How NO2 and OH Combine (IMAGE) California Institute of Technology Caption Nitrogen dioxide and the hydroxyl radical combine in the atmosphere to make either nitric acid or peroxynitrous acid; the so-called branching ratio of these two chemicals is important in models of ozone production. Credit Caltech/Mitchio Okumura Usage Restrictions With credit as given above License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.