Peroxynitrous acid (HNO₃) is a reactive nitrogen-containing species (RNS). It is the conjugate acid of peroxynitrite (ONOO). It has a pK_a of ~6.8. It is formed in vivo from the diffusion-controlled reaction of nitrogen monoxide (NO) and superoxide (O•−
2). It isomerises with k = 1.2 s, 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.