Global Distributions of Aerosol Types Determined Using CALIPSO Measurements and an Automated Aerosol Classification Scheme

One month of CALIPSO Level II data are analyzed to assess the veracity of the CALIPSO aerosol type identification algorithm and generate distributions of aerosol types and their respective optical characteristics. Aerosol classification has several purposes: attribution of aerosol radiative forcing to natural or anthropogenic emissions requires the determination of the source of the aerosol; aerosol radiative properties vary significantly by type; and, most directly, determination of aerosol type allows an estimate of extinction-to-backscatter ratio. In the CALIPSO classification scheme, each aerosol type is assumed to be a mixture of different species, where the mixing can be internal, external or both. The underlying paradigm is that the variety of emission sources and atmospheric processes will act to produce airmasses that can be characterized as consisting of a single, generic aerosol type. This is an idealization, but one that allows us to classify aerosols based on observations and location, and thus gain insight into the geographic distribution of aerosols and constrain the possible values of extinction-to- backscatter ratios for use in the CALIPSO aerosol extinction retrievals. The CALIPSO models define six aerosol types: desert dust, biomass burning, background, polluted continental, marine and polluted dust. While this set does not cover all possible aerosol mixing scenarios, it accounts for a majority of mesoscale aerosol layers. In essence the algorithm trades off complex transient multi-component mixtures for relatively stable layers with large horizontal extent (10-1000 km). For this initial assessment of algorithm performance, we produce global distributions of the CALIPSO aerosol types, along with the complementary distributions of integrated attenuated backscatter, backscatter color ratio, and volume depolarization ratio for each type. The aerosol type distributions are further partitioned according to various geophysical discriminators (e.g., geographic region, land vs. ocean, and day vs. night). For selected geographic regions, we compare the CALIPSO type distributions to distributions obtained from the MODIS aerosol products.