Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Investigating NOx Concentrations on an Urban University Campus Using Passive Air Samplers and UV−Vis Spectroscopy Cole M. Crosby,† Richard A. Maldonado,† Ahyun Hong,† Ryan L. Caylor,† Kristine L. Kuhn,‡ and Matthew E. Wise*,† †
Math and Science Department, Concordia University, Portland, Oregon 97211, United States College of Arts and Sciences, Concordia University, Portland, Oregon 97211, United States
‡
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S Supporting Information *
ABSTRACT: Gas-phase nitrogen oxides are important in the formation of tropospheric ozone. NOx (NO and NO2) as well as tropospheric ozone have been shown to have negative effects on human health. Therefore, accurately measuring NOx concentrations in the atmosphere is important. In this laboratory experience, students measured ambient NOx concentrations using a relatively simple and inexpensive passive sampling/UV−vis spectroscopy technique. The students used two different types of spectrophotometers to determine limits of detection and ambient NOx concentrations. Data demonstrated both spectrometers behaved similarly, proving laboratories utilizing different spectrophotometers could accurately perform the experiment. Although not statistically verified due to the limited number of passive samplers employed in the pilot experiment, measured NOx concentrations were similar to those calculated by a local air quality model (within approximately 50 parts per billion). At the end of the laboratory experience, students compared their measured NO2 concentrations to the United States Environmental Protection Agency’s primary and secondary standard of 53 parts per billion (annual mean). The initial learning goals of the experiment included the following: the successful creation of calibration curves, the determination of spectrophotometer limit of detection, and the calculation of ambient NOx concentrations. The experiment is appropriate for students enrolled in analytical and environmental chemistry courses. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Environmental Chemistry, Atmospheric Chemistry, UV−vis Spectroscopy, Laboratory Instruction, Hands-On Leaning/Manipulatives
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BACKGROUND Air quality impacts human health. However, the average person is likely unaware and/or unconcerned with chemical compounds present in the atmosphere and concentrations detrimental to their well-being. In urban areas, combustion engines significantly contribute to air pollution. Particulate matter (PM), carbon monoxide (CO), nitrogen oxides (NO and NO2), and volatile organic compounds (VOCs) are major components of motor vehicle exhaust. Many studies show a correlation between NO2 and CO concentrations and increased morbidity and mortality rates.1 The Clean Air Act requires the United States Environmental Protection Agency (USEPA) to set National Ambient Air Quality Standards (NAAQS) for pollutants such as NO2. The primary (health-based) and secondary (welfare-based) standard for NO2 is set at an annual mean of 53 parts per billion (ppb).2 Therefore, an accurate analytical method to measure atmospheric NO2 concentrations is critical. Under specific atmospheric conditions, tropospheric ozone (O3) formation is controlled by NO and NO2 (NOx). Reactions involving gas-phase NO, NO2, and O3 are shown in eqs 1 and 2: NO + O3 → NO2 + O2 (1) NO2 + hν + O2 → NO + O3
with O3. If VOCs and atmospheric peroxides are present, this conversion can take place resulting in the accumulation of O3. Tropospheric O3 is detrimental to human health because it can reduce the function of the lungs and ultimately cause lung disease.3 The NAAQS primary and secondary standard for O3 is 70 ppb averaged over an 8 h period.2 It is essential to have accurate measurements of atmospheric NO/NO2 ratios to understand tropospheric O3 concentrations. NOx is present in the atmosphere at ppb levels. Atmospheric variables such as temperature, water vapor, and other gas-phase species can induce interferences in many analytical instruments used to measure it.4 However, NOx can be quantified using relatively inexpensive passive sampling techniques. In this method, a pollutant is collected within a sampling system as air naturally flows through it. Once the pollutant is collected, it is quantified using an analytical technique such as ultraviolet− visible (UV−vis) spectroscopy. This technique (employed here) provides a time averaged, spatial analysis of air quality. Shooter5 developed a passive diffusion tube sampler for the measurement of NO2. The sampler contained a mesh coated with triethanolamine (TEA), an NO2 absorbing reagent. After sufficient exposure to NO2, Shooter5 used the following chemistry to quantify it. TEA and gas-phase NO2 reacted to
(2)
If these were the only reactions occurring in the atmosphere, there would be no net production or destruction of O3. In order to produce O3, NO must be converted to NO2 before reacting © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: March 9, 2018 Revised: July 9, 2018
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DOI: 10.1021/acs.jchemed.8b00175 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
application, providing students with direct knowledge of the chemical composition of the air they breathe.
form nitrosodiethanolamine, which was then hydrolyzed in aqueous solution to form nitrite ions. In order to determine the concentration of nitrite ions in solution (and by extension the concentration of NO2), the solution was reacted with sulfanilamide and N-(1-naphthyl)-ethylenediamene dihydrochloride (NEDA) to form a purple azo dye. The resulting solution absorbance was quantified using a UV−vis spectrophotometer set at 540 nm. Ambient NO2 concentration was calculated from absorbance using a nitrite calibration curve. Xiao et al.6 fabricated a passive sampler for the determination of NO2 in ambient air. The passive sampler contained a reservoir, which held approximately 4 mL of an absorbing reagent (sulfanilamide, tartaric acid, ethylenediaminetetraaceitc acid, NEDA, and water). Similar to the report by Shooter,5 the solution absorbance was quantified using a UV−vis spectrophotometer set at 540 nm, and ambient NO2 concentration was determined using a nitrite calibration curve. The experiment in this paper was designed to extend the work of Shooter5 and Xiao et al.6 This work differs from previous experiments in that ambient NOx concentrations were measured simultaneously with NO2 using commercially available passive air samplers. The samplers contain one NO2 collection pad impregnated with TEA. They also contain one NOx collection pad impregnated with TEA and 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO). The TEA and PTIO on the NOx collection pad convert gas-phase NOx into nitrite ions. The pad is then exposed to a solution of sulfanilamide and NEDA to form a purple azo dye. The absorbance of the solution can then be converted into ambient NOx concentration. Passive air samplers for NO2 and NOx (NO determined by the difference) developed by Ogawa & Co., USA, Inc., were selected due to their low cost and reusability. The sampling technique can simultaneously determine atmospheric NO and NO2 concentrations. This is important if the concentration of tropospheric O3 is to be fully understood. Therefore, this system is an appropriate choice for accurate measurements of NOx.
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MATERIALS Ambient air passive samplers (part PS-100) and NOx (part PS124) and NO2 (part PS-134) precoated collection pads were purchased from Ogawa & Co., USA, Inc. The reader is referred to the Ogawa & Co., USA, Inc. Web site for frequently asked questions regarding passive samplers.8 All chemicals were purchased from Sigma-Aldrich. Hazards
Appropriate safety precautions were followed throughout the experiment (e.g., wearing safety goggles). In this experiment sulfanilamide, 85 wt % phosphoric acid, NEDA, and sodium nitrite were used. The effects of sulfanilamide have not been studied in humans; however, it has been shown to cause thyroid cancer in mice. NEDA and sodium nitrite can be corrosive and can cause serious eye damage. Sodium nitrite has been shown to cause genetic defects in fetuses and may cause other pregnancy related damage. Direct skin/eye contact with phosphoric acid should be avoided. All chemicals used in this experiment have warnings for targeted organ toxicity. Additionally, single exposure specific organ toxicity is a concern with all chemicals used. Specific hazard information was not reported for the passive samplers and the collection pads. A complete list of safety concerns and chemical CAS numbers are found in the Supporting Information of this paper. Reagent Preparation
A 2 h laboratory period is not an adequate amount of time for students to prepare the reagents described below (along with other necessary procedures). However, students can perform reagent preparation if they are experienced or the laboratory period exceeds 2 h. In this experiment, teaching assistants (TAs) prepared several aqueous solutions prior to each laboratory period. The Ogawa & Co., USA, Inc., sampling protocol does not report a shelf life for aqueous reagents. However, it is good practice to make the reagents as close to the analysis period as possible. The solutions described below were used for the experiments. A sulfanilamide solution was prepared by dissolving 80 g of crystalline sulfanilamide in a solution of 85 wt % phosphoric acid (200 mL) and distilled water (700 mL). The solution was then diluted with distilled water to create a 1000 mL solution. A NEDA solution was prepared by dissolving 0.56 g of crystalline NEDA in 100 mL of distilled water. The color-producing reagent was prepared by mixing the sulfanilamide and the NEDA solution in a 10:1 ratio. Prior to experimentation, the TAs dried crystalline sodium nitrite for approximately 4 h at a temperature of 110 °C. The TAs prepared a stock solution of aqueous nitrite ions by dissolving 1.5 g of sodium nitrite in 1000 mL of distilled water. Using the stock nitrite solution, the TAs prepared various concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 μg nitrite/mL) of dilute nitrite solutions for calibration curves. After the reagents were prepared, the TAs refrigerated the color-producing reagent, the nitrite calibration solutions, and 500 mL of distilled water to a temperature of approximately 6 °C.
Course Applicability and Learning Goals
The experiment is appropriate for students enrolled in analytical and environmental chemistry courses. Instrumental methods used to determine solute concentrations are important in an analytical chemistry course, making this experiment suitable for any unit that discusses UV−vis spectroscopy. Many environmental chemistry courses have units devoted to air quality. Therefore, the experiment is suitable when discussing anthropogenic atmospheric pollutants. If the experiment is carried out in advanced courses, NOx photochemistry can be discussed prior to experimentation. NOx photochemistry is complex and is therefore outside the scope of this paper. If the reader requires additional information concerning NOx photochemistry, an excellent resource is Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 3rd ed., by Seinfeld and Pandis.7 The laboratory outcomes of the experiment are the following: (1) construct calibration curves using solutions with known nitrite concentrations (2) mastery of use of a spectrophotometer including the determination of limit of detection (3) the ability to calculate NO, NO2, and NOx concentrations in ambient air The primary objective of the experiment is the determination of ambient NOx concentrations. Therefore, the UV−vis spectroscopy experiment is utilized in a “real world” research
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EXPERIMENTAL SECTION The experiment described here was performed over a 2 week time period in a laboratory section composed of 23 students. The students met once a week for 2 h; each student spent 4 h B
DOI: 10.1021/acs.jchemed.8b00175 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
performing the experiment. During the first week, the students were separated into six groups (not exceeding four students). The passive samplers, each containing one NOx and one NO2 collection pad (referred to as “sample” pads), were preassembled by the TAs. Students can assemble the samplers if time is not a consideration. Each student group (SG) was assigned a passive sampler containing one NOx and one NO2 sample pad. SGs were also assigned an additional NOx and NO2 collection pad to act as “field blanks”. SGs took the beginning of the laboratory period to select an outdoor location for their samplers and field blanks. The sampler and the field blanks (sealed in separate brown plastic container) were left outdoors for a period of 1 week. In this experiment, the samplers were placed in positions to measure background NOx concentrations (not point source emissions). Since rainwater contamination was anticipated, the passive samplers were attached to the underside of a PVC endcap. After deploying the samplers and field blanks, SGs created a nitrite calibration curve for two spectrophotometers: a Vernier Spectrovis-Plus spectrophotometer controlled with a Vernier Labquest 2 and a WPA Biowave S2100 diode array spectrophotometer. An 8 mL portion of each chilled dilute nitrite solution (0.2, 0.4, 0.6, 0.8, and 1.0 μg nitrite/mL) and four 8 mL aliquots of chilled distilled water were added to separate vials (9 vials total). A 2 mL portion of chilled colorproducing reagent was added to each vial, and the solutions were mixed thoroughly. Each solution was then refrigerated for 30 min. After refrigeration, the solutions were warmed to room temperature, and the absorbance of each solution was determined using the UV−vis spectrophotometers set at 545 nm. SGs then created two nitrite calibration curves forced through the origin (according to the protocol provided by Ogawa & Co., USA, Inc.9). If the R2 values were
