Reactive nitrogen species play an important role in the chemistryof the troposphere. For example, photooxidation processes involvingreactive organic carbon and NOx (i.e., NO plus NO2)produce ozone (O3) and a host of oxidized forms ofnitrogen. The reactive nitrogen reservoir, NOy, isthus defined as the sum of NOx and all other oxidizednitrogen, excluding N2O. Measurement of NOycan provide useful information on the production and transportof O3. Numerous studies have shown strong relationshipsbetween atmospheric O3 and NOy concentrationsat rural sites. Trainer et al. (1993) observed that midday ozoneconcentrations increased with NOy at three sites innorthern Georgia, southwestern Virginia and central Pennsylvania.The ozone-NOy relationship was similar at all threesites, and exhibited a slope of about 5-10 parts per billion (ppb)of ozone per ppb of NOy. Olszyna et al. (1994) andKleinman et al. (1994) reported similar observations for ruralsites in central Tennessee and southeastern Georgia, respectively.Murphy et al. (1993) observed strong correlations between O3and NOy over a broad range of latitudes in the uppertroposphere and lower stratosphere. Hartsell and Edgerton (1995)reported strong, but variable, correlations of afternoon ozoneand NOy at three rural sites in southern Mississippi,central Alabama and northern Georgia. Taken together, these studiessuggest a general phenomenon of covariance between ozone and NOyat rural sites across eastern North America. In other words, theozone signal is accompanied by fairly predictable NOysignal.
The purpose of this analysis was to examine the behavior of O3and other trace gases at three rural sites in the southeast, andto determine whether other long-lived atmospheric species (namelyCO and SO2) could be used as tracers of NOy.This paper describes initial findings of the analysis of 1995data from three rural monitoring sites in Georgia, Alabama andMississippi. It describes the general relationship between O3and NOy under conditions favorable for high photochemicalactivity. Periods of elevated O3 are then identifiedand classified based on the abundance of the tracer species CO(automotive emissions) and SO2 (point source emissions)relative to NOy. Results indicate that 54 periods ofelevated O3 occurred during the summer of 1995. ElevatedCO (with or without elevated SO2) was associated withabout 80 percent of the 1995 episodes. Significant variabilityin source composition was observed across sites. For the networkas a whole, approximately 40 percent of episodes were classifiedas "predominantly urban", 40 percent were classifiedas "mixed" (elevated CO and SO2) and 20 percentwere classified as predominantly "major point source".
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The SCION network was established in 1991 and 1992 under the auspicesof the Southern Oxidants Study (SOS). The purpose of SCION wasto collect and interpret continuous O3, NO, NOy,CO and/or SO2 data, plus intermittent VOC data, atup to 12 rural sites across the southeastern U.S. Three SCIONsites were operated from 1992 through 1995 by Environmental Science& Engineering, Inc., under contract with the Electric PowerResearch Institute. These sites were: Yorkville, GA; Centreville,AL; and Oak Grove, MS (see Figure 1). Brief site descriptionsare provided below.
This site was located in Paulding County (latitude 33.93, longitude85.05) approximately 65 kilometers (km) northwest of Atlanta,GA, 40 km south-southwest of Rome, GA and 130 km south of Chattanooga,TN. The area around the site consisted of rolling terrain coveredpredominantly with hardwood oak forest interspersed with tilledfarmland and open pasture. Monitoring equipment was situated ina 50 hectare pasture, on relatively high ground, with no contiguousforest within 150 m. No secondary road was located within 300mand no highway was located within 5 km of the site. Numerous majorpoint sources of NOx existed within 100 km of the site;however, no major point source was within 20 km.
The Centreville, AL site was located Bibb County (latitude 32.89,longitude 87.23) approximately 85 km southwest of Birmingham,AL, 110 km northwest of Montgomery, AL and 55 km southeast ofTuscaloosa, AL. The surrounding area was heavily forested withmixed deciduous (oak-hickory) and coniferous (loblolly pine) forest.Monitoring equipment was in a 10-hectare clearing adjacent toa National Weather Service station, roughly 200 m southeast ofa lightly traveled county road (CR25). No contiguous forest canopywas within 100 m of the site. No major point source was within40 km of the site and only three minor point sources (annual NOxemissions <100 tons per year) were within 20 km.
Oak Grove, MS
This site was located in Perry County (latitude 30.99, longitude88.93) approximately 40 km southeast of Hattiesburg, MS, 120 kmnorthwest of Mobile, AL and 160 km northeast of New Orleans, LA.Land use surrounding the site was primarily forested (De SotoNational Forest) with interspersed areas of cropland and pasture.Equipment was located in a 15hectare pasture at least 100m from the nearest tree line. A lightly traveled dirt road wasapproximately 125 m north of the site, but no secondary roadsor highways were within 5 km of the site. No major or minor sourcesof NOx were within 20 km of Oak Grove.
Continuous measurements of O3, NO, NOy,CO, SO2 and various meteorological parameters weremade from March 1, 1995 through October 31, 1995 at each site.All data were collected, validated and reported on 1-minute, 15-minuteand 60-minute averaging intervals. Gas measurements were madeat a reference height of 10 m above ground level to avoid gradientsassociated with deposition and/or emission of reactive species,and Teflon inlet lines were used to transmit gases from the 10-mtower to their respective analyzers. O3 measurementswere made with a Thermo-Environmental (TE) Model 49-103 ozoneanalyzer equipped with an internal O3 generation systemand operated on the 0-500 parts per billion (ppb) range. Multi-pointgas replacement calibrations of the O3 analyzer wereperformed using a NIST-traceable transfer standard.
NO and NOy measurements were made with a modified TEModel 42S NO/NOy analyzer operated on the 0-200 ppbrange. Instrument modifications included relocation of the molybdenumconverter assembly from inside the instrument to the gas inlet,increase of system flow to approximately 3.0 liters per minuteand external control of converter temperature. The NO/NOyanalyzer was calibrated by gas replacement at the beginning andend of the field season and by method of addition at least daily(once per day for NOy and four times per day for NO).Method of addition calibrations were used to correct data formatrix effects, including NO titration by O3 and quenchingof the chemiluminescent signal by water vapor.
CO and SO2 measurements were made with TE Model 48Sand 43S analyzers operated on the 0-5000 ppb and 0-200 ppb ranges,respectively. The CO analyzer was subjected to internal, catalyticzeroing every three hours to compensate for instrument drift andchanges in temperature and water vapor. The SO2 analyzer was subjectedto daily zero checks using an internally mounted annular denudercoated with sodium carbonate. Both the CO and SO2 analyzerswere calibrated daily using method of additions. Based on thirdparty audit results and other information, the trace gas datahave an estimated accuracy on the order of +/- 10 percent (Olszyna,1996).
Episode Identification and Classification
For the purposes of this study, an episode of elevated O3was defined as any day with an eight-hour rolling average concentrationgreater than or equal to 80 ppb. This threshold is in the rangeunder consideration for revision of the national ambient air qualitystandard (NAAQS) for O3. Eight-hour rolling averageswere calculated for each hour of the day during the 1995 fieldseason. The maximum value for each day was then compared withthe 80 ppb threshold for determination of episode days.
Episodes identified by the above procedure were then classifiedinto potential source categories using observed ratios of CO*to NOy and SO2 to NOy, where CO* is observedCO minus the northern hemispheric background of 80 ppb (Parrish,1993). The three source categories were defined as urban, majorpoint source and mixed (i.e., both urban and point source). Differentiationbetween source categories was based on differences in emissionratios between automobile emissions and major point source emissions(see Table 1). Automotive emissions contain a typical CO:NOxratio of about 8:1, on a molar basis, and SO2:NOx ofless than 0.1:1 (NRC, 1991). Emission ratios from major pointsources are considerably more variable, but inspection of emissionsdata from major point sources (i.e., >1000 tons per year ofSO2) in the southeastern U.S. indicates overall SO2:NOx and CO:NOx ratios of 2.4:1 and<1:1, respectively (USEPA, 1996). Given mandated emission reductionswhich were scheduled to occur after 1994, a SO2:NOx ratioof 2:1 was taken to be representative of major point sources.
It should be noted that the source classification approach shownin Table 1 is intended to identify the probable dominant contributor(s)of NOy as opposed to the sole contributor(s) of NOy.For the purposes of this study, a dominant contributor has beendefined as one that could provide at least 75 percent of observedNOy, based on tabulated tracer:NOy ratios.It is also clear that atmospheric processing will influence speciesratios. Under clear sky conditions typical of high photochemicalactivity, the atmospheric lifetimes of key species rank as follows:CO >> SO2 > NOy (Calvert and Stockwell,1984; Hidy, 1994; Seinfeld, 1986). Observed ratios of speciesshould, therefore, increase as the chemical age of emissions increases.
Table 1. Classification Approach for Ozone Episodes.
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Scattergrams of O3 versus NOy for Oak Grove,Centreville and Yorkville are shown in Figure 2. Plots were generatedfrom 15-minute average values taken during clear air, mid-afternoonconditions from May 15, 1995 through October 15, 1995. Selectionof data was intended to identify periods of maximal photochemicalactivity and minimal vertical stratification of the atmosphere.In general, a strong relationship between O3 and NOywas observed at all three sites; however there was clear variabilityboth within sites and between sites. Oak Grove and Centrevilledata covered similar ranges in NOy (i.e., 1-9 ppb)and exhibited regression slopes of about 10. Yorkville data covereda much broader range in NOy (i.e. 2-18 ppb) and exhibitedsubstantially lower regression slope and coefficient of determinationthan the other sites.
There was appreciable scatter at all sites and instances of markeddeparture from a general linear trend. Results for Yorkville,for example, suggest that the relationship is nonlinear, withsignificantly lower slope in the range of 5-10 ppb versus 1-5ppb of NOy. Intersite and day to day variability maybe related, in part, to specific meteorological conditions, theproximity of NOy sources and availability of reactivehydrocarbons. Despite the apparent variability, overall resultsare consistent with previous investigations for widely distributedrural sites across the eastern U.S. (Trainer et al., 1993; Olszynaet al. 1994). Given the short atmospheric lifetime of NOy(i.e., 1-2 days) these results suggest that O3 is formedrapidly and in fairly close proximity to sources of NOx.Otherwise, atmospheric processing would eradicate any relationshipbetween relatively long lived O3 and its precursors.
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Episodes of elevated O3 at the three SCION sites areshown in Table 2. Also shown, for reference purposes, are exceedancesof the current 1-hr standard (i.e., >124 ppb) and data forthe years 1992 through 1994. Results show that 1-hour episodeswere rare, or nonexistent, at all sites and that none ofthe sites was in violation of the current standard. The frequencyof 8-hour episodes was much higher, but quite variable from siteto site and year to year. Yorkville exhibited the highest frequencyof 8-hour episodes followed by Centreville, then Oak Grove. Thedata in Table 2 also indicate that, although 1995 was more orless typical from the standpoint of 1-hour episodes, this yearproduced an unprecedented number of 8-hour episodes. In fact,each site reported nearly as many 8-hour episodes in 1995 as inthe three previous years combined.
Table 2. Number of ozone episodes at three SCION sites, 1992-1995.
The distribution of 8-hour episodes during the 1995 ozone seasonis depicted in Figure 3. Yorkville reported a total of 29 episodesspanning the period May 23 through October 9. Roughly half ofthe episodes occurred during two 2-week periods in mid-July andmid-August. The earlier of these corresponded roughly to the OTAG1995 model period and included ten consecutive episode days. Otherepisodes occurred singly or in pairs throughout the ozone season.
The distribution of episode days at Centreville was noticeablysparser than at Yorkville, but covered about the same time framefrom late May to early October. There was some evidence of elevatedO3 during mid-July and mid-August; however, most episodesoccurred singly or in pairs. Oak Grove reported only five episodedays during 1995 and these occurred during the period late Maythrough late August. There was little evidence of the mid-Julyepisodes seen at Yorkville and Centreville (1 day) and no evidenceof the mid-August episode. Three of the five episode days at thissite were observed during the last week in August.
Some sense of the geographic extent of 8-hour episodes can beobtained by examining co-occurrences across sites. Table 3 showsthe number of days that an episode was observed at one, two orall three sites. A total of 54 episodes was reported on 39 individualcalendar dates. Of the 39 days, 25 (64 percent) involved onlyone site, 13 (33 percent) involved two sites and only 1 (3 percent)involved all three sites. In other words, episodes were quitevariable in space and time, and rarely affected all three sites.August 30 was the only day in 1995 for which all three sites reportedan 8-hour episode. This suggests that the spatial scale of 1995episodes was typically less than the distance between any twosites (about 250-300 km).
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Following identification of episode days, each day was classifiedaccording to the approach described earlier (see Table 1). Exampletime series plots and pollutant scattergrams for days classifiedas predominantly "major point source", "urban"and "mixed" are depicted in Figure 4 through Figure9. The "major point source" episode occurred at Yorkvilleon 8/13/95. Time series data for that day indicated a strong pulseof O3, NOy and SO2 beginningaround 1200 and ending at 1800 (see Figure 4). The maximum 1-minuteO3 observed during the episode was approximately105 ppb. Short-term oscillations in O3 werealso evident, and these generally coincided with oscillationsin SO2 and NOy. Scattergrams of O3,CO and SO2 versus NOy show moderate to strongcorrelations for O3, and SO2, but essentiallynone for CO. In other words, the SO2 and NOybehave largely as though they originate together, while CO behavesas though it originates elsewhere.
Table 3. Co-occurrences of 8-hour ozone episodes during 1995.
The "urban" episode occurred at Centreville on June14, 1995. Time series data for this episode show that O3concentrations were around 65 ppb between 1200 and 1400, rosesharply to about 110 ppb shortly before 1500 and then declinedslowly through the afternoon and early evening (see Figure 6).O3 values remained elevated for almost eight hoursand did not return to early afternoon levels until roughly 2200.Time series information for NOy and CO* (CO minus troposphericbackground) revealed several major excursions just after midnight,followed by a relatively stable signal between 1000 and 1200.This was followed by a broad peak, coincident with the increasein O3, and then a decline to more or less constant,but elevated, levels. SO2 concentrations exhibiteda small (1-2 ppb) increase during the afternoon hours, possiblyreflecting a variety of sources entrained within the urban plume.
Scattergrams of O3, CO and SO2 versus NOyshow that there were strong covariances among species during thisepisode. Regression analysis showed that O3 variedin a highly linear fashion with NOy and that the slopeof the regression line was about 6. The CO* versus NOyrelationship was also quite linear with slope of about 24, orsubstantially higher than fresh automotive emissions. This regressionslope suggests that the urban plume had undergone significantprocessing prior to its arrival at Centreville. The SO2 versusNOy relationship was highly linear; however, the slopeof the regression line was far below that expected from a majorpoint source.
The "mixed" episode occurred at Oak Grove on July 13,1995. O3 concentrations for this day started around70 ppb at midnight, decreased to about 35 ppb by 0600 and thenrebounded to 70 ppb by 1100 (see Figure 8). A sharp rise in O3occurred between 1100 and 1200, with peak concentrations of about85 ppb, and remained nearly constant until 1600. Time series datafor NOy show marked similarity to O3, withminimum values around 0600 followed by a late morning maximumand declining values through the afternoon.
CO* and SO2 concentrations also showed marked variabilityduring the episode day. Both species exhibited peak concentrationsbetween midnight and 0400, followed by a significant decreasearound sunrise. Numerous SO2 plumes in the severalppb range were observed between 0700 and 1200. These plumes appearedto be superimposed on a plateau of about 3 ppb which lasted fromaround 0700 to 1600 and then rapidly transitioned to near baselineconcentrations. Concentrations of CO* were around 130 from 0700to 1200, then increased slightly between 1200 and 1600, followedby a sharp reduction just after 1600. In general the highest O3concentrations occurred during the afternoon (secondary) maximain CO* and SO2.
Scattergrams of O3, CO* and SO2 versus NOyduring the afternoon hours of July 13 showed relatively weak correlations,perhaps due to the narrow range of NOy concentrations(see Figure 9). The overall slope of the SO2 versusNOy regression line was 1.2; however some portionsof the scattergram show no apparent correlation while others suggesta fairly strong correlation with a slope closer to 3 or 4. Clearly,the episode included a variety of chemical regimes, sometimesdominated by CO* and other times dominated by SO2.Typical of all "mixed" episodes was elevated CO* throughoutthe episode, punctuated by occasional excursions in SO2.
Episode classifications for the three sites for 1995 are summarizedin Table 4. Yorkville, GA and Centreville, AL reported the mostepisode days and experienced similar distributions of episodetypes. For example, Yorkville reported 29 episode days, of which12 (41 percent) were predominantly "urban", 10 (34 percent)were "mixed" and 7 (25 percent) were major point source.In contrast, Oak Grove, MS reported only 5 episode days and allof these were classified as "mixed". For the networkas a whole, "urban" and "mixed" episodes accountedfor about 80 percent of episodes, while "major point source"episodes accounted for only 20 percent of episodes.
Table 4. Classification of 1995 Ozone Episodes.
The unique source classification for Oak Grove may reflect theremoteness of the site to large urban areas and major point sources.Another factor that could influence Oak Grove is a weak sea breezeeffect. Winds at the site often exhibited a diurnal cycle reminiscentof a sea breeze-land breeze circulation on days with light winds.Under these conditions, emissions from a variety of sources alongthe Gulf coast of the might become intermingled as they are transportedto the site. Additional analysis of meteorological conditionsare needed to determine if this occurred on the episode days ofinterest.
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Atmospheric sampling for O3, CO, SO2 andNOy was conducted at Yorkville, GA, Centreville, ALand Oak Grove, MS during the ozone season of 1995. Analysis ofpollutant relationships showed strong correlations between O3and NOy, as expected, as well as significant correlationsbetween tracer species and NOy. Days with elevatedO3 (i.e., 8-hour average > 80 ppb) were identifiedand classified based on relative abundances of tracers to NOy.A total of 54 days with elevated O3 was observed in1995. Yorkville, the nearest site to major urban and point sourcesreported the most episode days in 1995, while Oak Grove, the remotestsite, reported the fewest. Of the 54 episode days, 25 (64 percent)involved a single site with elevated O3, 13 involvedtwo sites and only one day involved all three sites. In otherwords, almost all days with elevated O3 exhibited significantspatial variability, as well as temporal variability.
Analysis of tracer data (CO and SO2) showed that alldays with elevated O3 were associated with urban and/ormajor point source signatures. The distribution of episode typesdiffered from site to site, but, over the entire network, themost frequent signature was "urban", followed by "mixed",then by "major point source". Episodes of elevated O3,are thus associated with distinct and identifiable plumes originatingfrom urban areas and/or major point sources. The nature of theseplumes and the strong covariances among short-lived and long-livedspecies, in turn, suggests that we are observing near field transport( i.e., intra-regional) of O3 and its precursors ratherthan long range transport (i.e., inter-regional) of these species.Additional research is needed to determine if this phenomenonis: 1) unique to the southeast; 2) unique to 1995; or 3) consistentwith observed meteorological conditions.
Overall conclusions of the study are as follows:
1) Elevated O3 concentrations (8-hour maximum >80 ppb) in the rural southeast are spatially and temporally variable.Episodes were rarely observed simultaneously across the rural3-site SCION network during 1995.
2) Episodes of elevated O3 can be classified basedon tracer information. Results for 1995 showed that transportfrom urban areas (with and without embedded point sources) accountedfor the majority of episode days. By virtue of their detectability,NOy plumes (and associated tracer species) are likelyof recent and local origin (e.g., tens of kilometers), ratherthan of distant origin (hundreds of kilometers).
3) Data from additional locations and years are needed to evaluatethe generality of these findings.
Taken together, the conclusions of this analysis raise importantquestions about the frequency, magnitude and sources of intra-regionaland inter-regional transport of O3 and its precursors.Modeling results and policy conclusions regarding intra-regionaland inter-regional transport must be reconciled with these observations.
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This research is part of the Southern Oxidants Study (SOS) --a collaborative university, government and private industry studyto improve the scientific understanding of the accumulation andeffects of photochemical oxidants. Financial and in-kind supportfor SOS is provided by the USEPA, NOAA, DOE, TVA, EPRI, SouthernCompany, Coordinating Research Council, Duke Power Company andnumerous states. SCION is one of three regional oxidant networksinitiated by SOS. The goal of SCION is to elucidate relationshipsbetween ozone and related pollutants (precursors and tracers)across broad areas of the rural southeast.
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Calvert. J. G. and W. R. Stockwell. 1984. Mechanism and ratesof the gas phase oxidations of sulphur dioxide and nitrogen oxidesin the atmosphere. In SO2, NO and NO2Oxidation Mechanisms: Atmospheric Considerations,Vol. 3. Butterworth, Boston, MA, pp. 1-62.
Chameides, W. L., F. C. Fehsenfeld, M. O. Rodgers, C. Cardelino,J. Martinez, D. D. Parrish, W. Lonneman, D. R. Lawson, R. A. Rasmussen,P. Zimmerman, J. Greenberg, P. Middleton and T. Wang. 1992. OzonePrecursor Relationships in the Ambient Atmosphere. J. Geophys.Res. 97, 6037-6055.
Edgerton, E. S. and B. E. Hartsell. 1996. Ozone/Tracer Relationshipsat Three Rural Sites in the Southeastern U.S. Presented at theSOS/Nashville Data Analysis Workshop. May 5, 1996. Raleigh, NC.
Hartsell, B. E. and E. S. Edgerton. 1995. 1992-1993 Data Reportfor SCS-SCION Sites. Environmental Science & Engineering,Inc. Durham, NC.
Hartsell, B. E. and E. S. Edgerton. 1996. A Comparison of Modeledand Measured Ozone, NOy and CO at Nine Regional Monitoring Stationsduring the 1995 OTAG Episode. Presented at the OTAG Air QualityAnalysis Workshop, September 25, 1996, Norfolk, VA.
Hidy, G. M. 1994. Atmospheric Sulfur and Nitrogen Oxides.Academic Press, San Diego, CA.
Kleinman, L., Y. N. Lee, S. Springston, L. Nunnermacker, X. Zhou,R. Brown, K. Hallock, P. Klotz, D. Leahy, J. Lee and L. Newman.1994. Ozone Formation at a Rural Site in the Southeastern UnitedStates. J. Geophys. Res., 99, 3469-3482.
Murphy, D. M., D. W. Fahey, M. H. Proggitt, S. C. Liu, K. R. Chan,C. S. Eubank, S. R. Kawa and K. K. Kelly. 1993. Reactive Nitrogenand its Correlation with Ozone in the Lower Stratosphere and UpperTroposphere. J. Geophys. Res., 98, 8751-8773.
National Research Council. 1991. Rethinking the Ozone Problemin Urban and Regional Air Pollution. Chapter 9, EmissionsInventories. National Academy Press, Washington, D.C. pp. 251-302.
Olszyna, K. J. 1996. Performance Audit Report for the SoutheasternConsortium: Intermediate Oxidants Network. Yorkville. Draft reportdated August 13, 1996.
Olszyna, K. J., E. M. Bailey, R. Simonaitis and J. F. Meagher.1994. O3 and NOy Relationships at a RuralSite. J. Geophys. Res., 99, 14557-14563.
Parrish, D. D., J. S. Holloway, M. Trainer, P. C. Forbes and F.C. Fehsenfeld. 1993. Science, 259, 1436-1439.
Seinfeld, J. H. 1986. Atmospheric Chemistry and Physics ofAir Pollution. Wiley, New York, NY.
Trainer, M. et al. 1993. Correlation of Ozone with NOyin Photochemically Aged Air. J. Geophys. Res. 98,2917-2925.
U. S. EPA. 1996. AIRS Executive version 3.0 AIRS Data Summaryfor September 1996. Office of Air Quality Planning and Standards.Research Triangle Park, NC.
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Figure 1. Locations of SOS-SCION Surface Monitoring Sites, 1995.
Figure 2. O3 versus NOy under photochemicallyconducive conditions during 1995.
Figure 3. Daily maximum 8-hour average O3 for 1995(black bars indicate episode days).
Figure 4. Time Series Data for "Major Point Source"Episode at Yorkville, GA (8/13/95).
Figure 5. Pollutant versus NOy Scattergrams for "MajorPoint Source" Episode of 8/13/95.
Figure 6. Time Series Data for "Urban" Episode at Centreville,AL (6/14/95).
Figure 7. Pollutant versus NOy Scattergrams for "Urban"Episode of 6/14/95.
Figure 8. Time Series Data for "Mixed" Episode at OakGrove, MS (7/13/95).
Figure 9. Pollutant versus NOy Scattergrams for "Mixed"Episode of 7/13/95.
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