title: Ionization of N2O4 in contact with water: mechanism, time scales and atmospheric implications.
abstract: Ionization of N(2)O(4) in and on thin water films on surfaces is believed to be a key step in the hydrolysis of NO(2) which generates HONO, a significant precursor to the OH free radical in the lower atmosphere. Molecular dynamics simulations using "on the fly" high-level MP2 potentials are carried out for ONONO(2) x (H(2)O)(n) clusters, n < or = 8, used to mimic the surface reaction, in order to investigate the ionization process and determine its time-scale and mechanism around room temperature. The results are (i) the isolated molecule does not convert to the NO(+)NO(3)(-) ion pair, even for long times; (ii) for ONONO(2) x (H(2)O)(n) with n = 1 and 2, ionization takes place in several picoseconds; (iii) for n > or = 3, ionization is essentially immediate, implying that the neutral species does not have sufficient lifetime to be considered a significant intermediate in the reaction; and (iv) even at ice temperatures, T < or = 250 K, ionization for n > or = 3 is immediate. The implications for hydrolysis of oxides of nitrogen on surfaces in the atmosphere are discussed.
authors.affiliation: Department of Physical Chemistry, and The Fritz Haber Research Center, Hebrew University, Jerusalem 91904, Israel.
authors.forename: Yifat
authors.initials: Y
authors.lastname: Miller
authors.forename: Barbara J
authors.initials: BJ
authors.lastname: Finlayson-Pitts
authors.forename: R Benny
authors.initials: RB
authors.lastname: Gerber