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OH, the detergent of the atmosphere, and OH-, the ion that cleanses our homes’ air

Did you know that earth’s atmosphere is self-cleaning, to an extent?  We would be A LOT worse off if it wasn’t.

OH, the hydroxyl radical, is the most important oxidizing species in the atmosphere.  In this article, we’re going to discuss how it’s formed in nature, what it does, and how it’s different from the hydroxide ion OH- that is formed in bipolar ionizers.  

You’ve probably heard that there is ozone in the earth’s atmosphere.  The majority of ozone is found in the stratosphere (about 10-25 miles above the earth), shielding us from the sun’s UV light and cosmic radiation.  This is where ozone can be destroyed by molecules that contain chlorine and bromine, such as chlorofluorocarbons (CFCs). (EPA.gov) About 10% of the ozone is found a bit lower, however, in the troposphere (where clouds are formed and planes fly).  In the troposphere, ozone performs a very important function by being a primary ingredient for the production of OH.  Here, UV energy from the sun (mostly in the UV-B range of 290-310nm)(Treatise on GeoChemistry, ch.5.5.9.1 Chemistry of the Hydroxyl Radical (OH) in the Troposphere), breaks down ozone (O3) into O + O2.  Then, in the presence of water vapor (there’s very little water vapor in the stratosphere, so this has to happen in the troposphere), the lone O molecule reacts with H2O to form 2 molecules of OH (hydroxyl radical).  

In chemistry, a radical, also called free radical, is a molecule that contains at least one unpaired electron.  OH is a radical which is highly reactive because of the configuration of electrons in its outermost shell.  Normally, atoms and molecules prefer to have 8 electrons in their outermost shell, making them most stable (called the Octet Rule), but they will compromise and share electrons if necessary.  OH has 7 electrons in its outer shell: 6 electrons are from the O atom and 1 electron from the H atom.  Each electron has a negative charge, but it is balanced by the same number of protons in the nuclei of the atoms, so that the total “charge” of the molecule remains neutral.  Electrons also like to be “paired”, and although each has a negative charge, they have opposing spin directions which causes them to seek to be “paired” with another electron.  The OH molecule constantly seeks one more electron to “pair” with the 7th electron in its outer shell.  OH only survives for nanoseconds after it is formed–because it can immediately steal that missing electron from most of the chemicals found in the troposphere.  This reaction of the OH with other molecules is called oxidation. 

(Oxidation: Despite the name, the presence of oxygen is not a requirement in an oxidation reaction.  The reaction is part of a transfer of electrons between two substances.  Oxidation occurs simultaneously with reduction in a type of chemical reaction called a reduction-oxidation or “redox” reaction.  The oxidized atom loses electrons, while the reduced atom gains electrons.  On earth, oxidation is usually an undesirable reaction.  Oxidation is another name for rust, corrosion, and breakdown of materials around us and in us.  Our bodies produce “anti-oxidants” to prevent breakdown of our cells. ) 

There are limitless reactions that can happen in the atmosphere, but OH reacts primarily with carbon monoxide (40%) to form carbon dioxide. Around 30% of the OH produced is removed from the atmosphere in reactions with organic compounds and 15% reacts with methane (CH4). The remaining 15% reacts with ozone (O3), hydroperoxy radicals (HO2) and hydrogen gas (H2). (Oxidation and OH Radicals)  With its supreme oxidation potential, hydroxyl radicals can react with molecules and chemicals that are otherwise extremely stubborn and resist oxidation. (Hydrogenlink.com)

Since OH is primarily formed with energy from the sun, OH production mainly happens during daylight hours.  The following map is a snapshot of a model showing how OH is generated as sunlight illuminates a rotating earth.  (The Atmosphere: Earth’s Security Blanket)  Because OH is so short-lived, it’s really hard to detect, so the formation or degradation of other chemicals is used to determine how much OH is in the atmosphere at any one time.  For example, this model is generated from the Tropospheric Emissions Spectrometer (TES) equipment on a NASA satellite.  TES measurements of a number of other chemical elements influenced by OH, such as ozone, carbon monoxide and nitrogen dioxide, have enabled scientists to better represent OH in these models.

Did you know that humans also generate OH indoors? (Science Daily)  Indoor air can have higher (but not dangerous) levels of ozone, which reacts with certain oils on our skin. The reaction releases a host of gas phase chemicals containing double bonds that react further in the air with ozone to generate substantial levels of OH radicals.  It’s a very new discovery (2022), which was aided with extensive computer modeling.  This is important to know, because although they are great to have in the upper level of the troposphere,  we don’t want high levels of hydroxyl radicals indoors.  They can damage tissue and frequently initiate chain reactions with other radicals and VOCs, able to produce harmful chemicals like formaldehyde.