OCl

OCl, in chemical notation, commonly refers to chlorine monoxide, a diatomic radical consisting of chlorine bonded to oxygen. In atmospheric chemistry, the species is usually written as ClO, but some sources use OCl as an alternative orientation of the same atoms. It is a highly reactive, short‑lived radical that plays a central role in ozone chemistry in both the stratosphere and troposphere.

Formation and reactivity: ClO forms primarily from reactions involving chlorine atoms and ozone, such as Cl + O3 → ClO + O2. It can also arise from photolysis of chlorine-containing reservoir species, including ClONO2 and Cl2O2, under solar radiation. ClO participates in catalytic cycles that destroy ozone, notably through reactions with atomic oxygen (ClO + O → Cl + O2) and by combining with other ClO units to form intermediates like ClOOCl, which contribute to further ozone depletion. The net effect of these pathways is the conversion of ozone and reactive oxygen species into less reactive forms, reducing stratospheric ozone concentration.

Occurrence and significance: ClO is most prominent in the upper atmosphere where chlorine chemistry driven by human-made compounds enhances ozone depletion, especially during polar spring, contributing to the ozone hole. Its abundance is controlled by photochemical activity, volcanic and anthropogenic chlorine sources, and atmospheric transport. ClO is a key indicator in atmospheric models and remote sensing studies aiming to understand and predict changes in ozone layer health.

Detection and study: ClO concentrations are measured with spectroscopic techniques from ground, airborne, and satellite platforms, including ultraviolet and infrared methods. The chemistry of ClO is fundamental to understanding human‑related impacts on ozone and the broader oxidative capacity of the atmosphere.