Effects of aircraft wake dynamics on measured and simulated NOx and HOx wake chemistry

High-resolution numerical large-eddy simulations of the near wake of a B757 including simplified NOx and HOx chemistry were performed to explore the e ffects of dynamics on chemistry in wakes of ages from a few seconds to seve ral minutes. Dilution plays an important basic role in the NOx - O-3 chemis try in the wake, while a more interesting interaction between the chemistry and dynamics occurs for the HOX species. These simulation results are comp ared with published measurements of OH and HO2 within a B757 wake under cru ise conditions in the upper troposphere taken during the Subsonic Aircraft Contrail and Cloud Effects Special Study (SUCCESS) mission in May 1996. The simulation provides a much finer grained representation of the chemistry a nd dynamics of the early wake than is possible from the 1 s data samples ta ken in situ. The comparison suggests that the previously reported discrepan cy of up to a factor of 20 - 50 between the SUCCESS measurements of the [HO 2]/[OH] ratio and that predicted by simplified theoretical computations is due to the combined effects of large mixing rates around the wake plume edg es and averaging over volumes containing large species fluctuations. The re sults demonstrate the feasibility of using three-dimensional unsteady large -eddy simulations with coupled chemistry to study such phenomena.