Lagrangian evolution of an aerosol column during the Atlantic Stratocumulus Transition Experiment

Two Lagrangian experiments were carried out during the Atlantic Stratocumulus Transition Experiment (ASTEX) with the intent of looking at aerosol evolution in the marine boundary layer (MBL). The second Lagrangian (L2) took place below broken stratus clouds and was more successful since little precipitation reached the surface. Aerosol below the inversion was primarily an aged pollution aerosol from central Europe. Vertical variability in aerosol concentrations was generally characterized by highest concentrations in the moist surface and transition layers. Concentrations were a factor of 2 or more lower in the dry subcloud layer and cloud layer and dropped to less than one tenth of surface values in the free troposphere above cloud. This behavior reflected the decoupled boundary layer below the main inversion and complicated the assessment of Lagrangian aerosol evolution. No evidence for new particle formation was observed during L2, and aerosol evolution proceeded only by mixing, coagulation, and removal mechanisms. An entrainment rate of about 0.6 cm s(-1) from the free troposphere into the MBL was a key parameter affecting aerosol evolution and resulted in about a 35% column dilution during the 34-hour L2 measurement period. Aerosol evolution in the decoupled subcloud and surface marine layer is consistent with an entrainment rate of about 0.45 cm s(-1) into the surface layer and about 0.25 cm s(-1) out of the layer. The ability to resolve the effects of separate processes influencing both gas and aerosol during Lagrangian evolution will depend upon (1) an adequate assessment of the variability in the air mass, (2) the ability to characterize this variability relative to the uncertainties in resampling the air mass, and (3) the extent to which the substantial changes due to entrainment alone can be reliably determined.