The global aerosol synthesis and science project (GASSP) : Measurements and modeling to reduce uncertainty

The largest uncertainty in the historical radiative forcingof climate is caused by changes in aerosolparticles due to anthropogenic activity. Sophisticated aerosol microphysicsprocesses have been included in many climate models in an effort to reduce the uncertainty. However, the models are very challenging to evaluate and constrain because they require extensive in-situ measurements of the particle size distribution, numberconcentration and chemical composition that are not available from global satellite observations. The Global AerosolSynthesis and Science Project (GASSP) aims to improve the robustness of global aerosolmodels by combining new methodologies for quantifying model uncertainty, an extensive global dataset of aerosolin-situ microphysicaland chemical measurements, and new ways to assess the uncertainty associated with comparing sparse point measurements with low resolution models. GASSP has assembled over 45,000 hours of measurements from ships and aircraft as well as data from over 350 ground stations. The measurements have been harmonized into a standardized format that is easily used by modellers and non-specialist users. Available measurements are extensive, but they biased to polluted regions of the northern hemisphere, leaving large pristine regions and many continental areas poorly sampled. The aerosol radiative forcinguncertainty can be reduced using a rigorous model-data synthesis approach. Nevertheless, our research highlights significant remaining challenges because of the difficulty of constraining many interwoven model uncertainties simultaneously. Although the physical realism of global aerosolmodels still needs to be improved, the uncertainty in aerosol radiative forcingwill be reduced most effectively by systematically and rigorously constraining the models using extensive syntheses of measurements.