The rate of formation of pollutants including ozone, nitric acid, and fine particulate nitrate is influenced by the reaction of hydroxyl radical with nitrogen dioxide to form nitric acid (HONO₂). The rate coefficient for this reaction is poorly defined due to practical limitations and the presence of a secondary pathway which forms peroxynitrous acid (HOONO).

A collaboration led by Mitchio Okumura, California Institute of Technology, USA, has used cavity ringdown spectroscopy to detect and monitor the formation of both HONO₂ and HOONO in real time. The rate constant for HONO₂ formation was shown to be lower than previously established, at 9.2 (±0.4) x 10^–12 cm³ molecule^–1 s^–1 at 760 torr of air.

Using the new rate constant, the team applied a 3D Eulerian photochemical model to predict ozone formation in southern California under summertime conditions. Their results suggest that current models underestimate ozone levels by between 5 to 10 % when pollution is at its highest.

• Rate of Gas Phase Association of Hydroxyl Radical and Nitrogen Dioxide
  A. K. Mollner, S. Valluvadasan, L. Feng, M. K. Sprague, M. Okumura et al.,
  Science 2010, 330 (6004), 646–649.
  DOI: 10.1126/science.1193030