Peroxynitrous acid (HNO<sub>3</sub>) is a reactive nitrogen species (RNS). It is the conjugate acid of peroxynitrite (ONOO<sup>âÂÂ</sup>). It has a pK<sub>a</sub> of approximately 6.8. It is formed in vivo from the diffusion-controlled reaction of nitrogen monoxide (ON<sup>â¢</sup>) and superoxide (). It is an isomer of nitric acid and isomerises with a rate constant of k = 1.2 s<sup>âÂÂ1</sup>, a process whereby up to 5% of hydroxyl and nitrogen dioxide radicals may be formed. It oxidises and nitrates aromatic compounds in low yield. The mechanism may involve a complex between the aromatic compound and ONOOH, and a transition from the cis- to the trans-configuration of ONOOH. Peroxynitrous acid is also important in atmospheric chemistry.