Optical excitation induces a transition in NO2 from a ground state vertical bar a> to an excited state vertical bar b>. The state vertical bar b> is not stable. The molecule evolves irreversibly from vertical bar b> to a state vertical bar c> in a time tau(0) approximate to 3 mu s. We investigate the transition from vertical bar b> to vertical bar c> by analysing results of molecular beam and static gas experiments, and we conclude that the irreversible evolution is either an intramolecular radiationless transition of a symmetric to an asymmetric configuration of the isolated NO2 or the effect of a background cosmic field inducing a parity nonconserving transition from vertical bar b> to vertical bar c>. (C) 2015 Elsevier B.V. All rights reserved.