Five integrated field projects were performed in the sub-tropical North East Atlantic during June and July of 1997. They were supported by continuous monitoring of the aerosol properties and trace gas precursors, and by a smaller scale pre-campaign on Tenerife in 1996. The basic framework of this experiment, entitled ACE-2, was funded by the European Union. Collaboration with other European (EU funded) and US groups enabled a much more comprehensive experiment, which encompassed the objectives of ACSOE ACE. The strategy of ACE-2 was implemented in a series of complimentary activities.
ACSOE's major contribution was to two of the experiments designed to look at the modification of cloud microphysics in relation to the aerosol entering cloud and particularly to the changes in the aerosol properties resulting from the chemical processing of aerosol by and in the vicinity of cloud.
In the first experiment, HILLCLOUD, a hill cap cloud which forms over a ridge on the NE of the island of Tenerife was used as a natural flow through reactor. An additional objective of this experiment was to characterise the size distribution, size dependent chemical composition and hygroscopic properties of the marine and modified continental aerosol arriving at the North coast of the island. The trace gas concentrations at the same site were similarly characterised.
The second experiment, LAGRANGIAN, was a Lagrangian experiment which looked at the modification of aerosol as it is advected away from the Portuguese coast over the Atlantic ocean in the cloudy Marine Boundary Layer (MBL). In this experiment the UK Meteorological Office C-130 followed and sampled the same air parcel in order to investigate the evolution of the aerosol and stratocumulus cloud microphysics, as the aerosol in the parcel was subjected to repeated cloud processing. In doing so, each of the processes studied in detail for a single cloud pass in the HILLCLOUD experiment were investigated.
These experiments were conducted in a variety of airmasses including those which had a long trajectory over the North Atlantic and which contained background levels of pollution (e.g. from North America), in airmasses which crossed anthropogenic sources over Europe, and finally, in airmasses which crossed the Sahara desert and which were transporting large amounts of desert dust into the north Atlantic region. More than one of these airmasses may be found at the same time at different levels in the atmosphere, and hence a detailed treatment of the entrainment process was essential. Modelling of the boundary layer structure, the dynamics, the cloud formation processes, and the homogeneous and heterogeneous chemical processes were used to interpret the data in both experiments.
Complementary to these studies was a series of C-130 flights through hill cap clouds on Great Dun Fell, UK and during ‘round-Britain’ flights to examine the effect on aerosols, CCN, and cloud particles during passage of air masses across the UK (MODCLOD).