Carbon monoxide (\(\text{CO}\)) is an invisible, odorless gas. This makes the gas insidious, as people cannot sense its presence. \(\text{CO}\) is recognized as a major atmospheric pollutant, playing a role in air quality and atmospheric chemistry. Its reactivity directly affects the concentration of other important atmospheric compounds. Unlike stable greenhouse gases that persist for centuries, \(\text{CO}\) is a relatively transient compound in the Earth’s lower atmosphere, or troposphere.
Atmospheric Residence Time of Carbon Monoxide
The atmospheric residence time determines how long carbon monoxide stays in the atmosphere. This term refers to the average duration a molecule remains in the atmosphere before removal by a chemical reaction or physical process. Globally, \(\text{CO}\) has a relatively short lifespan, typically ranging from a few weeks to a few months. The accepted global average lifetime is often cited as being between one and three months.
This short lifespan prevents the gas from becoming uniformly mixed globally. Instead, \(\text{CO}\) concentrations are highly variable, often concentrated near production regions. Despite its transient nature, the gas persists long enough to be transported over great distances by global wind patterns. This transport allows pollution plumes originating from one continent to impact air quality on another continent across an ocean.
Global Sources of Carbon Monoxide
Carbon monoxide enters the atmosphere from both human activities and natural processes. The primary origin of \(\text{CO}\) is the incomplete combustion of carbon-containing materials, which results from a lack of sufficient oxygen during burning. In urban and industrialized areas, the main sources are overwhelmingly anthropogenic. These include emissions from motor vehicle exhaust, industrial processes, and residential heating where carbon-based fuels are burned.
Globally, the single largest source of carbon monoxide is the atmospheric oxidation of methane (\(\text{CH}_4\)) and other naturally emitted volatile organic compounds (VOCs). These compounds form \(\text{CO}\) as a byproduct when they react chemically in the air. Natural events such as wildfires and volcanic eruptions also release substantial amounts of the gas. While biomass burning is often influenced by human activity, large-scale natural wildfires remain a significant global source of \(\text{CO}\).
Chemical Removal Processes in the Atmosphere
The short residence time of carbon monoxide is determined by its extremely efficient removal mechanism in the atmosphere. The gas is mostly destroyed through a chemical reaction with a molecule known as the hydroxyl radical (\(\text{OH}\)). The \(\text{OH}\) radical is a highly reactive molecule often referred to as the “detergent of the atmosphere” because it cleanses the air of many pollutants.
The reaction with \(\text{OH}\) converts the pollutant carbon monoxide into carbon dioxide (\(\text{CO}_2\)). This overall reaction is the dominant “sink,” or removal pathway, for \(\text{CO}\). A secondary, much smaller removal process also occurs when carbon monoxide is absorbed directly by microorganisms found in soil. These soil bacteria metabolize the gas, contributing a minor fraction to the total global removal budget.