A good description of the atmospheric aerosols is needed in order to address at some environmental issues, mainly the role of aerosols in the Earth radiative balance and consequently the aerosol effect on global warming. The uncertainty on the radiative forcing due to suspended particulate matter is indeed large in magnitude (-0.6±0.4 Wm-2 for direct effect; IPCC, 2007) if compared to the understanding of radiative forcing due to greenhouses gases such as CO2 (1.8±0.2 Wm-2; IPCC, 2007). This value and its related uncertainty are large and must be more accurately determined before the actual climate models can be used reliably to predict changes in the future climate.
Compared to greenhouse gases, a great deal of these uncertainties is stemmed from the spatial and temporal inhomogeneities of the aerosols properties. This variability is mainly related to their short average life (between 5 and 10 days) and the heterogeneous distribution of sources and their strengths (for example urban and industrial areas), but also to complex interactions with the atmosphere and the sun radiation.
Aerosols contribute to the climate forcing through different mechanisms: on one hand, they interact with the incident solar radiation by reflecting it back to space or by absorbing it. This direct interaction is in fact strongly dependent on the origin of the particles and the way they are produced and aged. The net effect they produce (cooling or heating the atmosphere) depends on their optical properties. In turn, these optical properties are difficult to measure and model.
On the other hand, they also act as condensation nuclei, modifying the properties of the clouds. Cloud properties are very sensitive to changes in their mean droplet size and number, affecting in turn the interaction with down welling and upwelling radiation (indirect effect). Change in cloud properties also affects precipitation efficiency.
In fact the main concern is addressed to anthropogenic aerosol emission. Usually the urban and industrial areas are the strongest sources of anthropogenic aerosols in Europe. The rising global economic development brings an increase of population in urban environments. Therefore, aerosol monitoring activities in/around urban areas are key for understanding the anthropogenic forcing on the climate in Europe.
For studying the radiative forcing of aerosols, the sky – sun photometry (or solar aureole) technique is the most accurate. Sky - sun photometry is a well known technique for measuring aerosol properties in the atmospheric column. It consists of measuring two different variables at ground level: direct irradiance from the sun, and diffuse radiance from the sky.
These measurements are performed for a set of different wavelengths in the UV-VIS-NIR bands, but mainly centered in the VIS and selected in order to minimize the absorption of atmospheric gases (such as ozone, water vapor, NO2, CO2…). The sky radiance measurements are taken from a section of the sky dome in order to simplify the computation (the so called almucantar plane, defined by a constant zenithal angle, and a variable azimuthal angle). This almucantar plane is chosen to cross the sun, so we can later combine the direct and diffuse radiance measurements.
Once the measurements are compiled, an inversion algorithm is applied in order to retrieve aerosol optical and radiative properties. The inversion algorithm cancels the multi scattering effects from the sky radiance through the application of a radiative transfer model. This model is also used for retrieving the aerosol parameters, by minimizing the difference between the measurements and the simulated outputs.
In this way, the called aureole technique can retrieve a rather complete set of aerosol optical and physical parameters representative of the whole column: aerosol single scattering albedo, optical depth, scattering phase function, size distribution and refractive index, only by means of ground based measurements of solar direct and diffuse radiation.
In the ESR websites ring, we automatically provide retrievals of these properties for the ESR network sites. The sun and sky radiance measurements are automatically stored, formatted, inverted, filtered and postprocessed. Then they are stored in a instantaneous, daily and monthly databases, that are retrieved in the websites as plots (the raw and processed databases are only available by direct request to the site responsable).