A photochemical trajectory model was used to reproduce the concentrations of several trace gases measured at Iza¿a/Tenerife during a selected episode of August 1993 with long-range transport from the North American continent across the Atlantic Ocean. The set of initial conditions necessary to match the observations is analyzed and compared with available data in the literature. Initial and final concentrations of several hydrocarbons are in reasonable agreement with other measurements made at similar latitudes, and the CO/NOy and NMHC/NOy ratios are within the range of those observed from airborne measurements over the eastern United States and Canada. Important exceptions are the light olefines ethene and propene which may originate from oceanic emissions. The simulated concentration of ozone at Iza¿a is very similar to the initial concentration used, and there is no indication for photochemical net ozone production during long-range transport. The abundance of NOx at the end of the simulation is largely due to decomposition of peroxyacetyl nitrate during the descent of the air before reaching Iza¿a. The results differ significantly from those of a steady state calculation which was made for comparison, and the height evolution of the trajectory was found to be a critical parameter. A parameter variation study revealed that significant net ozone production only occurs for very low initial ozone concentrations and/or NOy levels that are well above those observed during the period at Iza¿a at night. This study also emphasizes the important role of water vapor for the tropospheric ozone budget and the role ozone itself plays in its photochemical balance. ¿ 1998 American Geophysical Union