Hydroperoxy Radical - HO₂, this radical readily reacts with nitrogen oxides and hydrocarbons in the atmosphere; in urban atmospheres, the major source of the hydroperoxy radical is formaldehyde photolysis.
[Atmospheric Chemistry; Finlayson-Pitts and Pitts; 34; 1986; Wiley Publishers; New York]
[Journal of Chromatography; v579; 25; 1992.] 
Atmospheric Chemistry Glossary