ClOOCl

ClOOCl, also written Cl2O2, is the chlorine monoxide dimer, a reactive, intermediate species in stratospheric ozone chemistry. It consists of two ClO units connected by an O–O bond and is formed when two ClO radicals combine.

Formation and photochemistry: In the gas phase, ClO radicals recombine to form ClOOCl via ClO + ClO → ClOOCl. Under sunlight, ClOOCl absorbs solar radiation and photolyzes to yield two ClO radicals: ClOOCl + hv → 2 ClO. This photodissociation regenerates reactive chlorine monoxide and closes a subset of catalytic cycles that destroy ozone.

Role in ozone depletion: The ClOOCl dimer acts as a reservoir for chlorine in the stratosphere. Its formation stores reactive chlorine as ClOOCl, and its daylight photolysis releases ClO, feeding the catalytic destruction cycle of ozone that involves ClO, O3, and related species. The chemistry of ClOOCl is especially relevant to polar stratospheric regions during spring, when sunlight returns and ozone loss accelerates.

Detection and significance: ClOOCl is detected in the atmosphere by spectroscopic methods from satellites, balloons, and ground-based instruments. Its absorption of light in the near-UV and visible ranges enables its photolysis in daytime conditions. Laboratory studies of its photolysis rates and reactions are used to improve atmospheric models of ozone depletion and to understand the timing and magnitude of chlorine-catalyzed ozone loss.

See also: Ozone depletion, ClO radical, chlorine-catalyzed ozone destruction cycles.