Aircraft- and ground-based assessment of the CCN-AOD relationship and implications on model analysis of ACI and underlying aerosol processes

Yohei Shinozuka, Bay Area Environmental Research Institute Sonoma, Sonoma, CA, United States, Antony D Clarke, Univ Hawaii/Dept Oceanography, Honolulu, HI, United States, Athanasios Nenes, Ecole Polytechnique Federale de Lausanne, School of Architecture, Civil and Environmental Engineering (ENAC), Laboratory of Atmospheric Processes and their Impacts (LAPI), Lausanne, Switzerland, Terry L Lathem, Georgia Institute of Technology, Atlanta, GA, United States, Jens Redemann, University of Oklahoma, School of Meteorology, Norman, United States, Anne Jefferson, University of Colorado at Boulder, Boulder, CO, United States and Robert Wood, University of Washington, Department of Atmospheric Sciences, Seattle, United States
Abstract:
Contrary to common assumptions in satellite-based modeling of aerosol-cloud interactions, ∂logCCN/∂logAOD is less than unity, i.e., the number concentration of cloud condensation nuclei (CCN) less than doubles as aerosol optical depth (AOD) doubles. This can be explained by omnipresent aerosol processes. Condensation, coagulation and cloud processing, for example, generally make particles scatter more light while hardly increasing their number. This paper reports on the relationship in local air masses between CCN concentration, aerosol size distribution and light extinction observed from aircraft and the ground at diverse locations. The CCN-to-local-extinction relationship, when averaged over ~1 km distance and sorted by the wavelength dependence of extinction, varies approximately by a factor of 2, reflecting the variability in aerosol intensive properties. This, together with retrieval uncertainties and the variability in aerosol spatio-temporal distribution and hygroscopic growth, challenges satellite-based CCN estimates. However, the large differences in estimated CCN may correspond to a considerably lower uncertainty in cloud drop number concentration (CDNC), given the sublinear response of CDNC to CCN. Overall, our findings from airborne and ground-based observations call for model-based reexamination of aerosol-cloud interactions and underlying aerosol processes.
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