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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).
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.
| Ratio | Urban | Mixed | Major Pt. Source |
| CO*/NOy | >10:1 | >5:1 | <5:1 |
| and | and | and | |
| SO2 /NOy | <0.5:1 | >0.5:1 | >1.5:1 |
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.
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.
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.
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
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in the atmosphere. In SO2, NO and NO2
Oxidation Mechanisms: Atmospheric Considerations, Vol.
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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.
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Olszyna, K. J., E. M. Bailey, R. Simonaitis and J. F. Meagher.
1994. O3 and NOy Relationships at a Rural
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Chameides, W. L., F. C. Fehsenfeld, M. O. Rodgers, C. Cardelino,
J. Martinez, D. D. Parrish, W. Lonneman, D. R. Lawson, R. A. Rasmussen,
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Edgerton, E. S. and B. E. Hartsell. 1996. Ozone/Tracer Relationships
at Three Rural Sites in the Southeastern U.S. Presented at the
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Table 2. Summary of 1995 Ozone Episodes at Arendtsville, PA.
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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