4.3 Other Trace Gases

4.3 Other Trace Gases

Hundreds of different trace gases have been measured in the atmosphere and perhaps thousands more have yet to be measured. Many of these are volatile organic compounds (VOCs). Volatile means that the compound may exist in the liquid or solid phase but that it easily evaporates. Organic means that the compound contains carbon but is not carbon dioxide, carbon monoxide, or carbides and carbonates found in rocks. There are also other chemicals like the nitrogen oxides (e.g., nitric oxide (NO), nitrogen dioxide (NO₂), nitric acid (HNO₃)), sulfur compounds (e.g., sulfur dioxide (SO₂), sulfuric acid (H₂SO₄)) and halogen compounds (e.g., natural methyl chloride (CH₃Cl), human-made chlorofluorocarbons (CCl₂F₂)). If we pay attention, we can often smell and identify many of these chemicals, even at trace levels, although some, like methane, carbon monoxide (CO), and chlorofluorocarbons, are odorless. We enjoy smelling the VOCs emitted by trees in a forest—aah, that fresh pine smell—but we hold our nose to escape the smells of a stagnant swamp.

In addition to these thousands of chemicals that are emitted into the atmosphere every day, there are also some very reactive compounds that are created by atmospheric chemistry and play the important role of cleaning the atmosphere of many gases. The most important reactive gases are ozone (O₃) and hydroxyl (OH). We will focus the discussion of atmospheric chemistry on these two.

The Atmosphere’s Oxidizing Capacity

Earth’s atmosphere is an oxidizing environment. This term means what you think it would: gases that are emitted into the atmosphere react in a way that increases their oxygen content. Gases that contain oxygen tend to be “stickier” on surfaces and more water soluble, which means that they stick when they hit a surface or they can be readily taken up in clouds and rain drops and be deposited on Earth’s surface. We call gases hitting the surface and sticking “dry deposition” and gases being taken up in precipitation and rained out “wet deposition.”

Let’s consider a natural gas that is very important in our lives, methane, also known as natural gas. More and more methane is being extracted from below Earth’s surface and used to run our electrical power plants, heat our homes, cook our food, and, increasingly, to run our transportation vehicles. Methane is a simple molecule, CH₄, in which each of carbon’s four bonds is made with a hydrogen atom. Energy comes from heating methane to high enough temperatures that cause it to react, giving off energy as more stable molecules are formed. In complete combustion, each methane molecule is converted into one CO₂ molecule and two H₂O molecules. In the process, four oxygen atoms or two oxygen molecules are consumed.

This same process occurs in the atmosphere, but at much lower temperatures and at a much slower rate. In both cases, the first step in the methane oxidation sequence is the reaction with the hydroxyl radical (OH). In water, hydroxyl loses an electron and is ionized (OH^–), but in the atmosphere, hydroxyl is not ionized. We call OH a free radical because it has an odd number of electrons (eight for oxygen and one for hydrogen). Any gas with an odd number of electrons is reactive because the electrons want to be paired up in molecules because that makes them more stable.

Often, combustion is inefficient, resulting in the formation of carbon monoxide (CO). Examples include forest fires, humans burning fields to clear them for planting, poorly tuned vehicles, inefficient industrial processes, and other human-caused processes. The primary way that CO is removed from the atmosphere is by reacting with atmospheric OH. It takes a while for CO to be removed from the atmosphere by the reaction with OH, so that satellite instruments can track CO plumes as they emerge from their sources and flow around the world.

Where does OH come from?

Before we tackle this question, let’s first look at where ozone (O₃) comes from. We will start with stratospheric ozone (a.k.a. good ozone because it blocks solar UV that harms humans, other animals, agriculture, and ecosystems) and then eventually we will consider tropospheric ozone (a.k.a. bad ozone, which is the ozone that hurts our health when we breathe it and that damages plants and their fruit).