Analysis of Ozone, NOy and Tracer Data from a

Site in South-Central Pennsylvania

Eric S. Edgerton

and

Benjamin E. Hartsell

ESE Environmental, Inc.

Durham, NC 27713

919 544-3903 (v)

919 544-3882 (f)

for Submittal to the

Ozone Transport Assessment Group

Air Quality Analysis Subgroup

October 1996


CONTENTS:


INTRODUCTION

Reactive nitrogen species play an important role in the chemistry of the troposphere. For example, photooxidation processes involving reactive organic carbon and NOx (i.e., NO plus NO2) produce ozone (O3) and a host of oxidized forms of nitrogen. The reactive nitrogen reservoir, NOy, is thus defined as the sum of NOx and all other oxidized nitrogen, excluding N2O. Measurement of NOy can provide useful information on the production and transport of O3. Numerous studies have shown strong relationships between atmospheric O3 and NOy concentrations at rural sites. Trainer et al. (1993) observed that midday ozone concentrations increased with NOy at three sites in northern Georgia, southwestern Virginia and central Pennsylvania. The ozone-NOy relationship was similar at all three sites, and exhibited a slope of about 5-10 parts per billion (ppb) of ozone per ppb of NOy. Olszyna et al. (1994) and Kleinman et al. (1994) reported similar observations for rural sites in central Tennessee and southeastern Georgia, respectively. Murphy et al. (1993) observed strong correlations between O3 and NOy over a broad range of latitudes in the upper troposphere and lower stratosphere. Hartsell and Edgerton (1995) reported strong, but variable, correlations of afternoon ozone and NOy at three rural sites in southern Mississippi, central Alabama and northern Georgia. Taken together, these studies suggest a general phenomenon of covariance between ozone and NOy at rural sites across eastern North America. In other words, the ozone signal is accompanied by fairly predictable NOy signal.

The purpose of this analysis is to examine the behavior of O3 and other trace gases at a rural site in south-central Pennsylvania, and to determine whether other long-lived atmospheric species (namely CO and SO2 ) covary with NOy. This paper describes initial findings of the analysis of 1995 data from a NARSTO-NE surface monitoring site near Arendtsville, Pennsylvania. It describes the general relationship between O3, CO, SO2 and NOy under conditions favorable for high photochemical activity. Periods of elevated O3 are then identified and classified based on the abundance of the tracer species CO (automotive emissions) and SO2 (point source emissions) relative to NOy. Results indicate that 21 periods of elevated O3 occured during the summer of 1995. Elevated CO was associated with almost all of the 1995 episodes. One-third of episodes (7) could be classified as "predominantly urban", and all of the remaining episodes were classified as "mixed" (elevated CO and SO2).


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SITE DESCRIPTION AND METHODS

The site is part of the EPA-sponsored Clean Air Status and Trends Network (CASTNet) and the NARSTO-Northeast study. It is located in south-central Pennsylvania (latitude 39.92N, longitude 77.31W) on the Arendtsville Fruit Research Farm of Pennsylvania State University (see Figure 1). There are no large population centers in the vicinity of the site, which is approximately 175 km west of Philadelphia, 90 km northwest of Baltimore, 110 km north-northwest of Washington, DC and 50 km south-southwest of Harrisburg. No major point source of SO2 or NOx is within 40 km of the site, and no secondary road is within 200m of the site.

Monitoring equipment occupies a 50 meter by 50 meter clearing atop a knoll with good meteorological fetch in all directions. Tall grass is the dominant vegetation to the south through north, and peach trees (<5 meters tall) are the dominant vegetation from the north to south. Other than the equipment shelter and associated sampling towers, there are structures or obstacles to wind flow within 100m of the site.

Continuous measurements of O3, NO, NOy, CO, SO2 and various meteorological parameters were made from June 17, 1995 through September 30, 1995. All data were collected, validated and reported on 1-minute, 15-minute and 60-minute averaging intervals. Gas measurements were made at a reference height of 10 m above ground level to avoid gradients associated with deposition and/or emission of reactive species, and Teflon inlet lines were used to transmit gases from the 10-m tower to their respective analyzers. O3 measurements were made with a Thermo-Environmental (TE) Model 49-103 ozone analyzer equipped with an internal O3 generation system and operated on the 0-500 parts per billion (ppb) range. Multi-point gas replacement calibrations of the O3 analyzer were performed using a NIST-traceable transfer standard.

NO and NOy measurements were made with a modified TE Model 42S NO/NOy analyzer operated on the 0-200 ppb range. Instrument modifications included relocation of the molybdenum converter assembly from inside the instrument to the gas inlet, increase of system flow to approximately 3.0 liters per minute and external control of converter temperature. The NO/NOy analyzer was calibrated by gas replacement at the beginning and end 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 for matrix effects, including NO titration by O3 and quenching of the chemiluminescent signal by water vapor.

CO and SO2 measurements were made with TE Model 48S and 43S analyzers operated on the 0-5000 ppb and 0-200 ppb ranges, respectively. The CO analyzer was subjected to internal, catalytic zeroing every three hours to compensate for instrument drift and changes in temperature and water vapor. The SO2 analyzer was subjected to daily zero checks using an internally mounted annular denuder coated with sodium carbonate. Both the CO and SO2 analyzers were calibrated daily using method of additions.

Third party audits of gas analyzers were performed twice during the measurement period. Based on audit results and other information, the trace gas data has an estimated accuracy on the order of +/- 10 percent (Mueller and Roberts, 1996).

For the purposes of this study, an episode of elevated O3 was defined as any day with an eight-hour rolling average concentration greater than or equal to 80 ppb. This threshold is in the range under consideration for revision of the national ambient air quality standard (NAAQS) for O3. Eight-hour rolling averages were calculated for each hour of the day during the 1995 field season. The maximum value for each day was then compared with the 80 ppb threshold for determination of episode days.

Episodes identified by the above procedure were then classified into potential source categories using observed ratios of CO* to NOy and SO2 to noy, where CO* is observed CO minus the northern hemispheric background of 80 ppb (Parrish, 1993). The three source categories were defined as urban, major point source and mixed (i.e., both urban and point source). Differentiation between source categories was based on differences in emission ratios between automobile emissions and major point source emissions (see Table 1). Automotive emissions contain a typical CO:NOx ratio of about 8:1, on a molar basis, and SO2 :NOx of less than 0.1:1 (NRC, 1991). Emission ratios from major point sources are considerably more variable, but inspection of emissions data from major point sources (i.e., >1000 tons per year of SO2) in Pennsylvania indicates overall SO2 :NOx and CO:NOx ratios of 2.4:1 and <1:1, respectively, during 1994 (USEPA, 1996). Given mandated emission reductions which were scheduled to occur after 1994, a SO2:NOx ratio of 2:1 was taken to be representative of major point sources. It should be noted that the source classification approach shown in 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 been defined as one that could provide at least 75 percent of observed NOy , based on tabulated tracer:NOy ratios. It is also clear that atmospheric processing will influence species ratios. Under clear sky conditions typical of high photochemical activity, the atmospheric lifetimes of key species rank as follows: CO >> SO2 > NOy (Calvert and Stockwell, 1984; Hidy, 1994; Seinfeld, 1986). Observed ratios of species should, therefore, increase as the chemical age of emissions increases.

Table 1. Classification Approach for Ozone Episodes.
RatioUrban MixedMajor Pt. Source
CO*/NOy>10:1 >5:1<5:1
andand and
SO2 /NOy<0.5:1 >0.5:1>1.5:1


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RESULTS AND DISCUSSION

Scattergrams of binned O3 , CO and SO2 versus NOy are shown in Figure 2. Plotted points represent the average value for each of ten data bins (1920 1-minute data points per bin). Bins were generated by selecting all data between 1100 and 1600 local standard time with no precipitation and UV radiation >20 watts per square meter. The subset was sorted on NOy , divided into ten bins of equal number of data points, then the average of each variable was calculated for each bin (Trainer, 1993). Results show significant correlations with NOy for all three variables. The linear regression slope of O3 versus NOy is approximately 4.1, and is near the low end of previously reported values (Olszyna et al.,1994; Trainer et al., 1993; and Hartsell and Edgerton, 1995). This could reflect the proximity of the site to sources (i.e., relatively recent emissions), however insufficient NOy speciation data were collected to determine the extent of chemical processing.

CO* and SO2 also show a marked tendency to increase with increasing NOy . The slope of the CO* versus NOy line is similar to the CO:NOx ratio in automobile emissions, while the slope of the SO2 versus NOy line is less than half the estimated SO2:NOx ratio in major point source emissions. In general, the strong correlations among species suggests that tracer data may provide some insight into the history of NOy on time scales relevant to ozone episodes.

Daily maximum 8-hour average O3 concentrations for Arendtsville are shown in Figure 3. Over the 1995 field season (101 total days) there were 21 days with 8-hour average O3 greater than 80 ppb. The majority of episode days occurred in groups of three or more consecutive days, but there were also a number of one-day and two-day episodes. Two five-day events in June and July accounted for nearly half of the days with elevated O3. The highest recorded hourly average O3 concentration at Arendtsville during 1995 was 118 ppb.

Table 2 lists 8-hour maximum O3 concentrations for all episode days, as well as corresponding NOy, CO* and SO2. Also shown are tracer:NOy ratios and dominant source classifications for each episode day. Eight-hour O3 ranged from 80.1 ppb to 105.4 ppb with a median value of 86.8 ppb. CO*:NOy ratios ranged from 8.4 to 22.3 (median 12.6) and were greater than 10 on all but four episode days. SO2:NOy ratios ranged from 0.3 to 1.5 (median 0.67) and were less than unity on all but three episode days. The source classifications in Table 2 show that 15 of the episodes were "mixed", six were "urban" dominated and that none was "major point source" dominated. One episode (9/8/95) approached the SO2:NOy ratio for designation as "major point source" dominated, but also had an elevated CO*:NOy ratio.

As defined, the source classification results imply that urban emissions contributed greater than 75 percent of the observed NOy for six episode days and never contributed less than 25 percent. Major point sources, on the other hand, never contributed as much as 75 percent and usually contributed substantially less than 50 percent of observed NOy.

The majority of days classified as predominantly urban occurred during the period July 12-16. This time frame coincides with the latter part of the OTAG 1995 modeling episode. Examination of UAM-V model output suggests at least qualitative agreement with the urban designation on these days. Time series data from UAM-V show significantly elevated CO (200 to 300 ppb above background) from July 12 through July 16. Predicted CO concentrations are distinctly higher than observed concentrations (Hartsell and Edgerton, 1996), but this serves only to underscore the importance of urban emissions on these episode days.

O3, NOy and tracer data (1-minute values) for typical "urban" and "mixed" episode days are shown in Figure 4 and Figure 5, respectively. The "urban" episode (8/14/95) exhibited elevated NOy (i.e., >10 ppb) throughout the morning, followed by a slight decline through the afternoon hours, and then by a strong pulse late in the afternoon. O3 concentration exhibited a minimum between 0600 and 0900, fol.lowed by a steady increase to approximately 90 ppb, then a further increase coincident with the late afternoon NOy pulse. Scattergrams of CO* versus NOy and SO2 versus NOy for the afternoon of 8/14/95 show a significant relationship between CO* and NOy. The slope of the CO* versus NOy regression line was approximately 25, or well above the value expected from fresh automotive emissions. In contrast almost no relationship was observed between SO2 and NOy during this particular episode.

The "mixed" episode of 8/21/95 also showed complex behavior of O3 and NOy (see Figure 6). O3 concentration showed a minimum of approximately 20 ppb at 0800, followed by a rapid increase from 50 ppb to around 90 ppb at 1100. This was followed by a broad secondary peak (80-90 ppb) that declined slowly throughout the afternoon and evening hours. NOy concentration was initially around 5 ppb, then increased to about 25 ppb during the rush hour period and fell to 10 ppb by noon. A mid-afternoon pulse of NOy coincided with the secondary O3 peak, followed by a decline of NOy in parallel with O3. Scattergrams of CO* and SO2 versus NOy show significant covariance among all three species. Both CO* and SO2 increased with NOy, the former with a slope of about 7 and the latter with a slope of about 2.5.

Tracer and NOy data are plotted for all 1995 episodes in Figure 6. Also shown are reference lines indicating hypothetical relationships between tracer and NOy concentrations for 0 hours (fresh) and 24 hours of atmospheric processing, without precipitation. The fresh and 24 hour lines for CO* versus NOy reflect ratios of 8:1 and 15:1, respectively, assuming atmospheric lifetimes for CO and NOy of 30 days and 1.5 days, respectively. The fresh and 24 hour lines for SO2 versus NOy reflect ratios of 2:1 and 2.8:1, respectively, assuming an atmospheric lifetime for SO2 of 3 days. The majority of episodes showed a CO/NOy signature that was bracketed by the two reference lines for automotive emissions. No point fell below the fresh emission line and four points exceeded the 24 hour line. In contrast, the values plotted for SO2 and NOy showed values well below the reference line for fresh emissions. These results indicate a strong linkage between CO and NOy during episodes of elevated O3. This, in turn, suggests that urban sources play an important, and possibly dominant, role in the atmospheric NOy budget at Arendtsville.


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SUMMARY AND CONCLUSIONS

Atmospheric sampling for O3, CO, SO2 and NOy was conducted at Arendtsville, PA during the ozone season of 1995. Analysis of pollutant relationships showed a fairly strong correlation between O3 and NOy, as expected, as well as significant correlations between tracer species and NOy. Days with elevated O3 (i.e., 8-hour average > 80 ppb) were then identified and classified based on relative abundances of tracers to NOy. Of 21 days with elevated O3, 15 exhibited a "mixed" signature and 6 exhibited an "urban" signature for observed NOy. No episode was associated with a "major point source" signature. Examination of tracer:NOy ratios also suggested that relatively fresh emissions of CO and NOy were coincident with O3 episodes; that is, less than or equal to about 24 hours of transport and processing. In other words, the principal sources of NOy at Arendtsville appeared to be both urban and local. Similar observations have also been made for three rural sites in the southeastern U.S (Edgereton and Hartsell, 1996). Additional data and analyses are needed to: 1) evaluate source signatures in other regions and years; 2) calculate the chemical age of NOy and the production of O3 as a function of chemical age; and 3) link meteorological observations with chemical observations to form a more complete picture of transport and processing.


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ACKNOWLEDGEMENT

Data used for this study were obtained under the sponsorship of NARSTO-NE (1-minute data) and the USEPA Clean Air Status and Trends Network (hourly ozone). The material presented has not been reviewed by members of the NARSTO-NE community or the USEPA. Opinions and conclusions reflect the views of the authors only.


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References

Mueller, P. K. and P. T. Roberts. 1996. Memorandum to the NARSTO-NE Quality Systems Team. Data Qualification Statement, 1995 Surface Air Quality and Meteorological Data: Environmental Science and Engineering, Version 3, September 29, 1996 (draft).

Hartsell, B. E. and E. S. Edgerton. 1996. A Comparison of Modeled and Measured Ozone, NOy and CO at Nine Regional Monitoring Stations during the 1995 OTAG Episode. Presented at the OTAG Air Quality Analysis Workshop, September 25, 1996, Norfolk, VA.

Hartsell, B. E. and E. S. Edgerton. 1995. 1992-1993 Data Report for SCS-SCION Sites. Environmental Science & Engineering, Inc. Durham, NC.

Calvert. J. G. and W. R. Stockwell. 1984. Mechanism and rates of the gas phase oxidations of sulphur dioxide and nitrogen oxides in the atmosphere. In SO2, NO and NO2 Oxidation Mechanisms: Atmospheric Considerations, Vol. 3. Butterworth, Boston, MA, pp. 1-62.

Hidy, G. M. 1994. Atmospheric Sulfur and Nitrogen Oxides. Academic Press, San Diego, CA.

Seinfeld, J. H. 1986. Atmospheric Chemistry and Physics of Air Pollution. Wiley, New York, NY.

U. S. EPA. 1996. AIRS Executive version 3.0 AIRS Data Summary for September 1996. Office of Air Quality Planning and Standards. Research Triangle Park, NC.

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 Nitrogen and its Correlation with Ozone in the Lower Stratosphere and Upper Troposphere. J. Geophys. Res., 98, 8751-8773.

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 United States. J. Geophys. Res., 99, 3469-3482.

Olszyna, K. J., E. M. Bailey, R. Simonaitis and J. F. Meagher. 1994. O3 and NOy Relationships at a Rural Site. J. Geophys. Res., 99, 14557-14563.

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. Ozone Precursor Relationships in the Ambient Atmosphere. J. Geophys. Res. 97, 6037-6055.

Edgerton, E. S. and B. E. Hartsell. 1996. Ozone/Tracer Relationships at Three Rural Sites in the Southeastern U.S. Presented at the SOS/Nashville Data Analysis Workshop. May 5, 1996. Raleigh, NC.

Trainer, M. et al. 1993. Correlation of Ozone with NOy in Photochemically Aged Air. J. Geophys. Res. 98, 2917-2925.

National Research Council. 1991. Rethinking the Ozone Problem in Urban and Regional Air Pollution. Chapter 9, Emissions Inventories. National Academy Press, Washington, D.C. pp. 251-302.

Parrish, D. D., J. S. Holloway, M. Trainer, P. C. Forbes and F. C. Fehsenfeld. 1993. Science, 259, 1436-1439.


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FIGURES AND TABLES:


Table 2. Summary of 1995 Ozone Episodes at Arendtsville, PA.
Episode
DATE
O3
NOy
CO*
SO2
CO*/NOy
SO2/NOy
Class
6/17/95
80.88
9.70
143
7.31
14.79
0.75
M
6/18/95
80.09
7.17
131
5.02
18.21
0.70
M
6/19/95
105.41
14.33
167
17.46
11.64
1.22
M
6/20/95
80.65
8.96
172
5.64
19.19
0.63
M
6/21/95
85.78
24.79
209
16.15
8.42
0.65
M
7/12/95
80.75
7.03
157
3.02
22.31
0.43
U
7/13/95
92.54
9.58
148
3.98
15.41
0.42
U
7/14/95
90.31
8.82
119
3.92
13.44
0.44
U
7/15/95
90.01
8.98
95
5.52
10.62
0.62
M
7/16/95
81.86
8.30
89
2.45
10.73
0.30
U
7/22/95
84.80
11.64
178
5.29
15.33
0.45
U
7/31/95
91.23
8.70
91
5.81
10.41
0.67
M
8/1/95
88.73
7.77
123
4.77
15.81
0.61
M
8/15/95
86.83
11.90
122
5.14
10.25
0.43
U
8/16/95
103.45
14.64
150
11.64
10.25
0.80
M
8/17/95
87.05
8.34
70
5.73
8.39
0.69
M
8/21/95
87.98
8.04
120
7.30
14.97
0.91
M
8/24/95
91.14
10.00
126
13.56
12.63
1.36
M
8/26/95
85.35
9.87
124
6.74
12.55
0.68
M
8/31/95
86.21
8.35
83
7.74
9.98
0.93
M
9/8/95
86.54
14.33
126
21.55
8.79
1.50
M


Figure 1. Location of Arendtsville, PA CASTNet/NARSTO-NE Surface Monitoring Site.





Figure 2. Scattergrams of Binned O3, CO and SO2 versus NOy.





Figure 3. Daily maximum 8-hr Average O3 (values > 80 ppb in black).





Figure 4. Time Series and Tracer:NOy Data for an "Urban" Episode (8/14/95).





Figure 5. Time Series and Tracer:NOy Data for a "Mixed" Episode (8/21/95).





Figure 6. Tracer:NOy Scattergrams for all 1995 Episodes.


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