Influence of biomass combustion emissions on the distribution of ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D5, PAGES 5623-5634,MARCH 20, 1999

Influence

of biomass

combustion

emissions

on the distribution

of acidic trace gasesover the southern Pacific basin during austral springtime R. W. Talbot,• J. E. Dibb,• E. M. Scheuer, • D. R. Blake,2N.J. Blake,2G. L. Gregory, 3 G. W. Sachse, 3J. D. Bradshaw, 4,sS. T. Sandholm, 4andH. B. Singh6 Abstract. This paperdescribes the large-scaledistributions of HNO3,HCOOH, andCH3COOH overthe centraland SouthPacificbasinsduringthe PacificExploratoryMission-Tropics(PEMTropics)in australspringtime.Becauseof the remoteness of thisregionfrom continentalareas,low partper trillion by volume(pptv)mixingratiosof acidicgaseswereanticipatedto be pervasive overthe SouthPacific basin.However,at altitudesof 2-12 km over the SouthPacific,air parcels wereencountered frequentlywith significantlyenhancedmixingratios(up to 1200pptv) of acidic gases.Most of theseair parcelswerecenteredin the 3-7 km altituderangeandoccurredwithinthe 15ø-65øSlatitudinalband.The acidicgasesexhibitedan overallgeneralcorrelation with CH3C1, PAN, and03, suggestive of photochemical andbiomassburningsources.Therewasno correlation or trendof acidicgaseswith commonindustrialtracercompounds (e.g.,C2C14 or CH3CC13). The combustion emissions sampledoverthe SouthPacificbasinwererelativelyagedexhibiting C2H2/COratiosin the rangeof 0.2-2.2 pptv/ppbv.The relationships betweenacidicgasesandthis ratioweresimilarto whatwasobservedin agedair parcels(i.e., >3-5 dayssincetheywereovera continental area)overthewesternNorthPacificduringthePacificExploratoryMission-West Phases A andB (PEM-WestA andB). In the SouthPacificmarineboundarylayera median C2H2/COratioof 0.6 suggested thatthisregionwasgenerallynot influencedby directinputsof biomass combustion emissions. Herewe observed the lowestmixingratiosof acidicgases,with medianvaluesof 14 pptvfor HNO3, 19 pptvfor HCOOH, and 18 pptvfor CH3COOH.These valueswerecoincidentwith low mixingratiosof NOx(

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valuesof CH3CI. Thesecorrelations potentiallyindicatean

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03, ppbv [Madronich andCalvert,1990].Thisresultappears to support a significant photochemical source ofHCOOHrather thana predominanceof decomposition of CH3COOH in agedbiomass burning Figure7. Relationships between mixingratiosof acidicgasesand plumesovertheSouthPacific.We cannotruleout,however,some O3in thealtituderange2-12 km. Ther2valuesfor thesecorrela-

photochemical productionof CH3COOHas well. Becauseof tionswere•0.40. Thesegeneralcorrelations potentiallyindicatea potentiallycomplex(andunknown)chemistry in thesebiomass photochemical sourcefor HCOOH andCH3COOH.

TALBOT

ET AL.' ACIDIC

GASES OVER THE SOUTHERN

PACIFIC BASIN

5631

altitude.In both layers,aerosolNO3' mixing ratioswere abouta factor of 2 greaterthan those of HNO3 [Dibb et al., this issue], presumablydue to uptakeof HNO3 onto sea-saltparticlesin the marineboundarylayer[Huebert,1980] andpossiblyproductionof aerosol-NO3' from cloudprocessing in the transitionlayer. The mixing ratiosof carboxylicacidsin the marineboundary layeroverthe SouthPacificwere aboutan orderof magnitudeless thanthosepreviouslydeterminedfrom shipboardsamplingin the centralNorthPacificregion[Arianderet al., 1990].This probably

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areas and restricteddownward mixing acrossthe trade wind

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Formic

and acetic acid did not exhibit a difference

in

their mixing ratiosbetweenthe marine boundaryand transition layers.In the marineboundarylayer they had the largestmixing ratiosnoah of the ITCZ. This may reflectthe closerproximityof 1000

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Figure8. Relationships betweenmixingratiosof acidicgasesand

PAN in the altituderange2-12 km. The r2 valuesfor these correlations were •0.40. Thesegeneralcorrelations potentially indicatea photochemical sourcefor HCOOH and CH3COOH. Mixing ratiosof PAN below• 5 pptvwereobservedin the altitude range2-4 km,wherethermaldecomposition of PAN wasapparently

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appeartobeanysystematic variationof HNO3mixingratiosat