon
J.D. 108 (minimum solarzenith angle of 2.0ø) than on J.D. 96
showsan increase in the late morning hours with a maximum
in the afternoonfollowed by a decline that reachesa minimum
(minimum zenith angle of 19.4ø). Table 3 showsthe results of
the aerosoloptical thickness(AOT) measurementsand aerosol
aboutan hour after sunset.The rise in the morningmay be ex- loadingsthat were collectedduringthe morninghoursof these
plainedby the increasein the conversionof NOx to other com- 2 days. The AOT valuesobservedon J.D. 96 of •- 0.1 (500 nm)
ponentsof NOywith increasingsolarradianceandthe growth comparewell with backgroundvalues determinedin California
of the boundarylayer promotingthe downwardmixing of pho- underclear sky conditions [Kaufmanet al., 1994]. The AOT
tochemically processedair from aloft. In the afternoon, sur- measurements
collectedon J.D. 108 are systematicallyhigher
face depositiondominatesproduction,leading to the observed as a result of the increasedaerosolloading in the outflow from
India.
decrease
in NOr
The filters
removed from the aerosol collector
Histogramsof the nitrogendata shown in Figure 8 illustrate
the distribution of the minute averages in the four regimes.
on the
morningand eveningof J.D. 108 were visibly blackened. Resultsfrom observations
and calculationsof AOT andj(NO2) in
Each bar shows the total number of values within the concenMarylandsuggestthat sucha reductionat low zenith anglesis
tration range of the bin and is broken down by regime. The the resultof a greaterdegreeof absorption(lower single-scatwidedistributions
of theNO andNOydatafor the SHmXregime tering albedo)in the aerosolcollectedin the plume from India
O.OO8
0.006
j (NO2)
S
-1
0.004
0.002
o.ooo
0
4
8
12
16
Time (UTC)
Figure 9. Comparisonof NO2 photolysisratesfor J.D. 96 and 108 measuredwith an upwardlooking chemical actinometer. The observedaerosoloptical depthat 500 nm was 0.13 on J.D. 96 and 0.26 on J.D. 108. The
11% reductionin NO2 photolysisand the two fold increasein optical depth are believed to be the result of increasedloading of aerosolwith a high imaginary index of refractionin the outflow from India.
RHOADS ET AL.: TROPOSPHEREOVER THE INDIAN OCEAN
18,993
Table 3. Aerosol optical thickness(AOT) measurements
J.D. 96
J.D. 108
(J.D. 108 - J.D. 96)
AOT
at 500,
at 675,
at 875,
at 945,
nm
nm
nm
nm
0.129
0.099
0.065
0.654
+
+
+
+
0.001
0.004
0.003
0.007
0.262 +
0.173 +
0.100 +
0.993 +
0.011
0.003
0.004
0.029
0.133
0.074
0.035
0.339
NSS aerosols
NSSSO4
'2,gg m'3
NH4+,gg m'3
Mineraldust,Bgm'3
Total,Bgm'3
0.096
0.01
< 0.01
2.80
0.74
9.28
2.70
0.73
9.28
0.1
12.82
12.7
J.D., Julianday; NSS, non-sea-salt.
than is commonly present in aerosol of the eastern United
States[Kondraguntaet al., 1996].
An important questionin accessingthe scopeof the climate
impact of such emissions is whether the aerosol plume was
confined to the boundary layer or was carried by zonal flow
with the potential of influencingthe radiative propertiesof the
entire region? A crudeestimateof the thicknessof a uniformly
polluted layer can be madeusing the differencein the optical
depths (0.13 at 500 nm) and the change in the aerosol loadings. Assumingthat the extinctionwas due totally to non-seasalt particles, the differencein the total (ammonium, sulfate,
and mineral) aerosol loading (L) for the two time periods is
12.7 gg m'3. Using a genericaerosolmassscatteringefficiency(S)of 3 m2g-1[Waggoner
et al., 1981],thethickness
of
the layer (D) can be estimatedas,
D = (AOT) / (S x L),
(3)
2. Despite the increasedCO and aerosol concentrations
foundin the SHmE regime,back trajectoriesindicate that both
it and the SHmX regime sharethe remote Indian Ocean as the
immediatesourceregion. As the ITCZ wasapproached,back
trajectoriesshow northernhemisphereair circulatingto the
eastof localizeddisturbances
along the ITCZ andmixing with
southernhemisphereair. This cross-ITCZ flow is believed to
be the causeof the increasedlevels of pollutantsmeasured
in
this region.
3. Strong evidenceof direct transportof anthropogenic
emissionswas seen 1500 km from southernIndia. Sharpin-
creases
in CO, CO2,03, NOy,NSSsulfate,ammonium,
andnitrate aerosolsweredetected. A comparisonof the pollutant
levels observedin the plume from India with North American
emission inventories suggeststhat the air massessampled
were substantiallydepletedin solublespecies.
4. Total gaseousreactivenitrogen paralleledsurfaceozone
in its latitudinal profile with maxima in the most southernand
northernregionssampled. In the southernhemisphere
the cor-
which gives a layer of about 4.7 km in depth. Despite the
simplistic nature of the estimate, the result suggeststhat the
relationwith ozonemay be attributableto downward
transport
polluted airmass extended well above the marine boundary
from the stratosphere or lightning, while the increase oblayer which was typically observedat about 1-2 km in the
servednearthe coastof India can be accountedfor by convensonde data.
tional photochemistry. A strong diurnal variation in ozone
wasobservedin the equatorialregions of both hemispheres.
Conclusions
The low levels of NO encountered indicate that most of marine
boundarylayer over the Indian Oceanis an region of active
A number of atmospherictrace species were measuredover photochemicalozone destruction.
the Indian Ocean in order to assessthe potential for anthro5. The nitrogen dioxide photolysis rate coefficient measpogenic emissions to modify tropospheric chemistry across uredwith an upwardlooking chemical actinometershowedan
the ITCZ. Major findings are as follows.
11% reduction in an air mass encountered downwind of India
1. In transectingthe Indian Ocean from Durban, South Afwhen comparedto values obtainedfarther south during the
rica, to Colombo, Sri Lanka, four distinct meteorological re- cruise. The aerosol loadings in the region near India were 4
gimes were found in the springof 1995. Average CO increased timesgreaterthanin the moresouthernregime. A comparison
withlatitude
goingfrom33øSto 9øNandwascloselymirrored,of theoptical
depthmeasurements
forthese
regions
showed
an
by increasesin aerosol loading. Across the ITCZ at 6øS the
increaseof 0.13, suggestingthat the aerosol had a substantial
NSSSO4
2-concentration
increased
bya factorof 4. Thestrong absorptiveelement suchas mineral dust or soot.
latitudinal gradient in aerosolconcentrationsmeasuredon this
cruiseis consistentwith the areal distributionof aerosoloptical thickness (AOT)as measuredby AVHRR [Husar et al.,
1997] in the spring. AVHRR shows a strong gradient in AOT
with highest valuesover the ArabianSea, decreasingsharply
in the region 10øNto the equator;this is the region wherethe
ship experienceda shift in air mass trajectories from NHmE to
NHcT (Figure 1) and a sharp increasein the concentrationof
aerosols and CO.
Acknowledglnents.Thanksgo to the captainand crew of the R/V
MalcolmBaMrige;theyprovidedtransportation,
food,equipment,
help,
andhumorfor 33 long days. We are gratefulto R. Pinkerand F. Miskolczi for providingtheir assistance
in the opticaldepthmeasurements.
We thankRik Wanninkhofand Dave Ho of AOML for providingthe
CO2 datathat they collectedduringthe voyage. We extendthanksto T.
Kfishnamurti
for providinginsightintotheuniquemeteorology
of the Indian Ocean. We alsoextendour appreciation
to P. J. Crutzenfor his insightfulcomments
duringthepreparation
of thismanuscript.This work
18,994
RHOADSET AL.:TROPOSPHERE
OVERTHE INDIAN OCEAN
was fundedprimarily by the Center for Clouds,Chemistryand Climate,
which is sponsoredby the National Science Foundationand through
grantsNSF ATM9204047 andATM9414846.
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(ReceivedNovember 13, 1996; revisedApril 4, 1997;
acceptedApril 9, 1997.)