Particulate Matter in the Atmosphere

Hydroxyl radicals in the troposphere

The chemistry of the troposphere is governed by reactions with the hydroxyl radical. The hydroxyl radical consists of one oxygen atom and one hydrogen, atom chemically bonded to form a diatomic molecule. Oxygen brings eight electrons (six in the valence orbitals) and hydrogen brings one electron to the hydroxyl radical. The total electrons in the hydroxyl radical is nine (an odd number), and therefore the hydroxyl radical will always have an unpaired electron. The unpaired electron is usually associated with the oxygen atom, and this is represented in formulas by placing a single dot near the oxygen as shown below:

Figure 33.1 The hydroxyl radical

The hydroxyl radical reacts with atmospheric gases through several different pathways. The most common reaction of the hydroxyl radical is called "hydrogen abstraction." In hydrogen abstraction reactions a hydrogen atom from another substance is added to the hydroxyl radical, converting the hydroxyl radical to a water molecule and simultaneously generating a new radical on the substance which lost the hydrogen atom. The first step in the reaction of hydroxyl radical with methane (shown below) is an example of a hydrogen abstraction reaction.

Hydroxyl radicals can combine directly with another substance to form a new molecule that includes all the atoms from the two initial molecules. The reaction between the hydroxyl radical and nitrogen dioxide (shown below) is an example of this type of direct combination reaction.

Finally, hydroxyl radicals can add directly to a double bond in a larger molecule to create a substance that contains the hydroxyl group covalently bonded to the larger molecule, and also contains an unpaired electron to continue to react in a chain reaction process. This is the reaction that leads to formation of larger molecules through a "chain-reaction." The term chain-reaction is used because the new, larger molecule will continue to react (and grow larger) until the free radical is "quenched." Chain-reactions are quenched when two free radicals react with each other to form a single covalent bond that contains no unpaired electrons.

Important reactions of the hydroxyl radical in the troposphere:

Primary reactions that generate hydroxyl radicals in the troposphere.

Reaction between hydroxyl radical and carbon monoxide to form carbon dioxide and the hydroperoxyl radical.

Reactions of the hydroperoxyl radical and the hydroxyl radical with nitrogen oxide (NO) and nitrogen dioxide (NO₂) to form nitric acid.

Reactions of the hydroxyl radical with methane to ultimately form the hydroperoxyl radical and formaldehyde.

Reactions of the hydroperoxyl radical with nitrogen oxide to form nitrogen dioxide and hydrogen peroxide.

Gas phase chemistry of the troposphere is governed by free radical chemistry, and the tendency for substances to become more highly oxidized. Most atoms form covalent molecules when their oxidation state is in low of intermediate oxidation states. Covalent molecules of low molecular weight are gases and would continue to build up in the atmosphere indefinitely if they were transformed to higher oxidation states. The highest oxidation states of most elements lead to ionic solids. Ionic solids form "condensation nuclei" and are washed out of the atmosphere by rainfall. These atmospheric transformations for carbon, nitrogen and sulfur are shown below.

Successive oxidations of carbon, sulfur and nitrogen in the atmosphere.

Atmospheric particulate matter comes from several sources. Physical processes can breakdown larger material into smaller particles. Coal grinding, fugitive road dust and dust from rock quarries are examples of physical processes that release particulate matter to the atmosphere. These particles are usually large (>100 m m diameter), do not have a long residence time in the atmosphere, and are not taken into the body during respiration.

Particulate matter formed through chemical reactions are typically much smaller (<10 m m diameter) than particulates from physical processes (>100 m m diameter). Chemical processes that release particulate matter to the atmosphere include all forms of combustion (automobiles, fossil fuel power plants, forest fires and residential fireplaces) and atmospheric emissions from volcanoes.

The smallest particles in the atmosphere are the result of high temperature combustion and gas to aerosol conversions. The atmospheric conversion of sulfur dioxide to ammonium sulfate is an example of a gas to aerosol conversion process. Depending on the relative humidity and the presence of other atmospheric gases, the sulfuric acid formed will be removed directly in precipitation, or neutralized and converted to ammonium sulfate. These particles will range in size from 50 to as many as 10,000 molecules.

In water aerosol H₂SO₄ dissociates:

If NH₃ dissolves in same H₂O

NH₃ + H₂O à NH₄⁺ + ^-OH

Thus, particles form through complex gas phase, heterogeneous, and liquid phase reactions.

Atmospheric organic molecules are released to the atmosphere by incomplete combustion, and react through free radical reactions to form larger molecules that can coat the surface of smaller, inorganic particles. The most toxic of these substances are the polyaromatic hydrocarbons, often called PAHs. Benzopyrene, shown below, is a common substance found in combustion products and is a known carcinogen.

Original page created by Dr.Richard Foust, Professor of Chemistry and Environmental Science, Northern Arizona University