Monthly global maps have been developed (a) to define aerosol optical properties (also as a function of wavelength), (b) to distinguish between natural and anthropogenic contributions, and (c) to derive cloud-relevant estimates for CCN and CCN enhancement as a result of anthropogenic activities. In developing these maps, the preferential use of quality measurements from ground-based monitoring networks over pure model simulations was promoted. This study demonstrated the value of coordinated ground-based aerosol monitoring, especially when data were extended with modeling. Modeling and satellite retrievals will also benefit from complementary ground network data on aerosol, more accurate satellite datasets, or better model parameterization and simulations. Many sources have had to rely completely on modeling, because data were either unavailable or lacking in accuracy. Future efforts should explore new measurement-based dataset sources to constrain the freedom in

Figure 3.11 Monthly global maps showing anthropogenic enhancement of CCN concentrations over natural CCN at 0.1% supersaturation at 1 km above the ground.

modeling or to replace simulations. Examples are vertical profiling from active remote sensing.

The various global maps created for this chapter relied on many assumptions and applied often rather simple methods. There is obviously room for improvement and fine tuning; the values indicated for (tropospheric) aerosols were intended to provide general numbers on amount, spatial distribution, and seasonality. Despite providing estimates for CCN concentrations, the cloud-related aspect of aerosols remains unclear. Little is known about what happens in cloudy environments to aerosol or cloud particles as a result of interactions, as almost all aerosol data are associated with cloud-free conditions. New measurements (e.g., active remote sensing, high temporal resolution data) and modeling concepts (e.g., bridging modeling scales) are needed to move beyond questionable statistical associations.

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