Criegee Intermediates (CIs) are highly reactive, short-lived molecules that have emerged as significant players in Earth’s atmospheric chemistry. These species, chemically classified as carbonyl oxides, possess a unique electronic structure that makes them extremely powerful oxidizing agents in the air we breathe. Despite being first postulated over half a century ago, the direct observation and detailed study of CIs only became possible in the early 2010s, revolutionizing how scientists model the composition of the troposphere. The transient nature of these compounds means they participate in chemical reactions almost instantaneously, thereby influencing the formation of various secondary pollutants. Understanding the life cycle of Criegee Intermediates is important for accurately predicting air quality and the long-term impact of human and natural emissions on the global climate system.
The Genesis of Criegee Intermediates
The primary source of Criegee Intermediates in the atmosphere is a chemical process known as ozonolysis, which involves the reaction between ozone (\(\text{O}_3\)) and unsaturated hydrocarbons, or alkenes. These hydrocarbons are released into the air from both biogenic sources, like isoprene from trees, and anthropogenic sources, such as vehicle exhaust. When an ozone molecule encounters a carbon-carbon double bond in an alkene, it quickly attaches itself in a highly exothermic reaction.
This initial attachment forms an unstable, ring-shaped structure called a primary ozonide, or molozonide, which exists for only a fraction of a second. This molozonide rapidly breaks apart due to its inherent instability. The decomposition yields two distinct fragments: a stable carbonyl compound, which is typically an aldehyde or a ketone, and the highly energetic carbonyl oxide itself, the Criegee Intermediate.
The concept of this intermediate was first proposed in the 1950s by German chemist Rudolf Criegee, whose mechanism provided a comprehensive explanation for the products of ozonolysis reactions.
Highly Reactive Atmospheric Players
The extreme reactivity of Criegee Intermediates stems from their unique electronic configuration, which can be described as having both zwitterionic and biradical character. This structure means the molecule possesses separated positive and negative charge centers, making it highly susceptible to reaction with other atmospheric species. The nascent Criegee Intermediate is incredibly short-lived, often existing for only milliseconds.
A significant portion of the CIs formed initially are chemically activated with high internal energy, leading to rapid unimolecular decay pathways. This decay often results in the formation of hydroxyl radicals (\(\text{OH}\)), which are powerful oxidizers that play a major role in cleaning the troposphere.
However, a fraction of these intermediates loses excess energy through collisions with nitrogen and oxygen molecules, forming “stabilized” Criegee Intermediates (sCIs) that can survive long enough to participate in bimolecular reactions. These stabilized intermediates drive many secondary atmospheric processes by reacting with common atmospheric gases. The most important reaction partners include water vapor (\(\text{H}_2\text{O}\)), sulfur dioxide (\(\text{SO}_2\)), and various nitrogen oxides (\(\text{NO}_x\)). Reaction with water vapor is a major sink for CIs globally, forming hydroxy-hydroperoxides, especially in humid environments. The reaction rate constants for the simplest CIs with water vapor span a wide range, but they are consistently fast enough to significantly limit the CIs’ atmospheric lifetime.
Influence on Air Quality and Climate
The reactions of Criegee Intermediates have measurable consequences for both regional air quality and the global climate system. Their ability to rapidly oxidize certain trace gases allows them to influence the formation of secondary pollutants that directly affect human health and environmental conditions. The reaction with sulfur dioxide (\(\text{SO}_2\)) is perhaps the most consequential of their atmospheric processes.
Criegee Intermediates react with \(\text{SO}_2\) to produce sulfur trioxide (\(\text{SO}_3\)), which then quickly combines with water vapor to form sulfuric acid (\(\text{H}_2\text{SO}_4\)). This pathway is significant because it represents a non-photolytic, or daylight-independent, source of sulfuric acid, challenging the long-held view that the \(\text{OH}\) radical was the sole gas-phase mechanism for \(\text{SO}_2\) oxidation. In certain environments, particularly those rich in biogenic alkenes, the CI pathway can dominate the production of \(\text{H}_2\text{SO}_4\).
The newly formed sulfuric acid is a major precursor for the nucleation and growth of sulfate aerosols, which are tiny airborne particles. These aerosols have a direct impact on public health, as fine particulate matter is linked to respiratory illnesses and cardiovascular problems. They also contribute to atmospheric haze and are a component of acid rain. From a climate perspective, the sulfate aerosols generated through CI chemistry scatter incoming solar radiation back into space, exerting a cooling influence on the planet.
Furthermore, CIs react with nitrogen oxides, such as nitrogen dioxide (\(\text{NO}_2\)), and various organic acids, leading to the formation of organic nitrates and other oxidized compounds. These reactions contribute to the formation of secondary organic aerosols (SOA), which are complex mixtures that affect air quality and climate. The overall impact of Criegee Intermediates is to significantly increase the oxidative capacity of the atmosphere, driving the removal of certain pollutants while simultaneously creating new ones.