Atmospheric Environment 33 (1999) 3023}3036

The evolution of photochemical smog in a power plant plume

Menachem Luria!,", Ralph J. Valente!, Roger L. Tanner!,*, Noor V. Gillani!,#,Robert E. Imho!!, Stephen F. Mueller!, Kenneth J. Olszyna!, James F. Meagher!,1

!Atmospheric Sciences and Environmental Assessments, Tennessee Valley Authority, P.O. Box 1010, Muscle Shoals AL 35662-1010, USA"Environmental Sciences Division, School of Applied Science, The Hebrew University, Jerusalem 91904, Israel

#Earth System Science Lab., University of Alabama, Huntsville AL, 35899 USA

Received 1 August 1998; received in revised form 22 October 1998; accepted 14 December 1998

Abstract

The evolution of photochemical smog in a plant plume was investigated with the aid of an instrumented helicopter.Air samples were taken in the plume of the Cumberland Power Plant, located in central Tennessee, during the afternoonof 16 July 1995 as part of the Southern Oxidants Study } Nashville Middle Tennessee Ozone Study. Twelve cross-windair sampling traverses were made at six distance groups from 35 to 116 km from the source. During the sampling periodthe winds were from the west}northwest and the plume drifted towards the city of Nashville TN. Ten of the traverseswere made upwind of the city, where the power plant plume was isolated, and two traverses downwind of the citywhen the plumes were possibly mixed. The results revealed that even six hours after the release, excess ozone productionwas limited to the edges of the plume. Only when the plume was su$ciently dispersed, but still upwind of Nashville,was excess ozone (up to 109 ppbv, 50}60 ppbv above background levels) produced in the center of the plume.The concentrations image of the plume and a Lagrangian particle model suggests that portions of the power plantplume mixed with the urban plume. The mixed urban power plant plume began to regenerate O

3that peaked at 120 ppbv

at a short distance (15}25 km) downwind of Nashville. Ozone productivity (the ratio of excess O3to NO

yand NO

z) in the

isolated plume was signi"cantly lower compared with that found in the city plume. The production of nitrate, achain termination product, was signi"cantly higher in the power plant plume compared to the mixed plume,indicating shorter chain length of the photochemical smog chain reaction mechanism. ( 1999 Published by ElsevierScience Ltd. All rights reserved.

Keywords: Power plant plume; Ozone; Nitric acid and nitrate

1. Introduction

The concerns about human health e!ects, agriculturalcrop reduction, and damages to natural vegetation thatresult from elevated ozone levels in rural southeasternUS motivated the establishment of a coordinated re-search program named the Southern Oxidant Study

*Corresponding author.1Present address: Aeronomy Laboratory, NOAA, 325 Broad-

way, Boulder CO 80303, USA.

(SOS). Under the umbrella of SOS, universities, indus-tries and government agencies joined forces to investigatethe mechanisms leading to the formation of ozone andother secondary photochemical products. The ultimategoal of SOS is the development of e$cient and economi-cal strategies for ozone reduction.

During the last decade high sensitivity measurementsof trace gases (O

3, SO

2, CO, speciated nitrogen oxides,

organic compounds, peroxides, aldehydes, speciated ni-trogen oxide [NO

y] species, and aerosol composition)

have been performed at various locations in south-eastern USA. Measuring sites were operated in Alabama

1352-2310/99/$ - see front matter ( 1999 Published by Elsevier Science Ltd. All rights reserved.PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 0 7 2 - 2

(Cantrell et al., 1992), Georgia (Kleinman et al., 1994),Virginia (Poulida et al., 1991; Doddridge et al., 1992;Jacob et al., 1995) North Carolina (Hartsell et al.,1994) and Tennessee (Olszyna et al., 1994, 1997). Oneof the main objectives of these studies was to deter-mine the e$ciency with which O

3is produced in

these rural environments. The number of excess O3

molecules (O3

concentrations above the background)produced per molecule of NO

x(NO#NO

2) emitted

and per NOx

oxidation product NOz

(NOz"

NOy!NO

x) formed, provide measures of the O

3formation e$ciency.

The aforementioned observations indicated that theslope of the linear relationship between O

3and NO

zunder NO

xlimiting condition ranges from 18 at the

Virginia site (Jacob et al., 1995), to 11}12 at the Tennes-see and the Georgia sites. The data from the Tennesseesite also showed that the slope of O

3}NO

ylinear "t was

approximately 10. Airborne data collected during theafternoon hours in the urban plume of Atlanta during thesummer of 1992 (Imho! et al., 1995), yielded O

3/NO

yslopes that varied from 4 to 10. Another important indi-cator in evaluating the photo-oxidation process is theratio NO

z/NO

y(or the reciprocal of this ratio NO

x/NO

yTrainer et al., 1993), which can be considered the `chem-ical agea of an air mass. Based on measurements at therural Tennessee site, Olszyna et al. (1994) concluded thatnet production of O

3(the balance between formation

and removal processes) ceases when NOz/NO

y'&0.7

for NOy

levels of up to 7 ppbv. Analysis of O3/NO

yrelationships obtained at those sites strongly suggeststhat the atmosphere in rural southeastern USA is NO

xlimited.

These previous studies inspired the utilization of air-craft to selectively sample air masses that are known tocontain an excess of one component, i.e., urban plumesthat are high in VOC and power plant plumes that arerelatively VOC-lean. It is well documented that O

3pro-

duction in power plant plumes is delayed until theplumes are quite diluted. However, O

3production is

known to be enhanced when the plumes mix with ex-ternal sources of VOC, such as urban plumes (forexample see Luria et al., 1983).

During the summer of 1995, a coordinated e!ort utiliz-ing six research aircrafts, together with an intensiveground-sampling program, took place under the SOSprogram. The TVA helicopter was assigned to investigatethe chemical transformations in power plant plumes asthey mixed with rural and urban air masses, and toevaluate the contribution of power plants to the forma-tion of O

3in rural southeastern US. In our previous

publication (Gillani et al., 1998a) the relative productionof ozone and nitrate in urbane and rural power plantplumes was examined. Based on 10 "eld measurementdays it was estimated that 3.1$0.7 molecules of O

3and

more than 0.6 molecules of NOzare produced per mol-

ecule of NOx

released from large isolated power plants.The data suggest that large power plant in the vicinity ofNashville can contribute as much as 50 ppbv of excessozone to the urban area. It was further reported that therate of NO

zproduction is on the order of 10}15% h~1.

In another publication the issue of extremely rapid NOy

removal rate was examined (Gillani et al., 1998b). Datafrom one sampling day performed in 1994 in the plume ofthe Cumberland Power Plant suggested that the di!eren-tial loss rate of NO

y(the loss of NO

yrelative to the loss of

SO2) is 0.09 h~1. This high value implies that NO

yis

removed from power plant plume at an extremely rapidrate of'0.13 h~1. However, the 1995 data did not con-"rm this observation. Based on four sampling days per-formed in the plume of the same power plant it was foundthat the di!erential loss rate is on the order of 0$0.03 h~1. The conclusion of this report was that the issueof possibly rapid NO

yloss rate need to be further ex-

plored.In this paper we report the results of an in-depth

analysis of one sampling day (16 July 1995). On this daythe plume from the Cumberland power plant was trackedduring the entire afternoon. The plume was isolated overa rural area during the "rst part of the day and passednear the city of Nashville TN before blending into thebackground. Additionally, the sampling sequence, mov-ing from near the source progressively downwind, en-abled the construction of a Lagrangian picture of theplume.

2. Experimental details

Air samples were taken using the TVA Bell 205 heli-copter, operated for the Atmospheric Sciences Depart-ment. The Bell 205 platform was used because its slow#ying speed (less than 180 km h~1) allowed detailedmeasurements of the rapid concentrations changes inplumes from point sources. The base of operation wasNashville International Airport (BNA), although thehelicopter landed for refueling at various local airports asdictated by the mission. Detailed information regardingthe instruments on board the TVA helicopter their sensi-tivity accuracy and response time was given elsewhere(Hubler et al., 1998). The gas analyzers on board were allmanufactured by Thermo Environmental InstrumentsInc., Franklin, MA (TEII). They included a model 49 O

3monitor, a model 43B SO

2monitor, four model 42S NO}

NOxmonitors, and one model 42 NO}NO

xmonitor. All

the 42S and 42 instruments had fast response chips instal-led (0.5 s updates) and had the factory "lter holdersreplaced with low dead volume "lters to reduce responsetime. One model 42S was modi"ed to continuouslymeasure only NO

y. The modi"cation included the re-

placement of the factory-installed molybdenum NOx

toNO converter with a custom built gold tube converter,

3024 M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036

mounted at the sample inlet on the nose of the helicopter.This converter selectively reduces NO

yto NO with CO

on a gold catalytic surface heated to 3003C (Bollinger etal., 1983). Before every #ight the gold tube system wascleaned for three hours by sampling zero air with theconverter temperature raised to 5003C.

A second model 42S was similarly modi"ed, and inaddition, a 47 mm nylon "lter (Nylasorb, GelmanSciences Inc., Ann Arbor, MI) was installed ahead of theconverter. The nylon "lter, which was replaced aftereach #ight, removed nitric acid and nitrate particulate.The results from this 42S instrument were designated asNOH

yto represent all NO

yspecies except HNO

3and

NO~3. The di!erence between these "rst two 42S instru-

ments (NOy!NOH

y) is assumed to represent the total

nitrate in the air sample (HNO3plus NO~

3, for details see

Tanner et al., 1998). In pre#ight tests it was demonstratedthat the nylon "lter removed less than 3% of NO orNO

2, yet it detected no signal when a sample of approx-

imately 20 ppbv of HNO3

was introduced. The thirdmodel 42S instrument was used for continuous samplingof NO.

The fourth model 42S was modi"ed to provide selec-tive and continuous measurement of NO

2. The modi"ca-

tion included the replacement of the factory installedmolybdenum converter with a photolytic cell, operatedat 0.5 atm, continuously illuminated by a 300W xenonlamp. The NO

2conversion e$ciency of the cell was

measured prior to each #ight and data processing algo-rithms were developed that correct for reactions betweenNO, O

3and NO

2in the cell and tubing runs in order to

calculate the ambient NO2

concentration. The model 42instrument was modi"ed to measure O

3rather than

NO/NOx, in order to obtain fast time response data

comparable to the nitrogen oxides measurements. Themodi"cation included the removal of the NO

x-to-NO

converter and the replacement of the O3

input to thereaction cell with an external source of 100 cm3min~1

NO in N2

(1500 ppm, Scott Specialty Gases Inc.). Thechemiluminescence from the modi"ed instrument isproportional to the ambient O

3levels. Under conditions

when the ambient O3

levels were relatively constant,the results from the modi"ed model 42 and the model49 were within $3%, but only the modi"ed 42 couldprovide O

3measurements fast enough for the NO

2algorithm calculations during rapid concentration chan-ges accompanying plume traverses. Ozone data present-ed in this paper are those of the modi"ed model 42instrument.

Airborne air samples for all gas analyzers were drawnthrough a Te#on tube (0.635 cm outside diameter) thatextended forward to a point ahead of the rotor down-wash regime and 50 cm directly below the nose of thehelicopter. Multi-point calibration was done twice eachday during #ight days, or whenever a zero/span checkshowed a deviation of more than 10% of full scale, using

a TEII model 146 calibration system and an automatedcalibrator control program written for the on-board datalogger. The gas analyzers were zeroed and spanned be-fore each #ight using certi"ed master gas mixtures (ScottSpecialty Gases, Plumsteadville, PA). The gas analyzerswere operated continuously between #ights, samplingzero air in order to clean and minimize contamination ofthe inlets at the airport.

Other continuous measurements included temper-ature, relative humidity and pressure (altitude). Positionand heading were derived from a long range aid tonavigation (loran C) system. Data from the continuousmonitors were recorded at a frequency of 1 Hz using twoCampbell Scienti"c (Logan, Utah) model 21X data log-gers. For the purpose of the data analysis, the raw 1 sdata were averaged over 5 s intervals. Because of therapid changes in the concentrations of trace gases insideand outside the plume, because of the di!erent responsetimes of the instruments, and to allow the calculation ofdi!erences and ratios between variables, it was necessaryto calculate time displaced values (tds) in order tosynchronize all the measurements. The tds values werenumerically calculated by comparing the rise times ofa slower analyzer and a comparable faster one to aninstantaneous change in the level of calibration gas. Inthe data reduction process, corrections were also madein the 1 s raw data for O

3, NO and NO

2. This correction

was needed to account for the reaction between NO withO

3that occurred during the time elapsed between the

sample entrance to the manifold and arrival at the de-tector. The tds 1 s raw data were then averaged over 5 sintervals to obtain the "nal data set analyzed herein.Upper air wind data, as measured by wind pro"lerslocated through the study area, were used for the analysisof the data (for details see Meagher et al. (1998) andMcNider et al. (1998)).

3. Meteorological information

During the week preceding 16 July, a high pressureridge dominated the weather. This high pressure systemcaused elevated O

3levels throughout the region. On 15

July, the high pressure ridge weakened and a strongerwesterly #ow aloft gradually moved to the south. On the16th, a cold front, which extended from the Great Lakesinto Western Texas, continued to move southeast behindthe surface high pressure ridge which moved to theeast. Widespread prefrontal convection formed dur-ing the mid-afternoon in the unstable atmosphereover the Southeast. The wind #ow at the lower layersof the atmosphere was westerly with an occasional nort-herly component.

To determine the characteristics of the boundarylayer, an altitude pro"le was taken at the beginningof the #ight. The O

3and temperature data indicated

M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036 3025

Table 1A summary of Cumberland plume air sampling traverses on 16 July 1995

Traverse Local time!(start}end)

Distance from thesource (km)

Sampling altitude(m)

Estimated releasetime

NOy

max(ppbv)

O3

max/min(ppbv)"

1 1250}1254 37 510 0930 66 80/352 1312}1316 34 1209 0930}1000 55 65/363 1332}1339 47 601 0930 39 85/554 1348}1355 49 316 0930}1000 45 90/505 1430}1439 36 305 1130 53 80/406 1445}1450 36 820 1130}1200 90 85/257 1523}1530 59 814 0930 25 94/658 1537}1544 61 377 1000 16 100/709 1555}1600 84 412 0930 16 109/102

10 1653}1711 105 497 0830}0900 6 11811 1716}1733 116 505 0830 9 11912 1806}1816 58 519 1330 37 80/45

!Local time is 5 h behind UT."Minimum values are given for the traverses near the source where O

3is depleted in the center of the plume. Maximum values are

given for all other traverses.

a well mixed boundary layer and that the #ue gasesfrom the Cumberland power plant were capped underthe inversion layer located at around 1250 m abovesea level.

4. Results

On 16 July 1995 air sampling was performed from1200 until 1830 CST, with two brief refueling stops.During this period, samples were taken in the plume ofthe Cumberland power plant located in north-centralTennessee (36.393N, 87.653W). After the plume waslocated in the east}southeast sector from the powerplant, a series of cross wind traverses was initiated.Twelve traverses were made at 6 distances ranging from35 to 120 km from the source. All of the air samplingtraverses were performed within the boundary layer ata constant altitude. Eleven samples were taken withinthe range of 320}820 m ASL. One traverse was takenat a higher altitude (T2, taken at 1210 m; see below),still within the boundary layer. A summary of the#ight information is given in Table 1. The time of releaseof the sampled air mass was estimated with the aidof trajectory calculations based on a Lagrangianparticle transport model using the pro"ler wind data,in the manner described by Gillani (1986). Fig. 1shows a map of the sampling area with the location ofthe 12 traverses. Also shown is the grid between 35.83N,87.33W and 36.53N, 86.43W, for which the Lagrangianconcentrations image of the plume was estimated(see below).

The crosswind pro"le of the major components mea-sured in the plume at "ve downwind distances is given in

Fig. 2. No measurements were made near the source onthis day where O

3was assumed to be completely titrated

by NO in the center of the plume (based on measure-ments performed during other days). In the data from the"rst downwind traverses, about 35 km from the source orapproximately 3}3.5 h after the plume release, the pro-cess of ozone recovery had already become very notable.A clear increase in O

3at the edges of the plume and levels

that were signi"cantly lower than background in thecenter of the plume was observed for all six traversestaken near the source at a distance of up to 50 km (about4 h of travel time). At the next downwind sampling dis-tance, around 60 km, O

3peak levels were still occurring

at the edges of the plume (see Fig. 2), and in the center theO

3mixing ratio had only recovered to near background

levels. Even at the fourth sampling distance, 84 km fromthe power plant, O

3in the center of the plume was

slightly lower than in the edges of the plume but signi"-cantly higher than the background. At the last distancesampled (105}116 km from the source), downwind ofNashville, O

3peaked at the center of the plume. Back-

ground O3

and NOy

levels were determined from themeasurements outside the plume in the relatively cleannortheastern sector. These values were 56 and 1.4 ppbv,respectively.

As shown in Table 1, the estimated times of release ofthe #ue gases sampled during 9 of the 12 plume traverseswere between 0830 and 1000, with seven of these beingwithin 30 min time period. The results from these ninetraverses were used in the analyses below. The fact thatthe samples taken at the two furthest distances werereleased earlier than those sampled at the nearer distan-ces is relatively less signi"cant for the purpose of the dataanalysis.

3026 M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036

Fig. 1. The geographical location of TVA's helicopter air sampling traverses, performed on 16 July 1995 in the plume of the Cumberlandpower plant, displayed over a map of Central Tennessee. The traverses indicated are only those used to construct the concentrationsimage plots. The numbers identify the traverse as listed in Table 1 and the location of the numbers are at the beginning of each traverse.The grid box represents the area for which the plume concentrations image was generated.

The data obtained from the aforementioned ninecross-plume traverses, including background observa-tions on both sides of the plume, were used to constructapproximate concentrations image and concentrationsratio image plots of the various parameters measured inthe plume. Fig. 3 shows the distribution of the majorprimary plume gases (SO

2, NO, NO

2, NO

y) for a grid of

approximately 5000 km2 between 35.83N, 86.43W and36.53N, 87.43W. Likewise, Fig. 4 shows similar plotsfor NO

z, nitrate, O

3, and for the background corrected

ratio of O3/NO

y(SO

3T/SNO

yT"[O

3!56]/[NO

y!1.4])

for the same geographical grid. It must be noted thatthese distribution plots represent a semi-Lagrangianview of the plume constituents between the "rst samplingtime at around 1250 h (on the left) and the last traversearound 1810 h (on the right). The horizontal axis onthese plots represents a time lapse of approximately 5.3 h(at an average wind speed of 3.5 m s~1 from the WNWdirection). Obviously, the accuracy of the concentra-tions image plots decreases with distance fromactual measurements. This is the case speci"cally overdowntown Nashville where airborne data were not col-lected.

5. Discussion

5.1. The power plant plume and the Nashville urban plume} do they mix?

Because SO2

is scrubbed from the #ue gases of theCumberland power plant, the ratio SO

2/NO

yat the

source, and in the plume, is relatively low (approximately0.1 at the source). But, due to the fact that there are noother major SO

2sources along the plume path, the SO

2signature (Fig. 3) is a reasonably good tracer of the powerplant plume. The SO

2concentrations image indicates

that the bulk of the plume released on the morning of 16July disintegrated north of downtown Nashville. How-ever, these data do not support nor eliminate the possi-bility that some portions of the power plant plume mixedwith the urban plume through a process of lateral di!u-sion. The traces of NO and, to a lesser extent, of NO

2(see

Fig. 3), become indistinguishable from the backgroundlong before the power plant plume passes near Nashville.NO and NO

2levels over the city and in the urban plume

are very low. On the other hand, the trace of NOy

thatextends further to the east and southeast may support the

M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036 3027

Fig. 2. Mixing ratios of O3, O

3#NO

2and NO

yas a function of latitude for cross wind plume samples, taken in the plume of the

Cumberland power plant on 16 July, 1995 at six downwind distances.

possibility of mixing of the power plant plume with theurban plume. It also shows that the levels of NO

yin the

urban plume (less than 10 ppbv) are signi"cantly lowerthan values measured in the power plant plume and thatthe urban plume is spread over a much larger area.

To further examine pollutant transport, a numericalsimulation of transport conditions was performed for the16 July episode using the RAMS meteorological model(Pielke et al., 1992) and a Lagrangian particle dispersionmodel (LPDM) developed by McNider et al. (1988) and

3028 M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036

Fig. 3. A Lagrangian simulation of SO2, NO, NO

2, and NO

ymixing ratios (in ppbv) in the Cumberland power plant plume. The

concentrations image plots were estimated from the afternoon #ight measurements of 16 July 1995. The data from the 9 traverses weregridded using the Kriging linear interpolation method. The gridding and plotting were performed with the aid of commercially availablesoftware (Surfare for Windows by Golden Software Inc., Golden CO).

further described in Mueller et al. (1996). Both the RAMSand subsequent LPDM simulations were made usinga high-resolution computational grid with 4-km horizon-tal grid cell spacing. Mueller et al. (1996) previously usedthese models to examine pollutant transport during highozone episodes over the southern Appalachian Moun-tains in eastern Tennessee.

A simulation of pollutant transport from Nashville on16 July indicated that near-ground urban pollutantstraveled toward the east}southeast. This coincides close-ly with the estimated urban plume location based onmeasurements made during helicopter traverses 10 and11. Simulated elevated particles released from the buoy-ant Cumberland source traveled in an easterly directionpassing just north of downtown Nashville. Successivecomputed locations of a pu! emitted from Cumberlandat 0930 are illustrated in Fig. 5. This release time waschosen because it corresponds closely with the estimated

emission time of the plume material sampled at 1600along #ight path 9.

Model particles were vertically mixed throughout theboundary layer before arriving in the vicinity of Nash-ville. The simulated location of the 0930 pu! at 1600agreed quite well with the helicopter plume traverse thatoccurred at about the same time along path 9 (compareFigs. 1 and 5). By 1700 the computed location of the 0930pu! was nearly due east of Nashville. Thus, these modelresults suggest that mixing of the Cumberland and Nash-ville urban plumes occurred mostly east of Nashville, andinvolved power plant plume material that was 8 or morehours old.

5.2. The formation of O3 and nitrate in the isolated plume

The chemical evolution of the plume is demonstratedin Fig. 6 for both secondary pollutants, O

3and nitrate

M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036 3029

Fig. 4. A Lagrangian simulation of the distribution of NOz, nitrate, O

3(in ppbv) and the ratio O

3/NO

yin the plume of the Cumberland

power plant for 16 July 1995. The data from the 9 traverses were gridded using the Kriging linear interpolation method. The griddingand plotting were performed with the aid of commercially available software (Surfare for Windows by Golden Software Inc., GoldenCO).

(HNO3#NO~

3). There are certain similarities in the

formation of these two oxidation products. The plots onboth sides of Fig. 6 (left for O

3, right for nitrate) show

that the crosssection of the plume consists of two zones asindicated by the change in slope. In one zone (the maturezone) the formation of the secondary products increasesas a function of NO

yup to a certain peak level. In the

other zone the photochemical production of O3

has notbeen completed. The mature zone extends acrossa greater portion of the plume as a function of plume age(downwind distance). At the furthest downwind distancethe entire plume consists of this zone. In the case of O

3,

the picture is quite clear. The reactive zone appears at theedges of the plume where VOC rich ambient air mixeswith the NO

yrich plume. The depth of the mature zone

(as illustrated in Fig. 6 from the number of data points)is determined by dispersion and by the concentrationof VOCs in the ambient air. When a NO

y/VOC ratio

favorable to O3

formation exists throughout the plume,a linear relationship between O

3and NO

y(and NO

z) is

achieved all the way across the entire plume. This linearrelationship is consistent with many other "eld observa-tions (for example, Trainer et al., 1993) and is typical forNO

xlimiting conditions.

Because the nitrate is obtained by subtractingmeasurements from two di!erent instruments, the datanear the source, where NO

yand NOH

yare relatively high

and highly variable, are less accurate. However, at anintermediate distance the di!erences between O

3and

nitrate are evident as shown in Fig. 7. At 61 km from the

3030 M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036

Fig. 5. Temporally varying locations of a computed particle pu! released from the Cumberland Power Plant at 0930 local time on 16July 1995. Results are for RAMS-based winds. County and state boundaries are shown along with the pu! footprint at one-hourintervals (alternating pu! positions are labeled by the time with which they are associated). Only locations of particles computed to begreater than 450 m above ground level are shown to correspond most closely with the helicopter sampling elevations (#ight pathsindicated by bold black/white lines). A portion of the simulated Nashville urban plume at 1700 is also shown.

source O3

still peaks in the plume edges while nitratepeaks in the center. This phenomenon is consistent withthe role of HNO

3in the photochemical oxidation pro-

cess. HNO3

is a termination product in the free radicalchain mechanism and is produced in the reaction be-tween NO

2and OH. This termination reaction path

becomes relatively more dominant in VOC-lean airmasses compared with chain propagation reactions(OH#VOC) under these conditions.

The di!erences between O3and nitrate were found not

only across the plume but also as a function of downwinddistance. Fig. 8 shows the peak mixing ratios of O

3, O

x(the sum of O

3#NO

2) and nitrate in the center of the

plume and at the plume's wings as a function of theestimated travel time. As shown, O

3increases all the way

to the last sampling distance, while nitrate peaks after&7 h and decreases slightly thereafter.

Thus, the observations are consistent with expecta-tions, i.e., nitrate formation occurs earlier in the life of theplume and therefore is found closer to the center of theplume relative to O

3. However, nitrate is not present in

the core of the plume during the early stages due to thefact that the concentration of HO radicals in this zone isvery low.

5.3. Plume characteristics upwind and downwind ofNashville

Another tool that was used to estimate the track of theCumberland plume was the regional surface ozonemeasurements. Fig. 9 gives the maximum 1-h O

3mixing

ratios observed at 9 sites in the vicinity of Nashville. Thedisplayed daily maximum values were observed between1300 and 1600 h which is approximately the same timethat the Cumberland plume was tracked from thehelicopter over the same area. The information given inFig. 9 shows that high O

3levels were observed all around

Nashville while lower values were observed north, southand west of the city. The surface data, shown in Fig. 9provide an additional con"rmation to the hypothesisthat the power plant plume passed over downtownNashville and is responsible for the measured excess O

3.

Because of the proximity to the urban emission sourcesof NO (which titrates O

3), it is highly unlikely that the

high O3

value observed at the downtown sampling sitecould be related to local sources. This observation is incontrast with an earlier O

3episode noticed over Nash-

ville four days earlier (Valente et al., 1998). The di!erencebetween the two events is that the early episode was

M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036 3031

Fig. 6. O3

and nitrate as a function of NOyand NO

zin the plume of the Cumberland Plume, at six downwind distances. Each point

represents a 5-s measurement.

3032 M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036

Fig. 7. Across the plume O3, nitrate and NO

ymixing ratios as

a function of time, 61 km downwind from Cumberland powerplant.

related to pollution from local sources that remainedover the city for an extended time period because ofextremely stagnant atmospheric conditions (winds were(2 m s~1 and variable). Conversely, on 16 July thewind speed was faster at '3.5 m s~1 and the winddirection was consistent from the WNW. Thus, under theconditions prevailed on 16 July the polluted downtownair mass traveled signi"cant distance (20}40 km, approx-imately 3 h of travel time) before it could e!ectivelygenerate excess O

3. Also, the size of the area a!ected by

the high O3

during the earlier episode was so small(&10]20 km) that it was not seen by the regulatorymonitors located throughout the greater Nashville area.

A comparison of the various parameters and ratiosupwind and downwind of Nashville demonstrates howthe VOC-lean power plant plume is di!erent from thepossibly mixed power plant/urban plume. The traces ofNO

zand nitrate show that the maximum levels of these

compounds occur in the isolated power plant plume,before it had the opportunity to mix with the urbanplume. As shown in Figs. 3 and 4, the maximum values ofthese parameters are co-located with the tail end of thecontinuous SO

2trace. At this location, the air mass

chemical age (the ratio NOz/NO

y) reaches 0.7 indicating

the end (or near the end) of the photochemically activeregime (Olszyna et al., 1994). The O

3concentrations

image (Fig. 4) is somewhat similar to that of nitrate inthat it also shows two peak areas, but the di!erence isthat unlike the nitrate, the main O

3peak occurs in the

southeast sector and downwind of Nashville. In the "rstpeak, O

3levels only reach about 110 ppbv over a limited

area while in the second peak the levels reach almost120 ppbv over a much larger area.

Fig. 8. Center peak and wing mixing ratios of O3, O

3#NO

2and NO~

3as a function of estimated plume travel time.

The ratio SO3T/SNO

yT (Fig. 4), which approximately

represents the number of excess O3

molecules producedper molecule of NO

xemitted (i.e., ozone production

e$ciency), is relatively low as long as the power plant

M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036 3033

Fig. 9. Daily 1-h maximum surface O3

measurements for 16 July 1995, illustrated over a map of the central Tennessee.

plume is isolated. For the isolated plume, even when the`chemical agea is greater than 0.7, the ratio SO

3T/SNO

yT

is lower than 5. The SO3T/SNO

yT ratio only increases

up to 11 in the region where the impact of the urbanplume is most evident. Other indications of the second-ary impact of the power plant plume on O

3forma-

tion downwind of Nashville are the variations inSO

2traces and the O

3production e$ciency along the

traverses T10 and T11. As stated earlier, the mixingof the power plant plume with the city plume mayhave occurred in the northern sector of the samplingarea. Indeed, higher SO

2levels were observed in

the northern parts of the traverses T10 and T11 whilethe maximum O

3levels were observed at the southern

parts of the traverses (see Fig. 10). Fig. 10 also showsthat both O

3mixing ratio and O

3production e$-

ciency are signi"cantly lower in the northern part ofT10 and T11 where the impact of the power plantplume is more likely. The contribution of the cityplume was essential for the generation of an additional10 ppbv of O

3.

It is almost impossible to exactly apportion the relativecontributions of the urban plume and the power plantplume to O

3formation downwind of Nashville. Never-

theless, an upper limit for the potential contribution ofthe power plant can be estimated from the ratio betweenO

3and SO

2upwind and downwind of Nashville. This

upper limit estimate is based on the assumption that allof the O

3precursors that originate from the Cumberland

Power Plant plume are associated with SO2

while thosethat originate from the city sources do not contain SO

2.

Obviously, this estimate provides an upper limit becausecertain non-Cumberland sources also emit SO

2. For the

9th traverse (upwind of Nashville) the average SO2

mix-ing ratio, during the period that O

3exceeded 100 ppbv,

was 2.4 ppbv and the average O3

was 105 ppbv. If weassume that background O

3was 56 ppbv, then the ex-

cess, 49 ppbv can be associated with emission from thepower plant. Likewise, for the 11th traverse (down-wind of Nashville) the average mixing ratios were 1.1and 110 ppbv, respectively. If the upwind values repres-ent the contribution of the power plant then it can beestimated that 1 ppbv of SO

2is associated with excess

formation of 20.5 ppbv of O3. If this ratio is applied for

the downwind measurements, the upper limit for thecontribution of all SO

2emitting sources is 22.5 ppbv of

excess O3, less than half (approximately 42%) of the

additional O3.

3034 M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036

Fig. 10. Mixing ratios of O3, SO

2and NO

yand the ratio SO

3T/SNO

yT as a function of latitude for the cross wind samples T10 and T11,

taken downwind of Nashville on 16 July 1995.

6. Conclusions

Airborne sampling of the Cumberland power plantplume upwind and downwind of Nashville revealed thatO

3production e$ciency in the isolated power plant plume

was signi"cantly lower than in the mixed power planturban plume (see Fig. 4d). The lower O

3production e$-

ciency in the isolated power plant plume can be explainedby the enhanced formation of nitrate, a termination prod-uct, in the isolated plume. The highest nitrate values wereobserved closer to the source whereas the highestO

3values were found further downwind of Nashville (see

Fig. 4a and c). The airborne measurements as well asa particle dispersion model strongly suggested that thetwo plumes mixed, and that the high O

3levels observed

downwind of Nashville resulted from the mixed plume(see Fig. 5). An upper limit of 42% of the observed excessO

3production was estimated to be derived from all SO

2emitting sources including the Cumberland Power Plant.

Acknowledgements

The authors wish to acknowledge Dr. H. Copelandand Mr. L. Gautney of TVA for their e!ort in collecting

the data. This research is part of the Southern OxidantStudy (SOS) } a collaborative university, government,and private industry study to improve scienti"c under-standing of the accumulation and e!ects of photochemi-cal oxidants. Financial and in-kind support of SOSresearch and assessment activities is provided by theEnvironmental Protection Agency, National Oceanicand Atmospheric Administration, Department of En-ergy, Tennessee Valley Authority, Electric Power Re-search Institute, The Southern Company and the Statesof Alabama, Florida, Georgia, Kentucky, Louisiana,Mississippi, North Carolina, South Carolina, Tennesseeand Texas.

References

Cantrell, C.A., Lind, J.A., Shetter, R.E., Calvert, J.G.,Goldan, P.D., Kuster, W., Fehsenfeld, F.C., Montzka,S.A., Parrish, D.D., Williams, E.J., Buhr, M.P.,Westberg, H.H., Allwine, G., Martin, R., 1992. Peroxyradicals in the ROSE experiment: measurement andtheory. Journal of Geophysical Research 97, 20,671}20,686.

M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036 3035

Doddridge, B.G., Dickerson, R.R., Wardell, R.G., Civerolo, K.L.,Nunnermacker, L.J., 1992. Trace gas concentrations andmeteorology in rural Virginia, 2, Reactive nitrogen com-pounds, Journal of Geophysical Research 97, 20,631}20,646.

Gillani, N.V., 1986. Ozone formation in pollutant plumes: develop-ment and application of reactive plume model with arbitrarycross wind resolution. EPA Report dEPA-600-S3-86-051.

Gillani, N.V., Meagher, J.F., Valente, R.J., Imho!, R.E., Tanner,R.L., Luria, M., 1998a. Relative production of ozone andnitrates in urban and rural power plant plumes, 1. Com-posite results based on data of ten measurement days.Journal of Geophysical Research 103, 22,593}22,615.

Gillani, N.V., Luria, M., Valente, R.J., Tanner, R.L., Imho!,R.E., Meagher, J.F., 1998b. Loss rate of NO

yfrom a power

plant plumes based on aircraft measurements. Journal ofGeophysical Research 103, 22,585}22,592.

Hartsell, B.E., Aneja, V.P., Lonneman, W.A., 1994. Relationshipbetween peroxyacetyl nitrate, O

3and NO

yat the rural

southern oxidants study site in the central Piedmont,North Carolina, site SONIA. Journal of Geophysical Re-search 99, 21,033}21,041.

Hubler, G., Alvarez, R., Daum, P., Dennis, R., Gillani, N.,Kleinman, L., Luke, W., Meagher, J., Rider M., TrainerM., Valente R., 1998. An overview of the airborne activitiesduring the Southern Oxidant Study (SOS) 1995 Nash-ville/Middle Tennessee Ozone Study. Journal of Geo-physical Research 103, 22,245}22,260.

Imho!, R.E., Valente, R.J., Meagher, J.F., Luria, M., 1995. Theproduction of O

3in an urban plume: airborne sampling of

the Atlanta urban plume. Atmospheric Environment 27,2349}2358.

Jacob, D.J., Horowitz, L.W., Munger, W., Heikes, B.G., Dicker-son, R.R., Artz, R.S., Keene, W.C., 1995. Seasonaltransition from NO

x- to hydrocarbon-limited conditions

for ozone production over the eastern United States inSeptember. Journal of Geophysical Research 100, 931}932.

Kleinman, L., Lee, Y.-N., Springston, S.R., Nunnermacker, L.,Zhou, X., Brown, R., Hallock, K., Klotz, P., Leahy, D., Lee,J.H., Newman, L., 1994. Ozone formation at a rural site inthe southern United States. Journal of Geophysical Re-search 99, 3469}3482.

Luria, M., Olszyna, K.J., Meagher, J.F., 1983. The atmosphericoxidation of #ue gases from a coal-"red power plant:a comparison between smog chamber and airborne plumesampling. Journal of Air Pollution Control Association 33,483}487.

McNider, R.T., Moran, M.D., Pielke, R.A., 1988. In#uenceof diurnal and inertial boundary-layer oscillations onlong-range dispersion. Atmospheric Environment 22,2445}2462.

McNider, R.T., Norris, W.B., Song, A.I., Clymer, R.L., Gupta, S.,Babta, R.M., Zamora, R.J., White, A.B., Trainer, M.,1998. Meteorological conditions during the 1995 South-ern Oxidant Study Nashville/Middle Tennessee FieldIntensive. Journal of Geophysical Research 103,22,225}22,244.

Meagher, J.F., Cowling, E.B., Fehsenfeld, F.C., Pakhurst W.J.,1998. Ozone formation and transport in southern UnitedStates: overview of the SOS Nashville/Middle Tennesseeozone study. Journal of Geophysical Research 103,22,213}22,224.

Mueller, S.F., Song, A., Norris, W.J., Gupta, B.S., McNider, R.T.,1996. Modeling pollutant transport during high-ozoneepisodes in the southern Appalachian Mountains. Journalof Applied Meteorology 35, 2105}2120.

Olszyna, K.J., Bailey, E.M., Simonaitis, R., Meagher, J.F., 1994.O

3and NO

yrelationships at a rural site. Journal of Geo-

physical Research 99, 14,557}14,563.Olszyna, K.J., Luria, M., Meagher, J.F., 1997. The correlation of

temperature with rural ozone levels in southeastern USA.Atmospheric Environment 31, 3011}3022.

Pielke, R., Cotton, W.R., Walko, R.L., Tremback, C.J., Lyons,W.A., Grasso, L.D., Nicholls, M.E., Moran, M.D., Wesley,D.A., Lee, T.J., Copeland, J.H., 1992. A comprehensivemeteorological modeling system-RAMS. Meteorologyand Atmospheric Physics 49, 69}91.

Poulida, O., Dickerson, R.R., Doddridge, B.G., Holland, J.C.,Wardell, R.G., Watkins, J.G., 1991. Trace gas concentra-tions and meteorology in rural Virginia, 1. Ozone andcarbon monoxide. Journal of Geophysical Research 96,22,461}22,475.

Tanner, R.L., Valente, R.J., Meagher, J.F., 1998. Measuringinorganic nitrate species with short time resolutionfrom an aircraft platform by dual channel ozone chemi-luminescence. Journal of Geophysical Research 103,22,387}22,396.

Trainer, M., et al., 1993. Correlation of ozone with NOy

inphotochemically aged air. Journal of Geophysical Re-search 98, 16,745}16,753.

Valente, R.J., et al., 1998. Ozone Production during an urban airstagnation episode over Nashville Tennessee. Journal ofGeophysical Research 103, 22,555}22,568.

3036 M. Luria et al. / Atmospheric Environment 33 (1999) 3023}3036