For the determination of the concentration of atomic nitrogen at atmospheric pressure and temperature, chemical titration with molecular oxygen has been studied by spectroscopic experiments and chemical simulations. The formation of oxygen atoms (by the reaction between nitrogen atoms and molecular oxygen) leads to the rapid production of ozone which can be easily observed and quantified using its strong ultraviolet absorption. Chemical simulations indicate that, although the amount of produced ozone provides only a lower limit of the atomic nitrogen concentrations, the observed ozone concentrations are only 10-15% below the atomic nitrogen concentrations over several orders of magnitude. (c) 2005 Elsevier B.V. All rights reserved.