BIDIRECTIONAL MIXING IN AN ACE-1 MARINE BOUNDARY-LAYER OVERLAIN BY A 2ND TURBULENT LAYER

In the Lagrangian B flights of the First Aerosol Characterization Expe riment (ACE 1), the chemistry and dynamics of the postfrontal air mass were characterized by tracking a constant-level balloon launched into the air mass for three consecutive 8-hour flights of the instrumented National Center for Atmospheric Research C-130 aircraft during a 33-h our period. The boundary layer extended to a height of 400 to 700 m du ring this period, with its top;defined by changes in the amount of tur bulent mixing measured rather than by an inversion. Above the planetar y boundary layer to a height of 1400 to 1900 m, a second layer was cap ped with a more pronounced temperature inversion and contained only in termittent turbulence. Since this layer served as a reservoir and mixi ng zone for boundary layer and free tropospheric air, we have called i t a buffer layer to emphasize its differences from previous concepts o f a residual or intermediate layer. Estimates of the entrainment rate of dimethyl sulfide (DMS) and aerosol particles between the boundary l ayer and the buffer layer demonstrated that exchange occurred across t he interface between these two layers in both upward and downward dire ctions, In situ measurements of aerosol particles revealed highly conc entrated, nucleation-mode aerosol particles between 10 and 30 nm diame ter at the beginning of the first Lagrangian B flight in the buffer la yer, while few were present in the boundary layer. Observations during the second and third flights indicate that aerosol particles of this size were mixing downward into the boundary layer from the buffer laye r while DMS was transported upward. This fortuitous enhancement of aer osol particles in the buffer layer allowed simultaneous use of DMS and aerosol particle budgets to track the bidirectional entrainment rates . These estimates were compared to those from measurements of mean ver tical motion and boundary layer growth rate, and from estimates of the fluxes and changes in concentration across the layer interface. In ad dition, three different techniques were used to estimate DMS emission rates from the ocean surface and showed good agreement: (1) evalulatio n of the DMS and aerosol mean concentration budgets, (2) seawater DMS concentrations and an air-sea exchange velocity, and (3) the mixed-lay er gradient technique.