POLIPHON conversion factors for retrieving dust-related cloud condensation nuclei and ice-nucleating particle concentration profiles at oceanic sites

Abstract

Abstract. Aerosol-Cloud interactions (ACI) result in the largest uncertainties in the global radiation budget so far. To improve the current consideration of ACI in global circulation models, it is necessary to characterize the 3-D distribution of dust-related Cloud Condensation Nuclei Concentration (CCNC) and Ice Nucleating Particle Concentration (INPC) globally. This can potentially be realized using the POLIPHON (POlarization LIdar PHOtometer Networking) method together with spaceborne lidar observations. However, dust-related conversion factors to convert bulk aerosol optical properties from lidar measurements to aerosol microphysical properties, are still less constrained in many regions, which limits the applications of POLIPHON method. Here we retrieve the essential dust-related conversion factors at the remote oceanic/coast sites using the historical AERONET (AErosol RObotic NETwork) databases. Depolarization-ratio-based dust ratios Rd at 1020 nm are applied to identify the dust-occurring cases so that it can be possible to contain fine-mode dust dominated cases (after the preferential removal of large-size dust particles during transport), study the evolution of dust microphysical properties along the transoceanic pathway, and mitigate occasional interference of large-size marine aerosols. The newly proposed scheme is proven to be valid and feasible by intercomparisons with previous studies at nine sites in/near the deserts. The dust-related conversion factors are calculated at 20 oceanic/coast sites using both PD (pure dust) and PD+DDM (dust-dominated mixture) datasets. At nearly half sites, the conversion factors are solely calculated using the PD data sets; while at the rest sites, the participation of DDM datasets is required to ensure enough data points in the calculation. Evident variation trends in conversion factors are found for cv,d (extinction-to-volume, gradually decrease), c250,d (extinction-to-particle (with radius >250 nm) number concentration, gradually increase) and cs,d (extinction-to-surface area concentration, plunge of decrease) along both the transpacific and transatlantic dust transport pathways. The retrieved dust-related conversion factors are anticipated in inversing 3-D dust-related CCNC and INPC distribution globally to improve the understanding of ACI in atmospheric circulation models.