Observations of the evolution of the aerosol, cloud and boundary-layer characteristics during the 1st ACE-2 Lagrangian experiment

During the Ist Lagrangian experiment of the North Atlantic Regional Aerosol Characterisation Experiment (ACE-2), a parcel of air was tagged by releasi ng a smart, constant level balloon into it from the Research Vessel Vodyani tskiy. The Meteorological Research Flight's C-130 aircraft then followed th is parcel over a period of 30 h characterising the marine boundary layer (M BL), the cloud and the physical and chemical aerosol evolution. The air mas s had originated over the northern North Atlantic and thus was clean and ha d low aerosol concentrations. At the beginning of the experiment the MBL wa s over 1500 m deep and made up of a surface mixed layer (SML) underlying a layer containing cloud beneath a subsidence inversion. Subsidence in the fr ee troposphere caused the depth of the MBL to almost halve during the exper iment and, after 26 h, the MBL became well mixed throughout its whole depth . Salt particle mass in the MBL increased as the surface wind speed increas ed from 8 m s(-1) to 16 m s(-1) and the accumulation mode (0.1 mu m to 3.0 mu m) aerosol concentrations quadrupled from 50 cm(-3) to 200 cm(-3). Howev er, at the same time the total condensation nuclei (> 3 nm) decreased from over 1000 cm(-3) to 750 cm(-3). The changes in the accumulation mode aeroso l concentrations had a significant effect on the observed cloud microphysic s. Observational evidence suggests that the important processes in controll ing the Aitken mode concentration which, dominated the total CN concentrati on, included, scavenging of interstitial aerosol by cloud droplets, enhance d coagulation of Aitken mode aerosol and accumulation mode aerosol due to t he increased sea salt aerosol surface area, and dilution of the MBL by free tropospheric air.