Environmental Engineering Laboratory

Photochemical Cycle of NO2NO_2, NONO, and O3O_3

⏻ Learning Objectives

  • Perform photochemical reactions inside environmental chambers.

  • Explain the principle of operation of O3O_3 and NOxNO_x gas-phase monitors.

  • Explain the photochemical cycle of O3O_3, NONO, and NO2NO_2.

  • Measure and calculate the ozone produced at the photostationary state.

Background

Ozone Photochemistry

Ozone is a gas composed of three atoms of oxygen (O3O_3). Ozone in the air we breathe can harm our health, especially on hot sunny days when ozone can reach unhealthy levels. People at greatest risk of harm from breathing air containing ozone include people with asthma. Elevated exposures to ozone can affect sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas. In particular, ozone can harm sensitive vegetation during the growing season. (Source: EPA). Tropospheric O3O_3 is generated from to major classes of precursor molecules: volatile organic compounds (VOCs) and nitrogen oxides NOxNO_x, which is the sum of NONO and NO2NO_2. Most of the direct emissions on NOxNO_x are in the form of NONO. Ozone levels exceeding 200 ppb are considered severe air pollution episodes. The current U.S. National Ambient Air Quality Standard for O3O_3 is an 8-hour average of 80 ppb. Understanding VOC chemistry is critical to explain the total ambient O3O_3 concentrations. Nevertheless, the photochemical cycle of NO2NO_2, NONO, and O3O_3 remains the starting point for modeling tropospheric O3O_3 production.

Photochemical O3O_3 Production from NOxNO_x

Sunlight at wavelength < 424 nm photolizes NO2NO_2 into NONO and atomic OO

NO2+hν→NO+O NO_2 + h\nu \rightarrow NO + O

The atomic OO reacts with oxygen O2O_2 to form O3O_3. Reaction (2) is the only source of atmospheric O3O_3.

O+O2+M→O3+M O + O_2 + M \rightarrow O_3 + M

where MM is a third body required to stabilize the excited product OO2⋆OO_2^\star by collision. Finally O3O_3 reacts with NONO to regenerate NO2NO_2

O3+NO→NO2+O2 O_3 + NO \rightarrow NO_2 + O_2

Reactions (1)-(3) form the basic photochemical NOxNO_x cycle. Cycling between NONO and NO2NO_2 takes place in the troposphere on a time scale of a minute in the daytime. There is no net production of O3O_3, but some O3O_3 is present. The steady state O3O_3 concentration is

[O3]=−12([NO]0−[O3]0+jNO2k3)+12[([NO]0−[O3]0+jNO2k3)2+4jNO2k3([NO2]0+[O3]0)]1/2 [O_3] = -\frac{1}{2} \left ( [NO]_0 - [O_3]_0 + \frac{j_{NO_2}}{k_3} \right ) \\ + \frac{1}{2} \left [ \left ( [NO]_0 - [O_3]_0 + \frac{j_{NO_2}}{k_3} \right )^2 + 4 \frac{j_{NO_2}}{k_3} \left ( [NO_2]_0 + [O_3]_0 \right ) \right ]^{1/2}

The ration jNO2/k3j_{NO_2}/k_3 depends on sunlight or blacklight intensity. For full blacklight intensity in the chamber used in this lab, jNO2/k3≈10.2 ppbj_{NO_2}/k_3 \approx 10.2\; ppb. The characteristic relaxation time to steady state is

τ=1k3[NO] \tau = \frac{1}{k_3 [NO]}

where k3=1.9×10−14 cm3 molecule−1 s−1k_3 = 1.9\times 10^{-14} \; cm^{3}\; molecule^{-1} \; s^{-1} at T=298KT = 298K.

Reaction Kinetics

The photolysis rate jNO2j_{NO_2} depends on the actinic flux (intensity of sunlight or intensity of blacklights). At noon in the cloud-free atmosphere jNO2≈4×10−22 molecule−1 s−1j_{NO_2} \approx 4\times 10^{-22}\; molecule^{-1}\; s^{-1} and otherwise lower. The value for k3=1.9×10−14 cm3 molecule−1 s−1k_3 = 1.9\times 10^{-14} \; cm^{3}\; molecule^{-1} \; s^{-1} at T=298KT = 298K. The figure below shows a typical evolution of NO2NO_2, NONO, and O3O_3 for the experiment you are going to perform. At t=0t = 0 the conditions are [O3]0=0 ppb[O_3]_0 = 0\; ppb, [NO]0=60 ppb[NO]_0 = 60\; ppb, and [NO2]0=450 ppb[NO_2]_0 = 450\; ppb. The blacklights are turned on at t=0t = 0. Predictions for the photostationary state concentration and relaxation time based on Eqs. (4) and (5) are provided. After 5 min, the lights are turned off and the system restores to the initial state.

Simulation Results

⌨ Prompt

The goal is to measure the photostationary steady state of the NO2NO_2, NONO, and O3O_3 system. The environmental chamber will be filled with zero air that is free of VOCs and other chemicals. A mix of [NO]0[NO]_0 [NO2]0[NO_2]_0 and [O3]0[O_3]_0 will be added to the chamber and the system will be allowed to equilibrate. Concentrations of NO2NO_2, NONO, and O3O_3 will be monitored in real time using gas-analyzers. After the system is equilibrated, blacklights will be turned on. The blacklights provide photons λ<424 nm\lambda < 424\; nm, thus initiating the photolysis of NO2NO_2 and the production of O3O_3. Observe the formation of O3O_3 until the photostationary state of the system is reached. Then turn off the light and observe the reaction of NONO with O3O_3, returning the system back to the initial state. Repeat the cycle at least once, preferably twice to ensure consistent results.

During the Lab

  • Make sure the O3O_3 and NOxNO_x gas analyzers are running. Make sure that the data acquisition system is turned on and recording data. Compare the computer logged data on the screen to the values reported on the analyzers. Fill the chamber with zero air. Verify that initial concentrations are approximately zero. Add the NONO/NO2NO_2 mixture to the chamber. Record the initial NONO, NO2NO_2 and O3O_3 concentrations. Turn on the light and let the system equilibrate to the photostationary steady state. Turn off the light and observe reversion to the initial state.

  • Make a draft of the schematic drawing of the experimental setup. Carefully indicate (1) All of the instruments involved, including vacuum pumps, instruments, flow rates, flow paths, light sources (location and number), compressed air sources, and data acquisition system. (2) How the reactants are added to the chamber, (3) timings of reactants added and lights turning on and off. You should make a sketch during the experiment.

  • Make a draft of the schematic drawing of the O3 analyzer and discuss with your group and/or TA.

  • Download the data from the computer for later analysis.

Report

Please follow the report outline.

Don't forget to print and paste the grading rubric at the end of your report. Not providing the grading rubric will lead to point deductions. Use the grading rubric as a checklist for how to prepare a proper report.

Methods

This lab uses an environmental chamber, a photometric UV absorption O3O_3 analyzer and a chemiluminescence NOxNO_x analyzer.

Chamber POM airbeam

The environmental chamber consists of a Teflon bag housed inside an enclosure that is lined with blacklights. The blacklights can be turned on/off using switches. An injection port is available to add reagents. You will receive a glass bulb that contains a nominal amount of NONO. You will add the NONO with the help of the TA and/or the instructor. Ozone is measured using a gas-analyzer. A 254 nm UV light signal is passed through the sample cell where it is absorbed in proportion to the amount of ozone present. Periodically, a switching valve alternates measurement between the sample stream and a sample that has been scrubbed of ozone. NOxNO_x is measured using a second gas analyzer. The NOxNO_x instrument determines the concentration of nitric oxide (NONO), total nitrogen oxides (NOxNO_x) , the sum of NONO and NO2NO_2) and nitrogen dioxide (NO2NO_2) in a sample stream. The principle of operation is chemiluminescence. Chemiluminescence is the emission of light from a chemical reaction and is triggered by the reaction of NONO with ozone O3O_3. The amount of light produced is linear with NONO concentration. NO2NO_2 is measured by converting NO2NO_2 with to NONO using heated molybdenum converter chip.

Resources

Teledyne Product Manuals (O3O_3 analyzer and NOxNO_x analyzer) link

CC BY-NC 4.0 Don Collins, David Cocker, and Markus Petters.