Ambient Nitrogen Oxide Concentrations During Peak Tourist ...
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Ambient Nitrogen Oxide Concentrations During Peak Tourist Season in Yosemite National Park
Bianca Auble
2011 School of Science Summer Research
Abstract
This study assessed the concentrations of nitrogen oxides and surface-‐level ozone in heavy traffic areas of Yosemite Valley in Yosemite National Park. Measurements were conducted in July and August, when visitor totals peak in the Park, and when ozone concentrations tend to be at their maximum. Ozone and nitrogen oxides were measured at two Valley sites–Camp 4 and Falls Lodge−using monitors deployed on a mobile trailer. Raw data were collected as 1-‐minute averages, which were later converted into 1-‐hour averages in order to improve the signal to noise ratio. Ozone measurements at an additional stationary site (the Schoolyard) were used for comparison purposes among the sites. Diurnal cycles showed that ozone at Camp 4, Falls Lodge, and the Schoolyard was low in the early morning hours and higher in the morning and afternoon hours. Nitrogen oxides at Camp 4 showed a trend with lower concentrations in the morning and higher concentrations in the afternoon. Falls Lodge showed opposite results, with the highest NOx values in the morning hours and the lowest in the afternoon. Nitrogen oxide and ozone concentrations were plotted against one another to visualize titration effects. Titration of ozone was not apparent at Camp 4, but may be occurring at Falls Lodge.
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Introduction:
Tropospheric ozone is a primary component of photochemical smog. Due to its
detrimental effects on flora, fauna, and humans, ozone is categorized as one of the six
criteria pollutants that are regulated via a National Ambient Air Quality Standard set by the
Environmental Protection Agency (EPA). In humans, high levels of ozone can cause
respiratory problems, headaches, and irritation to the eyes, especially in high-‐risk
individuals (i.e. children, those with respiratory conditions, sensitive adults, etc.), and
permanent lung damage remains a possibility with long-‐term exposure (Six).
Ozone is created through a chemical reaction that combines its two main
precursors⎯volatile organic compounds (VOCs) and nitrogen oxides (NOx = NO +
NO2)⎯with sunlight (hν):
(1) VOC + NOx + hν O3 + other pollutants
Anthropogenic VOCs – hydrocarbons produced by factories and power plants – and
biogenic VOCs–natural sources produced by plants−contribute to the total VOC
concentration in a given area (Volatile). Sources for NO include byproducts of fuel
combustion, such as vehicular emissions; once in the atmosphere, NO is converted to NO2
on a timescale of a few hours. Areas with a significant amount of both reactants (VOCs and
NOx) and exposure to warm, sunny conditions are conducive to ozone formation.
Although NOx can contribute to ozone formation through the photochemical
reaction above, fresh NO plumes can also remove ozone by titration. Titration in this
fashion produces NO2 and O2:
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(2) NO + O3 NO2 + O2
During daytime hours, it is difficult to observe titration effects because the available NOx is
used in the formation of ozone via photochemical equation (1). Titration can be more
readily observed during nighttime hours, however, because the absence of sunlight
prevents the formation of ozone, removing (1) as a complicating factor (Sillman).
Significant titration is most often observed in urban areas with substantial sources of fresh
NO plumes (i.e. vehicles, power plants).
In remote areas where ozone precursors are scarce, such as Yosemite National Park
located in the Sierra Nevada Mountain Range, it would seem intuitive that ozone levels
should be low. However, increased vehicular traffic due to tourism brings an influx of
nitrogen oxide and VOC emissions, especially in the summer months – June, July, August,
and September. Since most park visitors limit their stay to the Valley, this traffic increase
primarily affects the air quality in the Yosemite Valley region rather than backcountry
terrain.
According to data compiled by the National Park Service Public Use Statistics Office,
a total of 3,901,408 visitors traveled to Yosemite for recreational purposes in 2010. 60% of
these tourists entered the Park within the four summer months of June-‐September; nearly
80% of the total Park visitors were counted in only the six-‐month span of May through
October (Figure 1, NPS Stats).
To date, monitoring of ambient nitrogen oxide levels in Yosemite Valley has been
very limited, and the extent to which high visit totals and elevated vehicular traffic might
correlate with increased air pollution has not been well understood. To address this
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problem, the California Air Resources Board and Yosemite National Park developed a
mobile monitoring platform that could be deployed in multiple locations in Yosemite Valley
to measure air quality data during the summer of 2011.
Methods:
A mobile trailer (Figure 2) was constructed for the purpose of measuring ambient
nitrogen oxide levels at multiple sites within Yosemite Valley. Four solar panels were
attached to a long, flat trailer bed to allow deployment of the trailer in areas with no AC
power capabilities. The solar panels were used to recharge three 12 V, deep cycle batteries
enclosed in a weatherproof steel box which supplied power for the system via a Samplex
Power 600 W DC-‐AC Power Inverter. A second weatherproof steel box was mounted on the
trailer to house the Inverter and the field monitors. A sample inlet approximately 2.5
meters high was attached to metal rod secured to the outside of the trailer and sheltered by
a metal rain shield. The air sample travelled into a filter and through 2 meters of Teflon
tubing, an unreactive material to prevent absorbance of the air sample components, before
entering the monitors. The inlet tubing was plumbed into the all the monitors so they all
measured data on the same air sample.
The field monitors included one Model 202 2B Technologies Ozone Monitor, one
Model 410 2B Technologies Nitric Oxide Monitor, and one Model 401 2B Technologies
Nitrogen Dioxide Converter, all of which transmitted their data to a CR23X Campbell
Scientific Micrologger. The Model 410 was attached to the Model 401 and set to collect NOx
rather than NO or NO2, exclusively. The Model 201 O3 monitor and the Model 410/401 NOx
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monitor measured values for their respective pollutants at 10-‐second intervals, and
recorded these data as 1-‐minute averages. These data were late converted to 1-‐hour
averages to allow analysis of trends.
A Model 408 2B Technologies Nitric Oxide Calibration Source was used to calibrate
the Nitric Oxide Monitor multiple times during the deployment at each site.
The deployment sites included a dirt turnout alongside Valley Loop Road across
from Camp 4 and a dirt plot at the intersection of Valley Loop Road and the road leading to
Yosemite Lodge near the base of Lower Yosemite Falls. (These sites will be hereafter
referred to as Camp 4 and Falls Lodge.) Measurements were taken at Camp 4 for Julian
days 194 -‐ 201 (July 13 – July 20) and at Falls Lodge from Julian days 208 – 213 (July 27 –
August 1). Frequent equipment failures limited the amount of useable data acquired, which
resulted in short monitoring periods at each site.
A stationary monitoring site – YOSE Schoolyard -‐ was located in Yosemite Valley off
of the main road, and had reduced exposure to ambient nitrogen oxides from vehicular
emissions. This station transmitted ozone data (via satellite) that are publically available
via an online database: http://www.nature.nps.gov/air/monitoring/network.cfm. Ozone
hourly averages for Julian days 194-‐213 (July 13 – August 1) were used for comparison
purposes with the ozone data collected from Camp 4 and Falls Lodge.
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Results:
Ozone Concentrations:
Hourly ozone concentrations at Camp 4 over Julian Days 194-‐201 did not exceed 60
ppb (Figure 3), and Schoolyard displayed similar patterns: an increase throughout the late
morning, peaking in the early to mid afternoon, followed by a subsequent decrease in
ozone concentration in the evening to the early morning hours. The most significant
variations between the sites occurred for maximum and minimum ozone. Concentrations
of ozone at Falls Lodge and Schoolyard over Julian Days 208-‐213 differed by a greater
margin at the maxima and minima than Camp 4 and Schoolyard. Falls Lodge showed a
greater range of ozone than Schoolyard. Maximum ozone concentrations at Falls Lodge
were most often greater than maximum values at Schoolyard, but only by 3-‐5 ppb.
Similarly, minimum ozone values at Falls Lodge were more often than not lower than
Schoolyard by 2-‐4 ppb.
Average Diurnal Variation in NOx and O3:
Average diurnal curves were graphed based on the 1-‐hour averaged data of NOx and
O3. Figure 5 depicts the diurnal curve for NOx at Camp 4. Maximum NOx concentrations
occurred between hours 15:00 and 16:00 PST, with lower levels observed in the evening
and early morning. Interestingly, the diurnal curve for NOx at Falls Lodge (Figure 6)
showed drastically different results than Camp 4. Maximum NOx concentrations were
observed between hours 6:00 and 8:00 PST with the much lower values in the afternoon.
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The average diurnal curves of O3 concentration for Camp 4, Falls Lodge, and
Schoolyard (Figure 7) followed similar patterns. Low concentrations of ozone were found
in the evening and early morning hours, while concentrations between 40 and 50 ppb
occurred from the late morning into the early evening, 10:00 to 20:00 PST.
Titration:
Hourly averages of NOx and O3 from Camp 4 were plotted against one another
(Figure 8), and a similar graph was plotted for Falls Lodge (Figure 9). These plots were
examined for instances when spikes of nitrogen oxides might coincide with decreases in
ozone concentration, indicating a possible titration effect. Camp 4 showed little to no
evidence of titration, but Falls Lodge may have had a few instances of titration where
increases in nitrogen oxides occurred with decreases in ozone levels. The morning hours of
Julian days 209, 210, and 212 showed a dip in ozone that corresponded to a peak of NOx
(Figure 9). The diurnal cycle of NOx at Falls Lodge (Figure 6) showed high concentrations
during the morning hours, the same time of day when ozone in the area dips.
Discussion/Analysis:
Ozone measurements at Camp 4, Falls Lodge, and Schoolyard yielded similar 1-‐hr
average values, and displayed similar patterns in their daily cycles and diurnal variation.
The Schoolyard site was used as a basis for background ozone because it was close to the
two monitoring sites visited by the mobile monitoring platform, but it was located off of the
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main road, where vehicle emissions of NOx would be reduced. The observation of similar
ozone concentrations at all sites does not support the ozone titration effect due to fresh
plumes of nitrogen oxide molecules eliminating ozone. If significant titration were
occurring Camp 4 and Falls Lodge, then the ozone levels should show a dip when nitrogen
oxide levels are high at these locations.
Indication of titration during the daytime could be masked by the production of
ozone when sunlight is available. During nighttime hours, vehicular traffic (our assumed
source of fresh nitrogen oxide for titration) was considerably lower, as most visitors do not
travel in the middle of the night. Daytime traffic was the more common scenario, and thus
higher ambient nitrogen oxide concentrations were assumed more common in the daytime.
Thus, if NO was titrating ozone during the daytime hours when NOx levels were highest, the
production of additional ozone may make it difficult to observe the titration taking place
because ozone concentration would not dips as rapidly as expected.
Despite the apparent lack of titration at Camp 4, Falls Lodge showed the possibility
of a titration effect, although it was difficult to make clear conclusions. Ozone levels did
tend to dip when nitrogen oxide concentrations rose; however, this result was not
consistent throughout the data. A longer collection period should be utilized to obtain a
more robust assessment of the titration hypothesis.
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Conclusion:
Although total visitor data in Yosemite National Park supports the idea that more
vehicles enter the Park during the summer months, the data do not indicate significant
titration of ozone by NOx is occurring to any extent in the Valley region of Yosemite
National Park. Further testing will allow for a greater accumulation of data for analysis.
Resources:
"NPS: Explore Nature» Air Resources Division-‐Monitoring." Nature.nps.gov » Explore
Nature. Web. 30 Sept. 2011.
<http://www.nature.nps.gov/air/monitoring/network.cfm>.
NPS Stats. Rep. National Park Service Public Use Statistics Office. Web.
<http://www.nature.nps.gov/stats/>.
Sillman, Sandford. The Relation Between Ozone, NOx, and Hydrocarbons in Urban and
Polluted Rural Environements. Tech. Atmospheric Environment, 1999. Print.
"Six Common Air Pollutants | Air & Radiation | US EPA." US Environmental Protection
Agency. 01 July 2010. Web. 30 Sept. 2011.
<http://www.epa.gov/oaqps001/urbanair/>.
"Volatile Organic Compounds Emissions." United States Environmental Protection Agency.
17 Feb. 2010. Web.
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Figures:
Figure 1: “Visit by Month/Year (2010)”: National Park Service Public Use Statistics Office, Yosemite National Park Statistical Report. (NPS Stats).
Figure 2: Image of mobile monitoring station at Camp 4 site.
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Figure 3: O3 hourly averages comparison between Camp 4 and Schoolyard for Julian Days 194-‐201.
Figure 4: O3 hourly averages comparison between Falls Lodge and Schoolyard for Julian Days 208-‐213.
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Figure 5: Diurnal Curve of NOx concentration in ppb versus hour of day at Camp 4 from Julian Day 194-‐201.
Figure 6: Diurnal Curve of NOx concentration in ppb versus hour of day at Falls Lodge from Julian Day 208-‐213.
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Figure 7: Camp 4, Falls Lodge, and Schoolyard O3 Diurnal Curves in ppb
Figure 8: Hourly averages of O3 and NOx (ppb) at Camp 4.
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Figure 9: Hourly averages of O3 and NOx (ppb) at Falls Lodge.