Dec 20, 1997 - PEM-West B [Merrill et al., this issue]. Typical trajectories ...... satellite data pro- vided by Ross Swick and Steve Goodman of NASA Marshall are.
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 102, NO. D23, PAGES 28,367-28,384, DECEMBER
20, 1997
Impact of lightning and convection on reactive nitrogen in the tropical free troposphere S. Kawakami, •,2Y. Kondo,• M. Koike,• H. Nakajima,• G. L. Gregory,3 G. W. Sachse, 3 R. E. Newell,4 E. V. Browell,3 D. R. Blake/J. M. Rodriguez, 6 and J. T. Merrill7
Abstract. Latitudinaldistributions of NO, NOy,03, CO, CH3I, andH20 mixingratiosat 8.9-12 km were obtainedbetween30øN and 10øSby DC-8 aircraft measurementsmade in
February 1994duringPacificExploratory Mission-West B (PEM-WestB). VerylowNOy mixingratioswith a medianvalue of 51 partsper trillion by volume (pptv) were observed at 9.5-12 km at løN-14øN duringtwo flightsmade within 3 days.A very low median 03 mixingratio of 19 partsper billion by volume (ppbv) and high mixingratiosof H20 and
CH3Iweresimultaneously observed, suggesting thatthelowNOyvalueswereprobably due to the convectivetransportof air from the tropicalmarine boundarylayer to this
altitude.The medianNOy/O3 ratiobeinga factorof 2 smallerthanin the air masses in the tropicalmarine boundarylayer might suggestthe possibilitythat the heterogeneous
removalof HNO3duringconvective transport furtherreducedNOylevels.In additionto themeasurements between9.5and12 km,lowvaluesof NOyand03 wereobserved between4 and 12 km at iøN. Divergentwind fieldsat 200 and 1000 hPa and infrared (IR) cloudimagesshowthat there was large scaleconvection(>1000 km x 1000 km) in the northeastof New Guinea Islandcenteredaround0øSand 150øEas part of systematic convectiveactivityof the IntertropicalConvergenceZone (ITCZ) and the SouthPacific ConvergenceZone (SPCZ). This type of large scaleconvectioncouldhave transportedair
withlowlevelsof NOyand03 to themiddleanduppertroposphere overa wideareain thetropics. On theotherhand,NO mixingratiosof 50-200pptvandhighNOx/NOy ratiosof 0.4-0.6 were observedat 9.5 km between4øSand 10øS.High H20 mixingratios of 600-1200 partsper million by volume (ppmv) and low CO mixingratiosof 65 ppbv observedin the air massindicatedthat the high NO valueswere probablydue to NO productionby lightning.Satelliteobservationsshowedrelativelyfrequent lightningflashes over the New Guinea Island for 3 daysprior to the aircraft measurements. These results are consideredto be consistentwith the idea that, in general,marine convectionis not accompaniedby lightningactivity,whereasconvectionover land is. Becauseof the large
arealextentof theinfluences fromtheseprocesses, theconvective transport of lowNOy air and NO productionby lightningshouldplay critical roles in controllingthe abundance of reactivenitrogenin the equatorialregion. upper troposphere[Crawfordet al., this issue(a); Davis et al., 1996a;Ridleyet al., 1987; Carroll et al., 1992;Liu et al., 1987]. NOx (NO and NO2) in the free troposphereplaysa very A long lifetime of NOx in the upper tropospherecan resultin importantrole in controllingthe ozone(03) budget,especially a highertotal 03 productionover the lifetime of NO• [Ehhalt in regionsrelativelyremote from anthropogenicsources.Be- et al., 1992; Davis et al., 1996a]. NO• is further oxidized to causethe 03 lossrate is lower at higheraltitudes,primarilydue HNO3, CH3COO2NO2 (PAN), and HO2NO 2. Total reactive to lower H20 mixing ratios, positivephotochemical03 pronitrogen(NOy)in theuppertroposphere is definedasNOy = ductionrateshave generallybeen predictedin the middle and NO + NO 2 + HNO 2 + HNO3 + HO2NO 2 + NO3 + 2N205 + PAN + other organicnitrates + aerosolnitrate. Becausethe •Solar-Terrestrial EnvironmentLaboratory,NagoyaUniversity, lifetimeof NOyislongerthanthatof eachcomponent species, 1.
Introduction
Toyokawa,Aichi, Japan.
havebeenusedto studythe long-range 2Nowat theNationalSpaceDevelopment Agencyof Japan,Earth NOy measurements transportand budget of reactivenitrogen species. 3NASALangley Research Center,Hampton,Virginia. The important sourcesof NO• in the free troposphereare 4Department of Earth,Atmospheric, andPlanetary Sciences, Mas- NO productionby lightning,exhaustfrom aircraft, regenerasachusetts Institute of Technology,Cambridge. tion from HNO3, transportof polluted air from the boundary SDepartment of Chemistry, University of California, Irvine. 6Atmospheric andEnvironmental Research Inc.,Cambridge, Mas- layer, and transportfrom the stratosphere.The globalbudget sachusetts. of NO• estimateslightning generates5-8 TgN/year of the 7Centerfor Atmospheric Chemistry. Studies,GraduateSchoolof ObservationResearchCenter, Roppongi,Minato-ku, Tokyo, Japan.
Oceanography, Universityof Rhode Island,Narragansett. Copyright1997 by the AmericanGeophysicalUnion. Paper number 97JD02073. 0148-0227/97/97JD-02073509.00
zation(WMO), 1995;Pratheret al., 1995],althoughthe estimate has a large uncertainty[e.g.,Liaw et al., 1990]. Becauselightning activityis generallyhigherat lower latitudes[Turmanand Edgar, 1982; Kotaki and Katoh, 1983; Orville and Henderson, 28,367
28,368
KAWAKAMI ET AL.: IMPACT OF LIGHTNING
Table 1. Summaryof the Flights Flight
Date, UT
Description
mixingratiosof Nay, NO, andothertracespecies in theupper Altitude and Latitude
6
Feb. 11-12, 1994 local:Guam
7 8 9
Feb. 13-14, 1994 Feb. 17-18, 1994 Feb. 19, 1994
local: Guam local: Guam local: Guam
Feb. 21, 1994
transit: Guam
Feb. 25, 1994
to Hong Kong local:Hong Kong 8.2 km (17øN-15øN)
10 11
AND CONVECTION ON NO AND NOr
9.5 km (12øN-10øS), 12 km (0ø-4øS) 8.9 km (25øN-12øN) 9.5 km (30øN-16øN) 9.5 km (lløN-IøN), 12 km (11øN-2øN) 9.5 km (14øN-5øN)
troposphereobservedbetween 30øN and 10øSare presented. The effectsof largescaleconvectivetransportand lightningon the observedreactivenitrogen are discussed.
2.
Experiment and Data In thisstudyweusedthemixingratiosof NO, Nay, CO,03,
CH3I, and H2a obtained at 8.9-12.0 km at latitudesof 30øN-
10øS.For NO, Nay, 03, CO, andH2a , 10sdatawereused.
The valuesof CH3I were obtained every 3 to 4 min, and each samplingtook 0.5 to 1 min on average.To minimizethe effect of the diurnal variation of NO, only data obtainedat a solar 1986;Christianet al., 1996],NO productionhasbeenestimated zenithangle(SZA) of smallerthan70øwereused.Data for this to be largest in the tropical region [Kumeret al., 1995]. In paper were obtained during flights 6-11 made betweenFebaddition, influencesfrom anthropogenicemissions,aircraft ex- ruary 11 and 25, 1994.A brief summaryof theseflightsis given haust, and stratosphericair intrusion are consideredto be in Table 1. Photostationarystate Na2/Na ratioswere calcugenerally lower in the tropical region than at midlatitudes, lated usinga boxmodelwith measuredvaluesof 03, CO, H2a, resultingin a muchmore importantrole playedby lightningin atmospherictemperature,and UV flux as inputs [Crawfordet the tropics[Lawrenceet al., 1995;Kotamarthiet al., 1994;Levy al., 1996;Davis et al., 1996a].Nax mixingratioswere calcuet al., 1996].The spikesin NO of up to 900 partsper trillion by lated from the Na2/Na ratio and the observedNO mixing volume(pptv) observedon board an aircraft at an altitudeof ratiosand are labeled(Nax)mc. 9.3 km near Hawaii were interpretedto be NO productionby The NO andNay mixingratiosweremeasured by Nagoya lightning[Ridleyet al., 1987;fromDaviset al., 1987].Murphyet University(NU) [Kondoet al., this issue(a, b)] and Georgia
Instituteof Technology(GIT) [Sandholmet al., this issue].In the uppertroposphereat Darwin (12øS),Australia,to explain thisstudytheNO andNay valuesmeasured byNU wereused. theobserved highNay mixingratiosandtheirlargevariability. In general,the agreementbetweenthe NU andGIT NO values Detailed measurementsand analysesof NO production by was good. For example, based on 30 s sample integration thunderstormsover New Mexico were made by Ridleyet al. intervals,a standardregressionanalysisof approximately5000 [1996],and the NO productionratesper lightningflashor per paired NO measurementsgave a slope of 1.25 _+ 0.01, an thunderstormwere estimated. However, atmosphericmea- interceptof 1.2 _+0.2 pptv, and a squareof correlationcoefsurementsof NO productionby lightning are still limited in ficient(r2) valueof 0.95withNU having thehigherNO values spiteof its importancein the globalbudgetof reactivenitrogen. [Crawford etal.,thisissue (a)].Ontheotherhand,thetwoNay On theotherhand,mixingratiosof NO, Nay, and03 in the measurementswere occasionallyin significantdisagreement. tropicalregionwere observedto be generallymuchlower than This disagreementmight be due partly to the higher converthoseat midlatitudesduringPacificExploratoryMission-West sionefficiencyof HCN into NO by the goldcatalyticconverter PhasesA and B (PEM-West A and B) [Kondoet al., 1996; in the GIT instrument(J. Bradshaw,privatecommunication, Gregoryet al., 1996;Kondoet al., this issue(a)], the measure- 1996).HCN is generally not includedin the Nay family.Dementsmade over the westernPacific[Kondoet al., 1993],and taileddescriptions of the NU Na/Nay instrument andHCN Airborne Arctic Stratospheric Experiment 2 (AASE 2) interferencetestsare givenby Kondoet al. [thisissue(b)]. In [Folkinset al., 1995]. Tropical air is generallyisolatedfrom short,interferencefrom HCN was estimatedusingHCN mixmidlatitude air which is influencedby anthropogenicsources ing ratiosof 180-350 pptv,which correspondedto valuesoband stratosphericair intrusions.In addition,high humidityin served in the northern hemisphere [Rinslandet al., 1982; the tropicallower tropospherecanlead to a largenet lossof 03 Mahieu et al., 1995]. The conversionefficiencydependedon [Daviset al., 1996a],and rapid dry andwet depositionof HNa3 humidity, O3 concentration,and the surfacecondition of the
al. [1993]proposed lightning asan importantNay sourcein
can lowerthe Nay level.Frequentconvective transportof goldconverter. TheHCN conversion efficiency of theNU Nay marineboundary layerair shouldkeepthe levelsof Nay and instrumentused during PEM-West B was estimatedto be 4% 03 low in the free troposphere.In fact, 03 mixingratioslower than 10 parts per billion by volume (ppbv) were frequently observedby balloon-bornesoundingsbetween 10 km and the tropopauseover the equatorialPacific[Kleyet al., 1996].Although NO productionby lightningduring convectivetrans-
for dry air. The additionof H2a (0.4%) to dry air reducedthe conversionefficiencyto 2.5%. The conversionefficiencyincreasedwhen 50-300 ppbv of 03 was added.For 03 mixing ratioslower than 100 ppbv,the conversionefficiencywaslower than 5%. Interference by HCN causedan uncertaintyof 10
portwill augmentthe NO andNay levelsin the free tropo- pptvin theNU Nay measurements whenanHCN mixingratio sphere,convectionwithout lightningwill lower their levels.
of 200 pptv was assumed.The overall absoluteaccuracy(1
Aircraftmeasurements of NO, Nay, 03, CO, H2a, non- sigma)andprecision (1 sigma)of theNay measurement were methane hydrocarbons(NMHCs), and other trace species estimated to be 27% and 16 pptv,respectively, for an Nay were made over the western Pacific Ocean in February and March 1994 during NASA's Global TroposphericExperiment (GTE) PEM-West B. Detailed descriptions of the PEM-West B experimentare givenby Hoell et al. [thisissue].Time series plots of variousmeasurementdata for each flight as well as locationswhere thesedata were obtainedare givenby Gregory and Scott[1995]. In this paper, latitudinal distributionsof the
mixingratio of 100 pptv. The absoluteaccuracyand precision
of theNO measurements were21% and4 pptv,respectively, for an NO mixingratio of 20 pptv. The measurementtechniques,data samplingrate, accuracy, andprecisionof the othermeasurements usedin thisstudyare givenin the PEM-West A and B overviewpapers[Hoellet al., 1996, this issue].The absoluteaccuracyand precisionof 03
KAWAKAMI ET AL.: IMPACT OF LIGHTNING
AND CONVECTION
ON NO AND NOv
28,369
40 øN
30 oN
•,
.......;............. *'•i F8
........... ' .... "M7^ ,,-'-"•'••'• '"' ,,•a..,. ,, ,,,© SHC
20ON ,•......... • ................
-o
I,
-.e-,.CON
•
--I--lXlI'1U
I
20 øS
.
•
100 øE
•
120 øE
140 øE
160 øE
180 øE
Longitude
Figure 1. Flighttracks(flights6-10) andtrajectoriesfor air massessampledat 9.5 kin. Symbolson the flight tracksdenotethe locationwherethe air masseswere sampled.Symbolsalongthe trajectoriesdenotelocations at 0000 UT on each day. CON, continental air; MT, maritime tropical air; NHC, northern hemispheric convectiveair; SHC, southernhemisphericconvectiveair.
measurementsmade during PEM-West B were 3 and 2%, respectively,those of CO were 1 and 1%, and those of H20 were 2 to 4% in the troposphere. 3. 3.1.
Results Air Mass
and Discussion Classification
Ten-dayback trajectorieson isentropicsurfaceswere calculated for air massessampled on board the aircraft during PEM-West B [Merrillet al., this issue].Typical trajectoriesfor air massessampledat 9.5 km are shownin Figure 1. In this study,the observedair massesat 9.5 and 8.9 km between30øN and 10øSwere classifiedinto four groupsusingthesetrajectories and mixing ratios of H20 and CH3I. The latitudinal distributionsof thesetwo speciesare shownin Figures2a and 2b and approximaterelativehumidity(percent)is alsoshownin the right axisof Figure 2a. Most of the data were obtainedat 9.5 km, althoughthe data obtainedat 8.9 km were alsousedat latitudesbetween 12øNand 25øN.For simplicitywe will refer
these air masseswere stronglyaffected by marine boundary layer air transportedby deep convection.When an air massis strongly affected by convection,the trajectory beyond that point cannotbe tracedbackreliably.Becausetheseair masses were sampledin the southernhemisphere,theywere classified as southernhemisphericconvective(SHC) air. During flights9 and 10, air masseswere sampledat latitudes betweeniøN and 14øN.The trajectoriesindicatethat theseair massescame from the southern hemisphere and remained over the ocean for more than 10 daysprior to the measurements.The H20 mixing ratios were 300-1200 ppmv, and the median value of the relative humidities was 48%. The CH3I mixing ratios were between0.10 and 0.25 pptv and were systematically higher than in the other air masses,as seen in
100
1000 j• .
!
• CON A MT NHC ß SHC
to the values at these two altitudes as those at 9.5 km. The level
of H20 can be used as a good indicator of recent influence from convectivetransportover the ocean. Becausethe predominantsourceof CH3I is oceanicand its lifetime is about 3-4 days and 2 days in the lower and upper troposphere, respectively[Daviset al., 1996b],the level of CH3I canbe used in a similarway. Data between1.5øSand 10øSwere obtainedduring flight 6. The trajectoryfor an air masssampledat 4øSindicatesthat it originatedfrom the northern hemisphereand remained over the oceanfor more than 10 daysprior to the measurements,as shownin Figure 1. The trajectoryfor the air masssampledat 8øSalso originatedfrom the northern hemisphere,although the air massstagnatednear the locationwhere it was sampled for a few days.The H20 mixingratiosin theseair masseswere between 600 and i200 parts per million by volume (,ppmv). Thesevalueswere in the highestrangeat this altitude (Figure 2a). The medianvaluesof the H20 mixingratiosand relative humiditieswere 790 ppmv and 52%, respectively,as summarized in Table 2. The high H20 concentrationsuggeststhat
ß
10
._>
100
-'0
Latitude
Figure 2a. Latitudinal distributionof H20 mixingratio at 9.5 km. The data at 8.9 km are alqo]]qectRt 19øN-gqøN(the data
with 200-500 ppmv).Approximaterelativehumidity(percent) is also shownon the right axiscalculatedfrom a typical temperature of -31.5øC measuredat 9.5 km. CON, continental air; MT, maritime tropical air; NHC, northern hemispheric convectiveair; SHC, southernhemisphericconvectiveair.
28,370
KAWAKAMI ET AL.' IMPACT OF LIGHTNING AND CONVECTION ON NO AND NOr
0.30
o /x ß ß
0.25
CON MT NHC SHC
200 .... . ..... :
,
i
o CON
' ' ' ' ½ø''•' ' '/ o• •
'.. aMT : m NHC 0.20
• ß SHC
150
3o • o
oo
•$ o• .o •o o o
ß......
0.15 o
0.10
ee
o
oo
oo
e•
o ß
o
o
o
ß
o
,,
o
o
0.05
o
ß
o
o,o•o o
ooo
o
oo
o
o
o
o
oo
o
oo
oo
:
0.00 -10
0
Figure 2b.
lO Latitude
20
30
•
0 -
ßm
•m
ße• ....
-10
Same as Figure 2a, but for CH3I.
•. •
a,
0
,
,
,
10 Latitude
,
,
•
,
,
•
,
,
,
•
•
20
,
,
,
,
30
Figure 2c. Same as Figure 2a, but for NO. When an NO mixingratio higherthan 200 pptvwasobserved,the maximum mixingratio andthe numberof data(in parenthesis) are given.
Figure 2b. The H20 and CH3I concentrationssuggestthat these air masseswere stronglyaffected by deep convection. Becausetheseair masseswere sampledin the northernhemisphere,theywereclassified asnorthernhemispheric convective recentconvection.The trajectoriesof theseair massesindicate (NHC) air. that theyoriginatedfrom northernmidlatitudesand stagnated At latitudes between 12øN and 29øN, air masseswhich re- over the oceandue to the westerlywinds at midlatitudesand mainedover the oceanfor 3 or fewer dayssincehavingpassed easterlywinds at lower latitudes.These air massesremained overa landmasswere sampledduringflights7 and8. Theseair over the oceanfor more than 10 daysprior to the measuremasseswere classifiedas continentalair (CON). The H20 mentsand were classifiedas maritimetropical(MT) air. mixing ratios were between 30 and 500 ppmv, which were The latitudinaldistributions of thevaluesof NO, NOy, 03, systematically lower than thoseof the convectiveair described CO, theNOy/O3 ratio,andthe (NOx)mcfNOy ratioat 9.5km above.The centerof the Japanjet, maximumspeedof about70 are shownin Figures 2c-2h. Different symbolsare used to m/s,was locatedat 32øNand 12 km [Menill et al., this issue]. indicate the sources of the air masses. The median values of The CON air masseswere transportedfrom over the Asian thesequantitiesat 9.5 km in the four air massesare summacontinentby thesestrongwesterlywindsto the locationwhere rized in Table 2. measurementswere made within a few days. 3.2. CON and MT Air Masses During flight 6 the H20 mixing ratios in the air masses measuredat 1.5øS-8øNwere 80-300 ppmv. These low values In the CON and MT air masses the NOy mixingratios suggestthat these air masseswere not stronglyaffectedby generallyrangedfrom 150 to 800 pptv as shownin Figure2d.
Table 2. Median Values of the Mixing Ratios and Ratios at the Altitude of 9.5 km
N CON MT NHC SHC
CON MT NHC SHC
NO (pptv)
269 208 972 338
101 77 7 68
N
(NOx)mc/NOy (pptv/pptv)
221 183 780 254
0.32 0.29 0.15 0.44
(-21/+43) (-19/+12) (-6/+6) (-47/+32)
(-0.05/+0.04) (-0.05/+0.04) (-0.10/+0.19) (-0.12/+0.11)
N 269 208 973 338
NOy(pptv) 383 330 51 184
9.27 8.08 2.65 7.24
H20
N CON MT NHC SHC
420 345 1681 473
(ppmv) 39 101 656 786
(-6/+30) (-14/+72) (-204/+237) (-180/+183)
(-76/+219) (-79/+82) (-20/+25) (-95/+99)
0 3 (ppbv)
420 345 1662 473
NOy/O3 (pptv/ppbv)
N 269 208 952 338
N
41 41 19 24
N
(-1.16/+1.08) (-1.08/+1.32) (-0.98/+1.17) (-3.00/+3.21)
(-6/+21) (-5/+3) (-1/+2) (-3/+6)
CO (ppbv)
356 268 1457 396
82 86 86 65
(-12/+13) (-5/+5) (-4/+2) (-5/+11)
Relative
N 420 345 1681 473
Humidity,% 2 7 48 52
(-0/+2) (-1/+5) (-15/+15) (-12/+12)
N 20 14 73 21
CH3I (pptv) 0.04 0.07 0.16 0.09
(-0.02/+0.01) (-0.06/+0.07) (-0.04/+0.07) (-0.02/+0.03)
N is the numberof datapoints.The central___67% valuesare shownin parentheses. CON, continentalair; MT, maritimetropicalair; NHC, northernhemispheric convectiveair; SHC, southernhemispheric convective air.
KAWAKAMI ET AL.: IMPACT OF LIGHTNING AND CONVECTION ON NO AND NOr
28,371
120
ß
1200
o CON
ß a MT ß ß NHC
110
ß SHC 1 ooo
o a ß ß
,x
CON MT NHC SHC
o
100
ß
800 >
600
e
o
90
•o 880 0
0
80
4OO
70 ••
2OO
¸ ChristmasIs. (2 øN) X Samoa (14 øS) I i i i i i i l,l
-10
0
10
20
30
-10
0
• I I I I I I I I I I I II
10 Latitude
Latitude
I II
20
I I i I
30
Figure2d. SameasFigure2a, but for NOy.
Figure 21'. Same as Figure 2a, but for CO. The CO mixing ratio measuredby CMDL/NOAA at ChristmasIsland (2øN, 157øW)and Samoa(14øS,170øW)are alsoshown.Four-year Themedianvaluesof theNOy,03, andCO mixingratiosin the average(1990-1993)andfive-yearaverage(1989-1993)of the CON air masseswere 383 pptv,41 ppbv,and 82 ppbv,respec- monthlymean value in Februaryare presentedfor Christmas tively. These values in the MT air masseswere similar to the valuesin the CON air masses,althoughthe variabilityof these valueswas larger in the CON air than in the MT air.
Island and Samoa,respectively.
Vertical profilesof the medianvaluesof the variousspecies obtainedduring PEM-West B are presentedby Kondo et al. [thisissue(a)] for the differentair massesdefinedusingtrajectories.The continentalair definedby Kondo et al. included the CON air definedin this study,althoughit alsoincludedair massesobtained at 30øN-40øN.The vertical profiles of the
were sampled,as seenin Figure 1. This suggests that to some
mixing ratios of CO and NMHCs show that influencesfrom anthropogenicsourceswere generallylimited to altitudesbelow 4 km in the continental
air. The median values of the CO
extent the MT
air masses were mixed with the CON
air. The
relativelysmallinfluencesfrom anthropogenicsourceson the CON air and somemixingprocessbetweenthe CON air and MT air probablycausedthe similarityin the CO mixingratios between
the two air masses.
The mixingratiosof NO andNOr in the CON andMT air massesare consideredto representvalues in the air masses transportedfrom midlatitudesinto the tropicalregion during
mixingratioswere 176and 109ppbvin the lower(0-4 kin) and this season. Because the influences from the continental suruppertroposphere(7-12 kin), respectively. The medianvalue face were weak in the upper troposphere,intrusionof strato(and central67% range) of the CO mixingratiosin the CON sphericair, NO productionby lightning,and aircraft emission to be important sources for NO•candNOr at air massesdefinedin this studywas 82 ppbv (70-95 ppbv), wereconsidered which was systematicallylower than that of the continental air midlatitudes[Kondoet al., thisissue(a); Koikeet al., thisissue]. Regenerationof NOx from HNO 3 can also be an important as definedby Kondo et al. The MT air masses remained over the ocean for more than source of NO x both at low and middle latitudes. Higher 8 ratios in the upper tropospherethan in the 10 daysprior to the measurements,as describedabove.How- (NOx)m½/C3H the importanceof in situ ever,the locationof the air massesseveraldaysprior to sam- lowertropospherefurther suggested pling overlappedwith the locationwhere the CON air masses 8O
6O
l/
o CON
A MT
I
I
i
ßSHC NHC •[-ß
OCON
A MT
•o
' 15
•Xos
• 10
ß NHC
ßSHC
5
2O
ß
o1__
:,,,•
-10
-10
0
10
20
Latitude
Figure 2e. Same as Figure 2a, but for 03.
0
30
Figure 2g.
"!l",
,l
,
10 Latitude
20
3O
Sameas Figure2a, but for the NOy/O3 ratio
(pptv/ppbv).
28,372
KAWAKAMI ET AL.: IMPACT OF LIGHTNING AND CONVECTION ON NO AND NOr
were encounteredover the wide latitudinal rangeduringPEMWest B were unprecedentedly low. Very low 03 mixingratios,with a medianvalue of 19 ppbv, were observedin the NHC air simultaneously with very low
• CON /• MT
;
0.8
• .oø
ß NHC
.
ß SHC
e•
NOyvalues, asseeninFigure2e.Removal of 03 byreaction of O(•D) andH20 andsurface deposition occurs morerapidlyin
o
0.6•, .•.: _•t.
ø
,
ø
.:.
0.4 '•.'
;
!j'
0.0
-10
0
10 Latitude
20
30
the humid tropical marine boundarylayer than in the dryer free troposphere.The photochemical lifetime of 03 at 0øN18øNduringPEM-West A wascalculatedto be 12 and 56 days at 0 and 9 km, respectively[Daviset al., 1996a].Indeed,very low 03 mixingratiosof 9-12 ppbvwere observedin the tropical marineboundarylayer and lower free troposphereduring PEM-West A [Kondoet al., 1996;Gregoryet al., 1996;Singhet al., 1996].During PEM-West B, 03 mixingratiosin the tropical marineboundarylayerwere observedat iøN duringflight 9, 12øN-15øNduring flight 7, and 16øNduring flight 8. Each measurement
was made at 0.5 km for about 1 hour. The ob-
Figure2h. Sameas Figure2a, but for the (NOx)mc/NOy served03 mixingratioswere 20-25 ppbv,whichwasclosem ratio.
the medianvalue of 19 ppbvobservedat 9.5 km. The low 03 valuesin the NHC air were primarilydue to the convective transportof 0 3 poor air from the marine boundarylayer,
NOx sources[Kondoet al., this issue(a); Crawfordet al., this consistent withthe lowNOyvalues. issue(b)]. Altitudeprofilesof thevaluesof NOy, NO, the (NOx)mc/ 3.3. ConvectiveTransport Over the Pacific Ocean
NOy ratio,relativehumidity,03, CO, andthe NOy/O3ratio were obtainedduringa spiraldescentand a ramp ascentmade
3.3.1. NO, NOy,and0 3in NHC air. Themostprominent aroundiøNduringflight9 (Figure4). TheNOyandNO mixing feature in the latitudinal distributionswas the very low values
ratiosat 4-12 km were 30-80 pptv and below 15 pptv,respec-
ofNO andNOyobtained at9.5kmat løN-14øN intheNHC air tively. The 03 and CO mixing ratios were quite uniform at 4-12 km, about 18 and 80 ppbv, respectively.The relative andNOywere7 and51 pptv,respectively. TheNO andNOy humiditywasgreaterthan60% up to 12km. The highhumidity mixingratioswere quite uniform over theselatitudes,and the andverylowmixingratiosof NOyand03 between 4 and12km with stronginfluencesby deep convection.The variabilitywasmuchsmallerthan in the other air masses.The are consistent massasshownin Figures2c and 2d. The medianvaluesof NO
increased below4 km with de67%rangeof theNOymixingratioswas31-76pptv.In addi- NOy mixingratiosgradually tion to the valuesat 9.5 km, the samelevelof NOywasalso creasingaltitude,reaching130pptvat 0.5 km. The CO mixing observedat 12 km at the samelatitudes.(Thesevaluesare not ratioswere alsohigherbelow4 km (-100 ppbv)than thoseat plottedin Figure2d becausethe datacompletely overlapwith altitudes above 4 km. The wind direction measured on board thoseat 9.5km.) Thisindicates thattheverylowNOyair was the aircraft was easterlyand northwesterlyat altitudesabove distributed
at least between 9.5 and 12 km at these latitudes.
and below 4 km, respectively,suggestingdifferent air masses
The locations whereNOy mixingratioslowerthan80 pptv were observedat these two altitude regimes. were observedbetween8.2 and 12.0 km are shownin Figure 3.
TheselowNOyvaluesweremostlyobserved duringflights9 and 10, althoughsomedatawere alsoobservedduringflights6 and 11. As describedabove,the high mixingratiosof H20 and CH3I in the NHC air massessuggestthat they were recently transportedfrom the marineboundarylayerby deepconvection. Consideringthe higherremovalrate of HNO 3 by wet and dry depositionin the tropicalmarineboundarylayercompared to that in the free troposphere[e.g., Giorgiand Chameides,
30 øN
20 øN • .F11
xm
• F8
'o .• 10ON
1986],thelowNOymixingratiosin theNHC airwereprimarily dueto transport of NOypoorair fromthe marineboundary
'•F10
layer.
•
L •,F6
0 o
NOymeasurements weremadeat Hawaii(20øN,156øW) at 3.4 km during the Mauna Loa ObservatoryPhotochemistry
Experiment (MLOPEX).The medianandminimum NOyval-
10 øS
uesfor the free troposphericair were 262 and 65 pptv,respec-
120øE
130øE
, , 140øE
150øE
Longitude
tively[Htibleret al., 1992a].The medianandminimumNOy
values in the marine free troposphericair obtained during whereNOymixingratioslowerthan ChemicalInstrumentationTest and Evaluation2 (CITE 2) Figure3. Thelocations 80 pptv were observed at altitudesbetween9.5 and 12 km made near the west coast of the United States were 298 and
(solidcircles). Thelocations wherethevaluesof (NOx)mc/NOy
84 pptv,respectively [Htibleret al., 1992a,b]. In the tropicalair ratio and NO mixingratio higherthan 0.4 and 50 pptv,respecsampledbetweenCalifornia and Tahiti at 10-12 km during tively,were observedare shownas opensquares.Flight tracks AASE2, theNOyvaluesweregenerally between 100and300 of flights6 and 8-11 are alsoshownirrespectiveof the flight
pptv[Folkins etal., 1995].TheNOyvaluesin theNHC airthat
altitudes.
KAWAKAMI ET AL.: IMPACT OF LIGHTNING AND CONVECTION ON NO AND NOv
28,373
12
10
i 8
0
40
0
80
120
0
5
NOy(pptv)
A E
10
15
20
NO(pptv)
12
12
10
10
!
0.0
0.2
0.4
0.6
(NOx)m•NOy (pptv/pptv)
i
''
(f)
•"'
'
,•
,
0
20
40
60
80
100
Relativehumidity(%)
ig)
8
0
,
10
0
20
30
50
03 (ppbv)
t
70
,
I
90
,
,
110
CO (ppbv)
0
I
2
•
I,
4
,
I
6
,
8
NOy/O 3(pptv/ppbv)
Figure4. The verticalprofilesof NOy, NO, the (NOx)mc/NOy ratio,relativehumidity,03, CO, andthe NOy/O3ratioobtained duringflight9. Opencircles represent thedataobtained duringtherampascent at iøN, and solid circlesrepresentthe spiral descentdata obtainedat iøN. have been affectedby precipitationwere sampledduring flight 9 on February 19. Precipitationpersistentlyoccurrednorth of New Guinea Island betweenFebruary 17 and 22, althoughthe valueof the NOy/O3 ratioin theNHC air masswas2.7pptv/ correspondingplates are not shown.As describedlater, this ppbv. The air in the tropical marine boundarylayer was sam- region coincidedwith the large scale convectiveregion. Conpled for about 1 hour at iøN followingthe measurementsat 9.5 sideringthe trajectoriesof the air masses,the air massessamkm duringflight 9. The medianvalue (and 67% range) of the pled during flight 10 could have alsobeen affectedby precipNOy/O3 ratio in the boundarylayerwas 4.8 (and 3.5-6.2) itation (Figure 1). These data supportthe aboveproposition pptv/ppbv,whichwasa factor of 1.8 higherthan the value at 9.5 that a portion of the HNO 3 had been removedduring conveckm. The 03 mixing ratios were similar between the marine tive transport of the NHC air. As shown in Figure 4g, the boundarylayer air and NHC air at 9.5 km, while the median NOy/O3 ratio changed little at altitudesabove4 km andwas NOymixingratioin themarineboundary layerairwas91 pptv, 2-3 pptv/ppbv.Thesevalueswere closeto the medianvalue of which was a factor of 2 higher than those at 9.5 km. These 2.7 pptv/ppbvobservedat 9.5 km, suggestingthat the removal
3.3.2. Heterogeneous removalof NOy. The NOy/O3 ra-
tios in the NHC air masswere systematicallylower than those in the other air masses,as shownin Figure 2g. The median
resultssuggest thepossible removalof NOy duringconvective of NOy couldhaveoccurredmainlyat altitudesbelow4 kin. transportof tropicalmarine boundarylayer air. Marine boundary layer air was alsosampledat 13øNduringflight 7 whichwas made 4 days before flight 9. The median value (and 67%
On the other hand, it should be noted that although the
NOy/O3 ratiosin the marineboundarylayerair observed at iøN were used as a reference value, air massesbelow 4 km had
comparedto thoseat range)of the NOy/O3ratiowas4.2 (and3.2-5.0)pptv/ppbv, slightlydifferentchemicalcharacteristics whichwas also a factor of 1.6 higher than the value at 9.5 km. altitudes above 4 km, as describedabove. Consequently,the It wasfoundthatmorethan80% of NOywasin the formof NHC air observedat 9.5 km might have been affectedmore by HNO3 in the tropicalmarineboundarylayer [Kondoet al., this air originating from the marine boundary layer at different issue(a)]. Consequently, if precipitationoccurredduringthe locations.There is therefore a limitation in making a quanti-
convection, the NOy levelwouldhavebeenfurtherreduced tativeestimateof the degreeof the NOy removalduringthe throughefficientrainoutof HNO 3.A high degreeof additional 11i311JU¾O.l UI IlI•U 3 UUllll• tll• LZi:tIlS[OUIL CUUJLI IlaV½causedthe 1
•œ
T T't•.T/'"•
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z,............
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unprecedentedly lowNOylevelin theuppertroposphere. The precipitation rate measured by the Defense Meteorological Satellite Program (DMSP) satelliteon February 18, 1994, is shown in Plate
1. It can be seen that air masses which could
convectivetransport using only a comparison between the
NOy/O3 ratiosobserved at 9.5 km andin the boundary layer. There is another point which makes the aboveproposition lessconclusive.The removal of HNO 3 shouldhave increased
the (NOx)mc/NOy ratio and decreased the HNO3/NOyratio. However,the medianvalueof the (NOx)mc/NOy ratioin the
28,374
KAWAKAMI ET AL.: IMPACT OF LIGHTNING AND CONVECTION ON NO AND NOr
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