2.4.2 Developments Related to Aerosol Observations
Surface-based measurements of aerosol properties such as size distribution, chemical composition, scattering and absorption continue to be performed at a number of sites, either at long-term monitoring sites, or specifically as part of intensive field campaigns. These in situ measurements provide essential validation for global models, for example, by constraining aerosol concentrations at the surface and by providing high-quality information about chemical composition and local trends. In addition, they provide key information about variability on various time scales. Comparisons of in situ measurements against those from global atmospheric models are complicated by differences in meteorological conditions and because in situ measurements are representative of conditions mostly at or near the surface while the direct and indirect RFs depend on the aerosol vertical profile. For example, the spatial resolution of global model grid boxes is typically a few degrees of latitude and longitude and the time steps for the atmospheric dynamics and radiation calculations may be minutes to hours depending on the process to be studied; this poses limitations when comparing with observations conducted over smaller spatial extent and shorter time duration.
Combinations of satellite and surface-based observations provide near-global retrievals of aerosol properties. These are discussed in this subsection; the emissions estimates, trends and in situ measurements of the physical and optical properties are discussed with respect to their influence on RF in Section 2.4.4. Further detailed discussions of the recent satellite observations of aerosol properties and a satellite-measurement based assessment of the aerosol direct RF are given by Yu et al. (2006).