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, while the compound may exist in the liquid or solid phase, it easily evaporates or partitions from aqueous phase into the gas phase. 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., methylene chloride (CH2Cl2), chlorofluorocarbons (CCl2F2)).
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 radical (·OH). We will focus the discussion of atmospheric chemistry on these two.
The Atmosphere’s Oxidizing Capacity
Earth’s atmosphere is an oxidizing environment: gases that are emitted into the atmosphere react in a way that increases their oxygen content. Gases that contain oxygen tend to adsorb more readily onto surfaces and more water soluble, which means that they adsorb 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 adsorbing onto surfaces “dry deposition,” and gases being taken up in precipitation and rained out “wet deposition.”
Let’s consider methane, the primary gaseous constituent in natural gas and biogas. Methane is increasingly being extracted from below Earth’s surface and used to electrical power generation, residential heating, other stationary uses, and increasingly to fuel vehicles. In complete combustion, each methane molecule is converted into CO2 and two H2O. 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). A chemical radical is a molecule with an unpaired valence electron, often represented with the dot shown on the molecular formula. Such molecules are significantly reactive because the unpaired electron is less stable than if it were to gain or lose an electron.
Where does ·OH come from?
Before we tackle this question, let’s first look at where ozone (O3) comes from. We will start with the stratosphere (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).