JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 101, NO. D19, PAGES 24,043-24,068, OCTOBER 30, 1996
Ozone and aerosol distributions
and air mass characteristics
over
the South Atlantic Basin during the burning season E. V. Browell, 1M. A. Fenn, 2C.F. Butler, 2W. B. Grant, 1M. B. Clayton, 2J.Fishman, 1 A. S.Bachmeier, 1,3B. E. Anderson, 1G. L. Gregory, 1H. E. Fuelberg, 4 J.D. Bradshaw, 5 S.T. Sandholm, 5D. R. Blake, 6B. G.Heikes, 7G.W. Sachse, 1H. B. Singh, 8and R. W. Talbot 9 Abstract. In situand laserremotemeasurements of gasesandaerosolsweremadewith airborne instrumentation to investigatethe sourcesandsinksof tropospheric gasesandaerosolsoverthe tropicalSouthAtlanticduringtheNASA GlobalTropospheric Experiment(GTE)/Transport and Atmospheric ChemistryNear theEquator--Atlantic(TRACE A) field experimentconducted in September-October 1992. Gasesfromextensive firesin Brazilweretransported by convective stormsintotheuppertroposphere wheretropospheric ozone(03) wasphotochemically produced andadvectedeastwardoverthe SouthAtlantic. In centralAfrica, thefireswerewidespread, and in theabsenceof deepconvection,the fire plumeswereadvectedat low altitudes(below--6 km) overtheAtlantic. Therewasa positivecorrelationbetween03 andaerosolsfoundin theplumes thatwerenot involvedin convection.High 03 (>75 partsperbillionby volume(ppbv))was observedin the low-altitudeplumes,andalsoin theuppertroposphere where03 oftenexceeded 100ppbvwith low aerosolloading.Theaveragetropospheric 03 distributions weredetermined for the following: Brazil and westernSouthAtlantic,easternandcentralSouthAtlantic,central andeastcoastof Africa, andtheentireSouthAtlanticBasin. The tropopause heightsand03 columnsacrossthetroposphere werecalculatedfor individualflightsandfor the average03 distributionsin theaboveregions.A maximumtropospheric 03 columnof 56 Dobsonunits(DU) wasfoundoverthe biomassburningregionin Zambiaandin the subsidence regionoverthe central SouthAtlantic. Thehigh03 regionovertheSouthAtlanticfrom4ø to 18øScorresponded with the latitudinal extent of the fires in Africa. In situ and laser remote measurements were used
to determinethefrequencyof observation andchemicalcomposition of ninemajorair masstypes. Biomassburningemissionscontributedto mostof the air massesobservedoverthe SouthAtlantic Basin,andbiomassburningwasfoundto contributeup to half (28 DU) of the03 columnacross thisregion.
Introduction
The Transport and Atmospheric Chemistry Near the Equator--Atlantic(TRACE A) field experimentwasconducted in 1992 to investigatethe causeof the significanttropospheric ozone (03) enhancementthat forms over the tropical South Atlantic Oceanin the late winter in the southernhemisphere [Fishmanet al., 1990;Fishman1991]. This 03 featurewasfirst
1Atmospheric Sciences Division, NASALangley Research Center, Hampton,Virginia.
2Science Applications International Corporation, Hampton, Virginia. 3NowatUniversity ofWisconsin--Madison. 4Florida State University, Tallahassee. 5Georgia Institute ofTechnology, Atlanta. 6University ofCalifornia, Irvine.
7University ofRhode Island, Narragansett. 8NASAAmesResearch Center, MoffettField.
9University ofNewHampshire, Durham. Copyright 1996bytheAmerican Geophysical Union. Papernumber95JD02536. 0148-0227/96/95JD-02536509.00
observedfrom satellitedata and subsequentlyconfirmedthrough a seriesof periodic ozonesondelaunchesfrom AscensionIsland between 1990 and 1992 [Fishman et al., 1992]. Several theories were suggestedfor the buildupof 0 3in thisregionat this time of the year. Fishmanet al. [1991] suggestedthat it could be due to the photochemicalproductionof 03 in the plumesfrom biomass burningin Africa, while Krishnamurtiet al. [ 1993] indicatedthat downwardtransportof 03 from the stratosphere couldalsohave a significantcontributionto the enhancement of tropospheric 0 3 in this region. Two of the major scientificobjectivesof the TRACE A field experimentwere as follows' (1) to studythe vertical and horizontal variability of 03 concentrationsacrossthe South Atlantic Basin
from
Brazil
to
southern
Africa
and
to relate
these
observationsto sourcesand sinksof tropospheric0 3, and (2) to investigatethe distributionof natural and anthropogenicaerosols over the same region and to relate these observationsto the ail•lt aii n'tasses over the tropical Atlantic. TRACE A was designed to integrate measurements from aircraft,balloons,and satellitesto investigate the origin of the large-scale,seasonaltropospheric03 maximum that has been observedover the tropical South Atlantic Ocean west of Africa [Fishman, 1994]. The NASA Ames Research Center'sDC-8 aircraft was the primary platform for in situ and laser remote measurementsduring TRACE A. It contained
24,043
24,044
BROWELL ET AL.: OZONE AND AEROSOL DISTRIBUTIONS
instrumentsfor a broadrange of in situmeasurements of chemical species and meteorological parameters and a lidar system for remotemeasurements of 0 3 and aerosoldistributionsaboveand below the aircraft [Fishmaneta/., this issue(a)]. The airborne lidar systemprovided the only meansof determining the large-scalevariability of 03 and aerosolsthroughout the troposphere. These measurementsare necessaryto examine the sourcesand sinks of 0 3 as they are related to different air masses that are observed over the South Atlantic.
Contributions
from photochemicalprocessingof anthropogenicand naturally occurring trace gases, stratosphere-troposphere exchange,and long-range transport must be examined across the entire troposphereto determine the large-scalemagnitudeof their impact on the tropospheric03 budget. The airborne lidar systemcan determinethe 03 and aerosoldistributionsin regionsof the tropospherethat are not accessibleto the in situ instrumentson the DC-8, and becauseatmosphericcrosssectionsof 03 andaerosols can be obtained very rapidly on one pass of the aircraft, large distancescan be coveredto determinethe extent and homogeneity of air massesobservedduringthe field experiment.In regions where the air massessampledby the in situ instrumentswere also observedremotely with the lidar system, the interpretationof these air mass types could be extendedto larger horizontaland vertical scalesfor estimatesof the impact of transportand photochemicalprocesseson the tropospheric0 3 budget[e.g., Browell et al., 1992; 1994; 1996]. This paperreportsthe resultsof large-scalestudiesof the distributionof 03 and aerosolsover the SouthAtlanticBasinusing primarily data from an airborne lidar system. These results, together with airborne in situ measurementsof 03; aerosols
[Anderson etal., thisissue];NOx andNOy[Heikes etal., this issue;Smythet al., this issue; Talbot et al., this issue];carbon monoxide (CO) (J. E. Collins etal., personalcommunication, 1996); methane(CH4) and carbondioxide (CO2) [Smythetal., this issue];peroxyacetylnitrate (PAN) [Singhetal., this issue]; nonmethanehydrocarbons (NMHC) [Blakeetal., this issue];hydrogen peroxide (H202)[Heikes et al., this issue]; and other species[Fishman etal., this issue(a)] and with the meteorological analysesof atmospherictransport[Bachmeierand Fuelberg, this issue;Fishmanet al., 1994;Fuelbergeta/., this issue(a, b); Pickeringetal., this issue(a, b); Thompsonetal., 1996] provide insightsinto factorsdeterminingthe wintertimetropospheric03 enhancement over the tropicalSouthAtlantic.
Experimental Techniques
1983; Browellet al., 1983, 1985a,1985b]. Comparisons between airborne DIAL and in situ 0 3 measurements were made throughoutTRACE A to constantlyverify that their agreement was
consistent
with
the
above
stated
limits.
The
aerosol
backscattermeasurements were derivedfrom the range-corrected lidar signal at the 1064-nm laser wavelength,and the resolution of thesemeasurementswas defined by the vertical averaging intervalof 60 m andthe horizontalaveragingintervalof 1.75 s or about 408 m. The accuracy and precision of the aerosol measurements
than 1%.
with low aerosol extinction
Detailed
characteristics
at 1064 nm were better
of the current airborne DIAL
systemandthe 03 DIAL techniqueare givenby Browell[ 1989]. In addition to the airborne DIAL system,the DC-8 had instrumentationfor in situ measurements of trace gases,aerosols, and meteorologicalparameterssuchas temperature,dew point, and winds. A general description of these systemsand their measurementsis given by Fishman et al. [this issue (a)], and moredetailsareprovidedin companionpapersin this issue. Data Results
and Discussion
Fifteenflights/missions were conductedaspart of the TRACE A field experiment in the southern hemisphere between September22 and October 24, 1992. A list of the TRACE A flights, the flight objectives,and flight specific informationis given in the overviewpaperby Fishmanet al. [thisissue(a)], and the groundtracksfor theseflightsare shownin Figure1. During this field experiment, five flights were conductedover the westernAtlantic/easternBrazil, three flights were over southern Africa, six flights were over the central/eastern tropicalAtlantic, and one flight was across the South Atlantic at midlatitudes. Basesof operationfor multiple flights includedBrasilia,Brazil; Johannesburg, SouthAfrica; Windhoek,Namibia; andAscension Island. Data were collectedduringmore than 120 hoursover the 15 flights. Ozone and aerosoldistributionswere measuredremotely with the DIAL systemon all flights during TRACE A. Thesemeasurements providednearlycompletealtitudecoverage of 0 3 and aerosol distributions from near the surface to the tropopauseregion alongthe aircraftflight track. This sectiondiscusses examples of 03 and aerosol distributions observed over/nearBrazil, southernAfrica, and the tropical SouthAtlantic duringthe early australspringof 1992; the averagetropospheric 0 3 distributionsobservedacrossthe SouthAtlantic Basin during TRACE A; the different air masstypesthat were observedwith the DIAL system; the extent of these observations;and the chemicalcharacteristics of the major air masstypes.
An airborneDifferentialAbsorption Lidar(DIAL) systemwas usedto provideverticalprofilesof O 3andaerosols fromnearthe surfaceto abovethetropopause alongtheflighttrackof theDC-8 Aerosol and Ozone Distributions in the Southern aircraft.Simultaneous zenithandnadirlidarmeasurements of 03 Hemisphere and aerosolswere madefrom a rangeof about750 m abovethe aircraftto abovethetropopause in the zenithcaseandfrom about Tropical westernAtlantic Ocean.The surveyflightfromRe750 m below the aircraft to about 300 m above the surface in the
nadircase.The DIAL 0 3 measurements weremadeusinganonline wavelengthat 288.2 nm and an off-line wavelengthat 299.5 nm, and the aerosol backscatter measurementswere made
at laser wavelengthsof 1064, -600, and -300 nm. An 03 measurement accuracyof betterthan10% or 2 partsperbillionby volume(ppbv),whicheveris larger,with a verticalresolutionof
cife to Brasilia, Brazil, on September24, 1992 (TRACE A Flight5), revealedgeneralcharacteristics of the 0 3 and aerosol distributionsobservedoff the southeast coastof Brazil during TRACE A. Plate 1 showsthe aerosoland 03 distributions observed with the DIAL
above and below the DC-8
over this
region. On the northernpart of this surveyflight (left side of nadirdisplays)the amountof aerosolloading,asindicatedby the 300 m in the nadir and 450 m in the zenith and a horizontal relative aerosolscatteringlevels, was very low in the altitude resolutionin bothcasesof about70 km (given5-rain averaging range from about 2 to 9 kin. The boundarylayer (BL) had a time and assumingan aircraft speedof about 233 m/s) was depthof 0.8 to 1.0kin, andin thefreetroposphere (FT) abovethe obtainedwith a precisionof betterthan5% or 1 ppbv [Browell, BL, therewere significantaerosollayersup to -2 kin. In the
BROWELLET AL.: OZONEAND AEROSOLDISTRIBUTIONS
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region of low aerosolloading , or "clean" air, in the FT from about2 to 5 km, the 0 3 levels were generallyabove40 ppbv; however,above -5 km there were two different 0 3 conditions. Ozonelevels were found to exceed60 ppbv in the regionfrom about19ø to 24øS,and03 wasgenerallybelow40 ppbvfrom 24ø
abovethe tropopausewhich was at-16 kin. These aerosolswere
to 27øS. The air flow in the middle FT between 10 ø to 24øS came
1440and1508UT appeared to be associated witha stratospheric intrusionsincethe featurehad 03 exceeding150 ppbv and it appeared to have continuity to the tropopause. In situ
from Brazil [Bachmeierand Fuelberg,thisissue]wherebiomass burningwas occurring[Kirchhoffetal., thisissue]. The resultsof a 5-day, isentropicback trajectory analysisperformed for a clusterof points at the DC-8 altitude (11.5 km) near 20øS are shown in Figure 2 (see Bachmeierand Fuelberg [this issue], Fuelbergetal. [thisissue(b)], andPickeringetal. [thisissue(a)] for a discussion of thevarioustrajectoryanalysismethodsusedin the interpretationof TRACE A data). The back trajectories indicate that the air sampled by the DC-8 in the FT with 0 3 _• 65 ppbv and CO •_ 120 ppbv had possibly come from westernor southernBrazil. The low aerosolloading associated with thisair is thoughtto bedueto washoutof theaerosols during vertical convective transport [Gatz, 1977] of plumes from biomassburningthatcanphotochemically produce0 3 [Pickering etal., 1989; 1990; 1991; 1992; this issue(a)]. The region of lower 03 south of 24øS at the DC-8 altitude and in the 4.5- to 6-km altituderange (Plate 1) was associatedwith rapid air flow acrosssouthernSouthAmericafrom the SouthPacific(Figure2). The air masswithenhanced aerosols andenhanced 0 3 (?_50 ppbv) observedbelow 6 km and southof-26øS appearedto be coming from direct advectionof biomassburningplumesfrom southern Brazil or Argentina. The zenith lidar data (Plate 1) indicate that there were some thin aerosol layers above the aircraft (10.5-14 km). A cirrus cloud can also be seenon the right side of the display at 12 to 13 km. Sulfuricacid aerosolsin the lower stratosphere resulting from the Mount Pinatuboeruptionin June 1991 were observed
still foundin high concentrations in the tropicalstratosphere in September 1992dueto theinhibitionof transport outof thetropics by stratospheric dynamics[Grant et al., 1996]. The enhanced 03 (> 100 ppbv)in the uppertroposphere observed betweenabout
measurementsof 0 3, CO, and dew point temperatures(DPT) indicatedthat betweenabout1310 and 1345UT (-15 ø to 19øS)
the DC-8 flew through a broad region of stratospherically influencedair with 03>75 ppbv,CO27øS)the flow is predominantly
319K
offshorewith the air comingfrom the interiorof Brazil. The zenith 0 3 data obtainedby the DIAL were used to esti• ..• ..... ',, \ i [ •, • • ; "'¾... mate the tropopauseheight along the DC-8 flight track. The ,., .... •........ ........ ;................. ................ '"',.,',. "' ' ......... i .... j ! •,.......... ,, •. .: tropopauseheight was estimated to be at the altitude where a i i .• .....•.... 03 profile at .... \ •. • >.. ...........:. ........................ .. ! i ...... straightline passingthroughthe lower stratospheric 300 and 150 ppbv would intersectwith the averageupper tropospheric0 3 level defined to be the average 03 value in the altitude region 1 to 3 km below the 100 ppbv level. The tro..,--' ' popausealtitudewas limited to be at leastashigh as the 100 ppbv l \ >.-' ', :: t, t iI r/f/' I ' ' 03 level. After determiningthe tropopause height,the 03 column Figure 2. Isentropic back trajectoriesfor a cluster of end could be determinedbetweenthe groundand the tropopauseand pointsnearDC-8 flight trackat a potentialtemperature (PT) of between other altitude levels acrossthe troposphereusing the 342 K (-230 hPa or -11.5 km) near 20øS and PT of 319 K tropospheric0 3 distributions discussedabove (e.g., Plate 1). (-475 hPa or -6.3 km) near 28øS. Figure 4 showsthe tropopauseheights and 03 columnsfrom groundto 6 km, 6 km to tropopause,and groundto tropopause. The stratospheric intrusionindicatedby the enhanced03 in the Theseratios were then usedto piecewiselinearly extrapolatethe DIAL data from the lowest altitude DIAL measurement to the upper troposphere (Plate 1) can also be seen in the lower tropopauseheight of-10.5 km in the vicinity of the intrusion surface. To avoid unrepresentative03 distributions in the vicinity of airports,no takeoff or landingin situ 03 measurements (-1450 UT) comparedto the tropopauseheightof-16 km away were usedin estimatingthe trend in the low-altitude 03 profile. from the intrusion. The total tropospheric03 columnwas 36 DU outsidethe intrusion(near 1415 UT) with nearlyhalf of the 0 3 Whenno representative spiralsor rampswereavailablefor the03 (17 DU) below 6 km and the remainder(19 DU) from 6 km to the profile estimate,the averagelinear profiles from the TRACE A field experimentwere used. The average03 ratioswere foundto tropopause(-16 km). In the vicinity of the intrusion,the total 03
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Figure 3. National Meteorological Center's (NMC) analysisof wind vectors and streamlinesat 700 hPa (-3.2 km) for 1200UT on September24, 1992. DC-8 flight trackis alsoshown.
24,048 4O
BROWELL ET AL.: OZONE AND AEROSOL DISTRIBUTIONS 2O
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Figure 4. Tropopauseheightsand 0 3 columnsfor Flight 5 on September24, 1992.
columnand0 3 columnfrom 6 km to tropopause decreased to 26 and9 DU, respectively, dueto thelowertropopause height. The low 0 3 region centeredaround16øS(Plate 1) that resultedfrom theonshore flow fromtheAtlantichadan03 columnbelow6 km of only 13 DU. The highest0 3 columnin the lowertroposphere (below 6 km) found on this flight was 21.5 DU on the most southernpart of the flight track (-28øS) where the flow was offshore.
Biomassburning over Brazil. The sourceregion for the biomassburningin centralBrazil was investigated on Flight 6. Figure 5. Isentropic back trajectoriesfor a cluster of end pointsnearDC-8 flight trackon Flight6 at 346 K (-200 hPaor -12.4 km) and309 K (-800 hPaor -2.0 km).
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Plate 2. Aerosol and 03 distributionsassociated with biomass burningovercentralBrazil on September 27, 1992 (Flight6).
The nadir aerosoland 03 data from the lidar measurements for this mission are shown in Plate 2. On this flight the BL depth rangedfrom 2.7 to 4.0 km abovethe surfacewith many optically thick cloudsat the top of the BL. The aerosolloadingin the BL can be seento be greatestnear the fires on the westernpart of the flight track (left side of Plate 2). The air flow in the BL at a potential temperature(PT) level of 309 K (-2 km) was generally from the east as indicated by the trajectory analysis shown in Figure 5. The 0 3 levels in the BL were only moderate (40-60 ppbv); however, the CO was very high (>200 ppbv) indicatingthe generalpresenceof gasesfrom the biomassburning [see e.g., Crutzen et al., 1979; Andreae et al., 1988;Kirchhoff et al., 1989, 1990]. The relatively low amountof 0 3 in the BL could indicate that theseplumes are from nearby fires, and there has not been enoughtime for the photochemistryto substantially increasethe 0 3 levels. Above the B L there is also an indication of low aerosol loading up to about 7 km and then cleaner conditionsto above9 km. The 03 distributionshowssignificant 0 3 levels (>70 ppbv) above the BL. This again shows the negative correlation that is sometimesobservedbetween03 and aerosolsdue to the preferentiallossof aerosolsduringconvective cloud transportfrom the BL to the FT. The air flow changed from easterly to westerly from the BL to the DC-8 altitude of 12.4 km (PT of 346 K). A back trajectory analysisfor the air sampledby the DC-8 at this altitudeis shownin Figure 5. This slow-moving air was crossingthe most active biomassburning region in central and western Brazil, and it had time for
significantphotochemicalproductionof O3 in the FT. Kirchhoff
BROWELL ET AL.: OZONE AND AEROSOL DISTRIBUTIONS
Aerosol
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Plate 3. Zenith(top)andnadir(bottom)lidarmeasurements of aerosol(left) and0 3 (right)distributions on a flight from Brasiliato Rio de Janeiroon October1, 1992 (Flight8) (Note thedifferentcolorscalesusedfor nadir and zenith 0 3 data).
et al. [this issue] alsodiscussessomeof the characteristicsof the
and 1403-1408 UT). The averagenadir and zenith 0 3 profiles
foundfrom 25.0ø to 27.1øSare shownin Figure6. Includingthe 0 3 distributions observed overthecerradoduringTRACE A. on the DC-8, the 0 3 profile was nearly Clean southern hemispheric air. The lowest03 levels in situ 0 3 measurements acrossthe tropospherein the southernhemispherewere observed flat from 1 to 10 km with an averagevalue of 40 ppbv. From on October 1, 1992, on Flight 8 from Brasilia to Rio de Janeiro. 10 km to the tropopause(14.3 km), the 03 profile had a nearly Plate 3 showsthe nadir and zenith DIAL aerosoland 03 linear increasefrom about 40 to 140 ppbv. The tropospherein measurementson a portion of this flight. Enhancedaerosolsand
this region had the most homogeneousand lowest average 0 3
elevated0 3 (>50 ppby) below -5 km were observedonly over levels found in the southernhemisphereduring TRACE A, and Brazil on this flight. After passingthe coast of Brazil at the tropospheric0 3 column was the lowest at 28 DU. In situ on the DC-8 also showed low levels of CO 1350UT, the FT above2 km wasrelativelyfree of aerosolsand measurements had progressively lower 0 3. The westerlyto southwesterly air (60-70ppbv), CH4 (1675-1685 ppbv), and CO2 (354.2-355.0 flow into this region from the SouthPacific was similar to that
partsper million by volume (ppmv)) in this air mass.
Midlatitude observations. A midlatitude flight was made observed on Flight5 (Figure2). Below8 km theair appeared to havehadlittle continentalinfluencefrom SouthAmericaduring from Rio de Janeiro,Brazil, to Johannesburg,South Africa on October3, 1992 (Flight 9). This survey flight reached as far southas 38øS, and it traverseda deeptroughsystemthat induced a large-scale stratospheric intrusion. Plate 4 shows the tropospheric0 3 distribution for this flight. The center of the trough was at about-17øE (17øW), and the stratospheric clouds,is responsiblefor the over estimatein the aerosolscatter- intrusion can be readily seen in the enhanced0 3 distribution extending down in altitude to about 5.5 km and having a ing abovethe near-fieldclouds(1256-1310 UT, 1318-1325 UT, the transit acrossfrom the Pacific. Above the DC-8, there was an
abundance of cirrusover mostof the region,and the 03 had a gradualtransitionwith altitudeto the tropopauseat-14.5 km. Note that in the zenith aerosolscatteringplot, a detector-hang effect,whichwasdueto unusuallystrongnear-fieldscattering by
24,050
BROWELL ET AL.: OZONE AND AEROSOLDISTRIBUTIONS Zenith 18
.. •....
....
,
....
Ozone (ppbv)
.
17
.1o1:o,o 111:0.0 121:0.0 ? ß ß11:00 , .131:0 , , ß15:00 I_L
16 E
UT
.....
PT6
15
PT7
I
20-
I
PT8 PT9 I
I
PT1'
PT2'
PT3'
I
I
I
PT4'PTT'PT9' I
i
18'
14
16'
14.
13
12'
lO'
12
ß
e11
4'; 2-'
10
ß
....
. ....
0
ß ....
50
1 O0
,, ....
150
o
o
ß
250
20O
-31.78
3OO
'",'"
Ozone, ppbv
-27.86
-36.06
-37.80
-36.49
I"'I" -19.88
-.34.54
-31.43
N Lat
I::::'*' -9.57
1.76
12.94
23.03
E Lon
Plate 4. Ozone distribution between Rio de Janeiro, Brazil,
Nadir
andJohannesburg, SouthAfrica, on October3, 1992 (Flight9). 7
6
E
in thisanalysis.The centerof the troughfrom the European Center for Medium-RangeWeather Forecasting(ECMWF) analysiswascentered at about15øW,andthetropopause level
5 4
represented bya PVvalue ofabout 1.0x 10-4K m2kg-1s-1was
3
as low as 420 hPa near the center of the intrusion. The general
2 1
....
,
0
......
,
25
50
.
,
•
,
ß .
75
. lOO
Ozone, ppbv
Figure 6. Zenithandnadiraverage0 3 profilesfor 1340-1444 UT on October 1, 1992.
horizontal extent from about 6 ø to 25øW. The DC-8 entered the
morphologyof the intrusionfrom the ECMWF analysisagrees reasonablywell with the observed0 3 distributionfor PV> 0.8; however,the 0 3 distributionoutsideof the main intrusionregion (PV _100 ppbv) in the predominantwesterliesin the upper troposphere(10-15 km). There
does seem to be evidence
of an intrusion
at about 25øS
where the tropopausedecreasesin height acrossthe subtropical jet [Shapiroand Keyser,1988] from 16 km at 23øSto 12.5 km at 31øS(Figure16). The averagecolumn03 in the tropicsoverthis region rangedfrom 36 to 38 DU with about 53% (-19.5 DU) of the 03 columnbelow 6 km. This compareswith about28 DU for a clean0 3 profile of about40 ppbv acrossthe tropospherewith 59% (16.5 DU) of the03 columnbelow6 km. Over the central and easternAtlantic, the average03 distribution (Plate 8) showsthe impactof the flow from both Africa and Brazil in the mid to uppertroposphereand predominantlythe influence of Africa at the lower altitudes [Thompsonet al., this issue]. The influence of the clean SH air to the south of 20øS at low altitudescan be readily seen. Except for this clean air, the rest of the troposphereis dominatedby the transportand produc-
The averagelatitudinaland!ongimdina!03 disLxib,,tinn.• over the South Atlantic Basin are also presentedon the right side of Plate 8. The averagelatitudinal 03 distributiondeterminedfrom all the TRACE A flights shows significant levels of 03 throughoutthe tropospherein the SH. The NH air below about 5 km has similar 03 levels as the midlatitudeSH air. The 0 3 profile within the region from 25øS to 2øN generallyincreases with altitude from the 40 to 50 ppbv range near the surfaceto greaterthan 100 ppbv belowthe tropopause.Figure 17 showsthe average tropopausealtitudes and 03 columns as a function of latitude. The tropopauseheight varies from about 17.3 km near the equatorto about15 km at 32øS. The maximumaverage03 columnwas 52 DU at 9øS. The 03 columnexceeded45 DU over the range from 18ø to 4øS, which is consistentwith the latitude rangeof the biomassburningover Africa, and over 46% of the 03 columnwas below 6 lcm. This high SH 03 columncomparesto a NH 03 columnof 27 DU at 15øN and a minimum SH 03 column of 36 DU at 29øS. The lowesttropospheric 0 3 columnobserved during TRACE A was on Flight 8 when the level reached28 DU at 27øS. Below 6 km, the NH 03 columnof 11 DU at 15øN was also lower than the SH minimum
of 14 DU at 38øS.
30
15 ++
tion of 03 from the biomassburningregionsin Africa andBrazil. Figure 16 showsthe tropopausevariationover thisregionand the a• 20 +++++ +++ + + 03 column distributions. The tropopausedecreasesslowly from oN 17.2km nearthe equatorto 15.3km at 34øS. The rapiddecrease o in the averagetropopauseheight near 35øS indicatesthat the Ozone column + ++++ subtropicaljet is farther southover this region than was found 10 ognd-tropopause (2xscale) +gnd - 6 km over Brazil and the westernAtlantic. The maximum average03 [] 6 km- tropopause o Tropopause Altitude columnwas 51 DU at 9øS,which is 82% higherthanthe clean03 0 ................................... profile. The columnexceeded45 DU from 6ø to 18øSwith nearly equalcolumnsof 03 aboveandbelow6 km. The latitudebandof Time, UT the maximum 03 columnscorrespondsvery closely with the same latitude band where the fires were occurringin Africa Figure15. Tropopause heights and0 3 columns forFlight18 on October 24, 1992. [Justiceet al., thisissue;Thompson et al., thisissue]. +
9 i0 '1'1'i2' i3 i4 ;5
24,058
BROWELL ET AL.: OZONE AND AEROSOL DISTRIBUTIONS
Latitudinal
Ozone
Distribution
Average Latitudinal and Longitudinal Ozone
Average Ozone (ppbv)
Average Ozone (ppbv) 20 ,I,,
40 i
i
Brazil
60 .
80
i
& Western
.
Distrubutions
0
100
i
.
ß
i
Atlantic
.
20 i
40
.
i
.
60 !
80
.
i
100
•
i
Average Latitudinal Ozone Distribution -
20-
2o
2o 2018
18
.
I
•
12-
16
"I .,
•10
....
.
16
•4 12
• 12 •4-
10
•10-
•
õ•
*
12 10 ß
6
6' 4'
4 ß
2
0
.,h..---[ ....
I .... •0
-30
•
I ' ' ' ' [....i". -20
Central
& Estern
-•0
2 0
, . I 0
2
2 . 0 I '•
N Lat
ß
''1"I'
"'• "•
-30
-40
....
'l ' ' ' ' I ....
-20
-lO
o
I ' ' ' ' I
o
lO
2o
N
Average Longitudinal Ozone Distribution
Atlantic
-
20-
20-
20
2O
18
16
•'14-
12-
t
12
•10-
10
'--'10 •)
ß
8
8
6
r
4'
.
4
•_45DU inside this region). Over Africa the average 03 column also reached 48 DU in some locations. The lowest average03 columnwas associatedwith a 11i•lU•lUli [• •i Uli 11•L[ H-I ¾¾. tJ].]L•IU• ....................... ,,,-,o,,,, ........... t)l L[/lb 1• •i Uil•,
- 10
N Latitude,deg Central & Eastern Atlantic 4o
3o
observed byFishman etal.[1990, 1991] and lead totheTRACE
•' fromBrazilwaspredominantly inthemidtoupper troposphere.
++
+
o N
-40
excludingFlight 9 to emphasizethe longitudinaldistributionat the lower latitudes. The generalsubsidencein associationwith the anticyclonethat is generally located over the central South Atlantic duringthis seasoncanbe readily seenin the centerof the figure. This is the same enhanced03 region that was first
E A field experiment.The averagewesterlyflow of enhanced 03
o
10
24,059
15
o
E
•5 20
% ß
o N
Latitudinal Distribution
10•
40 ......... , ......... , ......... , ......... , ......... , ......... , 20
++
o
+
++ ++
Ozone column
10
5
o%00 oO øoøøøøøøøøøøøøøøøø o Ooooo
3O
o gnd-tropopause (2 x scale)
15
o
ß gnd - 6 km
[] 6 km - tropopause 0 ....................................... -40
-30
E
½TropopauseAltitude
' 0
-20
- 10
¾ 20
0
N Latitude,deg
.........
!
.........
!
.........
i
. +o•i••+
+ø++e+ +++++++++•ø.øq•o q• +++ + +
+++• + 10
Ozone column o gnd- tropopause(2 x scale)
2O
.........
+ gnd - 6 km
.
[] 6 km- tropopause .........
0oOO0%o0 oø
3O
10•
+%0
+.r++ + --
o
Central & East Coast of Africa 4o
o OooOO __ oo.o+
o N
-4O
15
! .........
-3O
o Tropopause Altitude
! ......... ! ......... -20 -10
! ......... 0
i...,,,,,,
10
,0 20
N Latitude,deg +4,O ø
•5 20
LongitudinalDistribution ++
øo
+++
•
++
+++++ +
o N
4O
20
o
10 Ozonecolumn
5
o gnd- tropopause(2 x scale)
+ km [] gnd6 km-6tropopause
30
15
•
o Tropopause Altitude
•
•i
10• •
0........................................ 0 •20[ ..+•+i•+o++. +•+oo+• +++ +'+T +•+ T+•+:• '• -40
-30
-20 -10 N Latitude, deg
0
o N O
•
Figure 16. Tropopause heights and O3 columns corresponding 10[ n•::: aloe'Tt5 OU•gnn•,
to averagelatitudinal 03 distributionsshownin Plate 8 for
2 x sc
Brazil and western Atlantic; central and eastern Atlantic; and central and eastern coast of Africa.
ropopauseAltitude
0
-60
Theaverage longitudinal 0 3 distribution across theSouth
-40
-20
0
20
40
0
E Longitude, deg
Atlantic Basin isalso shown inPlate 8. Thelatitudinal range of Figure 17.Tropopause heights and 03columns corresponding
theTRACEA dataincluded in theaverage longitudinal to average longitudinal andlatitudinal TRACEA 03
distributionwas in the region from about 35øS to the equator,
distributionsshownin Plate8.
24,060
BROWELLETAL.: OZONEANDAEROSOL DISTRIBUTIONS Table 1. Percentage of Troposphere withDifferentAir Masses Altitude
Range, Air MassType
km
Brazil and
Central and
Central and
Western
Eastern
EastCoast
Atlantic
Atlantic
of Africa
Brazil, Atlantic, and Africa
Reference
1-16
31.7
51.1
31.5
42.2
(R.EF)
5-16
34.1
60.8
41.9
49.7
Near Surface
1-16
6.8
1.9
6.1
4.3
(NS) Plumes-Bkg. 03
1-2 1-16
59.1 7.6
27.5 6.1
83.3 12.7
50.0 7.5
1-6
11.0 7.7
18.2 0.1
27.3 8.5
17.7 3.6
Plumes-Low 03
1-16
4.6
0.5
1.0
1.6
(LPLU)
1-6
9.1 3.5
1.5
0.0
2.6
4.2
Plumes-High 03
1-16
3.9
6.3
22.0
8.7
(HPLU)
1-6 8-13
4.7 5.8
18.8 0.0
30.6 32.1
16.0 9.1
All Plumes
1-16
16.1
12.9
35.7
17.8
1-6 8-13
24.8 17.0
38.5 0.1
60.5 40.7
37.9 13.1
1-16 13-16
13.0 61.0
6.0 24.8
6.7 29.6
6.8 30.2
High0 3
1-16
5.8
16.9
18.2
15.2
(j_iO3)
4-13
8.4
24.5
26.9
22.2
Low03
1-16
20.9
9.5
1.0
10.0
(LO3)
1-10
33.6
12.3
1.5
14.4
Low03and
1-16
5.8
1.5
0.8
3.7
10-16
13.4
4.0
2.0
9.2
(BPLU)
Stratospheric Influence (SINF)
Cirrus (LO-C)
8-13
8-13
0.1
0.4
thelowestaverage0 3 columnwas38 DU near42øW. Thislow low aerosolscattering(lessthan a factorof 2 abovethe estimated 03 columnis thought to be significantly influenced by theclean scatteringfrom aerosol-freeregion at same altitude);(2) nearPacific air observed over the western South Atlantic during TRACE
A.
Tropospheric Occurrenceof Different Air Masses The airborne DIAL
data are well suited to determine the verti-
cal extentof variousairmasstypesalongthe flight track. Nadir and zenith aerosoland 0 3 measurements are usedto categorize air into dominanttypesand thento describetheir spatialdistribution and contribution to the overall composition of the troposphere.Prior to establishingthe criteriafor the differentair masstypes,a referenceor discriminator0 3 profile had to be defined. The reference0 3 profile was established by averagingthe ozonesondeprofiles obtainedduring the TRACE A time period from Cuiba, Brazil; Pretoria, South Africa; Etosha, Namibia; and
Brazzaville, Congo. These ozonesondesites were thoughtto providea representative setof 0 3 profilesthatwerenot normally directly downwindof biomassburningregions. The average0 3 profile from these four sites was closely approximatedby a piecewiselinear 0 3 profile increasingfrom 23 ppbv at the surface to 52.5 ppbv at 2.5 km to 99 ppbv at 18 km. This reference0 3 profile, which clearly had someenhanced0 3 levelsin the mid to uppertroposphere,was consideredto be an estimateof the average background0 3 distributionover the SouthAtlantic Basin during the TRACE A field experiment,and it was usedas the 03 discriminatorin the air masscharacterizationanalysis. Nine air mass types were identified during this field experiment, and they were categorized using the following criteria in each altituderange: (1) referenceair in the free troposphere(REF): 0 3 within 80-120% of reference03 profileand
surfaceair (NS): enhancedaerosolscattering(morethanfactorof 2 abovethe estimatedscatteringfrom near-byaerosol-freeregion)
below about3 km agl and 0 3 lessthan 120% of reference0 3 profile; (3) plumeswith low 0 3 in the free troposphere (LPLU): enhancedaerosolscatteringand03 lessthan80% of reference03 profile; (4) plumeswith background0 3 in the free troposphere (BPLU): enhancedaerosolscatteringand0 3 within 80-120% of reference 0 3 profile; (5) plumes with high 03 (HPLU): enhancedaerosolscatteringand 0 3 greaterthan 120% of reference0 3 profile; (6) low 03 air (LO3): 0 3 lessthan 80% of reference 0 3 profile and low aerosol scattering; (7) stratospherically-influenced air (SINF): 0 3 more than 120% of reference0 3 profile, aerosolscatteringcomparableto levels abovethe tropopause; (8) high0 3 air (HO3): 0 3 morethan120% of reference0 3 proffie, low aerosolscattering,andno evidenceof stratosphericintrusionsin PV analysis;and (9) low 0 3 air with cirrusclouds(LO-C): 0 3 lessthan 80% of reference0 3 profile and in vicinity of cirrusclouds. It shouldbe notedthatnot all the plumesidentified above are from pollution sources. Layers containing enhancedaerosolscan result from naturalprocessessuch as low-altitudecloudpumpingof marine aerosolsandcontinental dust transport. The amountof O 3 in thesenaturallayersis usually low to moderate placing them in the LPLU and BPLU categories,respectively. The air mass analysiswas conductedfor each flight during TRACE A, and the resultswere combinedin the following four broadregions: (1) Brazil andwesternAriantic(30øto 0øS;60ø to 30øW); (2) central and easternAriantic (30 ø to 0øS; 20øW to 20øE); (3) central and east coast of Africa (30 ø to 10øS;20ø to 40øE); and (4) entire South Ariantic Basin (Brazil, Ariantic, and
BROWELL ET AL.' OZONE AND AEROSOL DISTRIBUTIONS
24,061
Percentage of TRACE-A Air Moss Types REF
BPLU
Brazil
LPLU
& Western
18 q
HPLU
NS
Atlantic
-
LO-C
16
L03
Centrol&
N --
!
SINF
36 57 60
18 -I 16 ß
75
14
72
77
E
6g 58
s• 156
241
H03
Eostern
Atlontic
N -211 175 165
164
150 106
12
95 167
•E 10
547
-•
755
8
778
257
6
786
277
4
•
2
,
599
25s
4
164 112
2
248
0
576 474
287
.,
5O
o 0
20
40
60
80
100
20
Percentage Central
&
East
Coast
40
60
80
lOO
Percentage of Africa
Brazil, Atlon[ic,&
N •
z6 86
18 16
lO3
14 ..:
94
106
lO5
12,
57
N • 341
33O 334 356
35O 303
E 12 •,
..•
56
•r 10 ,
Africo
16o
219
303
• 10
817
•
D
8 •
1190
2o9
6
19o 179
•:
6
1292
4
177
4
1051
62
2
220
8
140
2
11
0
1287
1114
894 532
173
o
20
40
60
80
100
Percentage
0
20
40
60
80
lOO
Percentage
Plate 9. Percentageof observations of differentair massesover the following: Brazil and westernAtlantic; centraland easternAtlantic; centraland eastcoastof Africa; andBrazil, Atlantic, and Africa (all TRACE A
flightsin SH northof 30øS). Thedifferentairmasstypesinclude:reference air (REF);plumeswithbackground 0 3 (BPLU);plumeswithlow 03 (LPLU);plumeswithhigh03 (HPLU);near-surface air (NS);low03 air with cirrusclouds(LO-C); strataspherically influencedair (SINF); low-O3 air (LO3); andhigh-O3 air (HO3). The numberof independent air masssamples(N) in the analysisat eachaltitudeis alsogiven.
Africa) (30ø to 0øS;60øWto 40øE). The frequencyof observa- face into the upper tropospherewithout the complete removal of tion of the differentair masstypeswerecalculatedfrom the sur- large aerosols. The near-surfaceair was usually confined to the boundarylayer below 2 km (59.1%). The low 03 air in the low to mid troposphere (1-10 km) was the result of clean Pacific typeobserved overthealtituderangefrom1 to 16 km (average air being frequently detected in this region (33.6%). The tropopause height)andovervariousaltituderegions(derived averageextent of the low 0 3 air over the entire tropospherewas fromdatain Plate9). Theaverage 03 profilesobtained in eachof 20.9%. Cirrus outflow was observedin the upper troposphere theseregions is shownin Figure18(derived fromdatapresented (10-16 km) 13.4% of the time. High 03 air with low aerosol in Plate8) alongwith the reference 03 profileusedin the air loading was found throughout the mid to upper troposphere (4-13 km) with a frequency of 8.4%. Strataspherically influmassanalysis. Over Brazil and the westernAtlantic, plumeswith enhanced encedair was observedthe most frequently(61.0%) in the upper aerosolsandvarying0 3 levelswereprimarilyobserved below troposphere(13-16 km), but this representedonly 13.0% of the 6 km (24.8%of thetime;percentage usedbelowimpliesobserva- entire troposphere. The reference/backgroundair was observed tionalfrequency) or in theuppertroposphere from8 to 13 km the most (31.7%) acrossthe tropospherein this region. The (17.0%).Thiswasdueto directadvection of plumesatthelower average0 3 profiles (Figure 18) show that most of the reference altitudes anddeepconvective pumpingof air fromnearthesur- air is at low latitudes (0 ø to 10øS) below 10 km. In the 10ø to
face to 18 km in 1-km increments,andthe resultsarepresentedin Plate9. Table 1 summarizes the averageamountof eachair mass
24,062
BROWELLET AL.: OZONEAND AEROSOLDISTRIBUTIONS Brazil & Western Atlantic
20øSregion,thereis a mixtureof air masseswith low 03 air below 10 km and high 03 air above 10 kin. Low 0 3 air
20Average Ozon; atLatitudes: ....
dominatesbelow about13 km in the 20ø to 30øSregion. This is due to the predominantlywesterly flow that brings air with low 0 3 (75 ppbv) with low aerosolloading was fre- the central and eastern Atlantic where there was time for the quently observedin the mid to upper troposphere(4-13 km) highly contaminatedair from the fires in Africa andBrazil to beacrossthe South Atlantic basin (22.2% of the time). Outsideof comedilntedtinringthe convectionandadvectionprocesses.The directstratospheric intrusions, the air containing high03 andlow chemical compositionof this air resemblesa slightly diluted aerosolswas attributedto vertical transportby wet convective version of the high 03 air with about the same O3/CO and storms of gases associatedwith extensive fires in Brazil and NOy/CO ratiosandlowaerosol andsoluble gasconcentrations. Africa. The elevatedO 3 was from photochemical productionasThe excess03 overthe SouthAtlanticBasincanbe estimated sociatedwith the biomassburningandthe low aerosolloadingin by comparingthe 03 columnin variousregionsagainstthe clean
BROWELL ET AL.: OZONE AND AEROSOL DISTRIBUTIONS
03 columnof 28 DU with about14.5 DU below 6 km. The maximum tropospheric 03 column of 56 DU (27 DU below 6 km) was observedduringTRACE A over the centerof the average subsidence regionover the SouthAtlantic. Biomassburning contributedup to 28 DU (100% increase)to the tropospheric03 column. Below 6 km the increase was 12.5 DU (86%), while above 6 km the increasewas 15.5 DU (115%). This indicatesthat
24,067
Browell, E. V., S. Ismail, andS. T. Shipley,UltravioletDIAL measurementsof 0 3 profilesin regionsof spatiallyinhomogeneous aerosols, Appl. Opt., 24, 2827-2836, 1985a.
Browell,E. V., S. T. Shipley,C. F. Butler,andS. Ismail, Airbornelidat measurements of aerosols,mixedlayer heights,and ozoneduringthe 1980 PEPE/NEROS summerfield experiment,NASA Ref. Publ., RP-1143, 1985b.
the impact on the 03 columnis slightly greaterfrom emissions Browell, E. V., G. L. Gregory,R. C. Harriss,andV. W. J. H. Kirchhoff, that are convectively transported into the mid to upper Troposphericozone and aerosol distributionsacrossthe Amazon tropospherethan are directly advectedin plumes in the lower Basin,J. Geophys.Res.,93, 1431-1451, 1988. troposphere. Browell, E. V., C. F. Buffer, S. A. Kooi, M. A. Fenn, R. C. Hatriss, and
Acknowledgements. The authorsexpresstheir appreciationto Bill McCabe, Jerry Williams, Neale Mayo, and Byron Meadows for their supportin operatingthe airborne DIAL system in the field for the measurement of 0 3 andaerosoldistributions. We alsothankSyedIsmail and Greg Nowicki for their assistance in the reductionof the DIAL data. We appreciatethe cooperationof the NASA AmesResearchCenter'sDC8 flight crew in conductingthis mission. This researchwas supportedby the NASA Global TroposphericChemistryProgram.
G. L. Gregory, Large-scalevariability of ozone and aerosolsin the
summertimeArctic and sub-Arctictroposphere, J. Geophys.Res.,97, 1992.
Browell, E. V., M. A. Fenn,C. F. Buffer,W. B. Grant,R. C. Harriss,and M. C. Shipham,Ozone and aerosoldistributionsin the summertime troposphere overCanada,J. Geophys.Res.,99, 1739-1755, 1994.
Browell,E. V., et al., Large-scaleair masscharacteristics observedover the westernPacific duringthe summertime, J. Geophys.Res., IOI,
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