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JOURNAL

OF GEOPHYSICAL

RESEARCH,

VOL. 97, NO. D15, PAGES 16,481-16,509, OCTOBER 30, 1992

Summertime Tropospheric Observations Relatedto N xOy Distributions and Partitioning Over Alaska: Arctic Boundary Layer Expedition 3A S. T. SANDHOLM,1 J. D. BR^DSH^W,1 G. CHEN, 1 H. B. SINGH,2 R. W. TALBOT,TMG. L. GREGORY,3 D. R. BLAKE, 5 G. W. SACHSE,5 E. V. BROWELL,3 J. D. W. BARRICK,3 M. A. SHIPHAM,3 A. S. BACHMEIER,6 AND D. OWEN7 Measurementsof the reactive odd nitrogencompoundsNO, NO 2, peroxyacetyl nitrate (PAN), and

NOy are presented for the summertime middle/lower troposphere (6.1-0.15km) overnorthernhigh latitudes. In addition, the chemical signaturesrevealed from concurrent measurementsof 03, CO, C2H2, C2H6, C3H 8, C2C14,and H20 are usedto further characterizefactors affectingthe budget and

distribution of NxOyin theArcticandsub-Arctic tropospheric air masses sampled overAlaskaduring the NASA Arctic Boundary Layer Expedition (ABLE 3A) field campaign. Many of the compounds listed above exhibited a general trend of median mixing ratios increasing in proportion with altitude within the lower 6-km column. However, median mixing ratios of NO and NOx (NO + NO2) were nearly independent of altitude, having values of about 8.5 and 25 pptv, respectively. Median mixing

ratiosof NOy variedfromabout350pptv withinthe lowestaltitudesto about600pptv withinthe highest altitudes sampled. PAN constituted thelargestfractionof NOy (--•50%)at thehighest altitudes. In addition, PAN mixing ratios accountedfor all of the approximate60 pptv/km altitudinal dependency

in NOy. Theanalyses presented implicate biomass burningin Siberiaastheprobablesourceof about one-third oftheNOyabundance withinthemiddle/lower troposphere overAlaska.Theseanalyses also implicate the downward transport of air from altitudes in the vicinity of the tropopause as a major

contributorto the abundance of NOy (-•30-50%) within the lower 6-km columnover Alaska. However,the exactoriginof thishigh-altitude NOy remainsuncertain.The impactof lowerlatitude industrial/urbanpollution also remains largely uncertain, although various chemical signaturesimply inputs from these regions would have been relatively well aged (15-30 days).

1.

INTRODUCTION

also be significantly influenced by naturally occurring sources of trace gases. Stratosphere-troposphere exchange During the summertime, Alaska is situated between the can occur through several mechanisms, such as tropopause Arctic and polar jets, resulting in air mass compositions folding associated with jet streaks and the Polar and the affected by the mixing of cold Arctic air with warmer North Arctic jet streams, large-scale subsidence, and the developPacific maritime air. Consequently, Alaska constantly re- ment of cut-off lows. All of these mechanisms can provide ceives air masseshaving 2-5 day transits from areas that are inputs of O3 and other trace gasesto the middle troposphere. relatively free of major anthropogenic pollution sources. The peak activity in such processes begins in early spring This suggeststhat the Alaska troposphere should be rela- and extends into summer [e.g., Danielson, 1968; Raatz et tively free of short-livedcompounds(e.g., NO, NO2, C3H6) al., 1985; Shapiro et al., 1987; Reiter, 1975; Vaughn and originating from anthropogenicsources [e.g., Miller, 1981; Price, 1989;Ebel et al., 1991]. Biomass burning in sub-Arctic Patterson and Husar, 1981; Shaw, 1981, 1988; Carlson, regions releases a variety of trace gases to the atmosphere. 1981;Rahn, 1981;Raatz et al., 1985]. However, longer-lived Most of the area burned in these regions is the result of compoundsthat typically have a free tropospheric lifetime of naturally occurring fires, which can be promoted during later weeks to months(e.g., 03, CO, CFCs, C2H2, C2C14)can be phasesof ecosystem succession[Van Wagner, 1988; Chapin transported to Alaska from anthropogenic source regions and Shaver, 1985; Stocks, 1991]. This natural source of trace located thousands of kilometers away. Transport times gases may significantly influence the chemical characterisranging from 1 to 3 weeks can spread such source influences tics of the Alaskan summer troposphere. Intermixing these from mesoscale to Arctic-scale dimensions [e.g., Shaw, near and distance sources can result in a complex matrix 1981; Patterson and Husar, 1981]. representing both natural and anthropogenic influences on The chemical composition of Alaska's troposphere can the troposphere over Arctic and sub-Arctic Alaska. The distribution and abundanceof "reactive" odd nitrogen com-

•School of EarthandAtmospheric Sciences, Georgia Institute of Technology, Atlanta.

2NASAAmesResearch Center,MoffettField,California. 3NASALangleyResearch Center,Hampton,Virginia. 4Nowat Institutefor the Studyof Earth, Oceans,and Space, University of New Hampshire, Durham.

5Department of Chemistry, University of California at Irvine. 6planning Research Corporation, Hampton,Virginia. 7S.T. Systems Corporation, Hampton,Virginia. Copyright 1992 by the American Geophysical Union. Paper number 92JD01491. 0148-0227/92/92JD-01491 $05.00

pounds(NxOy) originatingfrom thesesourcescan significantly influence tropospheric photochemistry in these environments.

The NxOy family of reactiveodd nitrogencompounds contains both highly reactive compounds (e.g., NO, NO2, and NO3), reservoir compounds(e.g., peroxyacetyl nitrate (PAN), HO2NO2, and N205), and compounds that act as virtual sinks (e.g., HNO3 and particulate-nitrate (p-NO•-)). In remote regions free from significant direct sources, tropospheric NO and NO 2 concentrations are often small (mixing ratios 1 x 10-5 K hPa-• s-1) in areasnear 4- to 6-km, and 6- to 8-km altituderegions,respectively.These the Gulf of Alaska low and a strong low-pressure system estimatesonly apply to the air massesoverflow, and their north of the Aleutians (183øW, 55øN). These regions of measurements were limited to cloud-free conditions. enhanced potential vorticity covered areas of about 2-4 x Several scientificissuesmerit clarificationprior to our use 105km2 andwerecentered neartheindicated originof the of these classificationsfor assessingfactors controlling the sampled air mass (15IøW, 5 IøN on July 8 and near 178øW, abundanceof reactive odd nitrogen.In particular, air sam- 49øN on July 10-11). Three-day isentropic back trajectories pled during this program that was characterizedas originat- along the O = 310øK potential temperature surface (--•500 ing from the stratosphereusually containedmixing ratios of mbar) indicated an almost due north flow of air along the H20 that are more typical of the middle/uppertroposphere high-pressureridge. These trajectories indicated the air mass (i.e., >> 15 ppmv). In addition, the representativeness of the originated near 168øW, 57øN at the northern end of the carefully characterized haze layers derived from Kuskok- Aleutians between the two low-pressuresystemson July 10, wim Delta biomass burning emissionsshould be addressed 1988. These back trajectories also crossed regions where prior to extrapolating emission factors to the majority of several large fires were burning in the southwesternfoothills Alaskan or other sub-Arctic fire emissions. In order to of the Brooks Range (near 160øW, 66øW) about 24-32 hours further define the possibleimpact of haze layers and strato- prior to reaching the Barrow area (see Shipham et al. [this spheric/tropospheric exchangeon the reactive odd nitrogen issue] for fire locations). budget within the middle/lower tropospheric column ( 70 ppbv) comparedto mixingratiosin backgroundair. The mission 18 (on July 31, 1988) near Bethel. In this case,

16,486

SANDHOLM ET AL ' NxOr DISTRIBUTIONS, ABLE 3A AEROSOL 0 0.1 I

I

1 71 9 I

SCATTERING 0.2 0.3 I

1 730 I

I

RATIO 0.4

(VIS) 0.5

I

1 740

RELATIVE

0I LT

I

1000 I

1 71 9

AEROSOL

2000 I

SCATTERING

3000 I

1730

I

4000I 1 74-0

I

LT

I

,½OASTiPTA

,,

(IR)

5000 m

,COASTiPTA •

lO

lO

_1

4.

3

2

0 5 72.34

I 151.02

71.62

I 1 52.05

70.90

N LAT.

I 1 53.00

w

LON.

72.35

71.62

I

70.90

I

I

151.01

iCOASTPTA

13-

•e_

:.,,-.%,

[13'

-.

12

11

10

OZONE

0

0i

OZONE MIXING RATIO (PPBV) 30i 60i 90i 1 20 1 50 i

Plate 1. Aerosol and 0 3 distribution measurementsmade on July 12, 1988, during mission 6. The zenith airborne differential absorptionlidar (DIAL) aerosol (top panel) and 03 (bottom panel) data are shown in false color display with the relative amount of atmosphericback scatteringand 03 mixing ratio in parts per billion by volume (ppbv) defined at the top or bottom of the respective displays. In either case, black represents values greater than the maximum given on the color scale. The altitude is in kilometers above sea level (ASL). Local time is at the top of the aerosol display, and the aircraft latitude and longitude information is given in degreesat the top of the 03 display.

i

Plate 2.

15 i

MIXING

30 i

RATIO

45 i

(PPBV)

60 i

75 i

Nadir aerosol and 0 3 distributions obtained by the

airborne DIAL system on July 12, 1988, at the same time as the zenith data presented in Plate 1.

north of Prudoe Bay. Calculated Epv values were larger than the stratospheric threshold value in the vicinity of the Aleutian low, near the region where this air mass appears to have originated. A complete meteorological analysis (in progress),capable of providing a detailed characterization of mechanisms affecting these and other ABLE 3A case studies, is beyond the scope of this paper and will be presented in a subsequentpaper(s).

The ratio NOy/O3 had a value of about 0.12 for air however, the O3-enhancedregionappearedin the form of a distendedtropopauserather than a more well-defined 03-

suspected of having a stratospheric origin, which was encountered sporadicallyalong the 6.1-km flight leg of mission

enhanced stratum (cf. Plate 3). Back trajectory analysis also indicatedthat movement of this air parcel was controlled by a ridge of high pressure,which had built up along a line from Norton Sound to Anchorage, concurrent with a low-pressure system over the Aleutians and a strong Polar low located

6. ThisNOy/O3 ratiowaslargerthanratiosexpectedfrom high-latitude middle-stratospheric air (--•0.005 [Murphy et al., 1992]). This larger ratio could suggest a large fraction

(--•50%)of theNOy abundance withinthe depleted-aerosol/ enhanced-O3air parcel may have originated from sources

SANDHOLM ET AL..' NxO r DISTRIBUTIONS, ABLE 3A

16,487

other than the stratosphere,unlesssignificantair massaging

resultedin anincreased lossof 03 relativeto NOy duringthe downward transport of this air from altitudes well above the tropopause (i.e., >4 km above the tropopause). Several compounds exhibited enhancements, which correlated with those of CO, within the haze layers on mission 6. Enhancement factors relative to CO (i.e., AM//XCO) for these layers differed somewhatfrom those characterizedfor several of the other haze layers encountered during the ABLE 3A program [Wofsy et al., this issue]. The enhance-

NOy(ppfv) 200

1200

2200

co

R.H

mentfactorANOy/ACOextrapolated fromthe6.1-kmflight legportionof mission 6, wheremixingratiosof NOy reached a maximum (1.9ppbv),yieldeda ANOy/ACOof about0.025 (_+0.01). For the chemically enhanced layers encountered

near5.0 and4.3 km duringthe spiraldescent,ANOy/ACO was approximately 0.015 (_+0.005). These enhancementfactors were larger than the 0.003-0.008 range of values reported for two layers producedfrom biomassburning within the Kuskokwim Delta region, and are closer to enhancement factors reported for biomass burning in other regions [see Wofsy et al., this issue]. Enhancementfactors for C2H 6 and

20

IO0

180

C3H8 relative to CO also differed from those found in the ß

i

i

t

i

i

i

haze layers over the Kuskokwim Delta. However, the mix0 50 100 ing ratios of these hydrocarbon compounds and CO found outside of the haze layer on mission 6 did fall within the R.H. range characterized as typical for background high-latitude Fig. 2. Vertical sounding taken near Barrow, Alaska, during air observed during the ABLE 3A program [Blake et al., this mission 6 (70.3øN, 153.7øW) on July 12, 1988, where 03, CO, issue]. These larger enhancement factors may have resulted relativehumidity(% RH), and NOy (crosses)are shownas a from differences in the ecosystem types burnt (e.g., taiga function of altitude. versustundra/Borealfores,t),or the ecosystemages, or the ages of the plumes, or a combination of all these factors. This suggests that a larger range of enhancement factors back trajectory analyses along the © = 310 K potential might be implied for high-latitude haze layers produced by temperature surface(--•500mbar) indicated northerly air flow biomass burning than the limited range found over the extending to about 83øN. Past this point, two possible air mass origins were indicated: the Taymyr Peninsula and the Kuskokwim Delta region. The thin O3-depletedstratum occurringnear 10-10.5 km Queen Elizabeth Islands. Analyses along the © = 300 K (Plate 1) was observed on several occasions. This type of surface (--•625 mbar) followed a similar path but only toward layer was usually observed just below, or in association the Queen Elizabeth Islands. As in mission 6, calculated with, a thin layer of enhanced aerosol scattering. These values of Epv (at the 500-mbar level) exceededthe threshold strata predominantly occurred within about 1 km of a value for stratospheric air in the vicinity of both of the generallywell-defined(via measured03 profile) tropopause. indicated air mass origins (i.e., the Taymyr Peninsula and This trend was also observed over high-latitude regions of the northern end of Baffin Bay, 75øW, 75øN). Although Canada during the ABLE 3B program (July/August 1990). certainly not conclusive, this tends to support the speculaOn several occasions during the ABLE 3B program, por- tion that the 4-km air parcel was of stratosphericorigin. Figure 3 illustrates the vertical soundingsobtained from tions of this type of stratum were believed to have been sampled following these air parcels' downward transport the in situ measurements of CO, 03, NOy, and relative intothemiddletroposphere. NOy wasfoundto be enhanced humidity (% RH) made during a spiral descent near 156.8øW, by about 1.5- to 2-fold within these O3-depletedair parcels, 72.2øNon mission12. Extrapolation of the ratioANOy/AO3 determined from the difference in mixing ratios between the whereas CO mixing ratios were near background values (R. W. Talbot et al., Summertime distribution and relations of O3-enhancedair parcel near 4 km and the air samplednear 3

3 --• 0.01. This valuewasonceagain reactivenitrogenspeciesandNOy in the troposphere over km yieldedANOy/AO Canada, submitted to Journal of Geophysical Research, 1992). The origin of these air parcels remains a mystery, although there was some indication (as is the case shown here for mission 6) that advection from high-altitude air traffic corridors may have been important. The chemical characteristicsof these O3-depleted strata certainly merit future investigation. 4.1.2. Case study mission 12. The air mass sampled near Barrow during mission 12 (on July 21-22, 1988) also contained an air parcel of suspected stratospheric origin, characterized by small values of aerosol scattering and

larger than that expected for stratospheric air. As in the

mission 6 casestudy,the magnitude of theNOy/O3ratioin

conjunction with the general wetness of the air parcel suggeststhat this air parcel was modified with tropospheric air during its downward transport. Such chemical modification would have been necessaryfor this air parcel's origin to have been from regions well above the tropopause. Mixing ratiosof C2H2, C2H6, C3H8, and CO within the air parcels near 4 and 3 km were near or slightly smaller than values characterized as typical for high-latitude background air [Blake et al., this issue]. This, in conjunction with the enhanced03 mixingratios(cf. Plate 4). Three-dayisentropic small NOx mixing ratios (--•20 pptv) measuredin the air

16,488

SANDHOLM ET AL.' NxO r DISTRIBUTIONS, ABLE 3A AEROSOL 0 0.3 ß

SCATTERING 0.6 0.9

RATIO 1.2

= 300 K potential temperature surface indicated descending northerly air flow for the 3-day period prior to this mission (13). This flow pattern was preceded by cyclonic air flow around a strong Polar low during the fourth and fifth days prior to the mission. These trajectories indicated an air mass origin near 105øW, 75øN (at---550 mbar, on July 20, 1988) that was transported to 162øW, 61øN (at ---700 mbar, on July 25, 1988). Calculated Epv values for the 500-mbar pressure level exceeded threshold values for stratosphericair, on July 21 and 23, 1988, near the center of the low located just west

(VIS) 1.5



1006

1020

I

I

1030

iPT3

LT

iPT2

6

O(3p)+ M, kq;

--• 4 x 10-12 exp (210/T) with kl-2 representing the

HO 2 + 03 --> HO + 202, k6;

and

0 3 --• surfaceremoval, kd.

The loss rate of 03, LR(O3), based upon reactions(R3)(R7), is given by

LR(O 3) = (k6[HO2] + BR JO3 + kd)[O3],

k6103]

[NOeql] kl[HO2 ]+k2[RO 2] 1.4kl_ 2,

versus

(R7)

[NOeq] = (kl[HO2] +k2[RO2]) . k6[HO2][O3]

followed by

(R6)

(k6[HO2] + BR JO3 + kd)[O3]

0 3q-hv-• O(1D)q-02, JO3,

(R3)

(R5)

(3)

[NOeq],neededfor a zero net rate of 03 production(i.e.,

(1)

The lossprocessesof 03, underconditionstypical of these air masses,is primarily controlledby the reactions

(R4)

BR = k4[H20]/(kq[M ] + k4[H20]).

(2)

average of kl and k 2 (rate coefficientsfrom DeMore et al. [1990]). Equation (5) represents the smallest abundance of NO necessaryfor net photochemicalproduction and provides a usefulreferencepoint for comparisonsinvolvingthe additionalassumptionsnecessaryto evaluate (4). For the NOx mixing ratios measuredover Alaska (i.e., NOx < 100 pptv), the concentration of peroxy radicals has

beenpredictedto be nearlyindependentof NOx concentra-

16,496

SANDHOLM ET AL.' NxOr DISTRIBUTIONS, ABLE 3A (,,)

450

6O

I 3OO

1,50

o lOO

' 560 ' 560 ' 760 ' 960 ' 1100

z20 t O/

.

0

IO0

400

300

2OO

PAN(pptv)

NOy(pptv)

(b)

5O

(d)

55

•35

•40

z 20

z 25

::

5

100

........

460

7(•0

1OhO

1300

Fig. 7.

10

80

'

160

'

1•:0

,

1•,0

,

160

CO(ppbv)

NOy(pptv)

Correlation plots of aggregateddata, where the horizontal and vertical lines represent _+1 sigmaabout the

mean oftheaggregate: (a) PANversus NOywithanaggregatesize (AS) of 15 measurements(r 2 = 0.80 and slope of 0.30-+0.05•,(b)NOxversus NOywithAS= 26(r2 = 0.52andslope of0.014 + 0.0049), (c)NOxversus PANwith AS = 15(r • = 0.59 andslopeof 0.034-+0.010),(d) NOx versusCO with AS = 31 (for the first six aggregates,r 2 = 0.63, and slope of 0.53 -+ 0.20).

tion [e.g., Logan et al., 1981]. These estimatesalso indicated addition, JO 3 values were also corrected for daily total 0 3 mixing ratios of peroxy radicals were relatively independent column over the study region using the tabulated total ozone of altitude within the lower 6-km column near 60øN, with mapping spectrometer (TOMS) 1988 data. For clear-sky ([HO2] + [RO2])/[M] --• 25 pptv. A simplificationof (4), conditions, comparison of values obtained from these pawhich neglectsk d, but includesan estimationof the direct rameterized photolysis rate equations to those derived from photolytic loss of 03, is given by a two-stream model have agreed to better than _+20%over the range of solar zenith anglescontained within this filtered k] [03] BR JO3103] data set (38 ø < x < 67.5ø). = + . (6)

[NOeq2] 1.4kl-2kl-2([HO2] + [RO2])

Equations (5) and (6) were evaluated using the assumptions described above and for the individual 03, H20, and temperature measurementswithin the composite data set. The 03 photolysisrate, JO3, was derivedfrom the estimates of Demerjian et al. [1980] using a parameterized fit, which was similar to that described for JNO 2 by Parrish et al. [1981], given by

JO3 = a(exp [b secX]),

(7)

where a and b are linearly fit functions of albedo (a) and altitude (2) (e.g., a = c + da + e2) and X is the solar zenith angle. These clear-skyJO3 valueswere normalizedto JNO 2 values calculated from the Eppley UV-photometers using similarly parameterized clear-sky estimatesof JNO 2. This normalization produced results equivalent to those described by Chameides et al. [1990] for the normalization of clear-sky two-stream model photolysisrate estimates.This normalization, based on nadir and zenith Eppley UVphotometers, was assumed to provide a first-order correction for varying albedo. This correction was also assumedto be wavelength independentover the range of 300-400 nm. In

Figures10a and 10bdepictthe ratiosNOeql/NOmeas and NOeq2/NOmeas , whereNOmeas was the measuredmixing ratio coincident with measurementsof 03, H20, temperature, pressure,and UV solarflux. A nearly constantvalue of

[NOeq]]with altitudecouldbe predictedfrom(5), basedon the small temperature dependenciesof the rate coefficients [DeMore et al., 1990] and the nearly constant median

concentration of 03 versusaltitude(--•1x 10]2O3/cm3; cf. Figure8a). MedianNOeql/NOmeas ratioswere nearunity and varied only slightly with altitude, indicating measured NO mixing ratios were close to the lower limit for net 03

production estimated from(5). MedianNOeq2/NOmeas ratios were about twofold larger and also varied only slightly with

altitude.The smallaltitudedependency impliedfor NOeq2 was due, in part, to the nearly constant terms (k and [03]) discussed for (5) and the nearly constant value for BR JO3/[HO2]. The nearly fourfold smaller value of BR occurringin the highestversus the lowest altitude range was almost completely offset by the nearly twofold smaller valuesof [HO2] + [RO2] (for constantmixing ratios of HO2 + RO2), and the nearly twofold larger values of JO 3. These results suggest that the photochemical 03 loss rate via

SANDHOLM ET AL.' NxOr DISTRIBUTIONS, ABLE 3A

16,497

(d)

(c)

(f)

35 0

35

:;'5

•5 I.

•o

F•l.t i'tud e

F•1 .tt'tude(.Am)

Fig. 8. Composites ofvertical profiles takenoverAlaska,where(a) 03, (b) NOy/O3,(c) CO,(d) C2H2,(e) C2H6,

and(f) C3H8 havebeenplottedin altituderanges0.1-1.5, 1.5-3.0, 3.0-4.5, and4.5-6.1 km. The relativeprobability representedon the z axis is describedin Figure 6.

16,498

SANDHOLM ET AL.' NxOr DISTRIBUTIONS, ABLE 3A (o)

(c)

lOO

9O 8o

.a 70 =.

60

o

o

50

4O

20 lOO

3O

0

20

50

40

50

•00

500

(b)

lOO

900

1 oo

(d)

lOO

+++

8o

700

NOy(pptv)

•x (•tv)

8o

=.

60

o

4O

20

0

100

200 ..300 PAN(pptv)

20

400

aO

.

11•0

.

1:•0 140 CO(ppbv)

160

Fig. 9. Correlation plotsforaggregated data,wherethehorizontal andverticallinesrepresent -+1 sigma aboutthe

mean oftheaggregate: (a)03versus NOxwithanaggregate size(AS)of26measurements (r2 = 0.79forallaguegates andr 2 = 0.84 for thefirstsevenaggregates witha slopeof 1.32-+0.26),(b) 03 versusPAN withAS = 19(r• = 0.92

forallaggregates andr 2 = 0.98forthefirstseven aggregates witha slopeof0.22-+0.015),(c) 03 versus NOywithAS - 20 r: - 0 71 for all a re atesandr 2 - 0 97 for the firstsevenaggregates with a slopeof 0 109+ 0 0091) (d) 03

-

( - '

gg g

- '

2

'

• '

'

versusCO withAS = 38(r2 = 0.74for all aggregates andr = 0.90for thefirstsevenaggregates w•tha slopeof 1.48 -+ 0.22).

(R3)-(R5) was of comparablemagnitudeto that occurringvia ratioof [NO2]/[NO]is controlledprimarilyby the reactions (R6). In addition, these estimatessuggestthat, on average, (R1), (R2); (RS), (R9), involving mixingratios of NO were abouttwofold smallerthan those (R8) NO + 0 3 • NO 2 + 0 2, ks; requiredto balancethe photochemical ratesof 03 lossand production.

(R9) NO2 + h v--> NO + O, JNO2. In the companionpaper of Jacob et al. [this issue], clear-skyphotochemical modelcalculations predictedsignif- The steadystateratio of [NO2]/[NO] is given by

icant net O3-photochemical loss throughoutmost of the lower 6-km columnover Alaska(-1.8 x 105molecules [NO2]/[NO]= (k8103]+ kl[HO2] + k2[RO2])/JN02 (8) cm-3 s-1 diurnallyaveraged). The estimates presented here Under atmosphericconditionswhere [HO2] and [RO2] are are in generallygoodagreementwith the modelcalculations suppressed (e.g., [NOx] > 1 ppbv),(8) canbe reducedto the of Jacobet al., althoughsomedifferencesoccurat the lowest simplephotostationarystaterelationship: and highestaltitudes.These differenceswere likely due to [NO2]/[NO] = k8103]/JN02. (9) differences in the data set analyzed (e.g., the data set analyzedby Jacob et al. includeda large fraction of data taken within the mixed layer over the tundra). Even so, their From (9) an estimateof the smallestabundancesof NO2 can be obtainedfrom measurements of NO, 03, estimates within the 1.5- to 5-km altitude range also indi- (NO2calc) temperature, pressure, and UV solar flux. Figure 10cdepicts cated that the median NO mixing ratios measured over the solarzenithanglefiltered(38ø < X < 67.5ø)composite for Alaska were approximatelytwofold smallerthan those rethe ratio NO2calc/NO2meas using (9) and measured mixing ratios quiredto balance03 photochemicalproductionand loss. of NO2. Medianvaluesof thisratiowerelessthanunity,as TheseNOeq/NOmeas estimates are,however, in apparent expected for exclusion of reactions (R1)and(R2)in describing disagreementwith the one-dimensionalmodel results preNO2calc/NO2meas for these relatively cleanair masses. sentedin the companionpaperof Singhet al. [thisissue(b)]. The measured ratio [NO2]/[NO] can be expressedby an This latter evaluationcomparedcalculatedmixingratios of equation of the form

NOx necessaryfor net photochemical productionto the medianvaluesof NOx measuredover Alaska.Comparison {[NO2]/[NO]}meas = k8([O3]meas + [O3]eq)/JNO2, (10) of their NOx-basedanalysisto the NO-basedanalysesdiscussedaboverequiresan examinationof the photostationary where[O3]½q represents the concentration of the often termed"missing-oxidant"in unitsequivalentto thoseof 03. state relationshipbetween NO2 and NO. the missingoxidants,O3½q 4.2.3. NO2/NO photostationarystate implication on For HO2 and RO2 representing peroxy radical concentrations. The daytime steady state can be expressedby

SANDHOLM ETAL.' NxOr DISTRIBUTIONS, ABLE 3A

(d)

.SB

(o) '

16,499

.45

35

.Q

0

.35



22)

.25 ß

.].5

'"•0 %

%o .

(o) (b) .35

200

.25

.15

100 ß 85

0

q'o%' •'"•.

-' %ø.oO:0.606 ' 0.61 ' 0.614 ' 0.018 JN02(1/sec)

.27 .45

.21 .35

.15

.;•5

.15

ß213

ß 1•5

0.0

/0• --3 Z.0

t'

ø*C_ %

'

e

(.p.m')

• 1.t,i.t, ocle(.V,m)

Fig. 10. Composite verticalsounding overAlaska,where(a) NOeql/NOmeas , (b) NOeq2/NOmeas , (c) NO2calc/

NO2mea s, (d) O3eq/O3meas, and(f) JNO2 havebeenplottedin altituderanges0.1-1.5, 1.5-3.0,3.0-4.5,and4.5-6.1km.

The relativeprobabilitywithinan altituderangeis representedon the z axis. (e) A correlationplot of aggregated data,

wherethehorizontal andvertical linesrepresent _+1 sigma aboutthemeanoftheaggregate forO3e q versus JNO2 with an aggregatesize of 24 measurements(r • = 0.79 and slopeof 8700 _+ 1600).

[O3]eq= (kl[HO2] + k2[RO2])/k 8.

(11)

mixing ratios needed to balance (10) were about one-third as

large as those measured.Based on (11) and for k• --• k2,

to mixingratiosof HO2 Figure10ddepictstheratioof [O3]eq/[O3]meas calculated theseO3eq mixingratioscorrespond from (10). Within the middle two altitude ranges (1.5-4.5

km), medianvaluesof this ratio suggestthe equivalent03

+ RO2 of about 20 pptv. This value is in the range of those taken from Logan et al. [1981] and used earlier in the

16,500

SANDHOLM ET AL.: NxOr DISTRIBUTIONS, ABLE 3A

assessment of NOeq2. Medianvaluescalculated for theratio the troposphericlifetime of C2H 6 would have been of the O3eq/O3mea s withinthelowestaltituderangesuggest a small order of 60 -+ 20 days and that of C3H8 about 14 _ 4 days. contribution from peroxy radicals on the photostationary state of the mixed layer. This estimate is in general agreement with the analyses by Bakwin et al. [this issue] for surface measurementsof NO 2 and NO near Bethel. Larger median mixing ratios of HO 2 + RO 2 (-75 pptv) were, however, implied from our calculations for the highest altitude range. This latter results, in conjunction with the

largedifference betweenmeanandmedianO3eq/O3mea s ratios, suggests a more complete understanding of the NO2/NO photostationary state relationships preliminarily investigated here may require (1) refined model calculations, which incorporate actual conditions (e.g., the use of actual hydrocarbon abundances, representation of actual cloud

The O atom addition reactions are far less temperature dependent, but as three-body reactions they do depend slightly (< ---20%) on pressure over the pressure range encountered during ABLE 3A (0.5-1 atm) [DeMore et al., 1990]. The tropospheric lifetime of CO (--•54 _ 12 days)

would have been similarto that estimatedfor C2H6, and the lifetime of C2H 2 (--•14 - 4 days) would have been similar to that estimated for C3H8. The overall uncertainty in these

lifetime estimates would most likely be much larger than indicated by the pressure and temperature dependenciesof the rate coefficients, due to uncertainties in the estimates of average OH concentrations and the exclusion of surface sink terms. Even so, these estimates could be useful in describing fields, and changes in total 03 column), and (2) re- the general trends that might be expected between cominvestigation of small possible interferences in measured pounds and the order of magnitude of their lifetimes in NO 2 (e.g., 5-10 pptv of thermally/photolytically derived relation to transport time (At) from distant sources. The interference from 100-fold larger mixing ratios of unac- relative differencein elapsedtime (equated here to transport countedfor NOy compounds). Theseresultsalsosuggest time At) for compoundswith different chemical lifetimes can that without such reanalyses, which are in progress, com- be approximated by parisons of the model-calculated mixing ratios of NOx In [(CA/CB)tl/(CA/CB)t2 ] necessaryfor photochemical03 productionto mixing ratios At = (12) of NOx that were measured may be less interpretable than (kA - kB)CoH similar analyses based on NO mixing ratios. The mixing ratios of NO were more directly measured where C^, CB are the concentration of compoundsA and B than those of NO 2, allowing higher immunity to interfer- at times t 1 and t2, having reaction rate coefficientsk^ and ences, and absolute accuracy. Based on the preceding kB, and Con is the OH concentration. discussions,we believe the NO-based analyses more accuMixingratiosof C2H2 weresignificantly correlated (r 2 rately describe the role of NO on the photochemical lifetime 0.87) with those of CO (slope --•1.7 pptv/ppbv; Figure 12a) of 03. We find that on average the NO mixing ratios as might be expected from compounds that share both measured over Alaska were about twofold smaller (within common sources (i.e., combustion) and sinks (i.e., oxidation the 1.5- to 5-km altitude range) than those necessary to via O atom addition reactions involving OH). The composite balancethe photochemicalrates of 03 productionand loss. of individual C2H2/CO ratios (Figure 11a) exhibited an 4.2.4. Distributions and trends of representative carbon- increase in median values in proportion with altitude. This containing compounds. The compounds CO and C2H2, trend could suggestan influx of less aged (At --20-30%) air which are primarily products of combustion, also exhibited into the 4- to 6-km altitude range. altitude dependencies in their median mixing ratios (--•1 Average mixing ratios of C2H 6 and C3H8 were also ppbv/km for CO and --•5 pptv/km for C2H2; cf. Figures 8c correlated with those of CO (cf. Figures 12b and 12c). In and 8d). However, the most probable portion of the mixing addition, the average ratio of C3Hs/C2H6 was correlated ratio distributions for these compoundsvaried only slightly with ratios of C2H2/CO, which suggestscommon factors with altitude. The pronounced tails and secondary maxima (either sourcesor sinks) were controlling the mixing ratios of in their distributions contributed to most of the differences all of these compounds.The regressionof In (C3H8/C2H6) (--•10-15%for CO and 20-30% for C2H2) betweenthe most versusIn (C2H2/CO) yielded about a threefold smaller slope probable and the median mixing ratios for these compounds. than expected from (12), based upon equivalent transport Similar tendencies were also measured for mixing ratios of times (At), OH concentrations, and rate coefficientsrepreC2H6, and C3H8 (cf. Figures 8e and 8f), even thoughthese sentative of the tropospheric conditions encountered (i.e., latter compounds have both combustion- and noncombus- expected slope --•1.1 versus --•0.4; cf. Figure 12d). Although tion-related sources. These similarities may represent the very qualitative, this could suggestvarious source emission effectsof either common sources(or sourceregions)or sinks factors (signatures) were a more important factor than the for these compounds. OH photochemical sink in controlling the average relationThe ratios formed from various carbon-containing com- ships observed for these compounds. Relative emission pounds are useful as indicators of relative air massage when factors(e.g., AC2H2/ACO and AC3H8/C2H6) were found to the compounds chosen share common sources and sinks vary significantly even for similar types of sources (e.g., [e.g., Singh and Zimmerman, 1992]. The primary tropo- high-latitudebiomassburningAC2H2/ACO '-• 0.002-0.03 and spheric sink for the carbon-containingcompoundsdiscussed AC3H8/AC2H6--•0.1-0.3 [Wofsyet al., thisissue](alsoD. R. here is their oxidation by OH radicals via either H atom Blake et al., Nonmethane hydrocarbons in the troposphere abstraction(C2H6, C3H8) or O atom addition(CO, C2H2). H over central Canada, submitted to Journal of Geophysical atom abstraction reactions are temperature dependent and Research, 1992) and similar variances for industrial/urban would have had rate coefficientsthat varied by about -+35% emissions[Warnek, 1988; Singh and Zimmerman, 1992]). over the temperature ranges encountered in ABLE 3A Even though significant uncertainties (factors of twofold [DeMore et al., 1990]. For a diurnally averaged OH concen- to threefold) exist in estimated transport times based on trationof about1 x 106OH/cm3 [Jacobet al., thisissue], hydrocarbonratios, the small values of C3H8/C2H6 mea-

SANDHOLM ET AL.' NxOr DISTRIBUTIONS, ABLE 3A

(o)

.45

.:,.

16,501

(d)

.45

.3S 15

.iS

.'3 .6

%

R•.t •.tocje c•'m)

(,)

i:ql .tt.tocte c•'m• (c)



o ß

.35

.

•J

•)

. 15

(f)

.35 . 15

L..j

&

C'•..j

Fig. 11. Compositesof vertical profilestaken over Alaska, where (a) C2H2/CO, (b) C3Hs/C2H6, (c) C2C14,(d) H20, (e) staticair temperature,and (f) potentialtemperaturehave been plotted in altituderangesof 0.1-1.5, 1.5-3.0, 3.0-4.5, and 4.5-6.1 km. The relative probability representedon the z axis is as describedin Figure 6.

16,502

SANDHOLM ETAL.:NxOr DISTRIBUTIONS, ABLE 3A (a)

(c)

200 160

'•

140 120

co 8o

80

20

'

80

160 '

li0

'

'

1•0

40

16o

80

100 '

co (ppbv)

'

1•,0

160

(b)

1200

(d)

-1.8

'•'1000

-r -2.1

co 800

ß•

600

1•0

CO(ppbv)

80

.'

.

li0

.

.

1•0 '

'

-2.4

-2.7 170

co (•)

-0.6

-0.2

0.2

In (C2H2/C0)

Fig. 12. Correlation plotsof aggregated data,wherethehorizontal andverticallinesrepresent _+1 sigma aboutthe meanof theaggregate. (a) C2H2 versus COwithanaggregate size(AS)of 25measurements (r2 = 0.99andslopeof 1.73_+0.065).Theinsertrepresents individual measurements withinteriorlabels:I (0.1-1.5km),L (1.5-3.0km),h

(3.0-4.5 km),andH (4.5-6.1 km).(b)C2H6 versus COwithAs= 24(r2 = 0.97andslope of5.01_+0.33).(c)C3H8

versus COwithAS= 24(r2 = 0.93andslope of1.26_+0.12). (d)PlotofIn(C3Ha/C2H6) versus In(C2H2/CO) with

AS = 25(r2 = 0.84andslopeof 0.43-+0.065).

sured over Alaska suggest mid-latitude industrial/urban the high-latitudeatmosphericconditionsencountered.This emissionstransported into this region would have been lifetime is significantlylonger than those estimatedfor the relativelywell aged. Transporttimes of 15-30 days were othercarbon-containing compoundsdiscussed,but it is close estimatedfrom the C3H8/C2H6(--•0.1)ratiosmeasuredover to the longest PAN lifetimes estimated for the summertime summertimeAlaska versus ratios reported for industrial tropospherenear 6 km at high latitudes[Singh et al., this

pollution(0.4--0.8[e.g., Warnek,1988;DoskeyandGaffney, issue(b)]. Averagemixingratiosof C2Cl4 were correlated 1992]), and assuminga constant OH concentration. These with thoseof PAN and 03 (cf. Figure 13). Thesesimilarities transporttimeswere of the order of thosepredictedby the in altitudinaltrendsmay suggestthesecompoundsshared analysesof Patterson and Husar [ 1981] for the summertime either commonsource(s)(e.g., the result of industrial/urban emissions)or commonsinks(e.g., lower altitude sinkssuch

transportof pollutantsto Alaskafrom populationcentersin easternNorth America, northernEurope, and the area near the Sea of Japan(on averageAt _>20 days). Less distant sources could have also influenced the mea-

assurfacedepositionandlow-altitudethermallyinducedloss of PAN). These trendscould also simplyrepresenta tendencyfor long-livedcompoundsto accumulatein the Arctic

suredhydrocarbonratios (e.g., biomassburningin Siberia middle troposphere. and Alaska,AC3H8/AC2H 6 --- 0.1-0.3, and gas/oilproducUnlikethe correlations with PAN andNOy, average tion/leakage fromregionssuchasRussia,AC3H8/AC2H6 mixing ratios of C2C14were only slightly correlatedwith 0.1-0.7 [Blake et al., this issue]). The influence of more

thoseof NOy andCO. In addition,averagemixingratiosof

regional-scale emissions with