Question

The hydroxyl radical, \(\mathrm{OH}\), is formed at low altitudes via the reaction of excited oxygen atoms with water: $$ \mathrm{O}^{*}(g)+\mathrm{H}_{2} \mathrm{O}(g) \longrightarrow 2 \mathrm{OH}(g) $$ (a) Write the Lewis structure for the hydroxyl radical. (Hint: It has one unpaired electron.) Once produced, the hydroxyl radical is very reactive. Explain why each of the following series of reactions affects the pollution in the troposphere: (b) \(\mathrm{OH}+\mathrm{NO}_{2} \longrightarrow \mathrm{HNO}_{3}\) (c) \(\mathrm{OH}+\mathrm{CO}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2}+\mathrm{OOH}\) \(\mathrm{OOH}+\mathrm{NO} \longrightarrow \mathrm{OH}+\mathrm{NO}_{2}\) (d) \(\mathrm{OH}+\mathrm{CH}_{4} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{CH}_{3}\) \(\mathrm{CH}_{3}+\mathrm{O}_{2} \longrightarrow \mathrm{OOCH}_{3}\) \(\mathrm{OOCH}_{3}+\mathrm{NO} \longrightarrow \mathrm{OCH}_{3}+\mathrm{NO}_{2}\) (e) The concentration of hydroxyl radicals in the troposphere is approximately \(2 \times 10^{6}\) radicals per \(\mathrm{cm}^{3}\). This estimate is based on a method called long path absorption spectroscopy (LPAS), similar in principle to the Beer's law measurement discussed in the Closer Look essay on p. 564 , except that the path length in the LPAS measurement is \(20 \mathrm{~km}\). Why must the path length be so large?

Short answer

The hydroxyl radical, OH, has the Lewis structure O-H with an additional unpaired electron on the oxygen atom. The reactions mentioned affect troposphere pollution as follows: (b) The reaction between OH and NO₂ forms nitric acid, contributing to acid rain; (c) OH, CO, and O₂ react to form CO₂ and peroxy radicals, which then react with NO to form NO₂, converting pollutants into other species; (d) The series of reactions involving OH and CH₄ leads to the formation of CO₂ and H₂O, partially mitigating methane's effects as a greenhouse gas. Lastly, the path length for LPAS has to be 20 km because hydroxyl radicals have low concentrations in the troposphere, and a long path length increases absorption for accurate measurements.

Step-by-step solution

(a) Draw the Lewis Structure for the Hydroxyl Radical, \(\mathrm{OH}\)

To draw the Lewis structure for the hydroxyl radical, begin by counting the total number of valence electrons. Oxygen has 6 valence electrons, and hydrogen has 1. Together, the radical has 7 valence electrons. Connect the oxygen and hydrogen atoms with a single bond (representing 2 electrons) and draw three lone pairs (2 electrons each) around the oxygen atom. The remaining one electron will be unpaired in the oxygen atom, representing the radical electron. The Lewis structure for the hydroxyl radical looks as follows: O / \ H • / \ • •

(b) Explain the Reaction \(\mathrm{OH}+\mathrm{NO}_{2} \longrightarrow \mathrm{HNO}_{3}\) in Relation to Troposphere Pollution

This reaction represents hydroxyl radicals reacting with nitrogen dioxide to form nitric acid. Nitric acid is a significant contributor to acid rain, which leads to various environmental problems such as acidification of soil and water bodies. By forming nitric acid, the reaction removes the highly reactive hydroxyl radicals and nitrogen dioxide species from the troposphere, potentially reducing pollution.

(c) Explain the Reactions Involving \(\mathrm{OH}+\mathrm{CO}+\mathrm{O}_{2}\) and \(\mathrm{OOH}+\mathrm{NO}\) in Tropospheric Pollution

The first reaction, \(\mathrm{OH}+\mathrm{CO}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2}+\mathrm{OOH}\), represents hydroxyl radicals reacting with carbon monoxide and oxygen to form carbon dioxide and peroxy radicals. Carbon dioxide is a greenhouse gas that contributes to global warming. In the second reaction, \(\mathrm{OOH}+\mathrm{NO} \longrightarrow \mathrm{OH}+\mathrm{NO}_{2}\), peroxy radicals react with nitrogen monoxide to form the hydroxyl radical and nitrogen dioxide. The sequence of these reactions ultimately converts carbon monoxide and nitrogen monoxide, both pollutants, into other species that may contribute to pollution.

(d) Analyze the Series of Reactions Involving \(\mathrm{OH}+\mathrm{CH}_{4}\), \(\mathrm{CH}_{3}+\mathrm{O}_{2}\), and \(\mathrm{OOCH}_{3}+\mathrm{NO}\)

These reactions represent the oxidation of methane by hydroxyl radicals. Methane is a significant greenhouse gas that contributes to global warming. The series of reactions eventually lead to the formation of carbon dioxide and water, partially mitigating the effects of methane as a greenhouse gas.

(e) Explain the Need for \(20\mathrm{~km}\) Path Length in LPAS for Hydroxyl Radicals Measurement

The path length in long path absorption spectroscopy (LPAS) must be large (20 km) to measure the concentration of hydroxyl radicals because hydroxyl radicals have relatively low concentrations in the troposphere (approximately \(2 \times 10^{6}\) radicals per \(\mathrm{cm}^{3}\)). A large path length increases absorption of light by the hydroxyl radicals, making the measurement more accurate and reliable.

Key concepts

Lewis structure

The Lewis structure is a way to represent molecules using symbols for atoms and dots for electron pairs. It is a simplified view of how atoms share electrons in a molecule and helps predict shape, reactivity, and other properties. For the hydroxyl radical, \(\mathrm{OH}\), we start by counting the valence electrons: oxygen has 6 and hydrogen has 1, making a total of 7 valence electrons. This number is odd, which is typical for radicals that have an unpaired electron.
To draw its Lewis structure:

• Place the oxygen and hydrogen atoms next to each other, connected by a single bond (2 electrons).
• Arrange 3 pairs of dots (6 electrons) around the oxygen to represent the remaining valence electrons, providing it with a complete octet if we include the bond.
• Add the unpaired electron as a single dot, which is common for radicals like \(\mathrm{OH}\).

This resulting structure: \(\mathrm{H}-\cdots \mathrm{O}\,\cdot\), shows oxygen bonded to hydrogen, with an unpaired electron on the oxygen making it a radical.

troposphere pollution

Troposphere pollution refers to pollutants present in the lowest layer of Earth’s atmosphere. This part of the atmosphere holds the air we breathe and it is where weather occurs. Pollution here directly affects human health as well as environmental systems.
The hydroxyl radical, \(\mathrm{OH}\), plays an essential role in mitigating tropospheric pollution. It acts like an "atmospheric detergent", reacting with various pollutants. For example:

• It reacts with nitrogen dioxide \(\mathrm{NO}_{2}\) to form nitric acid \(\mathrm{HNO}_3\), removing reactive substances from the air.
• In another reaction series, it converts carbon monoxide \(\mathrm{CO}\) into carbon dioxide \(\mathrm{CO}_2\) and oxidizes methane \(\mathrm{CH}_4\) into less harmful products, reducing their harmful effects.

Both actions remove pollutants that otherwise would contribute to issues like acid rain and greenhouse effects.

long path absorption spectroscopy

Long path absorption spectroscopy (LPAS) is a technique used to measure atmospheric components, like hydroxyl radicals, over long distances to detect small concentrations. This method makes use of Beer-Lambert law, which states that the absorption of light by a substance is directly proportional to its concentration and the distance the light travels through it.
To measure the hydroxyl radical in the troposphere, which is relatively scarce with a concentration of around \(2 \times 10^{6}\) radicals per \mathrm{cm}^{3}\, a 20 km path is required. This extended distance enhances the sensitivity by increasing the total absorptive interaction with the hydroxyl radicals, allowing even trace amounts to be detected accurately. Without such a lengthy path, detecting such low concentrations would be nearly impossible with traditional methods.

atmospheric chemistry

Atmospheric chemistry is the study of chemical processes that occur in the Earth's atmosphere. It is crucial for understanding issues like air pollution, ozone layer depletion, and climate change. The hydroxyl radical plays a pivotal role in atmospheric chemistry due to its high reactivity with many different molecules.
Through a series of chain reactions, hydroxyl radicals influence the concentration of greenhouse gases and pollutants:

• They initiate the breakdown of pollutants like methane and nitrous oxides, reducing their global warming potential.
• They also help in converting harmful gases into more benign substances, thus purifying the atmosphere.

These reactions are crucial for maintaining the balance of trace gases in the atmosphere and have significant environmental and health implications.