Closing the Aerosol Forcing Uncertainty Gap

Abstract:

In the past 20 years, we have learned much about the spatial distribution and temporal variations of airborne particles from satellite remote sensing and aerosol transport modeling. Yet the estimated uncertainties of aerosol direct and cloud-mediated (indirect) aerosol forcing remain virtually unchanged. Extensive aerosol remote-sensing measurement is limitated in the vicinity of clouds, over bright surfaces, covering the diurnal cycle and especially at night, so regional-to-global scale aerosol forcing must be calculated from models. Model diversity offers a clear measure of the current state. Aside from future anthropogenic activity, aerosol direct and indirect forcing still represent the largest uncertainties in climate prediction modeling according to the IPCC.

Among climate forcing model inputs, particle microphysical properties lead the list of poorly constrained quantities. Particle light-absorption can be constrained with space- or surface-based multi-angle and polarimetric remote sensing, but not with sufficient precision so aerosol forcing uncertainty is comparable to that of other factors. Particle hygroscopicity, needed to interpret ambient aerosol optical depth (AOD) retrieval results and required to model aerosol-cloud-interactions, cannot be retrieved remotely. Similarly, remote sensing cannot distinguish the small-size end of the cloud condensation nucleus (CCN) size distribution from atmospheric gas molecules. Even particle mass extinction efficiency (MEE), required to apply satellite derived AOD to climate and air quality models that bookkeep aerosol mass, cannot be retrieved from space.

Complementing the satellite and modeling contributions to the global aerosol forcing picture, a robust program of systematic aircraft in situ measurements capturing the airborne particle microphysical properties unobtainable from space is required to close the aerosol forcing uncertainty gap. Such a program would exceed typical field campaign efforts in duration and scope but could be accomplished with a relatively small aircraft flying routinely two-to-three times per week on a continuing basis. Such a program is feasible because, even as emission amounts might vary, particle microphysical properties tend to be repeatable for a given aerosol source in a given season.