Environmental Engineering Laboratory

Ozonolysis of Alkenes

⏻ Learning Objectives

At the end of this lab you will be

familiar with environmental chamber design and operation
explain the principle of operation of $O_3$ and $NO_x$ gas-phase monitors
able to carry out VOC oxidation experiments
able to estimate reaction rates from experimental data
able to explain the formation of secondary organic aerosol
able to contextualize measurements within the peer-reviewed literature

⌨ Prompt

The goal is to measure the reactions of alkenes with ozone inside an environmental chamber. The group will prepare a mix of

O_3

,

NO_x

, and reactive alkene inside an environmental chamber and monitor the formation and decay of

O_3

with time. You should first add

NO

, and turn on the light. Observe the formation of

O_3

and the photostationary state of the system. Then add the alkene and observe the change in

O_3

and

NO_x

. Before the experiment, you will receive reagents from your TA, including a glass bulb with

NO

, and a vial with an alkene. You need to calculate the liquid volume of alkene to add to the bulb and then chamber. The target amount is ~100 ppb of alkene. This will allow you to use the decay of

O_3

to estimate the reaction kinetics. You should investigate the change of

NO

and

NO_2

, but realize that the rate will be not only determined by SCI +

NO

and SCI +

NO_2

, but also by additional radical reactions occurring in the chamber. For the report you should quantitatively evaluate the formation of

O_3

in the photostationary state, detail the origin of alkenes in the atmosphere, the alkene ozonolysis mechanism, estimate products, estimate the reaction rate of the alkene with ozone, and of the stable Crigee intermediate with NO and

NO_2

. Contrast the rate with values found in the peer-reviewed literature. Explain the principle of operation of the gas-phase monitors. Discuss the role of wall loss and other potential experimental artifacts that may bias your results. Also discuss the implication for secondary organic aerosol formation, including estimates of particulate matter mass that may have formed in the you reaction.

POM

airbeam

This lab uses an environmental chamber, a photometric UV absorption $O_3$ analyzer and a chemiluminescence $NO_x$ analyzer. To measure $O_3$ , 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. The $NO_x$ instrument determines the concentration of nitric oxide ( $NO$ ), total nitrogen oxides ( $NO_x$ ) , the sum of $NO$ and $NO_2$ ) and nitrogen dioxide ( $NO_2$ ) 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 $NO$ with ozone $O_3$ . The amount of light produced is linear with $NO$ concentration. $NO_2$ is measured by converting $NO_2$ with to $NO$ using heated molybdenum converter chip.

Background/Motivation

Photochemical $O_3$ production

Sunlight at wavelength < 424 nm photolizes $NO_2$ into $NO$ and atomic $O$

NO_2 + h\nu \rightarrow NO + O

The atomic $O$ reacts with oxygen $O_2$ to form $O_3$ . Reaction (2) is the only source of atmospheric $O_3$ .

O + O_2 + M \rightarrow O_3 + M

where M is a third body required to stabilize the excited product $OO_2^\star$ by collision. Finally $O_3$ reacts with $NO$ to regenerate $NO_2$

O_3 + NO \rightarrow NO_2 + O_2

Reactions (1)-(3) form the basic photochemical $NO_x$ cycle. Cycling between $NO$ and $NO_2$ takes place in the troposphere on a time scale of a minute in the daytime. There is no net production of $O_3$ , but some $O_3$ is present.

In the chamber, we start with a mix of $[NO]_0$ $[NO_2]_0$ and $[O_3]_0$ and then turn on the light. The system will equilibrate fairly quickly. The steady state $O_3$ concentration is

[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}

where $j_{NO_2}$ $[molecule^{-1}\;s^{-1}]$ is the photoloyis rate of $NO_2$ and $k_3$ $[cm^{3}\; molecule^{-1}\; s^{-1}]$ is the rate for the reaction (3). The characteristic relaxation time to steady state is

\tau = \frac{1}{k_3 [NO]}

Reaction Kinetics

The photolysis rate $j_{NO_2}$ depends on the actinic flux (intensity of sunlight or intensity of blacklights). At noon in the cloud-free atmosphere $j_{NO_2} \approx 4\times 10^{-22}\; molecule^{-1}\; s^{-1}$ and otherwise lower. The value for $k_3 = 1.9\times 10^{-14} \; cm^{3}\; molecule^{-1} \; s^{-1}$ at $T = 298K$ . The figure below shows a typical evolution of $NO_2$ , $NO$ , and $O_3$ . At $t = 0$ the conditions are $[O_3]_0 = 0ppb$ , $[NO]_0 = 10 ppb$ , and $[NO_2]_0 = 100 ppb$ . The lights are turned on, corresponding to a photolosyis rate $j_{NO_2} \approx 2.02\times 10^{-22}\; molecule^{-1}\; s^{-1}$ . 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.

⌨ Prompt

For your report, plot the observed time series of

O_3

,

NO_2

, and

NO

as shown above for a light on/light off cycle. Evaluate

j_{NO_2}

by matching the steady-state concentration. Estimate the relaxation time using Eq. (5) and show it on your plot.

Alkene Oxidation

Alkenes are ubiquitous atmospheric VOCs that originate from both biogenic and anthropogenic sources. The double bond reacts quickly with $O_3$ and is one of the major degradation pathways of alkenes. The ozone alkene reaction starts with the $O_3$ addition to the double bond resulting in a primary ozonide. The primary ozonide decomposes into an aldehyde and a stabilized Crigee intermediate (SCI) biradical. The SCI then reacts with either (a) an adehyde, (b) an alcohol, (d) carbon monoxide, (e) sulfur dioxide, (f) water vapor, (g) $NO$ or $NO_2$ , or (h) with itself. Some of these compounds will then go on and contribute to PM2.5. Furthermore hydroxyl radicals are formed from the reaction at high yield. The alkene + $O_3$ reaction is therefore important for understanding the degradation and fate of alkenes in the atmosphere, for understanding the OH budget in the atmosphere, and for understanding PM2.5 formation from VOCs in the atmosphere.

Source. Suda et al. (2012, doi:10.1029/2011JD016823).

Resources

O3 Analyzer Manual (link)

NOx Analyzer Manual (link)

Gas-Phase Tropospheric Chemistry of Volatile Organic Compounds: 1. Alkanes and Alkenes (link)

Atmospheric Degradation of Volatile Organic Compounds (link)

Kinetic and mechanism studies of the ozonolysis of three unsaturated ketones (link)

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