• document the physical and chemical aerosol species and their characteristics affecting the region;
• determine the physical and chemical processes controlling these characteristics, the life cycles of the aerosol types and implications for radiative transfer;
• assess the regional direct and indirect aerosol radiative forcing in the region.

The major contribution of the MRF C-130 was to carry out three Lagrangian experiments. These involved tagging a parcel of air within the marine boundary layer (MBL) near the Portuguese coast from the ACE-2 ship with a perfluorocarbon inert tracer gas and constant-level smart balloons (fitted with GPS transponders). This tagged air parcel was then followed over three back-to-back flights that covered between 26-30 hours of evolution. This method is the only effective way of determining the processes influencing aerosol evolution, i.e. a Eulerian method of spot measurements in space and time cannot reveal the processes at work. The first Lagrangian was carried out in a very clean maritime air mass that had spent more than five days over the North Atlantic. The other two were within continental air masses originating over Europe.

 

Aerosol measurements

As an example of the data collected with the C-130, Fig. 1 below shows a series of three aerosol number-size distributions, one from each of the Lagrangians, covering the particle diameter (D) range 50 nm — 5.0 mm. The green curve shows a representative spectrum from the first Lagrangian. It can be seen immediately that concentrations are much lower than the other two polluted Lagrangians, except for particles smaller than 0.04 mm which may be a result of new nucleation in either the MBL or lower free troposphere (FT). The curve is bimodal with the Aitken mode and accumulation mode lying at 0.03 and 0.2 mm, respectively. The lower end of the coarse mode (consisting of sea-salt particles) can be seen above 1 mm. It is thought that this bimodality is the result of cloud processing with particles above this dip providing the cloud condensation nuclei (CCN) in stratocumulus and weak cumulus clouds.

The red curve shows a representative aerosol size distribution from the second Lagrangian. Concentrations are higher than the first Lagrangian at sizes above 0.03 mm. This is because of the source of particles over the European continent. The spectrum is bimodal with the Aitken and accumulation modes at 0.06 and 0.03 mm, respectively. These modes, together with the Hoppel dip in between, lie at a greater size than during the first Lagrangian. This is probably because the maximum supersaturation at cloud base is lower in a polluted cloud than a clean one as a result of more activating cloud droplets depleting the available water vapour and so limiting the attainable supersaturation. Therefore, the critical cut-off size for CCN tends to increase in a polluted cloud.  In the first and second Lagrangians, the aerosol spectrum in number terms is dominated by the Aitken mode.

The blue curve shows a representative aerosol size distribution from the third Lagrangian. One broad mode can be seen peaking at 0.1 mm, although there is a hint of a bulge at larger sizes that constitutes the accumulation mode as particles grew slowly by coagulation and other mainly non-cloudy processes over a number of days. The accumulation mode does not protrude to sizes as large as the second Lagrangian, probably due to the lack of cloud processing.

 

Continental outbreak

We have been able to put together a generalised description of a European continental outbreak of pollution over the sub-tropical North Atlantic from all the ACE-2 activities. Figure 2 shows a schematic diagram of the main features of such an event:

Figure 2. Main features of a continental outbreak of pollution over the sub-tropical North Atlantic

Over the continent, the surface heating in the summer months produces a deep (2 to 5 km) convectively-driven continental boundary layer (CBL). The anthropogenic pollution produced over the continent is relatively well mixed throughout the CBL. When this is advected over the relatively cold sea, the surface fluxes cause a new MBL to form which rapidly grows as the air moves away from the coast. Having lost the strong surface forcing, the CBL collapses under the influence of the subsidence in the FT. The MBL grows within the pollution layer and as the top of the MBL is generally very stable a part of the pollution is trapped in a layer close to the surface. The pollution associated with the residual CBL above the top of the developing MBL will be differentially advected away as there is generally a vertical wind shear associated with the temperature inversion capping the MBL.

Eventually, the subsidence in the FT limits the growth of the MBL (1 to 2 km) and clean tropospheric air can be entrained into the MBL. It is this process which has the greatest effect on diluting the pollution in the MBL and assisting in the evolution of the continental air mass into a clean maritime air mass. Washout of the pollution by drizzle produced by clouds in the MBL is thought to be minimal as the high levels of pollution and soluble aerosol limit the size of the cloud droplets below the size where coalescence can be effective.

The evolution of the MBL also has a modifying effect on the evolution of the aerosol characteristics. As the air moves over progressively warmer seas the MBL deepens and moisture builds up and stratocumulus clouds form at the top of the MBL. If the MBL gets deep enough, the generation of turbulent kinetic energy, either by wind shear or cloud top cooling, may not be enough to maintain a well-mixed MBL. The cloud layer then becomes decoupled from the moisture source at the surface. The MBL can become layered with a well-mixed surface layer and a layer containing the cloud. Moisture builds up in the surface mixed layer (SML), it becomes conditionally unstable and cumulus clouds will grow from the top, which will help maintain the stratocumulus layer by locally supplying it with moisture.